D. A. Perala
Quaking aspen (Populus tremuloides) is the most widely distributed tree in North America. It is known by many names: trembling aspen, golden aspen, mountain aspen, popple, poplar, trembling poplar, and in Spanish, álamo blanco, and álamo temblón (49). It grows on many soil types, especially sandy and gravelly slopes, and it is quick to pioneer disturbed sites where there is bare soil. This fast-growing tree is short lived and pure stands are gradually replaced by slower-growing species. The light, soft wood has very little shrinkage and high grades of aspen are used for lumber and wooden matches. Most aspen wood goes into pulp and flake-board, however. Many kinds of wildlife also benefit from this tree.
General: Willow Family (Salicaceae): This is a native tree 5-30 m high, typically less than 15 m, with a rounded crown; lateral roots may extend over 30 meters and vertical sinker roots from the laterals may extend downward for nearly 3 m; bark typically smooth, greenish-white to gray-white, often thin and peeling, becoming thicker and furrowed with age, especially toward the base. Leaves simple, deciduous, broadly ovate to nearly round, 4–6 cm long, with small, rounded teeth on the margins, on a slender, flattened petiole, dark green and shiny above, pale green below, turning bright yellow, yellow-orange, gold, or reddish after the first frosts. The male (staminate) and female (pistillate) flowers are on separate trees (the species dioecious – or ‘polygamodioecious,’ because bisexual flowers may be produced at low frequencies on staminate and pistillate trees), each type of flower borne in pendent catkins. The fruits are narrowly ovoid to flask-shaped capsules 5-7 mm long, splitting to release the seeds; seeds ca.2 mm long, each with a tuft of long, white, silky hairs, easily blown by the wind. The common name is in reference to the shaking of the leaves in light wind.
Variation within the species: Considerable genetic and morphological variation exists over the range of quaking aspen. A number of species and varieties have been described but none are currently recognized. Entire stands are often produced as a single clone from root sprouts – this sometimes easily observable on a single mountainside in different timing in leaf appearance or in different hues and timing of fall coloration. Distinctively large triploid trees are sometimes found.
Quaking aspen hybridizes naturally with bigtooth aspen (Populus grandidentata), narrowleaf cottonwood (P. angustifolia), curly poplar (P. canescens), balsam poplar (P. balsamifera), eastern cottonwood (P. deltoides), and white poplar (Populus alba, a naturalized European species), and hybrids with black cottonwood (P. trichocarpa) occur rarely in Alaska. Quaking aspen, bigtooth aspen, European aspen (P. tremula), and three Asian species are closely related and sometimes classed together as a single, circumglobal superspecies (see Peterson and Peterson 1992).
Trembling aspen, golden aspen, mountain aspen, trembling poplar, white poplar, popple; aspen
Range and Habitat in Illinois
Regularity: Regularly occurring
Regularity: Regularly occurring
Global Range: Newfoundland, Labrador to southern Alaska; British Columbia through Alberta to New Jersey; Virginia, Missouri and the mountains of western United States and northern Mexico (Fowells 1965). The most widely distributed native tree in the United States (Knotts, 1999).
Occurrence in North America
MA ME MD MI MN MO MT NE NV NH
NJ NM NY ND OH OR PA RI SD TX
UT VT VA WA WV WI WY AB BC MB
NB NF NT NS ON PE PQ SK YT MEXICO
Quaking aspen is the most widely distributed tree in North America. It
occurs from Newfoundland west to Alaska and south to Virginia, Missouri,
Nebraska, and northern Mexico. A few scattered populations occur
further south in Mexico to Guanajuato . Quaking aspen is
distributed fairly continuously in the East. Distribution is patchy in
the West, with trees confined to suitable sites. Density is greatest in
Minnesota, Wisconsin, Michigan, Colorado, and Alaska; each of those
states contains at least 2 million acres of commercial quaking aspen
forest. Maine, Utah, and central Canada also have large acreages of
quaking aspen [89,125].
Regional Distribution in the Western United States
This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands
-The native range of quaking aspen.
Quaking aspen occurs in a wide variety of habitats (including soil type and moisture conditions) and at a great range of elevation, matching its extensive geographic range. It characteristically forms pure stands or mixed stands with bigtooth aspen, but it occurs with scrub oaks and sagebrush at lower elevations and as a prostrate form above timberline and exists as a dominant species in many communities at mid elevations. It is a shade-intolerant, disturbed site species and is quickly replaced in succession by more tolerant species.
Some trees are self-pruning, dropping numerous small twigs with excess fall foliage and returning nutrients to the soil. Leaves decay relatively rapidly, and a characteristic "aspen soil," with a higher pH than on conifer-dominated soils, develops on sites that have supported aspen for a number of generations.
Flowering occurs March–April (East) or May–June (West), before the leaves appear and fruiting in May–June (–July), often before the leaves are fully expanded. Temperatures above 12 C for about 6 days apparently trigger flowering. Female trees generally flower and leaf out before male trees.
Quaking aspen is the most widely distributed tree species in North America. It grows from Alaska across the Northwest Territories to Quebec and Newfoundland, south to West Virginia and Virginia, and in all of the western North America US states (except Oklahoma and Kansas) -- in all Canadian provinces and all but 13 US states (absent from the Southeast). It occurs in both the eastern and western sierras of Mexico, into the south-central part of the country. Outside of the main range, it is represented by a huge number of disjunct populations. For current distribution, please consult the Plant Profile page for this species on the PLANTS Web site.
Quaking aspen is a native deciduous tree. It is small- to medium-sized,
typically less than 48 feet (15 m) in height and 16 inches (40 cm) dbh
. It has spreading branches and a pyramidal or rounded crown
[60,75,88,166]. The bark is thin. Leaves are orb- to ovately shaped,
with flattened petioles . The fruit is a tufted capsule bearing six
to eight seeds. A single female catkin usually bears 70 to 100 capsules
[88,166]. The root system is relatively shallow, with widespreading
lateral roots and vertical sinker roots descending from the laterals.
Laterals may extend over 100 feet (30 m) into open areas . Gifford
 found that vertical roots of quaking aspen in Utah extended more
than 9 feet (2.7 m) down, branching into fine, dense roots at their
Quaking aspen forms clones connected by a common parent root system. It
is typically dioecious, with a given clone being either male or female.
Some clones produce both stamens and pistils, however . Quaking
aspen stands may consist of a single clone or aggregates of clones
. Clones can be distinguished by differences in phenology, leaf
size and shape, branching habit, bark character, and by electrophoresis
. In the West, quaking aspen stands are often even-aged,
originating after a single top-killing event. Some stands, resulting
from sprouting of a gradually deteriorating stand, may be only broadly
even-aged . Clones east of the Rocky Mountains tend to encompass a
few acres at most , and aboveground stems are short lived. Maximum
age of stems in the Great Lakes States is 50 to 60 years. Clones in the
West tend to occupy more area, and aboveground stems may live up to 150
years . A male clone in the Wasatch Mountains of Utah occupies 17.2
acres (43 ha) and has more than 47,000 stems. To date, it is the
world's most massive known organism. Clone age can be great; the large
Utah clone is estimated to be 1 million years old .
Seedling morphology: Quaking aspen seedlings can easily be
misidentified as cottonwood (Populus spp.) or willow (Salix spp.)
seedlings because quaking aspen seedlings bear only a slight resemblance
to the adult form. Leaves of quaking aspen seedlings are nearly
lanceolate. During the first growing season, vertical flattening of the
leaf petioles is not obvious, and there is no lateral branching. By the
second growing season, leaves are characteristically orbicular to
ovate, and there is vertical branching. Renkin and others  have
published photographs of excavated quaking aspen seedlings.
Quaking aspen seedlings can be differentiated from root sprouts by leaf
morphology, lack of woody tissue, lack of vertical shoots, and presence
of a taproot [90,133]. There are a few visual clues that can
distinguish seedlings from sprouts without excavation. Seedlings have
paired cotyledons or cotyledon scars a few millimeters above the soil
surface. The first pair of true leaves is nearly opposite, at right
angles to, and directly above the cotyledons. Leaf pattern of sprouts
is strongly alternate .
Physiology: Quaking aspen is not shade tolerant [123,130]; neither does
it tolerate long-term flooding nor waterlogged soils . Even if
quaking aspen survives flooding in the short term, stems subjected to
prolonged flooding usually develop a fungus infection that greatly
reduces stem life (and renders the wood commercially useless) [37,118,126].
Sprouting is hormonally controlled in quaking aspen. Sprouting is
suppressed by auxin, which is transported from the stem to the roots.
Auxin therefore maintains apical dominance. When stems are killed and
apical dominance is removed, cytokinins in the roots initiate root
sprouting. Clones with a strong tendency to sprout probably have high
cytokinin:auxin ratios .
Catalog Number: US 1524538
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): Fr. Marie-Victorin
Year Collected: 1926
Locality: Des Deux-montagnes., Quebec, Canada, North America
- Isotype: Victorin, Fr. J. L. 1930. Contr. Inst. Bot. Univ. Montreal. 16: 8.
Catalog Number: US 1524536
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): Fr. Rolland-Germain
Year Collected: 1926
Locality: Hull, Que., Quebec, Canada, North America
- Isotype: Victorin, Fr. J. L. 1930. Contr. Inst. Bot. Univ. Montreal. 16: 10.
Range and Habitat in Illinois
Comments: Climatic conditions vary throughout the ranges, but are often characterized by low seasonal temperature provided by high altitudes or northern latitudes, and short growing seasons. P. tremuloides tolerates a wide range of soil conditions from rocky soils or loamy sands, to clay soils. The most favorable soils are porous, and loamy soils that have abundant lime and humus. Growth in clay soils is reduced because of poor aeration; growth in sand is poor because of low moisture and nutrient levels. Rocky soils can hinder the spread of lateral roots. P. tremuloides grows at elevations up to 5,800 feet (1768 m) in the north, and rarely below 8,000 feet (2438 m) in lower California, growing at sea level only as far south as Maine and Washington. In the southwest United States, it often grows in cool shaded mountain slopes, canyons, and on stream banks, at about 6,500 to 10,000 feet (1981 to 3048 m) in elevation. P. tremuloides grows with other aspens and often in the following forest types: Jack pine-aspen, white spruce, balsam fir-aspen, black spruce-aspen, aspen-paper birch (Fowells 1965).
Quaking aspen occurs on a wide variety of sites [40,111]. It grows on
moist upland woods, dry mountainsides, high plateaus, mesas, avalanche
chutes, talus, parklands, gentle slopes near valley bottoms, alluvial
terraces, and along watercourses [40,109,158,166].
Climate: Climatic conditions vary widely over the range of quaking
aspen, especially minimum winter temperatures and annual precipitation.
Generally, quaking aspen occurs where annual precipitation exceeds
evapotranspiration. In Alaska and northwestern Canada, quaking aspen is
common in the boreal zone and extends into the warmest, frost-free sites
of the permafrost zone. At the eastern edge of quaking aspen's range,
climate is humid, with snowfall exceeding 120 inches (3,050 mm) per
year. The southern limit of quaking aspen distribution in the East is
roughly delinated by the 75 degree Fahrenheit (24 deg C) mean July
temperature isotherm. In the central Rocky Mountains, altitude plays an
important role in quaking aspen distribution. The lower limit of its
range coincides with a mean annual temperature of 45 degrees Fahrenheit
(7 deg C) .
Soils: Quaking aspen grows on soils ranging from shallow and rocky to
deep loamy sands and heavy clays. Good quaking aspen sites are usually
well drained, loamy, and high in organic matter and nutrients .
Cryer and Murray  stated that stable quaking aspen stands are found
on only one soil order - mollisols - and a few soil subgroups of which
Agric Pachic Cryoborolls and Pachic Cryoborolls are dominant. The best
stands in the Rocky Mountains and Great Basin are on soils derived from
basic igneous rock such as basalt, and from neutral or calcareous shales
and limestones. The poorest stands are on soils derived from granite.
In the Great Lakes States, the best stands occur in lime-rich, gray
glacial drift .
Elevation: Quaking aspen spans an elevational range from sea level on
both coasts to 11,500 feet (3,505 m) in northern Colorado. At its
northern limit, quaking aspen is found only up to 3,000 feet (910 m). In
Baja California, it does not occur below 8,000 feet (2,440 m). In
Arizona and New Mexico is is most abundant between 6,500 and 10,000 feet
(1,980-3,050 m); in Colorado and Utah, it occurs about 1,000 feet (300
m) higher. At either either of its elevational limits, quaking aspen is
stunted. At its lower limit, it grows as a scrubby tree along streambanks;
at high elevations, its stems are bent or prostrate .
Aspect: In Alaska and western Canada, quaking aspen grows best on south
to southwesterly exposures. It is common on all aspects in the West,
except in the Southwest, where it is most common on northern aspects.
In the prairie provinces of Canada, particularly on the prairie-woodland
interface, quaking aspen occurs on cooler north and east slopes, and in
Key Plant Community Associations
Quaking aspen is a major cover type in North America. In Minnesota,
Wisconsin, and Utah, quaking aspen occupies more land than any other
forest type. Quaking aspen also occurs in a large number of other
forest cover types over its extensive range. It is common in spruce-fir
(Picea-Abies spp.) types of the Great Lakes States and central Canada
and in mixed northern hardwoods. Mixed jack pine (Pinus banksiana) and
quaking aspen occur on the Precambrain shield in Canada and Minnesota.
In the Rocky Mountains, quaking aspen groves are scattered throughout
Engelmann spruce-subalpine fir (Picea engelmannii-A. lasiocarpa)
forests. Quaking aspen is common in mixed conifer forests of New
Mexico, Arizona, and California. At its lower altitudinal limit in the
western United States, quaking aspen is associated with scrub oaks
(Quercus spp.) or sagebrush (Artemisia spp.). Prostrate quaking aspen
occur above timberline . Throughout its range, quaking aspen
occurs in mid- to upper riparian zones [56,123].
Quaking aspen is listed as a dominant species in over 100 habitat, plant
community, and vegetation typings. A comprehensive list of these
publications can be obtained by using the Citation Retrieval System
(CRS). In CRS, a combination search using the keywords POPTRE and HTS
(Populus tremuloides and habitat types), and a second search using the
keywords POPTRE and COMM TYPES (P. tremuloides and community types),
will produce a list of habitat, plant community, and vegetation typings
describing quaking aspen as a dominant species. The search can be
narrowed by including the keyword for the state or administrative unit
of interest (e.g., search: POPTRE and HTS and CO).
Associated shrub species: East - Shrub species commonly associated with
quaking aspen in the East include beaked hazel (Corylus cornuta),
American hazel (C. americana), mountain maple (Acer spicatum), speckled
alder (Alnus rugosa), American green alder (A. viridis spp. crispa),
dwarf bush-honeysuckle (Diervilla lonicera), raspberries and
blackberries (Rubus spp.), willows (Salix spp.), and gooseberries (Ribes
Great Plains - Additional species occurring with quaking aspen in the
prairie provinces included snowberry (Symphoricarpos spp.), highbush
cranberry (Viburnum edule), limber honeysuckle (Lonicera dioica),
red-osier dogwood (Cornus sericea), western serviceberry (Amelanchier
alnifolia), chokecherry (Prunus virginiana), Bebb willow (Salix
bebbiana), and roses (Rosa spp.).
Alaska - Bebb willow and roses are also associated with quaking aspen in
Alaska. Other common shrub associates are Scouler willow (S.
scouleriana), bearberry (Arctostaphylos uva-ursi), mountain cranberry
(Vaccinium vitis-idaea), and highbush cranberry.
Rocky Mountains - Mountain snowberry (Symphoricarpos oreophilus),
western serviceberry, chokecherry, common juniper (Juniperus communis),
Oregon-grape (Berberis repens), Wood's rose (R. woodsii), myrtle
pachistima (Pachistima myrsinites), redberry elder (Sambucus pubens),
and a number of Ribes species are associated with quaking aspen in the
Rocky Mountains .
Pacific Northwest - In valleys west of the Cascades in Oregon and
Washington, quaking aspen alternates dominance with Douglas hawthorn
(Crataegus douglasii). Quaking aspen grows through the Douglas hawthorn
overstory, resulting in reduced vigor of Douglas hawthorn. Quaking
aspen eventually dies back, releasing Douglas hawthorn in the understory
Associated herbaceous species: East - Herbs commonly found in the
understory of quaking aspen in the East include largeleaf aster (Aster
macrophyllus), wild sarsaparilla (Aralia nudicaulis), Canada beadruby
(Maianthemum canadense), bunchberry (Cornus canadensis), yellow beadlily
(Clintonia borealis), roughleaf ricegrass (Oryzopsis asperifolia),
sweet-scented bedstraw (Galium triflorum), sweetfern (Comptonia
perigrina), lady fern (Athyrium filix-femina), bracken fern (Pteridium
aquilinum), sedges (Carex spp.), and goldenrods (Solidago spp.).
West - The herbaceous component of quaking aspen communities in the West
is too diverse to list. Forbs dominate most sites .
Habitat: Rangeland Cover Types
This species is known to occur in association with the following Rangeland Cover Types (as classified by the Society for Range Management, SRM):
105 Antelope bitterbrush-Idaho fescue
107 Western juniper/big sagebrush/bluebunch wheatgrass
318 Bitterbrush-Idaho fescue
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
411 Aspen woodland
412 Juniper-pinyon woodland
413 Gambel oak
509 Transition between oak-juniper woodland and mahogany-oak association
920 White spruce-paper birch
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
1 Jack pine
5 Balsam fir
12 Black spruce
13 Black spruce-tamarack
15 Red pine
18 Paper birch
19 Gray birch-red maple
20 White pine-northern red oak-red maple
21 Eastern white pine
25 Sugar maple-beech-yellow birch
26 Sugar maple-basswood
27 Sugar maple
28 Black cherry-maple
30 Red spruce-yellow birch
31 Red spruce-sugar maple-beech
32 Red spruce
33 Red spruce-balsam fir
35 Paper birch-red spruce-balsam fir
37 Northern white-cedar
39 Black ash-American elm-red maple
42 Bur oak
51 White pine-chestnut oak
55 Northern red oak
60 Beech-sugar maple
107 White spruce
108 Red maple
201 White spruce
202 White spruce-paper birch
203 Balsam poplar
204 Black spruce
205 Mountain hemlock
206 Engelmann spruce-subalpine fir
207 Red fir
208 Whitebark pine
209 Bristlecone pine
210 Interior Douglas-fir
211 White fir
212 Western larch
213 Grand fir
215 Western white pine
216 Blue spruce
218 Lodgepole pine
219 Limber pine
220 Rocky Mountain juniper
238 Western juniper
252 Paper birch
256 California mixed subalpine
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K003 Silver fir-Douglas-fir forest
K005 Mixed conifer forest
K007 Red fir forest
K008 Lodgepole pine-subalpine forest
K011 Western ponderosa forest
K012 Douglas-fir forest
K013 Cedar-hemlock-pine forest
K014 Grand fir-Douglas-fir forest
K015 Western spruce-fir forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K019 Arizona pine forest
K020 Spruce-fir-Douglas-fir forest
K021 Southwestern spruce-fir forest
K022 Great Basin pine forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K029 California mixed evergreen forest
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K055 Sagebrush steppe
K095 Great Lakes pine forest
K096 Northeastern spruce-fir forest
K098 Northern floodplain forest
K100 Oak-hickory forest
K101 Elm-ash forest
K106 Northern hardwoods
K107 Northern hardwoods-fir forest
K108 Northern hardwoods-spruce forest
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES10 White-red-jack pine
FRES21 Ponderosa pine
FRES22 Western white pine
FRES24 Hemlock-Sitka spruce
FRES26 Lodgepole pine
FRES28 Western hardwoods
FRES34 Chaparral-mountain shrub
FRES36 Mountain grasslands
FRES37 Mountain meadows
FRES38 Plains grasslands
At the eastern end of the range, in the Maritime Provinces of Canada, the climate is mild, humid, and snowfall is extremely heavy, 300 cm (120 in) or more per year. Some representative climates for the northern and eastern limits of quaking aspen as well as for the warmer parts of its eastern range are as follows (78):
Interior Alaska Gander, NF Ft. Wayne, IN Temperature, C: Minimum -61° -34° -31° January average -30° -7° -3° Maximum 38° 32° 41° July average 16° 16° 23° Precipitation, mm: Total 180 1020 860 Growing season 80 250 330 Frost-free days 81 160 176 Temperature, F: Minimum -78° -29° -24° January average -22° 19° 27° Maximum 100° 90° 106° July average 61° 61° 73° Precipitation, in: Total 7 40 34 Growing season 3 10 13 Frost-free days 81 160 176 In the central Rocky Mountains, where altitude plays an important role in the distribution of aspen, the lower limit of its occurrence coincides roughly with a mean annual temperature of 7° C (45° F). In Colorado and southern Wyoming, quaking aspen grows in a narrow elevational belt of 2100 to 3350 m (6,900 to 11,000 ft). Average annual precipitation in this belt ranges from 410 to 1020 mm (16 to 40 in). The southern limit of the range of aspen in the Eastern United States is roughly delineated by the 24° C (75° F) mean July temperature isotherm. In Canada the mean annual degree-day sum of 700° C (1260° F) with a threshold temperature 5.6° C (42° F) coincides closely with the northern limit of the species (51,69, 70,78,80).
Quaking aspen occurs where annual precipitation exceeds evapotranspiration. It is abundant in interior Alaska where annual precipitation is only about 180 mm (7 in) because evapotranspiration is limited by cool summer temperatures. In the interior west the 2.5 cm (I in) average annual surface water runoff isopleth is more coincident with the range of aspen than is any isotherm. This isopleth also is coincident with the southern limit of aspen in the prairie provinces of Canada eastward to northwestern Minnesota and south to Iowa where high summer temperatures limit growth and longevity. In summary, the range of quaking aspen is limited first to areas of water surplus and then to minimum or maximum growing season temperatures (33,71,91).
Quaking aspen commonly establishes from seed in Alaska, northern Canada, and eastern North America. Seedling establishment is less common in the West but occurs there in moist sites such as kettles and other topographic depressions, seeps, springs, lake margins, and burnt-out riparian zones. Drought stress kills seedlings, as does standing water.
Young trees first flower at 2-3 years but production of large seed crops begins at about 10-20 years; maximum seed production occurs at 50-70 years. Heavy seed crops are produced at 4-5-year intervals. Seeds are wind-dispersed for distances of 500 meters to several kilometers.
Germination generally begins nearly immediately after moisture is received and can occur across a broad temperature range, with optimal germination at 15-25 C. Surface placement or a very shallow depth of burial on exposed mineral soil (such as burned or scarified sites) apparently provide the best environment for germination. Continuous moisture is required.
Asexual reproduction and clones
Reproduction of quaking aspen is primarily by root sprouts, and extensive clones of root-interconnected trees are characteristic of the species. Most root sprouts develop within 10 meters of the parent stem, although some are produced at 30 meters or more. They develop from roots within 2-10 centimeters of the surface. Growth in primordia and buds is suppressed by apical dominance but resumes after stems are top-killed by fire, harvest or wind-breakage, or after defoliation and many thousands of sprouts per acre may be produced. Removal of the above-ground plant portion in June or July after maximum auxin production (the chemical agent of apical dominance) results in fewer suckers than top-removal during the dormant season. Sprouts produced in a closed stand usually die unless in a canopy gap. Saplings may begin producing root sprouts at 1 year of age.
Stands of quaking aspen may consist of a single clone or represent a mosaic of different clones. Even in a small area, wide variation in genetic traits exists between clones – differences may be seen in leaf shape and size, bark colour and texture, branching habit, resistance to disease and insect attack, sexual expression, growth rate, and phenology. The most conspicuous differences may be in the timing of spring leaf flush and in autumn leaf coloration.
The staminate-pistillate ratio of clones is 1:1 in most localities, but in the eastern US staminate trees may outnumber pistillate ones by 3:1. Some clones alternate between staminate and pistillate forms in different years or produce combinations of perfect, staminate, and pistillate flowers.
Individual trees of quaking aspen are short-lived (maximum age in the Great Lakes states is 50–60 years, up to 150 years in the West). Stands may be even-aged (after a single top-kill event) or only broadly even-aged (from sprouting of a gradually deteriorating stand). The clones are much older: many in the Rocky Mountain and Great Basin regions are at least 8000 years old, persisting since the last glacial retreat. A male clone in the Wasatch Mountains of Utah occupies 17.2 acres (43 ha) and has more than 47,000 stems – this clone is estimated to be 1 million years old and may be the world's most massive known organism. Clones east of the Rocky Mountains usually cover no more than a few acres.
Flower-Visiting Insects of Quaking Aspen in Illinois
(honeybees collect pollen; this tree is wind-pollinated; observations are from Robertson)
Apidae (Apinae): Apis mellifera cp fq
Known Pests: MALACOSOMA DISSTRIA, MARSSONIA POPULI, FOMES IGNARIUS, HYPOXYLON CANKER
Associated Forest Cover
Shrub species commonly associated with quaking aspen in the eastern part of its range include beaked hazel (Corylus cornuta), American hazel (C. americana), mountain maple (Acer spicatum), speckled alder (Alnus rugosa), American green alder (A. crispa), dwarf bush -honeysuckle (Diervilla lonicera), raspberries and blackberries (Rubus spp.), and various species of gooseberry (Ribes) and willow (Salix). Additional species occurring with quaking aspen in the prairie provinces include: snowberry (Symphoricarpos spp.), highbush cranberry (Viburnum edule), limber honeysuckle (Lonicera dioica), red-osier dogwood (Cornus stolonifera), western serviceberry (Amelanchier alnifolia), chokecherry (Prunus virginiana), Bebb willow (Salix bebbiana), and several species of rose (Rosa). The latter two also occur in Alaska plus such additional species as Scouler willow (Salix scouleriana), bearberry (Arctostaphylos uva-ursi), russet buffaloberry (Shepherdia canadensis), mountain cranberry (Vaccinium vitisidaea), and highbush cranberry. In the Rocky Mountains, shrubs commonly occurring with quaking aspen include mountain snowberry (Symphoricarpos oreophilus), western serviceberry, chokecherry, common juniper (Juniperus communis), creeping hollygrape (Berberis repens), woods rose (Rosa woodsii), myrtle pachistima (Pachistima myrsinites), redberry elder (Sambucus pubens), and a number of Ribes (69,70,72,78,85,91).
Herbs characteristic of quaking aspen stands in the east include largeleaf aster (Aster macrophyllus), wild sarsaparilla (Aralia nudicaulis), Canada beadruby (Maianthemum canadense), bunchberry (Cornus canadensis), yellow beadlily (Clintonia borealis), roughleaf ricegrass (Oryzopsis asperifolia), sweetscented bedstraw (Galium triflorum), sweetfern (Comptonia perigrina), lady fern (Athyrium filix-femina), bracken (Pteridium aquilinum), and several species of sedges (Carex spp.) and goldenrods (Solidago spp.). In the West, the herbaceous component is too rich and diverse to describe. Forbs dominate most sites, and grasses and sedges dominate others (72).
Diseases and Parasites
Beaver feed on the young tender bark and shoots of aspen and often cut down large numbers of trees near their colonies. A high population of porcupines can greatly damage tree crowns both directly by feeding, and indirectly by increasing the trees' susceptibility to attack by insects and diseases.
The red-breasted and yellow-bellied sapsuckers may seriously sear trees with drill holes. Minor damage is caused by such woodland birds as the ruffed grouse and the sharp-tailed grouse, which feed on the buds of quaking aspen; ruffed grouse also feed on the leaves during the summer months (78).
Aspen is susceptible to a large number of diseases (28,39,41,81,82). Shoot blight of some aspen caused by Venturia macularis is periodically severe. Angular black spots appear on the leaves, enlarging until the leaf dies. If the infection occurs at the top of the tree, the entire new shoot may be infected, blackened, killed, and bent to form a "shepherd's crook." This disease is common in young stands. A similar leaf disease in Wisconsin is caused by Colletotrichum gloeosporioides.
Two or more species of Ciborinia cause a leaf spot on trees of all ages. When the disease is severe, small trees may be killed, but older ones rarely die. Marssonina populi causes a leaf spot and shoot blight that is especially prevalent and damaging in the western states. It is responsible for occasional severe defoliation. Severe, repeated infection can cause mortality, although susceptibility to this disease varies greatly among clones. Another leaf spot of aspen is caused by Septoria musiva.
Several leaf rust fungi of the genus Melampsora infect aspen. M. medusae is common east of the Rocky Mountains. M. abietis-canadensis occurs throughout the range of eastern hemlock (Tsuga canadensis) and M. albertensis in the West. All can discolor and kill aspen leaf tissue and cause premature autumn leaf drop, but their damage is not serious.
Powdery mildew, Erysiphe cichoracearum in the West and the widespread Uncinula salicis can be conspicuous on aspen leaves but probably do little damage.
Recently, viruses have been detected in a few quaking aspen clones. Once trees in the clone are infected, regeneration by suckering maintains the infection, which is then impossible to eliminate except by artificially culturing virus-free tissue. The full extent and seriousness of viruses in aspen is unknown but decline of some clones has been attributed to them in both the East and the West.
Stain and decay have the greatest direct impact of the many stem pathogens on wood production. The role of microorganisms frequently associated with discoloration is poorly understood because staining also develops in their absence. Bacteria and yeast organisms are commonly associated with "wetwood," a water-soaked condition of live trees that leads to wood collapse during lumber drying.
A number of different bacteria and fungi are found in aspen tissue, apparently interacting to follow one another successionally, with bacteria appearing first. Phellinus tremulae causes a white rot of the heartwood at first but may eventually invade the entire stem. It causes the greatest volume of aspen decay and is so prevalent it conceals rot caused by other fungi. Sporophores (fruiting bodies) are the most reliable external indicator of decay. They provide a means to estimate present and future decay. Resistance to this fungus is strongly genetically controlled. Incidence and extent of infection increases with tree age or size but is not strongly related to site (76).
Peniophora polygonia is the second most important trunk rot fungus in the West and in Alaska, but it causes little actual cull. Libertella spp. is also an important trunk rot fungus in the West. Other less important trunk rot fungi found on aspen include Radulodon caesearius, Peniophora polygonia, P. rufa, and Pholiota adiposa.
More fungi species cause butt and root rots than trunk rots-as much as one-third of the decay volume in Colorado. Collybia velutipes is found in Alaska and causes the greatest amount of butt cull in the West. Ganoderma applanatum may be as important because it also decays large roots, which leads to windthrow. Less important butt rot fungi include Pholiota squarrosa, Gymnopilus spectabilis, Peniophora polygonia, and Armillaria mellea. The latter is primarily a root rot which can infect a high proportion of the trees (74). Other locally important root rots in the West include Phialophora spp. and Coprinus atramentarius.
Stem cankers are common diseases of aspen that have a great impact on the aspen resource. Depending on the causal fungus, cankers can kill a tree within a few years or persist for decades. Hypoxylon canker caused by Hypoxylon mammatum is probably the most serious aspen disease east of the Rockies, killing 1 to 2 percent of the aspen annually. It is not an important disease in the West, nor has it been found in Alaska. The infection mode of Hypoxylon is poorly understood but seems to be related to ascospore germination inhibitors in bark. Most canker infections seem to originate in young branches with scars or galls formed by twig-boring insects (4). Once infected, the host bark tissue is rapidly invaded and the fungus girdles and kills the tree in a few years (5).
Ceratocystis canker is a target-shaped canker caused by Ceratocystis fimbriata, C. moniliformis, C. piceae, C. pluriannulata, C. ambrosia, C. cana, C. serpens, C. crassivaginata, C. populina, C. tremuloaurea, and C. alba. This canker is found throughout the range of aspen, with C. fimbriata the most common causal pathogen. These cankers seldom kill aspens but can reduce usable volume of the butt log. Infection is primarily through trunk wounds and insects are the primary vectors.
Sooty-bark canker of aspen is caused by Phibalis pruinosa and is common and a major cause of mortality in Alaska and the West. The fungus infects trunk wounds and spreads rapidly, killing trees of all sizes. The fungus has been found only as an innocuous bark saprophyte on quaking aspen in the East.
Cytospora canker is caused by Cytospora chrysosperma, a normal inhabitant of aspen bark. The fungus is not considered a primary pathogen and causes cankers, lesions, or bark necrosis only after the host tree has been stressed, such as by drought, fire, frost, suppression, or leaf diseases. The disease is most serious on young trees and is found throughout the range of aspen.
Dothichiza canker, caused by Dothichiza populea, occurs in the eastern range of aspen. It is an endemic disease of young or weakened trees and is not found in vigorous stands.
In Ontario, a canker caused by Neofabraea populi has been found on young aspen. Few trees have been killed by it, however, and the disease is not known in the United States.
Cryptosphaeria populina cause a long, narrow, vertical canker that may spiral around an aspen trunk for 1 to 6 in (3 to 20 ft) or more. It is common in the West as far north as Alaska. Trees with large cankers have extensive trunk rot and are frequently broken by wind.
Aspen is susceptible to three types of rough-bark which are caused by the fungi Diplodia tumefaciens, Rhytidiella baranyayi, and Cucurbitaria staphula. Rough, corky bark outgrowths persist for many years but do little harm.
Quaking aspen hosts a wide variety of insects (28,81). One Canadian survey recorded more than 300 species, but only a few are known to severely damage trees. They may be grouped into defoliators, borers, and sucking insects.
Defoliators of aspen belong primarily to the orders Lepidoptera and Coleoptera. The forest tent caterpillar (Malacasoma disstria) and the western tent caterpillar (M. californicum) have defoliated aspens over areas as large as 259 000 km² (100,000 mi²). Outbreaks usually persist for 2 to 3 years and may collapse as quickly as they begin (88). Aspen growth losses during defoliation have been as high as 90 percent and may take as long as 3 or 4 years for total growth recovery. Some trees never recover and die as much as 20 to 80 percent of them on poor sites (90). On good sites mortality may be restricted to suppressed trees (59).
The large aspen tortrix (Choristoneura conflictana) is found throughout the range of aspen. It has defoliated trees over an area as large as 25 900 km² (10,000 mi²) in Canada and Alaska. Caterpillars predominantly infest the leaves of early flushing clones (89). Outbreaks normally collapse in 2 or 3 years and, although aspen growth is reduced, few trees are killed.
In the East, aspen is a favored host for the gypsy moth (Lymantria dispar) and the satin moth (Leucoma salicis) (78).
A great number of leaf tiers defoliate aspen. Sciaphila duplex is one that is often associated with the large aspen tortrix and has been a major pest in Utah. Other Lepidopterous defoliators of aspen include the Bruce spanworm, Operophtera bruceata, and Lobophora nivigerata.
Three species of leaf-rolling sawflies of the genus Pontania sometimes erupt in local outbreaks in the Lake States. Anacampsis niveopulvella is a Lepidopterous leaf roller that causes local damage in the West. Sawflies of the Platycampus genus chew holes in leaves.
The more common leaf miners of aspen are aspen leaf miner (Phyllocnistis populiella), the aspen blotch miners (Phyllonorycter tremuloidiella and Lithocolletis salicifoliella), and a leaf-mining sawfly (Messa populifoliella).
Defoliating beetles include the aspen leaf beetle (Chrysomela crotchi), the cottonwood leaf beetle (C. scripta), the American aspen beetle (Gonioctena americana), and the gray willow leaf beetle (Pyrrhalta decora). All have similar feeding habits; the larvae skeletonize lower surfaces of leaves, and adults feed on whole leaves.
Wood-boring insects that attack aspen are primarily beetles of the Cerambycidae (round-headed borers or long-horned beetles) and Buprestidae (flatheaded borers or metallic beetles). The poplar borer (Soperda calcarata) is the most damaging. The larvae tunnel in the bole, weakening and degrading the wood. Breakage by wind increases and the tunnels serve as infection courts for wood-rotting fungi. S. moesta is a smaller related borer that attacks small suckers and aspen twigs. It is important only in the West. Xylotrechus obliteratus has killed large areas of aspen in the West above 2130 in (7,000 ft).
The root-boring saperda (Saperda calcarata) feeds on phloem and outer sapwood near the base of young aspen suckers. Oviposition incisions of the poplar gall saperda (S. inornata) frequently cause globose galls to form on the stems of young suckers and on small branches of larger trees. These oviposition wounds can serve as infection sites for Hypoxylon that can then grow from a branch gall down into the bole of the tree causing a canker (4). The poplar branch borer (Oberea schaumi) attacks larger suckers and tree limbs. Damage by all these insects can lead to stem breakage. Site quality is not an important variable, and maintaining high stocking density of vigorous suckers is the best practice to minimize loss.
Two flatheaded borers, the bronze poplar borer (Agrilus liragus) and the aspen root girdler (A. horni), bore galleries that disrupt nutrient and water movement. The former attacks sucker stems and makes zig-zag galleries; the latter girdles the sucker with a spiral gallery from the lower trunk to the roots and back. A. anxius also girdles and kills aspen twigs in the West.
Some other Buprestids attacking aspen in the East are the flatheaded apple tree borer (Chrysobothris femorata), the Pacific flatheaded borer (C. mali), and the flatheaded aspen borers (Dicerca tenebrica, D. divaricata, and Poecilonota cyanipes). The first two and the latter are also reported in the West, along with the aspen ambrosia beetle (Typodendron retusum). None of these cause serious injury in well-managed stands.
A widespread weevil, the poplar and willow borer, Cryptorhynchus lapathi, can riddle aspen stems with galleries, especially planted trees.
A clear-wing moth of the genus Aegeria, and willow shoot sawfly (Janus abbreviatus) are examples of borers from nonbeetle families.
In the West, the fungus Ceratocystis fimbriata is carried by Epurea spp., Nudobius spp., and Rhisophagus spp. (28).
Sucking insects are represented mainly by aphids and leafhoppers. The poplar vagabond aphid (Mordvilkoja vagabunda) causes a peculiar curled and twisted gall of leaves as large as 5 cm (2 in) in diameter at the tip of twigs. Poplar petiole gall and twig gall aphids of the genus Pernphigus produce swellings on leaf petioles. Increased forking of aspen suckers may be caused by high populations of the speckled poplar aphid (Chaitophorus populicola) and the spotted poplar aphid (Aphis maculatae). They are commonly found on expanding aspen sucker leaves (35,81).
The genera Idiocerus, Oncomtopia, Macropsis, Oncopsis, and Agallia have several species of leafhoppers that cause leaf browning and slitlike ruptures in the bark of twigs. Only Idiocerus spp. have been found in the West. Several species of scale insects such as the oystershell scale (Lepidosaphes u1mi) are found on aspen but do little damage to healthy trees. Cutworms (moth family Noctuidae larvae) sometimes can cut a large number of succulent new suckers at the ground line. Black carpenter ants (Camponotus pennsylvanicus) frequently use and extend the tunneling made by the poplar borer, causing further damage (35,78).
Aspen is highly susceptible to fire damage. Fires may kill trees outright or cause basal scars that serve as avenues of entrance for wood-rotting fungi. Intense fires can kill or injure surface roots and thereby reduce sucker regeneration (19,56,78).
Early spring frosts may kill new leaves and shoots and, when especially severe, some of the previous year's shoots. Overwinter freezing can cause frost cracks. Strong wind can uproot or break mature aspen and even moderate wind can crack the bole of trees with lopsided crowns. Hail can bruise the bark of young aspen and, in severe storms, kill entire sapling stands. Aspen suffers little from ice storms or heavy wet snow, except when in leaf. Snow creep on steep slopes can bend or break aspen suckers as tall as 1.2 in (4 ft) (28).
Aspen suddenly exposed to full sunlight may suffer sunscald. Pole-size trees are more susceptible than saplings (19,58).
Aspen growth and vigor suffer from drought (79), and drought- stressed trees become predisposed to secondary agents such as insects and disease. Mechanical injuries inflicted on aspen bark by thoughtless recreationists can lead to infection by canker disease and eventual death in as few as 10 to 20 years.
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 81 to >300
PESTS AND DISEASE: Populus spp. have many natural enemies. The forest tent caterpillar, Malacosoma disstria, is one of the most important insects to attack P. tremuloides. The fungus Marssonia populi induces a leaf and twig blight of P. tremuloides that periodically becomes epidemic over extensive areas of the northwestern U.S. Clones vary in their susceptibility to the disease, and may become anywhere from only slightly damaged to entirely defoliated. The last reported epidemic occurred over large areas of northeastern Utah, southeastern Idaho, and western Wyoming (Harniss 1984).
The most serious diseases affecting P. tremuloides are the wood-rotting fungi and cankers. Fomes ignarius is a wood-rotting fungus that attacks the species throughout its range, causing decay of heartwood and sapwood (Fowells 1965). Hypoxylon canker is widely distributed in the Northeast and Lake States and causes heavy losses of Populus spp. in these regions. French (pers. comm.) stated that perhaps 15-20% of all trees in the Lake States are infected with Hypoxylon. The Hypoxylon fungus attacks the phloem, and kills the tree within 2-4 years of initial infection (French pers. comm.).
Moderate browsing by mammals such as deer causes little permanent damage to suckers. Mice, voles, and rabbits can girdle suckers, and beaver frequently cut larger trees.
Broad-scale Impacts of Plant Response to Fire
These Research Project Summaries provide information on prescribed fire use and
postfire response of plant species associates in quaking aspen communities:
- The effects of experimental fires in an Alaskan black spruce/feather
- Forest floor and plant responses to experimental fires in an Alaskan
black spruce/feather moss community
- Vegetation recovery following a mixed-severity fire in aspen groves of
- Vegetation response to restoration treatments in ponderosa pine-Douglas-fir
forests of western Montana
- Vegetation changes following prescription fires in quaking aspen stands of
Colorado's Front Range
provides a more detailed description of prescribed fire's effects on quaking aspen.
For further information on quaking aspen response to fire, see the other Fire Case Studies in this species summary.
Fire Management Considerations
Prescribed fire is recommended for quaking aspen [2,25,123,143].
Currently, an estimated 600 acres (240 ha) of quaking aspen burns per
year in the Intermountain Region. At that rate, it will require 12,000
years to burn the entire quaking aspen type in that Region. It is
likely that seral quaking aspen will be replaced by conifers; stable
quaking aspen stands may become less productive . In many areas of
the West, quaking aspen stands have lived longer than they did prior to
fire exclusion, and many stands are in a state of decline due to
advanced age . Gruell and Loope  found that in Jackson Hole,
Wyoming, quaking aspen stands begin to deteriorate after about 80 years.
Houston  stated in 1973 that quaking aspen in Yellowstone National
Park were primarily large trees ranging from 75 to 120 years of age.
Applying fire: Prescribed fire is often difficult to apply in quaking
aspen stands because of the prominence of live fuels and often sparse
distribution of fine dead fuels . Even if fuels are plentiful, they
are usually too moist to burn easily. Prescribed fire may be possible,
however, when live vegetation cures enough to contribute to fire spread
rather than hinder it. The combination of dry weather and cured fuels
occurs most often in early spring, late summer, and fall [131,138]. The
forest floor of a quaking aspen stand immediately after snowmelt is
covered by matted, cured surface vegetation and deciduous leaf litter.
Before leaf-out this mat is directly exposed to drying by wind and sun,
which increase fuel temperature and decrease fuel moisture. Without
rain, the withered leaves in the litter begin to curl, resulting in a
more favorable fuelbed for combustion and heat transfer. In Alberta,
these moderately severe, early season burning conditions can persist
from snowmelt until the first week in June .
In most years, leaf fall and autumn precipitation coincide, making fall
burning difficult. If September and October are dry, however, burning
may be possible. Surface fuels are dead and sometimes frozen, with a
continuous layer of loosely packed leaves, making quaking aspen more
flammable than at any other time of year .
Live fuel moisture varies greatly between understory species throughout
the growing season, but can be estimated well enough to determine when
to light prescribed fires. Brown and others  estimated that when
herbaceous vegetation is the primary fine fuel, at least 50 percent
curing is needed to sustain fire spread. Less than 50 percent curing
may be sufficient in stands with substantial conifers. Brown and
Simmerman  provide a method for appraising fuels and flammability in
quaking aspen to assist managers in choosing when to apply prescribed
fire and help determine proper conditions for burning. Five fuel types
in 19 community types common in the Intermountain West are presented,
accompanied by color photographs.
Prescriptions: Aspen parkland and northern forest - Bailey [174,175]
found that in Alberta, prescribed burning in quaking aspen forests and
parklands in spring was usually not successful above relative humidity
of 35 to 40 percent. He recommended that prescribed burning be
conducted 8 to 10 drying days after snowmelt, when air temperature is at
least 64 degrees Fahrenheit (18 deg C), relative humidity is less than
30 percent, and 3.3-foot (10-m) open winds are 5.4 to 21 miles per hour
Bailey and Anderson  reported that in central Alberta, quaking
aspen forest in a grassland-shrub-quaking aspen forest mosaic was the
most difficult of the three vegetation types to prescribe burn. With
spring burning, backfires consistently gave poor results, frequently
going out within a few feet of ignition and yielding a maximum
temperature of only 550 degrees Fahrenheit (288 deg C). Headfires were
hotter but gave variable results. Most headfire temperatures ranged
from 700 to 900 degrees Fahrenheit (371-482 deg C), but 14 percent were
in excess of 1,112 degrees Fahrenheit (600 deg C). Fire and fuel data
from the quaking aspen sites follow.
| fire temperature 393 +/- 28* (deg C) |
| total fuel 13,436 +/- 354 (kg/ha) |
| ground fuel 11,704 +/- 337 (kg/ha) |
| standing woody fuel 1,732 +/- 181 (kg/ha) |
*standard error of the mean (SEM)
Perala  recommended this prescription for burning quaking aspen
slash in the Great Lake States:
Months for burning dormant season
(all but June, July, & August)
Fuel model* D
Air temperature > 65 degrees Fahrenheit (18 deg C)
Relative humidity < 35%
Ignition component* 40-50
Energy release component* 14-17
Spread component* 4-7
Burning index* 13-21
Wind** 2.5-5 m/s
Number of days with less
than 2.5 mm rain > 5
*from the National Fire-danger Rating System 
**measured 20 ft. above ground, or at average height of vegetation
cover, averaged over at least a 10-minute period
Canadian Forest Fire Behavior Prediction (FBP) System : Alexander and
Maffey  provide examples for predicting fire spread rate, fuel
consumption, and frontal intensity in quaking aspen types using the FBP
Forage quality and fire: Three burned quaking aspen/shrub/tall forb
communities on the Caribou National Forest, Wyoming, showed increased
forage quality (better Ca:P ratios, higher elk digestibility, and higher
crude protein and P levels) than adjacent unburned sites during the
first postfire year. By the second postfire year, there were no
significant differences between forage quality on burned and unburned
sites. Shrubs on the unburned sites were above browse level throughout
the study period, however, while shrubs on the burned site were still
accessible to elk in the second postfire year .
Plant Response to Fire
|A prescribed fire in a black spruce-paper birch-quaking aspen community in boreal Alaska. Photo courtesy of US Forest Service, FROSTFIRE.|
Quaking aspen sprouts from the roots and establishes from off-site,
wind-blown seed after fire [27,123,157]. It is the classic soboliferous
species described by Stickney : a plant that sprouts from
carbohydrate-storing lateral roots (sobols).
Sprouting: Quaking aspen generally sprouts vigorously after fire.
Long-term growth and survival of quaking aspen sprouts depend on a
variety of factors including prefire carbohydrate levels in roots,
sprouting ability of the clone(s), fire severity, and season of fire.
Moderate-severity fire generally results in dense sprouting. Fewer
sprouts may be produced after severe fire. Since quaking aspen is
self-thinning, however, sprouting densities are generally similar
several years after moderate and severe fire. A low-severity surface
fire may leave standing live trees that locally suppress sprouting,
resulting in an uneven-aged stand [12,13,28,123].
Quaking aspen burned in spring generally sprouts later in the growing
season and again the following year. Fires in mid-growing season
generally result in late-season sprouting. Quaking aspen burned in late
summer or fall usually sprouts the next spring .
Predicting postfire sprouting: Applying prescribed fire in exclosures
in Yellowstone National Park, Renkin and Despain  found that root
biomass can be estimated from basal area, and both can be used to
predict local response of quaking aspen to burning. Sprout biomass
produced in postfire year 1 was positively correlated (r2=0.90, p=0.013)
with both prefire basal area and root biomass. On average, 11.5 metric
tons per hectare of root mass were required to produce 0.1 metric ton
per hectare of sprouts. Average sprout height was positively correlated
with basal area and root biomass (r2=0.85, p=0.004). On average, 25
square meters per hectare of basal area and/or 19 metric tons per
hectare of root biomass were required to produce 0.5 meter of sprout
Examples of sprouting: After the 1988 fires in Yellowstone National
Park, percentage of sprouts produced in spring, 1989, was significantly
higher (p=0.030) in burned stands (mean 82%) than on unburned stands
(mean 60%). The percentage of sprouts in fall, 1989, was also higher
(p=0.103) on burned stands (mean 82%) than in unburned stands (mean
65%). In spring 1990, sprout density averaged 80,000 stems per hectare
in burned stands and 27,000 stems per hectare in unburned stands. By
fall 1991, density was 38,000 stems per hectare in burned and 25,000
stem per hectare in unburned stands, respectively. Mean heights were
9.6 inches (24 cm) in spring 1990 and 10.8 inches (27 cm) in spring
1991. Browsing intensity was much higher in winter and spring (45-55%
of sprouts browsed) than summer and fall (5-10%). There were no
significant differences in browsing among burned stands, unburned stands
adjacent to burned stands, and remote unburned stands: Sprouts were
heavily browsed in all stand types .
Birch-aspen: Following a 1944 summer wildfire in Maine, quaking aspen
and paper birch sprouted vigorously, forming a dense stand. In 1951,
there were 40,000 to 45,000 stems (both spp.) per acre. Quaking aspen
dominated the stand; it averaged 20 feet (6 m) in height while paper
birch averaged only 6 feet (1.8 m) .
Birch-aspen: Following a 1944 summer wildfire in Maine, quaking aspen
and paper birch sprouted vigorously, forming a dense stand. In 1951,
there were 40,000 to 45,000 stems (both spp.) per acre. Quaking aspen
dominated the stand; it averaged 20 feet (6 m) in height while paper
birch averaged only 6 feet (1.8 m) .
Southwestern mixed conifer: Quaking aspen sprouted after the Walker Wildfire
on the Santa Fe National Forest of New Mexico. The Walker Fire occurred in
mixed-conifer forest with an overstory of Engelmann spruce (P. engelmannii),
Douglas-fir (Pseudotsuga menziesii), quaking aspen (Populus tremuloides),
and ponderosa pine (Pinus ponderosa). The litter layer was deep prior to the
wildfire. The fire was a moderate-severity surface fire that consumed
understory conifers and hardwoods (mainly quaking aspen). Overstory foliage
was killed by heat from the surface fire. The Walker Fire top-killed most of the
quaking aspen stems. Eighteen months after the fire, 1 acre of the Walker Burn
was fenced to exclude deer, elk, and cattle. Wildfire significantly increased
the number of quaking aspen sprouts. Five-year average sprout density was 12,960
sprouts per acre on the burn compared to 100 sprouts per acre in adjacent unburned
forest and 200 and 500 sprouts per acre on similar spruce-fir types in Arizona.
In 1964, quaking aspen sprouts on the burn were less then 3 feet (0.9 m)
tall, so ungulates could browse them easily. By June 1968, sprouts were
8 to 10 feet (2.4-3 m) tall, and getting out of reach as a food supply.
Cattle and wildlife use on the burned area did not significantly affect
quaking aspen sprout density; the number of sprouts was similar inside
and outside the exclosure .
For further examples of quaking aspen sprouting response after fire,
refer to the FIRE CASE STUDIES section. Cases from Arizona,
Colorado, Wyoming, Minnesota, and Alberta are presented.
Seedling Establishment: Fire exposes mineral soil, which is an
excellent seedbed for quaking aspen . Quaking aspen seedlings have
been noted following severe fire in Canada. Six years after fire in
northeastern Wisconsin, quaking aspen seedlings composed 20 to 35
percent of seedlings of all species present on the burn . Kay 
reported good seedling establishment following 1986 fires in Grand Teton
National Park and 1988 fires in Yellowstone National Park. Height
growth was negligible, however, due to ungulate browsing. Density,
height, and ungulate use of quaking aspen seedlings on the Yancy's Hole
Burn, Yellowstone National Park, were :
Transect # Year Number/ha % browsed Mean height (cm)
1 1989 177,202 -- 62
1991 32,154 100 50
2 1989 141,362 -- 60
1991 46,148 100 57
3 1989 109,522 -- 53
1991 16,660 100 75
Mean 1989 142,695 -- 58
1991 31,654 100 47
Renkin and others  are conducting a similar seedling study on
forested and nonforested sites in Yellowstone National Park; only
preliminary data are available at this time. They found that quaking
aspen seedlings were concentrated on wet microsites but widely scattered
on other site types. In 1989, quaking aspen seedling density on 14
plots ranged from 0.6 to 1,014 per square meter; average height ranged
from 2.3 to 11.1 inches (mean=5.1 inches) (5.7-27.8 cm, mean= 12.8 cm).
Quaking aspen seedlings were two to four times taller than lodgepole
pine seedlings on forested plots. In 1990, all plots had persistent
quaking aspen seedlings; in some cases the stem had died back but the
1-year-old roots had produced suckers. Density of surviving seedlings
ranged from 0.05 to 332 per square meter. Average heights had
increased, ranging from 3.6 to 15.6 inches (mean=7.8 in) (9-39 cm,
mean=19.4 cm). Quaking aspen seedlings on fenced plots averaged 12
inches (30 cm) in height; seedlings on unfenced plots averaged 5.36
inches (13.4 cm). Seedling survival was significantly greater (p=0.004)
on forested than nonforested plots. Survival was also influenced by
presence of ungulates, spring flooding, disease, and intraspecific
competition. Ungulate presence negatively influenced seedling survival
on unfenced plots (r=0.97, p=0.004). Plots submerged in spring showed
high seedling mortality. A fungus (Venturia tremulae) also contributed
to seedling death or dieback .
Broad-scale Impacts of Fire
Fire may kill (as opposed to top-kill) a deteriorating stand of quaking
aspen. A deteriorating stand on the Sweetwater drainage of the Wind
River Mountains, Wyoming, failed to sprout following a 1963 wildfire.
However, another 1963 wildfire in the Wind River Mountains, near
Pinedale, had the opposite effect on a deteriorating stand of quaking
aspen. Although the site was considered poor for quaking aspen due to
dry, sandy soil, fire only top-killed the stand. Browsing pressure on
sprouts was light, and postfire stocking was "more than adequate" for
The position of an individual tree on a slope, or within a stand, can
influence the degree of damage caused by fire. Even when damaged, trees
located near the boundaries of a fire can often maintain a live crown.
These peripheral trees may receive food supplies from the roots of
unburned neighbors. Quaking aspen on slopes generally show greater
damage than do trees on flatter areas. Flames moving uphill often curl
up the lee side of trees when fanned by upslope wind, charring the stem
further up its bole. The effect of slope is particularly pronounced (up
to 31-44% higher char heights) after fires of higher severity. This
relationship is presented in the following table :
Probability of mortality
dbh (cm) Average char height -
10 5 12
15 14 21
20 23 30
25 32 39
Uphill char height -
10 6 16
15 19 29
20 31 42
25 44 55
Immediate Effect of Fire
Small-diameter quaking aspen is usually top-killed by low-severity
surface fire . Brown and DeByle  found that as dbh increases
beyond 6 inches (15 cm), quaking aspen becomes increasingly resistant to
fire mortality. Large quaking aspen may survive low-severity surface
fire, but usually shows fire damage [26,94]. Moderate-severity surface
fire top-kills most quaking aspen, although large-stemmed trees may
survive. Some charred stems that survived low- or moderate-severity
fire initially have been observed to die within 3 or 4 postfire years.
Severe fire top-kills quaking aspen of all size classes.
Moderate-severity fire does not damage quaking aspen roots insulated by
soil. Severe fire may kill roots near the soil surface or damage
meristematic tissue on shallow roots so that they cannot sprout. Deeper
roots are not damaged by severe fire and retain the ability to sucker
Mortality does not always occur immediately after fire. Sometimes buds
in the crown will survive and leaf out prior to the death of the tree
. Brown and DeByle  reported that quaking aspen trees died over
a 4-year period following fires in Wyoming and Idaho, although most
individuals succumbed by the second postfire year. Even when quaking
aspen is not killed outright by fire, the bole may be sufficiently
damaged to permit the entrance of wood-rotting fungi . According to
Jones and DeByle , basal scars which lead to destructive heart rot
can be made on even good-sized aspen by "the lightest of fires." Basal
fire scars may also permit entry of borers and other insects which can
further weaken the tree .
Tree with adventitious-bud root crown/soboliferous species root sucker
Initial-offsite colonizer (off-site, initial community)
Fire adaptations: Quaking aspen is highly competitive on burned sites
. Even where quaking aspen was a barely detectable component of the
prefire vegetation, it often dominates a site after fire. Quaking aspen
has adapted to fire in the following ways .
1. The thin bark has little heat resistance, and quaking aspen is
easily top-killed by fire.
2. Root systems of top-killed stems send up a profusion of sprouts for
several years after fire.
3. Sprouts grow rapidly by extracting water, nutrients, and
photosynthate from an extant root system, and may outcompete other woody
4. Following a fire, a new, even-aged quaking aspen stand can develop
within a decade.
5. In contrast to most trees, quaking aspen is self-thinning. Without
intervention, a mature forest of healthy trees can develop from dense
Fire releases sprout primorida on roots from hormonally controlled
growth inhibition; removes canopy shade; and blackens the soil surface,
increasing heat absorption. Increased soil temperatures aid sprout
production [22,83]. On cold sites, quaking aspen may be unable to
sprout until soil temperatures rise after fire .
Quaking aspen is able to naturally regenerate without fire or cutting on
some sites , but fire may be required for regeneration on others.
There are areas in Jackson Hole, Wyoming, where ungulate browsing has
been light, both historically and recently, yet stems have not attained
tree size since extensive fires in the 1800's .
Fuels and fire behavior: Fuels are usually more moist in quaking aspen
stands than in surrounding forest. Crown fires in coniferous forests
often drop to the surface in quaking aspen, or may extinguish after
burning into quaking aspen only a few meters [19,55,138]. Quaking aspen
stands often act as natural fuelbreaks during wildfires , and fires
sometimes bypass quaking aspen stands surrounded by conifers . In
an analysis of fires in quaking aspen in National Forests of the
Intermountain West (USFS Regions 2, 3, and 4) from 1970 through 1982,
Bevins  reported that wildfires that burned thousands of acres
during extreme weather conditions usually penetrated less than 65 feet
(20 m) into quaking aspen. Managers he interviewed used the terms
"asbestos type" and "firebreak" to describe quaking aspen stands. Bevins
reported that mixed quaking aspen-conifer types such as those on the
northern Kaibab and Dixie National Forests did sustain fires, however,
and burned substantial amounts of quaking aspen. Throughout all three
Regions, a relatively few, large fires (>100 acres burned) accounted for
93.2 percent (or 1.12 million acres) of all quaking aspen burned.
Fire history: Before and during the mid-nineteenth century, fires were
apparently more frequent, and larger acreages of quaking aspen and
quaking aspen-conifer mixes burned, than any time since. A large
majority of the quaking aspen stands in Jackson Hole, Wyoming, date from
fires between 1850 and 1890 . In central Utah, Baker  and
Meinecke  found few quaking aspen fire-scarred later than 1885.
Earlier fire scars were common and showed a 7- to 10-year fire
frequency. Since quaking aspen is fire-sensitive, the fires were
probably of low severity. Extensive sampling of quaking aspen in
Colorado found few fire scars dating later than about 1880 .
These data indicate that there has been a great reduction of fire
rejuvenation of quaking aspen in the West since about 1900. Extensive
young stands of quaking aspen are uncommon in the West [65,151,46].
Conifers now dominate many seral quaking aspen stands. Probable
contributing factors are:
1. highly effective direct control of wildfires in the last 50 years,
especially in the quaking aspen type ,
2. reduction of fine fuels in quaking aspen/grass and quaking
aspen/forb types due to grazing [28,46], and
3. cessation of deliberate burning by Native Americans [9,68,80].
Ungulates, fire, and quaking aspen: In most areas, ungulate browsing is
probably not a major factor restricting postfire quaking aspen
regeneration. Quaking aspen has increased in importance in the East
despite browsing pressure from large white-tailed deer populations. In
many areas of the United States, elk populations impact quaking aspen
very little. Browsing elk had no significant impact on quaking aspen
sprout density after wildfire in New Mexico . In some areas,
however, fire suppression coupled with heavy ungulate browsing has
reduced quaking aspen regeneration. Failure of some stands in the Great
Lakes States to regenerate has been attributed to overbrowsing of
sprouts by white-tailed deer . Overbrowsing has particularly been
noted in northwestern Wyoming, in Yellowstone and Grand Teton National
Parks and the Bridger-Teton National Forest. Elk are the primary
browsers of quaking aspen in this area, although where moose populations
are high, moose have also removed considerable quaking aspen
regeneration. Historic narratives and photographic evidence suggest
that ungulates were a major biotic influence on quaking aspen in this
region during the the exploration and settlement periods. However,
fires were extensive during this period, so postfire sprouting of
quaking aspen and growth of palatable grasses, shrubs, and herbs,
probably produced a forage supply that dispersed browsing ungulates
sufficiently for quaking aspen to regenerate .
Coring of old quaking aspen stems in Yellowstone National Park showed
that most live, large quaking aspen established in a brief period
between the 1870's and 1880's: a period of severe fires followed by
above-normal precipitation. Elk, moose, and beaver populations were at
a historic low, and some wolves were present. Neither this combination
of conditions nor significant quaking aspen regeneration has occurred
since then. Elk populations were low in the 1950's and 1960's, but
fires were suppressed and the climate was dry. In the 1910's, there
were numerous elk and beaver and few fires. After the 1988 fires, elk
numbers were high and climatic conditions were dry. In this region,
even large-scale burning does not seem sufficient for quaking aspen
Prairie: Frequent fires on prairies and plains grasslands historically
helped control quaking aspen invasion . Fire may have been only one
of several factors controlling quaking aspen, however. Drought  and
ungulate browsing may have worked in conjunction with fire to curtail
woody plant invasion. Fire alone may not control quaking aspen spread
. Anderson and Bailey  reported that 24 years of annual spring
burning checked quaking aspen invasion onto tallgrass prairie, but
actually increased the number and cover of quaking aspen sprouts in the
area. Elk Island National Park, Alberta, was described by early
settlers as a grassland with scattered quaking aspen groves. By 1895,
extirpation of bison and severe reduction of other ungulates was
followed by expansion of quaking aspen. Bison were reintroduced with
Park establishment, but fire was not. Ungulate populations rose rapidly
and were culled in the 1930's and 1950's. Grassland expanded with the
ungulates, while quaking aspen expanded when culling occurred .
More info for the terms: cover, forbs, succession, swamp, tree
Quaking aspen is shade intolerant and cannot reproduce beneath its own
canopy [23,40,98,123,126]. Beyond that, there is no single, generalized
pattern of succession in quaking aspen. Quaking aspen is seral to
conifers in most of its range in the West, and in some portions of its
eastern range. In the East, quaking aspen is also replaced by hardwoods
[23,98]. In the Great Lakes States, successional trends are toward
northern hardwoods, spruce-fir, ash-elm (Fraxinus-Ulmus spp.), oak
(Quercus spp.), swamp conifers, and pine (Pinus spp.) types, in
decreasing order of importance . Where it is seral, quaking aspen
usually persists as a minor tree in late seral stages .
The canopy closes rapidly in young aspen stands . A quaking aspen
stand in Ontario closed and reached maximum development (foliage/unit
area of soil surface) in 4 years [127,128]. If quaking aspen does not
remain stable, rate of succession to other species varies with with
soil, site, and invading species . Mueggler  stated that
succession to conifers may occur in a single generation, or take longer
than 1,000 years. Harper  found that in central Utah, quaking aspen
succeeded to conifers in 75 to 100 years on sandstone soils. On
limestone or alluvial soils, succession to conifers took 140 years or
Quaking aspen is apparently stable on some sites. On some former pine
stands in the East, extensive clearcutting of the conifer overstory has
removed the pine seed sources. Quaking aspen has formed an apparently
stable overstory on many of these sites . Quaking aspen stands are
also considered stable in parts of Canada and the western United States
. Some stands, however, remain stable for decades but eventually
deteriorate. Deteriorating stands are often succeeded by conifers, but
shrubs, grasses, and/or forbs gain dominance on some sites .
Succession to grasses and forbs is more likely on dry sites and is more
common in the West than in the East .
Quaking aspen readily colonizes after fire, clearcutting, or other
disturbance . In Emigrant Wilderness Area, California, red fir
(Abies magnifica) stands on north slopes have converted to quaking aspen
after fire . In the Great Lakes States, quaking aspen has
regenerated on cut/burned sites through sprouting and seedling
establishment, becoming the dominant forest cover type .
Quaking aspen regenerates from seed and by sprouting from the roots
. Stump and root crown sprouting is rare in older trees, but
saplings sometimes sprout from the stump and root crown as well as the
Vegetative reproduction: Root sprouting is the most common method of
regeneration. Root suckers originate from meristems in the root's cork
cambium and can develop anytime during secondary growth . Saplings
may begin producing root sprouts at 1 year of age . There are
thousands of suppressed shoot primoridia on the roots of most mature
quaking aspen clones. Recently initiated meristems or primordia usually
sprout and elongate more vigorously than older primorida or suppressed
root buds . Root suckering is affected by depth and diameter of
parent roots. In Utah and Wyoming, Schier and Campbell  found that
25 percent of sprouts came from roots within 1.6 inches (4 cm) of the
surface, 70 percent from within 3.2 inches (8 cm), and 92 percent within
4.7 inches (28 cm). Compared with parent roots of quaking aspen in the
Great Lakes States, those of quaking aspen in the West were deeper. On
a Utah burn site, high-severity fires increased the depth of the parent
roots from which sprouts originated. Range in diameter of roots
producing sprouts was 0.04 to 3.7 inches (0.1-9 cm). Sixty percent of
suckers grew from roots smaller than 0.4 inch (1 cm) in diameter, 88
percent from roots smaller than 0.8 inch (2 cm), and 93 percent from
roots smaller than 1.2 inches (3 cm) in diameter. On a Wyoming site,
the percentages were 38 percent, 68 percent, and 86 percent,
Sprout development is largely suppressed by apical dominance .
Closed stands produce a few inconspicuous sprouts each growing season;
the sprouts usually die unless they occur in a canopy gap. When stems
are removed by cutting, burning, girdling, or defoliation, suppressed
primoridia, buds, and shoots resume growth. Best sucker production
follows either a fire that kills all parent trees and brush or other
complete clearing . The number of suckers produced can vary
markedly among clones [7,159], but the potential for suckering is
enormous. Jones  indicated that 20,000 to 30,000 sprouts per acre
is typical the first year following top-kill. Natural thinning is heavy
and effective. The least vigorous suckers die within 1 to 2 years.
After 5 to 10 years, most sucker clumps reduce to a single stem .
Most stems are overtopped by more vigorous neighbors. Diseases,
insects, browsing mammals, and snow damage also reduce sprout density
[35,87,108]. Bella and De Franceschi  reported that in Alberta and
Saskatchewan, stem density averaged 280,000 per hectare at age 2;
190,000 per hectare at age 3; and 80,000 per hectare at age 5.
Seedling establishment: Quaking aspen commonly establishes from seed in
Alaska, northern Canada, and eastern North America. Seedling
establishment is less common in the West, where rainfall is often
followed by dry periods that kill newly germinated seedlings . Even
in the West, however, quaking aspen may establish from seed more
frequently than previously thought. Studies on frequency of seedling
establishment in the West are conflicting. Some researchers found
absolutely no quaking aspen seedling establishment despite diligent
searching [4,5,16]; others reported the presence of only one  or a
few  seedlings, while still other researchers documented the
presence of hundreds of seedlings [7,90,97,167]. Only since the
stand-replacement fires of the late 1980's have researchers used
permanent plots to monitor quaking aspen seedling establishment and
survival in the West. Data from one such study are summarized after the
following discussion of sexual reproduction in quaking aspen.
Sexual reproduction: The staminate-pistillate ratio of adult clones is
1:1 in most localities, although it may be as high as 3:1 or more .
Some clones alternate between staminate and pistillate forms in
different years, or produce various combinations of perfect, staminate,
and pistillate flowers . Quaking aspen first flowers at 2 to 3
years. Minimum tree age for production of large seed crops is 10 to 20
years, and maximum seed production occurs at about 50 years of age. In
Utah, one 23-year-old tree produced an estimated 1.6 million seeds in
one spring . There are 3- to 5-year intervals between heavy seed
crops [55,102,110,148]. Seeds disperse a few days after they ripen.
Dispersal lasts 2 to 3 weeks . The plumose seeds are dispersed by
wind for distances of 1,600 feet (500 m) to several miles with heavy
winds. Seeds also disperse by water, and can germinate while floating
or submerged . Viability of fresh seed is good; germination of 80
to 95 percent is reported under laboratory conditions [103,109,142].
Viability lasts 2 to 4 weeks under favorable conditions of low
temperature and humidity , but seed loses viability rapidly under
less than optimum conditions [54,171].
Optimum conditions for germination and seedling survival include a moist
mineral seedbed with adequate drainage, moderate temperature, and
freedom from competition . In various collections, seeds have
germinated at temperatures from 32 to 102 degrees Fahrenheit (0-39 deg
C), with germination sharply reduced from 35 to 41 degrees Fahrenheit
(2-5 deg C) and progressively curtailed above 77 degrees Fahrenheit (25
deg C) [54,172]. Quaking aspen seed from northern Utah showed optimal
germination between 59 and 68 degrees Fahrenheit (15-20 deg C), and had
no light requirement. Seeds germinated best on the soil surface, with
emergence decreased by shallow burial . Burned or scarified soil
is an excellent seedbed ; litter provides the poorest seedbed. The
primary root grows slowly the first few days following germination, and
during this critical period the seedling depends upon a brush of hairs
to absorb water and anchor the plant . Minor disturbances can
uproot surface-germinated seedlings, and a drying seedbed can rapidly
desiccate seedlings .
Seedlings may reach 6 to 24 inches (15-61 cm) in height by the end of
their first year, and roots may extend 6 to 10 inches (15-25 cm) in
depth and up to 16 inches (41 cm) laterally. Roots grow more rapidly
than shoots; some seedlings show little top-growth until about their
third year . During the first several years, natural seedlings grow
faster than planted seedlings but not as fast as sprouts. High
mortality characterizes young quaking aspen stands regardless of origin.
In both seedling and sprout stands natural thinning is rapid. Stems
that occur below a canopy die within a few years .
Seedling study: Kay  documented postfire quaking aspen seedling
establishment following 1986 and 1988 fires in Grand Teton and
Yellowstone National Parks, respectively. He found seedlings were
concentrated in kettles and other topographic depressions, seeps,
springs, lake margins, and burnt-out riparian zones. A few seedlings
were widely scattered throughout the burns. In Grand Teton National
Park, establishment was greatest (950-2,700 seedlings/ha) in 1989, a wet
year, but hundreds to thousands of seedlings established each year
despite drought conditions in 1986-1988 and 1990-1991. Seedlings
surviving past one season occurred almost exclusively on severely burned
surfaces. In Grand Teton National Park, where seedlings were monitored
for several years, surviving seedlings were associated with bare mineral
soil, ash, and the absence of competing vegetation. In both Parks, 100
percent of seedlings were browsed, and mean heights of seedlings at
postfire year 5 (Grand Teton) and postfire year 3 (Yellowstone) were
nearly equal to mean heights at postfire year 1. During the same
period, 0 percent of lodgepole pine seedlings were browsed. Kay
predicted that long-term survival of quaking aspen seedlings will be
low. Most seedlings established on depressions that are subject to
spring flooding. Since quaking aspen does not tolerate standing water,
seedlings on depressions such as kettles and lake margins will probably
die in the first prolonged flood. At postfire year 5, quaking aspen
seedlings in Grand Teton National Park attained only 5 percent more
height growth than attained in the first postfire year. In contrast,
lodgepole pine seedlings had increased in height by an average of 176
Growth Form (according to Raunkiær Life-form classification)
Reaction to Competition
The tree has a pronounced ability to express dominance, and overstocking to stagnation of growth is extremely rare.
Quaking aspen is an aggressive pioneer. It readily colonizes burns and can hold invaded land even though subjected to fires at intervals as short as 3 years. In the northeastern United States, it is an old-field type, and in Canada it invades grassland if fire is excluded. In the Central Rocky Mountains, it constitutes the typical fire climax at the lower elevations of the subalpine forest. The extensive stands of aspen in that area are usually attributed to repeated wildfires, and aspen is generally regarded as a successional species able to dominate a site until replaced by less fire-enduring but more shade-tolerant conifers, a process that may take only a single aspen generation or as long as 1,000 years of fire exclusion. Aspen is considered a permanent type on some sites in the intermountain region of Utah, Nevada, and southern Idaho, but conifers would invade the type if seed trees were available.
The uneven-aged character of some western aspen stands suggests that under certain conditions aspen is self-perpetuating without major disturbance. These stands are relatively stable and can be considered de facto climax. Seral and stable aspen stands seem to be associated with certain indicator species (28,78,82).
In its eastern range, aspen in the absence of disturbance is regarded as transient. Successional patterns are determined by soil water regime (61). Pure aspen stands gradually deteriorate to a "shrubwood" dominated by the shrub component of the stand and with only a few scattered aspen suckers. If intolerant associates are present, they will outlive the aspen and eventually dominate but in turn will be replaced again by the shrubwood type. If tolerant hardwoods or balsam fir (Abies balsamea) are associated with aspen, they will eventually dominate by their longevity and ability to regenerate in their own shade (81).
The deterioration of aspen stands begins earliest at the southern limits of its eastern range and seems to be related to summer temperatures. Deterioration begins when crowns in old stands can no longer grow fast enough to fill the voids in the canopy left by dying trees. Increased breakage accelerates the deterioration process, which may be completed in as few as 3 or 4 years (81). Deterioration is a much slower process in the West, where aspen often is replaced by conifers. Dry sites may revert to rangeland dominated by shrubs, forbs, and grasses. Sometimes suckers appear in a deteriorating stand and ultimately an all-age climax aspen forest develops (28).
The shallow and extensive laterals have cordlike branch roots that undulate and meander for great distances without tapering. These roots are the main producers of suckers, particularly when they are close to the soil surface. Roots tend to follow soil surface irregularities and may even grow into decaying stumps or logs. The fine feeding roots are found at all levels down to 0.6 to 0.9 in (2 to 3 ft) except in restrictive horizons. Sinker roots occur as frequently as every meter or so on the lateral roots. They may descend to depths of 3 in (10 ft) or more where they end in a dense fan-shaped fine root mass. Sinkers are capable of penetrating strongly massive soil horizons or cracks in bedrock and often use vacated root channels (28,78).
Life History and Behavior
Quaking aspen catkins elongate before the leaves expand. In New
England, catkins appear in mid-March to April; in the central Rockies,
flowering occurs in May to June. Sustained air temperatures above 54
degrees Fahrenheit (12 deg C) for about 6 days apparently trigger
flowering [55,123]. At high elevation, trees may flower before snow is
off the ground . Female trees generally flower and leaf out before
male trees. Local clonal variation produces early- and late-flowering
clones of either sex, however. Catkins mature in 4 to 6 weeks (usually
in May or June). Branches usually leaf out from early May to June
. Seed dispersal in the Great Lakes States occurs from early May
to mid-June, beginning earliest on protected sites and in southern
portions of the region .
Persistence: PERENNIAL, DECIDUOUS
Quaking aspen seedlings at 1 year of age are already capable of reproducing by root sprouts (suckers), and mature stands reproduce vigorously by this means (19,43). Root collar sprouts and stump sprouts are produced only occasionally by mature trees but saplings commonly produce them (77). Aspen clones vary widely in many characteristics, even over a small area. Members of a clone are indistinguishable but can be distinguished from those of a neighboring clone by electrophoresis and often by a variety of traits such as leaf shape and size, bark character, branching habit, resistance to disease and air pollution, sex, time of flushing, and autumn leaf color (9,10,11,17,22,23,57,87). Clones typically have many ramets over an area up to a few tenths of a hectare in stands east of the Rocky Mountains (45,76). In the Rockies, clones tend to be much larger-one Utah clone covered 43.3 ha (107 acres) and contained an estimated 47,000 ramets. Clone size in an aspen stand is primarily a function of clone age, number of seedlings initially established, and the frequency and degree of disturbance since seedling establishment (46).
The root suckers are produced from meristems on the shallow, cordlike lateral roots within 2 to 10 cm (1 to 4 in) of the soil surface (28,81). In response to clone disturbance, the meristems may develop into buds and then elongate into shoots. Frequently, however, they remain in the primordial stage until stimulated to develop further. These preexisting primordia are visible as small bumps when cork is peeled off an aspen root (63).
The development of suckers on aspen roots is suppressed by apical dominance exerted by auxin transported from growing shoots, while cytokinins, hormones synthesized in root tips, apparently initiate adventitious shoot development. When an aspen is cut, cytokinins accumulate in the roots, the supply of inhibitory auxins is eliminated, and suckers are initiated. If aspen is girdled, the downward transport of auxin again is stopped, but upward translocation of cytokinins via the xylem is unimpeded. Cytokinins in this case do not accumulate in the roots, with consequently less sucker production. Thus high cytokinin-to-auxin ratios favor shoot initiation while low ratios inhibit it. A gibberellic-acid-like growth regulator also stimulates shoot elongation after sucker initiation.
Carbohydrate reserves supply the energy needed by elongating suckers until they emerge at the soil surface to carry on their own photosynthesis. Therefore, the density of regeneration varies according to the level of these reserves. However, the number of suckers initiated by aspen roots is independent of variation in carbohydrate levels. Apical dominance by elongating suckers further limits the total amount of regeneration. Carbohydrates can be exhausted by grazing, repeated cropping or killing of sucker stands, or insect defoliation (63,77,82).
Soil temperature is the most critical environmental factor affecting suckering. Initiation and development of suckers is optimum at about 23° C (74° F). High temperatures tend to degrade auxin and promote cytokinin production, which may account in part for the aspen invasion of grassland without apparent clone disturbance (51,82).
Excess soil moisture (impeded aeration) or severe drought inhibit sucker production (25,57,82).
Light is not needed for sucker initiation but is essential for secondary growth (78). Large clonal. differences in ability to produce suckers may be due to differences in growth regulators, carbohydrate reserves, and developmental stages of shoot primordia (63). Some clones in the interior West are unevenaged, suggesting weak apical control or high concentration of growth-promoting hormones so that they sucker at the least disturbance (69,82).
Suckers are initially sustained by the root system of the parent tree, but they may form as much as 4.7 in (15.5 ft) of new main roots in 10 weeks. In contrast, suckers of some Utah clones produce only weak adventitious roots and depend on the distal parent root for sustenance. The parent root usually thickens at the point of sucker origin distal to the parent tree. This indicates that translocation of food produced by the sucker is toward the growing tip of the parent root, which usually becomes part of the new root system (28,51,78,81). These connections readily conduct water and solutes from tree to tree (27). True root grafts, in contrast, are rare in aspen.
Suckers from the roots of badly decayed trees are not infected by the parent stump. Heart rot usually terminates in the base of the stump. Deteriorating clones, however, produce few suckers.
In general, sucker regeneration is proportional to the degree of cutting, with most suckers arising after a complete clearcut (43,57,64,65,75,78). Typically, from 25,000 to 75,000 suckers per hectare (10,000 to 30,000/acre) are regenerated in Alaska and the Great Lakes region and about half as many in the Rockies (28,91).
Light burning on heavily cut areas increases the number of suckers and stimulates their initial growth. However, hot slash fires diminish sucker vigor. Repeated burning increases stand density because it stimulates sucker numbers and prepares mineral soil seedbeds for seedling establishment; however, it reduces stand growth (6,19,28,56,64,78). Surface fires in established aspen stands are not common because of aspen's inherently low flammability. When they do occur, fire wounds and loss of shallow feeder roots substantially reduce aspen productivity. Fire is a useful tool, however, to stimulate regeneration and to reduce competition if clearcutting is not practiced. It is especially valuable for regenerating deteriorated stands and for maintaining wildlife habitat (21,57).
Disking stimulates suckering, but sucker growth and survival are usually diminished because of injury to their sustaining parent roots. Rows of suckers often appear along furrows prepared for planting conifers.
Herbicides have been used to kill residual trees and to increase suckering without affecting sucker growth or vigor (19,57,78).
Dormant season cutting generally produces vigorous suckers the next growing season. Summer cutting produces a sparse stand initially, but the number of suckers after 2 years is usually the same regardless of cutting season (15). Suckering sometimes fails inexplicably after hay-vesting aspen on fine-textured soils during the growing season (59).
The number of suckers following cutting increases as stocking density of the parent stand increases up to full site utilization. The effect of age and site index on aspen suckering is not clear (35,81).
Age of wood is the most important factor in rooting quaking aspen cuttings. With rare exceptions, the species roots poorly from woody stem cuttings, even when treated with indolebutyric acid (IBA). However, newly initiated shoots can usually be induced to root by dipping in IBA or other commercially available rooting powders. These softwood stem cuttings should be taken from actively growing shoots except during the period of extremely rapid mid-season elongation (14,63,78). Propagation by excising succulent young sucker shoots from root cuttings is easily accomplished by treating the shoots with IBA and growing them in a suitable medium in a misting chamber until rooted, in about 2 to 3 weeks (62). Quaking aspen scions can be grafted onto balsam poplar (Populus balsamifera), willows (Salix spp.), or bigtooth aspen (P. grandidentata). Quaking aspen plantlets have been produced by tissue culture (81).
During the first year seedlings may attain a height of 15 to 30 cm (6 to 12 in) and develop a 20- to 25-cm (8- to 10-in) long taproot and from 30- to 40-cm (12 to 16-in) long laterals. During the second and third years, wide-spreading lateral roots are developed, reaching lengths of 2 m (6 ft) or more in the second year. Quaking aspen roots form ectomycorrhizae if suitable inoculum is present (28,78,86).
Despite the abundance of aspen seed and high germinative capacity, few aspen seedlings survive in nature because of the short period of seed viability, unfavorable moisture during seed dispersal, high soil surface temperatures, fungi, adverse diurnal temperature fluctuations during initial seedling growth, and the unfavorable chemical balance of some seedbeds (51,52).
Height growth of the young trees is rapid for about the first 20 years and slows thereafter. During the first several years, natural seedlings grow faster than planted seedlings but not as fast as suckers. High mortality characterizes young quaking aspen stands regardless of origin. In both seedling and sucker stands natural thinning is rapid, and trees that fall below the canopy stop growing and die within a few years (78,93).
Seed Production and Dissemination
Seeds begin to be dispersed within a few days after they ripen and seed dispersal may last from 3 to 5 weeks. The seeds, buoyed by the long silky hairs, can be carried for many kilometers by air currents. Water also serves as a dispersal agent (78,91).
The viability of fresh fertile seeds is high (usually greater than 95 percent) but normally of short duration. Under favorable conditions viability lasts only 2 to 4 weeks after maturity and may be much less under unfavorable conditions. When air dried and stored in polyethylene bags at -10° C (14° F), seed retains high viability for at least I year. Seedlings are sturdiest when germinated at 5° to 29° C (41° to 84° F) and grown at about 20° C (68° F). Ripe quaking aspen seeds are not dormant, and germination occurs within a day or two after dispersal if a suitably moist seedbed is reached. Because germination declines rapidly after water potential exceeds -4 bars (-.4 MPa), a water-saturated seedbed is critical. Seeds germinate even when totally submerged in water or in the absence of light (32,47,50,66,78,92).
Flowering and Fruiting
Growth and Yield
The tallest quaking aspen are found in a belt bordering the midcontinental prairie region at about latitude 55° N., and in north-central Minnesota, northern Michigan, and in the Southwest. Few quaking aspen exceed 26 or 27 in (85 to 90 ft) in Alaska (38).
Growth and decay are both generally slower in Alaska and the West than in the East, hence pathological rotations are longer-80 to 90 years in Utah and 110 to 120 years for Colorado and Wyoming. In northern Minnesota, the pathological rotation is about 55 to 60 years and even shorter in southern Wisconsin and Michigan (35,69,70).
Now and in the foreseeable future, most aspen will be extensively managed (complete clearing for site preparation, no thinning) for fiberboard, pulpwood, flakeboard, and some sawtimber. Aspen is harvested either as whole-tree chips or as bolewood to a nominal top size for pulpwood or sawtimber. Some of the very best stands can be thinned to increase the production of large bolts (57,58).
Site quality varies regionally, being highest in the Lake States, followed by Alaska and the West. Biomass mean annual increment on the better sites in the Lake States and Canada culminates at about age 30 and at 4.4 to 4.8 mg/ha (2-2.2 tons/acre) dry weight (16,60). Mature stands in Newfoundland typically carry 64 m²/ha (280 ft²/acre) basal area. This amounts to 376 mg/ha (167 tons/acre) at age 90 years, or 4.2 mg/ha/yr (1.9 tons/acre/year) (54). However, exceptionally good growth of quaking aspen is possible in Arizona and in Colorado and southern Wyoming (44,70). A natural triploid clone in Minnesota produced an annual yield of 14.6 m³/ha (208 ft³/acre) of bolewood over 38 years (59).
Aspen responds to intensive management. Production by thinned stands for a 50-year rotation, including thinnings removed at ages 10, 20, and 30, is about 511 m³/ha (7,300 ft³/acre), or 10.2 m³/ha (146 ft³/acre) per year. This is about 42 percent greater than for similar, but unthinned, stands (58). Quaking aspen growth can be further increased by fertilization and irrigation (24,26,29,59,84). Sub-optimal fiber yield and the threat of Armillaria mellea root rot limit the practicality of rotations shorter than 1520 years (77).
Molecular Biology and Genetics
Barcode data: Populus tremuloides
Statistics of barcoding coverage: Populus tremuloides
Public Records: 16
Specimens with Barcodes: 29
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: Huge distribution range (most of northern North America except Arctic areas) and great abundance, despite concerns about apparent failure to persist in some sites.
Please consult the PLANTS Web site and your State Department of Natural Resources for this plant’s current status, such as, state noxious status and wetland indicator values.
Global Short Term Trend: Increase of 10 to >25%
Comments: Still abundant in much of its range, although in some areas has been outcompeted by conifers following fire suppression, combined with aspen seedling/shoot consumption by unnaturally abundant deer and elk as well as livestock (cf. knotts, 1999).
Comments: Aspen invasion of grasslands especially at the prairie-forest border has increased primarily because of fire suppression (Buell 1959, Maini 1960, Blake 1963). In Saskatchewan, Maini (1960) found that the age of the oldest P. tremuloides corresponded to dates of post-settlement fire suppression. Aspen groves that were present in the prairie just prior to that time often were of small, brush-like trees instead of tall specimens. Increased wetland drainage probably also has encouraged invasion (Buell 1960)
Undisturbed clones expand into adjacent prairie when light, moisture and soil conditions are appropriate especially for vegetative growth (Maini 1966b). Vigorous root suckers emerge in the prairie at the periphery of a clone, where other woody plants also frequently invade the prairie. As these suckers grow, and crowns coalesce, aspen shades out desirable grassland species.
Rate of invasion is related to disturbance, clone phenotype, slope, wind, moisture, drainage, soil texture and climate. Some examples of invasion rates include:
1. 11 m in 15 years upslope into a dry prairie from a ravine woods, parallel with wind direction (Wisconsin) (Chavannes 1940).
2. An average 1.5 m per year for 23 years in a low rolling prairie near the prairie forest border in Minnesota (Buell 1959).
3. 18 m in about 25 years following a peat-burn in a southern Wisconsin marsh (Vogl 1969).
Aspen persists in prairie regions because of its preference for full sun and its vigorous vegetative reproduction and clonal growth that is well-adapted to top removal (fire, cutting, browsing) and drought.
It is somewhat unclear why some quaking aspen stands break up and die
while others remain stable. The age at which quaking aspen clones begin
to die probably has a genetic component. Site quality can also be a
major factor . Is it well documented in the Great Lakes States
that environmental variables affect quaking aspen longevity [63,93].
Stands in this region may deteriorate* rapidly; more than half the trees
in a well-stocked stand may die in 6 years . In Utah, however,
clone deterioration was found to occur over a number of generations of
sprouts . Schier and Campbell  found that on the Wasatch
National Forest near Logan, Utah, concentrations of phosphorus and
percent silt were significantly lower on soils with deteriorating clones
than on soils with healthy clones. Ten deteriorating clones and ten
healthy clones were studied.
*Deteriorating stands are defined as those stands with a low density of
stems that are younger and smaller in size, and with poorer form and
higher crown:stem ratios, than healthy stands .
Cryer and Murray  speculated that both soil type and disturbance are
important in quaking aspen stability. As a quaking aspen stand matures,
a humus-rich (mollic) soil layer develops. Quaking aspen thrive for a
time, but without disturbance gradually begin to age and deteriorate.
With deterioration, the soil loses organic matter and thickness. With
loss of humus and litter, rapid percolation leaches the soil, which
becomes thinner, more acidic, and lower in nutrients. Acidic,
low-nutrient soils support conifers more readily than quaking aspen.
Disturbances such as burning or clearcutting tend to maintain quaking
aspen. If soil is already thin and acidic, however, clearcutting will
probably convert the site to conifers. Quaking aspen on such sites has
been observed to sprout, grow to about 3 feet (0.9 m) in height, and
begin to die. A deteriorating stand that is burned may be more likely
to revert to quaking aspen because burning increases soil pH and adds
organic carbon and nutrients to the soil. However, fire will probably
not rejuvenate the stand if quaking aspen biomass is so low that burning
does not appreciably raise soil pH and nutrient levels. Sucker vigor
will probably be low.
Range: There is increasing concern that in the West, poor quaking aspen
regeneration is due, at least in part, to wildlife overbrowsing young
sprouts . Where browsing pressure is heavy, ungulates may remove
quaking aspen regeneration before it grows above browseline. To provide
for quaking aspen regeneration in such areas, enough quaking aspen must
be removed to create an unbrowsed surplus of new growth . A few
areas of the West have such large elk populations that even after
large-scale wildfires, quaking aspen sprouts attained little height
growth because of intense browsing. In such areas, quaking aspen
sprouts probably require protection from browsing .
Promoting quaking aspen: Prescribed burning is one method of promoting
quaking aspen (see FIRE MANAGEMENT). When prescribed burning is not
desired or feasible, clearcutting or bulldozing is recommended [77,177].
Clearcutting often results in a sucker stand of 50,000 to 100,000 stems
per hectare [17,35,49]. A basal area of less than 4 trees/sq m/ha is
recommended to promote sprouting [87,122]. Partial cuttings seriously
inhibit sprouting because apical dominance is retained in standing
stems, and shade from standing stems reduces vigor of the few suckers
that do appear .
Clearcutting in southeastern boreal forest: Lavertu and others 
found that in balsam fir-northern white-cedar (Abies balsamea-Thuja
occidentalis) forest in Quebec, quaking aspen showed strong sprouting
response regardless of forest seral stage, number of quaking aspen
present before cutting, quaking aspen stem age, or quaking aspen root
density. After clearcutting on sites that had burned 46, 74, 143, 167,
and 230 years earlier, quaking aspen sprouted vigorously even on the
site that had not burned for 230 years, had only a single, living
quaking aspen stem, and the lowest quaking aspen root density of all
five site types. Initial sprouting densities were greater in younger
stands, but due to greater mortality of sprouts in younger stands,
differences in sprouting density between different-aged stands were not
significant 3 years after clearcutting.
Bulldozing: Carefully done, whole-tree bulldozing can stimulate quaking
aspen suckering [177,178]. Operations that cause deep cutting or
compaction of soil will reduce sprouting . Shepperd  obtained
good quaking aspen regeneration by pushing over whole trees using a
rubber-tire skidder with the blade positioned above ground level. This
technique severed large roots to a distance of 3.3 to 5 feet (1-1.5 m)
from the stem. Five years after treatment, quaking aspen suckers
averaged 37,888 per hectare when slash was removed and 10,131 per
hectare with slash intact. In contrast, sites that were clearcut
averaged 17,544 stems per hectare (no slash) and 7,038 stems per hectare
Quaking aspen control: On some sites, it may be desirable to convert
quaking aspen to another vegetation type. Stand conversion may be
relatively easy on dry or poorly drained sites, or on sites were quaking
aspen is exposed to snow damage. Quaking aspen production is usually
low on such sites to begin with, and such stands are prone to breakup.
On other sites, it may not be possible to eliminate quaking aspen, but
quaking aspen can probably be reduced . Very small clearcuts reduce
quaking aspen abundance because sprouting response is weak after such
treatment . Girdling also reduces abundance; sprouting occurs
after girdling, but shade provided by standing dead stems increases
sprout mortality. Also, it is thought that girdling promotes decay of
the root system . Use of glyphosate after cutting has been shown
to control quaking aspen regeneration for some time [122,123].
In Quebec, quaking aspen in a quaking aspen-paper birch stand
originating after a 1944 fire was partially controlled by removing
overtopping quaking aspen when the stand was 7 and 14 years of age.
Stocking varied as follows at postfire year 34 .
Treatment | Stocking
control (no treatment) | 5% paper birch; 90% aspen; 5% mixed hardwoods
Aug. 1951 cut & Nov. 1958 cut | 90% paper birch; 10% aspen
Nov. 1951 cut & Nov. 1958 cut | 44% paper birch; 41% aspen; 15% mixed hardwoods
Nov. 1951 cut & May 1959 |
herbicide (injection in | 32% paper birch; 63% aspen; 5% mixed hardwoods
Cultivars, improved and selected materials (and area of origin)
Contact your local Natural Resources Conservation Service (formerly Soil Conservation Service) office for more information. Look in the phone book under ”United States Government.” The Natural Resources Conservation Service will be listed under the subheading “Department of Agriculture.”
The thin, soft bark of quaking aspen makes it susceptible to many diseases and insect infestations as well as mechanical and fire damage. Fires may kill trees or cause basal scars that serve as entry points for wood-rotting fungi, which are common in older stands. The wood decays easily. Fires may also kill surface roots that could reduce sucker regeneration.
The poplar borer beetle, one of the most common wood borers of aspen, weakens trees by boring galleries in the trunk near the lower portion of the crown. Outbreaks of forest tent caterpillar may last 4-5 years and result in serious defoliation -- cold weather in the spring shortly after the eggs hatch and above-average fall temperatures can cause a rapid decline in caterpillar populations by killing eggs and larvae. Overgrazing by livestock or big-game animals disturbs roots and compacts soil, limiting sucker formation. Heavy grazing of young sucker stands by cattle for three years in a row may destroy them.
Quaking aspen can be propagated by seed, following cold stratification. Germination of fresh seed may be 80-95%, but viability lasts only 2-4 weeks under favorable natural conditions (low temperature and humidity). Seeds dried for 3 days and stored at cool temperatures may retain good viability for up to a year.
The species roots poorly from woody stem cuttings, but newly initiated (softwood) shoots can usually be induced to root by dipping in IBA (indolebutyric acid) or other commercially available rooting powders. A more preferred method uses root sprouts. Collect dormant lateral roots in early spring -- plant root cuttings 1-2 in diameter and 3-5 centimeters long in vermiculite and place in the greenhouse for 6 weeks. Excise the young sucker shoots and root in perlite/vermiculite (2-3 weeks, using IBA), misting frequently. Transplant the developing plants to peat/vermiculite mix and grow at 15-25º C. Or, the root cuttings may be planted directly into the perlite mix, with the top of the cutting just below the media surface.
Relevance to Humans and Ecosystems
Other uses and values
Mountain slopes covered by quaking aspen provide high yields of
good-quality water. Quaking aspen intercepts less snow than conifers,
so snowpack is often greater under quaking aspen .
Well-stocked quaking aspen stands provide excellent watershed
protection. The trees, the shrub and herbaceous understories, and the
litter of quaking aspen stands provide nearly 100 percent soil cover.
Soil cover and the intermixture of herbaceous and woody roots protect
soil except during very intense rains .
Quaking aspen is valued for its aesthetic qualities at all times of the
year. The yellow, orange, and red foliage of autumn particularly
enhances recreational value of quaking aspen sites .
Quaking aspen is widely used in ornamental landscaping .
Value for rehabilitation of disturbed sites
Aspens (Trepidae) are unique in their ability to stabilize soil and
watersheds. Fire-killed stands are promptly revegetated by root sprouts
(suckers). The trees produce abundant litter that contains more
nitrogen, phosphorus, potash, and calcium than leaf litter of most other
hardwoods. The litter decays rapidly, forming a nutrient-rich humus
that may amount to 25 tons per acre (oven-dry basis). The humus reduces
runoff and aids in percolation and recharge of ground water. Litter and
humus layers reduce evaporation from the soil surface. Compared to
conifers, more snow accumulates under quaking aspen and snowmelt begins
earlier in the spring. Soil under quaking aspen thaws faster and
infiltrates snow more rapidly than soil under conifers .
Wide adaptability of quaking aspen makes it well-suited for restoration
and rehabilitation projects on a wide range of sites. Seedlings
transplanted onto disturbed sites have shown good establishment .
Seedlings have some advantages over vegetative cuttings. In large-scale
greenhouse production, quaking aspen seedlings are more economical to
establish and grow . Seedlings grow a taproot and secondary roots
quickly, while quaking aspen cuttings can be slow to establish an
adequate root system . Also, genetic diversity is greater among
seedlings than cuttings . Seed stored at 4 degrees Fahrenheit (-20
deg C) has retained viability for at least 2 years. Fung and Hamel 
and Schier and others  provide procedures for collecting and
processing quaking aspen seed.
The major advantage of using quaking aspen cuttings is that clones with
desirable traits can be selected as parent stock. Quaking aspen
vegetative cuttings are difficult to root, however [123,146]. Stem
cuttings are especially difficult to root unless taken from young
sprouts. Root cuttings taken from young sprouts are generally most
successful. Schier and others  provide information on growing
quaking aspen cuttings in the greenhouse.
Case examples - Riparian: In riparian and lodgepole pine (Pinus
contorta) zones of Lost Canyon near Fresno, California, restoration was
needed after a hydroelectric plant pipe broke, scouring part of the
canyon. Quaking aspen seedlings showed 99.2 percent survival (or 357
live seedlings) and had a mean height of 10.6 inches (26.6 cm) 1 year
after transplant .
Strip-mined sites: Some old strip-mined sites in Pennsylvania, Ontario,
and elsewhere have not revegetated due to extreme acidity of the soil.
Quaking aspen is one of the first native tree species to volunteer on
these soils after application of lime [81,168].
Mine spoils: Quaking aspen transplants were successfully established on
phosphate mine spoils in southeastern Idaho that received only 18 inches
(450 mm) of annual precipitation .
Wild and domestic ungulates use quaking aspen for summer shade, and
quaking aspen provides some thermal cover for ungulates in winter
[42,35,152]. Seral quaking aspen communities provide excellent hiding
cover for moose, elk, and deer [42,161]. Deer use quaking aspen stands
for fawning grounds in the West . Ungulates generally do not use
quaking aspen much in winter. Perala  reported that in the Great
Lake States, pure quaking aspen stands provided white-tailed deer with
relatively poor insulation and protection from winter winds compared to
adjacent stands of conifers.
Quaking aspen provides good hiding and thermal cover for many small
mammals . Snowshoe hare use it for hiding and resting cover in
summer [42,43]. Beaver use quaking aspen branches for dams and lodges.
A variety of bird species use quaking aspen for hiding, nesting, and
roosting cover . Sapling and pole-size stands provide especially
good winter cover for birds . Snow tends to accumulate earlier and
deeper in quaking aspen than in adjacent conifer stands, and ruffed
grouse use the deep snow for burrowing cover in winter . Dense
stands of fairly small diameter stems ( less than 6 inches [15cm]) provide the
best protection from predators. Overall cover value for ruffed grouse
is enhanced in stands containing several size classes .
Over 4 years, 22 to 65 pairs of breeding birds were found in 10 acres (4
ha) of quaking aspen in northern Utah. Species nesting in quaking aspen
included the broad-tailed hummingbird, northern flicker, house wren,
American robin, warbling vireo, yellow-rumped warbler, junco, western
wood pewee, and lazuli bunting . The following other species also
nest in mature quaking aspen communities :
canopy nesters - pewees, vireos, western tanager, Cassin's finch,
ground nesters - hermit thrush, Townsend`s solitaire, dark-eyed junco,
white-crowned and Lincoln`s sparrows, veery, ovenbird, nighthawk,
Connecticut and mourning warblers
shrub nesters - flycatchers (Empidonax spp.), rose-breasted and
black-headed grosbeaks, chipping, clay-colored, and song sparrows,
yellow and MacGillivray`s warblers, rufous-sided and
green-sided towhees, black-billed cuckoo
cavity nesters - chickadees, nuthatches, woodpeckers, owls,
sapsuckers, hairy and downy woodpeckers
General cover value of quaking aspen has been rated as follows :
CO MT ND OR UT WY
Pronghorn ---- ---- Poor ---- Poor Poor
Elk Fair Good ---- ---- Good Good
Mule deer Fair Good Poor ---- Good Good
White-tailed deer Fair Good Fair ---- ---- Good
Small mammals ---- Good ---- ---- Good Good
Small nongame birds Good Good Good ---- Good Good
Upland game birds Poor Good Good ---- Good Good
Waterfowl ---- ---- ---- ---- Poor Poor
. Nutritional content of quaking aspen browse varies seasonally, by
plant part, and by clone [11,40,159]. Protein content drops as the
growing season progresses [42,179]. On a Utah site, average leaf
protein dropped from 17 percent in early June to 3 percent at
abscission. Clonal variation in leaf protein ranged from 13.4 to 20.9
percent in June and from 10.1 to 14.6 percent in September. Average
twig protein dropped from 17 percent in spring to 6 to 7 percent in
winter. Twig nitrogen, phosphorus, and potassium levels dropped from
spring to winter, but twig calcium, magnesium, sodium, and fat levels
increased. Phosphorus values in September averaged only 58 percent of
those in June .
Mean composition of quaking aspen terminal shoots, collected in March
and April in Soldotna, Alaska, was as follows :
dry matter (%) 43.6
gross energy (kcal/g) 5.1
crude protein (%) 7.9
neutral-detergent fiber (%) 54.9
acid-detergent fiber (%) 40.1
lignin (%) 10.5
ash (%) 1.9
in-vitro digestibility for moose (%) 42.0
species [38,23,42,84,161,169]. The buds, flowers, and seeds are
palatable to many bird species including numerous songbirds and ruffed
and sharp-tailed grouse [42,168].
Palatability of quaking aspen for livestock and wildlife species has
been rated as follows :
CO MT ND OR UT WY
Cattle Fair Fair Fair ---- Fair Fair
Domestic sheep Fair Good Good ---- Fair Good
Horses Fair Fair Fair ---- Fair Fair
Pronghorn ---- ---- Poor ---- Fair Fair
Elk Good Fair ---- ---- Good Good
Mule deer Good Fair Fair ---- Good Good
White-tailed deer Good Fair Fair ---- ---- Good
Small mammals ---- Fair ---- ---- Fair Good
Small nongame birds ---- Fair Fair ---- Fair Fair
Upland game birds ---- Good Good ---- Fair Good
Waterfowl ---- ---- ---- ---- Poor Poor
Importance to Livestock and Wildlife
Quaking aspen forests provide important breeding, foraging, and resting
habitat for a variety of birds and mammals. Wildlife and livestock
utilization of quaking aspen communities varies with species composition
of the understory and relative age of the quaking aspen stand. Young
stands generally provide the most browse. Quaking aspen crowns can grow
out of reach of large ungulates in 6 to 8 years . Although many
animals browse quaking aspen year-round, it is especially valuable
during fall and winter, when protein levels are high relative to other
browse species .
Large wild ungulates: Elk browse quaking aspen year-round in much of
the West, feeding on bark, branch apices, and sprouts [38,42,102]. In
some areas, elk use it mainly in winter . In northwestern Wyoming,
elk begin browsing quaking aspen as soon as they move onto winter ranges
in November and continue to use it through March .
Quaking aspen is important forage for mule and white-tailed deer. Deer
consume the leaves, buds, twigs, bark, and sprouts [42,102,158]. New
growth on burns or clearcuts is especially palatable to deer [42,43].
Deer in many areas use quaking aspen year-round , although in some
western states, deer winter below the aspen zone [42,43]. Quaking aspen
communities are described as the major "deer-producing forest type" in
the north-central United States . In the Great Lakes States,
quaking aspen is primary browse for white-tailed deer and moose .
Stands less than 30 years of age provide optimum forage for deer in
Minnesota . In some locations, sprouts provide key summer forage
for deer after herbaceous species have cured [42,43]. Quaking aspen is
one of the most important items in the summer diet of mule deer on the
Kaibab National Forest of Arizona [159,161], and comprises up to 27
percent of the summer diet of mule deer in parts of central Utah .
However, it is relatively unimportant deer browse in parts of South
Dakota . Mule deer in Utah have been observed consuming large
amounts of quaking aspen leaves after autumn leaf fall [42,161].
Quaking aspen is valuable moose browse for much of the year . Moose
utilize it on summer  and winter ranges [23,42,135]. Quaking aspen,
paper birch (Betula papyrifera), and willows (Salix spp.) make up more
than 95 percent of the winter hardwood browse utilized by moose on
Alaska's Kenai Peninsula . Relatively high levels of moose use
have been reported from early summer through late fall in Minnesota 
and Idaho . Young stands generally provide the best quality moose
browse . However, researchers in Idaho found that in winter, moose
browsed mature stands of quaking aspen more heavily than nearby
clearcuts dominated by quaking aspen sprouts .
Bison once favored quaking aspen-grassland transition zones in Manitoba
and Saskatchewan [32,102]. However, little is known about the historic
importance of quaking aspen browse to bison. Meagher  found that
woody plants made up only 1 percent of the diet of bison in Yellowstone
National Park, and she did not list quaking aspen as one of the woody
species bison used.
Bears: Black and grizzly bears feed on forbs and berry-producing shrubs
in quaking aspen understories. Quaking aspen forests in Alberta provide
excellent denning and foraging sites for black bear .
Lagomorphs: Rabbits and hares feed on quaking aspen in summer and
winter [42,43]. In winter, snowshoe hare and cottontail rabbits eat
quaking aspen buds, twigs, and bark [42,43]. Quaking aspen is one of
the most important and nutritious summer browse species for rabbits in
Alberta , and is a preferred winter food of snowshoe hare in
Manitoba . Pikas also feed on quaking aspen buds, twigs, and bark
. Lagomorphs may girdle suckers or even mature trees [23,102]. In
some parts of Canada, fairly high quaking aspen mortality has been
attributed to rabbits and hares [20,102].
Rodents and shrews: Small rodents such as squirrels, pocket gophers,
mice, and voles feed on quaking aspen during at least part of the year
[43,88,158]. Mice and voles frequently consume quaking aspen bark below
snow level, and can girdle suckers and small trees [23,43,88,152]. The
southern red-backed vole, deer mouse, and white-footed mouse are
dominant small mammals in quaking aspen communities of northern
Minnesota and upper Michigan. Small mammal populations in quaking aspen
generally fluctuate widely with stand age and annual variation in animal
population size. Highest densities typically occur in mature quaking
aspen stands. Field mice (Peromyscus spp.), for example, are most
abundant in mature quaking aspen communities . The red-backed
vole, however, is most abundant in sapling stands, somewhat less
abundant in mature stands, and least common in clearcuts.
Quaking aspen provides food for porcupine in winter and spring
[23,42,43]. In winter, porcupine eat the smooth outer bark of the upper
trunk and branches. Porcupine girdling of quaking aspen has killed
large tracts of merchantable trees in Minnesota. In spring, porcupine
eat quaking aspen buds and twigs .
Beaver consume the leaves, bark, twigs, and all diameters of quaking
aspen branches . They use quaking aspen stems for constructing dams
and lodges [42,102]. At least temporarily, beaver can eliminate quaking
aspen from as far as 400 feet (122 m) from waterways [6,23]. An
individual beaver consumes 2 to 4 pounds (1-2 kg) of quaking aspen bark
daily, and it is estimated that as many as 200 quaking aspen stems are
required to support one beaver for a 1-year period [42,43].
Birds: Quaking aspen communities provide important feeding and nesting
sites for a diverse array of birds . Bird species using quaking
aspen habitat include sandhill crane, western wood pewee, six species of
ducks, blue, ruffed, and sharp-tailed grouse, band-tailed pigeon,
mourning dove, wild turkey, red-breasted nuthatch, and pine siskin.
Quaking aspen is host to a variety of insects that are food for
woodpeckers and sapsuckers . Generally, moist to mesic quaking
aspen sites have greater avian species diversity than quaking aspen
stands on dry sites [40,42].
Many bird species utilize quaking aspen communities of only a particular
seral stage. Research at a northern Utah site suggests that blue
grouse, yellow-rumped warbler, warbling vireo, dark-eyed junco, house
wren, and hermit thrush prefer mature quaking aspen stands. The
MacGillivray's warbler, chipping and song sparrows, and lazuli bunting
occur in younger stands [39,42]. Bluebirds, tree swallow, pine siskin,
yellow-bellied sapsucker, and black-headed grosbeak favor quaking aspen
community edges .
Ruffed grouse: Through most of its range, ruffed grouse depends on
quaking aspen for foraging, courting, breeding, and nesting sites
[23,42,70]. It uses quaking aspen communities of all ages. Favorable
ruffed grouse habitat includes quaking aspen stands of at least three
different size classes [23,70]. Young (2- to 10-year-old) stands
provide important brood habitat, and 10- to 25-year-old stands are
favored overwintering and breeding areas . Quaking aspen leaves
and buds are readily available in abundant quantities in stands greater
than 25 years of age, and such older stands are used for foraging
Ruffed grouse chicks find protection in dense, young aspen suckers as
early as 1 year after fire or other disturbance . Pole-size stands
appear to offer the best breeding habitat and may support one breeding
bird per 3 to 4 acres (1.2-1.6 ha). Breeding generally does not occur
in stands exceeding 25 years of age or with a density less than
approximately 2,000 stems per acre .
Quaking aspen buds, catkins, and leaves provide an abundant and
nutritious, year-long food source for ruffed grouse [23,70]. Vegetative
and flower buds are the primary winter and spring foods of the ruffed
grouse. Ruffed grouse eat 6 times more quaking aspen buds than buds
from all other species combined . It is estimated that ruffed
grouse can consume more than 45 quaking aspen buds per minute and can
satisfy their daily winter food needs in as little as 15 to 20 minutes
. Ruffed grouse generally begin feeding on staminate flower buds
from several weeks prior to the period of snow accumulation, and continue
well into early spring [23,70]. Male ruffed grouse feed on staminate
catkins until at least early May . Nesting hens consume large
quantities of new quaking aspen leaves early in the spring [23,70].
Ruffed grouse consume quaking aspen leaves throughout the summer ,
and the leaves are considered to be the second most important food
source during the fall. Ruffed grouse appear to prefer certain clones.
Buds from some clones may be up to 30 percent richer in protein than
buds from neighboring clones .
Livestock: Most classes of domestic livestock use quaking aspen.
Domestic sheep and cattle browse the leaves and twigs [158,161].
Domestic sheep browse quaking aspen more heavily than cattle [158,161].
It is estimated that domestic sheep consume 4 times more quaking aspen
sprouts than cattle. Heavy livestock browsing can adversely impact
quaking aspen growth and regeneration [42,43,161].
Wood Products Value
Quaking aspen is one of the most important timber trees in the East.
Its wood is used primarily for particleboard, especially waferboard and
oriented strandboard, and for pulp. In the Great Lakes States, quaking
aspen is the preferred species for making oriented strandboard. Quaking
aspen fibers are well suited for making fine paper. Some quaking aspen
is used for lumber. Quaking aspen lumber is used for making boxes,
crates, pallets, and furniture. A small but growing volume is made into
studs. Quaking aspen wood is little used in the West, except in
Colorado, where it is used for pulp and particleboard . Specialty
products from quaking aspen wood include excelsior, matchsticks, and
tongue depressors. Quaking aspen pellets are used for fuel [125,170].
The wood of quaking aspen is light, soft, and straight grained. It has
good dimensional stability and it turns, sands, and holds glue and paint
well. It has relatively low strength, however, and is moderately low in
shock resistance. Both sapwood and heartwood have low decay resistance
and are difficult for preservatives to penetrate [125,170]. Quaking
aspen wood warps with conventional processing, but saw-dry-rip
processing controls warping .
Aspen forests allow more water or ground water recharge and streamflow than do conifer forests. This is primarily due to lower seasonal water losses to interception and transpiration by aspen compared to conifers (34). Clearcutting the aspen type may increase streamflow by as much as 60 percent during the first year. Subsequently, water yields gradually decline to preharvest levels and stabilize when maximum leaf area is attained at about age 10 to 25 (53).
The aspen type is esthetically appealing. The light bark and autumn colors are a pleasing contrast to dark conifers. In the West in particular, the type is used by recreationists during all seasons of the year.
Aspen stands produce abundant forage-as much as 1100 to 2800 kg/ha (1,000 to 2,500 lb/acre) in the Rockies annually, or three to six times more than typical conifer stands. These amounts are comparable to forage production on some grasslands. Although the type is sought after for summer sheep and cattle range in the West, its use for grazing in the East is much more limited (28).
Aspen stands, because of low fuel accumulations, are low in flammability and make excellent firebreaks. Violent crown fires in conifers commonly drop to the ground and sometimes are even extinguished when they reach an aspen stand (28).
Whole-tree aspen chips can be processed into nutritious animal feed ("Muka") or biomass fuels (82). Aspen could be grown for such purposes in dense sucker stands on biological rotations of 26 to 30 years (16).
Wood products from aspen include pulp, flakeboard, particleboard, lumber, studs, veneer, plywood, excelsior, shingles, novelty items, and wood flour. Aspen makes particularly good sauna benches and playground structures because the wood surface does not splinter.
Industry: Quaking aspen is an important fiber source, especially for pulp, flake-board, and other composite products. The wood is light and soft with little shrinkage (see Wheeler 2000) and is used for pallets, boxes, veneer, and plywood. Higher grades are used for other solid wood products, such as paneling, furniture components, and flooring. The wood characteristics make it useful in miscellaneous products, including excelsior, animal bedding, matchsticks, toys, beehives, tongue depressors, spoons, and ice cream sticks. It makes good playground structures because the surface does not splinter, although the wood warps and susceptible to decay.
Conservation: Quaking aspen is valued for its white bark and brilliant fall color, especially when clustered. The species been widely used in landscaping but is best in sites away from structures that might be damaged by the aggressive roots. The trees provide good visual screening and noise abatement.
Aspen stands are good firebreaks, often dropping crown fires in conifer stands to the ground when they reach aspens and even sometimes extinguishing the fire because of the small amount of flammable accumulation. They allow more ground water recharge than do conifer forests and they also play a significant role in protecting against soil erosion. They have been used in restoration of riparian habitats.
Wildlife: Young quaking aspen provides food and habitat for a variety of wildlife: black bear, deer, beaver, porcupine, elk, moose, ruffed grouse and many smaller birds and animals, including small mammals such as mice, voles, shrews, chipmunks, and rabbits. Bark, buds, new sprouts, twigs from the tops of fallen or logged trees, and fallen leaves all are wildlife foods.
Ethnobotanic: Native Americans used Populus bark (including aspen) as a food source. They cut the inner bark into strips, dried and ground it into meal to be mixed with other starches for bread or mush. Catkins were eaten raw, and the cambium was eaten raw or in a soup.
Populus tremuloides is a deciduous tree native to cooler areas of North America, one of several species referred to by the common name Aspen. It is commonly called quaking aspen, trembling aspen, American aspen, Quakies, mountain or golden aspen, trembling poplar, white poplar, popple, and even more names. The trees have tall trunks, up to 25 meters (82 feet) tall, with smooth pale bark, scarred with black. The glossy green leaves, dull beneath, become golden to yellow, rarely red, in autumn. The species often propagates through its roots to form large groves based on a single rhizome.
The Quaking Aspen is the most widely distributed tree in North America, being found from Canada to central Mexico. It is the defining species of the aspen parkland biome in the Prairie Provinces of Canada.
The quaking or trembling of the leaves that is referred to in the common names is due to the flexible flattened petioles. The specific epithet, tremuloides, means similar to Populus tremula, the European aspen. Some species of Populus have petioles flattened partially along their length, while the aspens and some other poplars have them flattened from side to side along the entire length of the petiole.
A tall, fast growing tree, usually 20–25 m (66–82 ft) at maturity, with a trunk 20–80 cm (0.66–2.62 ft) in diameter; records are 36.5 m (120 ft) in height and 1.37 m (4.5 ft) in diameter.
The bark is relatively smooth, colored greenish-white to gray, and is marked by thick black horizontal scars and prominent black knots. Parallel vertical scars are tell-tale signs of elk, which strip off aspen bark with their front teeth.
The leaves on mature trees are nearly round, 4–8 centimeters (1.6–3.1 in) in diameter with small rounded teeth, and a 3–7 centimeters (1.2–2.8 in) long, flattened petiole. Young trees (including root sprouts) have much larger—10–20 centimeters (3.9–7.9 in) long—nearly triangular leaves.
The flowers are catkins 4–6 centimeters (1.6–2.4 in) long, produced in early spring before the leaves; it is dioecious, with male and female catkins on different trees. The fruit is a 10-centimeter-long (3.9 in) pendulous string of 6-millimeter (0.24 in) capsules, each capsule containing about ten minute seeds embedded in cottony fluff, which aids wind dispersal of the seeds when they are mature in early summer.
The quaking aspen is the State Tree of Utah.
The northern limit is determined by its intolerance of permafrost. It occurs across Canada in all provinces and territories, with the possible exception of Nunavut. In the United States, it can be found as far north as the southern slopes of the Brooks Range in Alaska, and it occurs at low elevations as far south as northern Nebraska and central Indiana. In the western United States, this tree rarely survives at elevations lower than 1,500 feet (460 m) due to the mild winters experienced below that elevation, and is generally found at 5,000–12,000 feet (1,500–3,700 m). It grows at high altitudes as far south as Guanajuato, Mexico.
Shrub-like dwarf clones exist in marginal environments too cold and dry to be hospitable to full-size trees, for example at the species' upper elevation limits in the White Mountains.
It propagates itself primarily through root sprouts, and extensive clonal colonies are common. Each colony is its own clone, and all trees in the clone have identical characteristics and share a single root structure. A clone may turn color earlier or later in the fall than its neighbouring aspen clones. Fall colors are usually bright tones of yellow; in some areas, red blushes may be occasionally seen. As all trees in a given clonal colony are considered part of the same organism, one clonal colony, named Pando, is considered the heaviest and oldest living organism at six million kilograms and approximately 80,000 years old. Aspens do produce seeds, but seldom grow from them. Pollination is inhibited by the fact that aspens are either male or female, and large stands are usually all clones of the same sex. Even if pollinated, the small seeds (three million per pound) are only viable a short time as they lack a stored food source or a protective coating.
Beginning in 1996, individual North American scientists noticed an increase in dead or dying aspen trees. As this accelerated in 2004, word spread and a debate over causes began. No insect, disease, or environmental condition is yet specifically identified as a joint cause. Trees adjacent to one another are often stricken or not. In other instances, entire groves have died.
Many areas of the Western US have experienced increased diebacks which are often attributed to ungulate grazing and wildfire suppression. At high altitudes where grasses can be rare, ungulates can browse young aspen sprouts and prevent those young trees from reaching maturity. As a result, some aspen groves close to cattle or other grazing animals, such as deer or elk, have very few young trees and can be invaded by conifers, which are not typically browsed. Another possible deterrent to aspen regeneration is widespread wildfire suppression. Aspens are vigorous resprouters and even though the above-ground portion of the organism may die in a wild-fire, the roots, which are often protected from lethal temperatures during a fire, will sprout new trees soon after a fire. Disturbances such as fires seem to be a necessary ecological event in order for aspens to compete with conifers, which tend to replace aspen over long, disturbance-free intervals. The current dieback in the American West may have roots in the strict fire suppression policy in the United States. On the other hand, the wide-spread decimation of conifer forests by the mountain pine beetle may provide increased opportunities for aspen groves to proliferate under the right conditions.
Because of the vegetative regeneration method of reproduction used by the aspen, where an entire group of trees are essentially clones, there is a concern that something that hits one will eventually kill all of the trees, presuming they share the same vulnerability. A conference was held in Utah in September 2006 to share notes and consider investigative methodology.
Like other poplars, aspens make poor fuel wood, as they dry slowly, rot quickly, and do not give off much heat. Yet they are still widely used in campgrounds because they are cheap and plentiful and not widely used in building lumber. Pioneers in the North American west used them to create log cabins and dugouts, though they were not the preferred species.
In Canada, it is used mainly for pulp products such as books, newsprint, and fine printing paper. Aspen is especially good for panel products such as oriented strand board and waferboard. Its lumber is light in weight and is used for furniture, boxes and crates, core stock in plywood, and wall panels.
Between logging for fuel, building, and pulp, and clearing for agriculture, the area of aspens declined dramatically in the nineteenth and twentieth centuries.
- Quaking Aspen by the Bryce Canyon National Park Service
- "USDA GRIN taxonomy".
- "technology transfer fact sheet: Populus spp.". Forest Products Laboratory: R&D USDA. Madison, Wisconsin: United States Department of Agriculture Forest Service, Center for Wood Anatomy Research. Retrieved 20 September 2010.
- "Aspen, Quaking (Populus tremuloides)". Arbor Day Foundation.
- "S.B. 41 State Tree Change". Utah State Legislature.
- Genetic Variation and the Natural History of Quaking Aspen, Jeffry B. Mitton; Michael C. Grant, BioScience, Vol. 46, No. 1. (Jan., 1996), pp. 25-31.
- Ewing, Susan. The Great Alaska Nature Factbook. Portland: Alaska Northwest Books, 1996.
- Kelley, Katie (26 September 2006). "Emblem of the West Is Dying, and No One Can Figure Out Why". The New York Times.
Names and Taxonomy
The scientific name of quaking aspen is Populus tremuloides Michx.
(Salicaceae) [60,64,75,78,82,79,165,166]. There are no currently
recognized subspecies or varieties [64,75,78,100,136,165,166]. Roland
and Smith  recognize a form with extremely broad leaves, P.
tremuloides forma reniformis Tidestr., that occurs in northeastern North
Quaking aspen is in subsection Trepidae of the genus Populus. Some
authorities consider the Trepidae aspens a single taxonomic entity.
Under this treatment, quaking aspen, bigtooth aspen (P. grandidentata),
European aspen (P. tremula), and three aspens occurring in Asia are
classed together as a single, circumglobal superspecies .
Quaking aspen hybridizes naturally with bigtooth aspen and white poplar
(P. alba), a naturalized European species. Hybrid quaking
aspen-bigtooth aspen swarms occur in the Niobrara River valley of
Wyoming and Nebraska , and quaking aspen-bigtooth aspen hybrids are
common in some eastern locales . Black cottonwood (P. trichocarpa)-
quaking aspen hybrids occur rarely in Alaska .
Quaking aspen has been crossed with several Populus species,
particularly the Eurasian species gray poplar (P. canescens), European
aspen, and white poplar, in tree breeding programs .
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