Regularity: Regularly occurring
Global Range: Juglans californica is endemic to California and occurs in Los Angeles, Orange, Riverside, Santa Barbara, San Bernardino, San Diego and Ventura counties (CNPS 2001). More specifically, J. californica occurs most extensively in Ventura, Los Angeles and Orange counties in the Santa Clarita River drainage near Sulphur Mountain, and smaller stands occur in the Simi Hills and Santa Susana Mountains, the northern slope of the Santa Monica Mountains, the San Jose Hills, Puente Hills and Chino Hills (Quinn 1990). In addition, J. californica does occur outside this range, but is usually found with other species of trees, and most often with oaks (Quinn 1990).
distribution of southern California walnut-dominated forests and
woodlands is limited to the Santa Clarita River drainage in the vicinity
of Sulphur Mountain, small stands in the Simi Hills and Santa Susana
Mountains, the north slope of the Santa Monica Mountains, and the San
Jose, Puente, and Chino hills. The best remaining stands are in the San
Jose Hills . Outside of this range, southern California walnut
occurs in Santa Barbara, western San Bernardino, and northern San Diego
counties . It is conspicuously absent from the coastal foothills of
the Santa Ana Mountains, San Diego County .
Southern california walnut is cultivated in Hawaii .
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):
3 Southern Pacific Border
7 Lower Basin and Range
Occurrence in North America
Southern California walnut is a native, deciduous tree that grows from
20 to 49 feet (6-15 m) tall [15,21]. It varies considerably in
morphology according to the age of the tree and site characteristics.
Trees in savanna woodland tend to have multiple trunks which grow
outward from a ring at the base, giving younger trees the appearance of
"V"-shaped shrubs. Trees in more dense stands tend to be single-stemmed
and taller [13,25]. The strongly scented trunk is blackish brown and
becomes deeply furrowed with age . The root system is extensive,
often with a deep taproot . The leaves are 1.5 to 3 inches (3.5-7.5
cm) long . Southern California walnut trees live to be about 100
years old .
Southern California walnut is monoecious [15,21]. Slender staminate
catkins develop on the wood of the previous year. Pistillate flowers
are borne singly or in clusters in short terminal spikes on the current
year's growth . The globose fruit is contained in an indehiscent
husk or shell that does not open at maturity [4,15].
Catalog Number: US 319794
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Original publication and alleged type specimen examined
Preparation: Pressed specimen
Collector(s): W. H. Brewer
Year Collected: 1860
Locality: Sierra Santa Monica., California, United States, North America
- Syntype: Watson, S. 1875. Proc. Amer. Acad. Arts. 10: 349.
Habitat and Ecology
Comments: Juglans californica occurs in chaparral, cismontane woodland, and coastal scrub on alluvial soils between 500 and 900 meters in elevation (CNPS 2001). Specifically, the best developed stands of J. californica occur on steep hills with northern exposures, and almost always on soils from Miocene-Pliocene shales (Quinn 1990).
Southern California walnut occurs in a mediterranean climate,
characterized by mild, wet winters and hot, dry summers . Trees
generally occur on mesic sites such as north slopes, creekbeds, canyon
bottoms, and alluvial terraces [5,19,25]. Trees grow best in deep,
alluvial soils with high water-holding capacity. Soils are high in
clay content [10,25]. At California State Polytechnic University, soils
beneath walnut forests are 3.3 feet (1 m) deep .
Elevation: Although southern California walnut has been successfully
planted at elevations up to 3,500 feet (1,066 m), it usually occurs from
500 to 2,500 feet (150-760 m) elevation [1,25].
Key Plant Community Associations
Southern California walnut woodland may be monospecific or mixed. Coast
live oak (Quercus agrifolia) frequently codominants in the walnut
woodland . Between Santa Barbara and Orange counties, southern
California walnut is locally dominant or codominant in the coast live
oak phase of oak woodland [1,8]. Narrow, isolated stands of southern
California walnut sometimes occur in chaparral . Occasionally,
southern California walnut is found in coastal sage scrub .
Classifications naming southern California walnut as a dominant or
indicator species are as follows:
Community ecology and distribution of California hardwood forests and
Californian evergreen forest and woodland 
Oak woodland 
Vegetation types of the San Gabriel Mountains 
Demographic structure of California black walnut (Juglans californica;
Juglandaceae) woodlands in southern California 
An introduction to the plant communities of the Santa Ana and San
Jacinto Mountains .
Associated species not previously mentioned include arroyo willow (Salix
lasiolepis), California sycamore (Platanus racemosa), white alder (Alnus
rhombifolia), California bay (Umbellularia californica), laurel sumac
(Malosma laurina), sugar sumac (Rhus ovata), toyon (Heteromeles
arbutifolia), Mexican elder (Sambucus mexicana), redberry (Rhamnus
crocea), coffeeberry (R. californica), hollyleaf cherry (Prunus
ilicifolia), birchleaf mountain-mahogany (Cercocarpus betuloides),
California scrub oak (Quercus dumosa), poison-oak (Toxicodendron
diversilobum), spiny ceanothus (Ceanothus spinosus), bigpod ceanothus
(C. megacarpus), California sagebrush (Artemisia californica),
California buckwheat (Eriogonum fasciculatum), black sage (Salvia
mellifera), fuschia-flower gooseberry (Ribes speciosum), brome (Bromus
spp.), wild oat (Avena fatua), sweetscented bedstraw (Galium triflorum),
rape mustard (Brassica rapa), wildrye (Elymus spp.), and Mexican whorled
milkweed (Asclepias fascicularis) [9,13,14,18,24,28].
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):
FRES28 Western hardwoods
FRES34 Chaparral - mountain shrub
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
246 California black oak
248 Knobcone pine
249 Canyon live oak
250 Blue oak - Digger pine
255 California coast live oak
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K030 California oakwoods
K035 Coastal sagebrush
Fire Management Considerations
is a special resource that managers usually protect from fire. However,
the understory of these forests can be burned during cool weather to
eliminate accumulated ground fuels and produce a shaded fuelbreak .
Quinn  suggested that prescribed fires of low intensity, at intervals
of several years, be tested for their effects on southern California
Plant Response to Fire
Southern California walnut recovers well from fire. It sprouts
vigorously from the trunk and root crown when top-killed by fire, but
does not produce seedlings, an indication that most seeds are killed by
fire . In Los Angeles County, 10-year-old southern California
walnuts were severely burned. Sprouts from the root crowns reached
5 feet (1.5 m) during postfire year 1 . Southern California walnut
was sprouting from the root crown 3 years and 8 months after a fire in
Big Sycamore Canyon, Ventura County, in the fall of 1973 .
Several hundred trees were burned in July 1989 at California State
Polytechnic University. One year after fire there was no evidence of
dead trees, even though most of the branches and stems had been
top-killed. Almost all of the trees sprouted from the root crown within
6 weeks of the fire .
Immediate Effect of Fire
Southern California walnut has large woody platforms at the soil
surface. The platforms shield the meristematic tissue beneath them from
fire. After fire, sprouts surround the platforms, resulting in multiple
trunks . According to Quinn , the basal platforms are an
adaptation to fire similar to the lignotuber.
Most southern California walnut woodlands are subject to periodic fires.
Fire is an annual possibility in most locations, where dead annual
grasses are present beneath and between the trees during the summer fire
Growth Form (according to Raunkiær Life-form classification)
years of age . Variations in precipitation from year to year can
affect fruit production and seedling establishment. In drought years
little or no fruit is produced . Seeds do not have a dormancy
period and usually germinate within 4 weeks of dispersal [4,13]. In the
spring in the San Jose Hills, densities of 4,742 seedlings per acre
(2,000/ha) have been reported . The western gray squirrel may be an
important dispersal agent for walnut seed .
Vegetative reproduction: Southern California walnut sprouts from the
root crown and trunk following cutting or burning [25,29].
Life History and Behavior
More info for the term: tree
Southern California walnut leaves appear in January and February, with
all trees in full leaf by March . Trees on warmer or drier sites
develop leaves several weeks earlier than those in cooler, more mesic
locations. Flowering begins about the same time as leaf production,
with fruits developing to full size during spring. By late summer
fruits have matured. Fruit abscission begins in October and November,
but some fruits remain on the tree throughout winter .
Molecular Biology and Genetics
Barcode data: Juglans californica
Statistics of barcoding coverage: Juglans californica
Public Records: 1
Specimens with Barcodes: 1
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
- Needs updating
- 1997Rare(Walter and Gillett 1998)
National NatureServe Conservation Status
Rounded National Status Rank: N3 - Vulnerable
NatureServe Conservation Status
Rounded Global Status Rank: G3 - Vulnerable
Reasons: Juglans californica is endemic to California and occurs in Los Angeles, Orange, Riverside, Santa Barbara, San Bernardino, San Diego and Ventura counties (CNPS 2001). While the California Native Plant Society considers Juglans californica rare, it is currently found in sufficient numbers and is distributed over a wide enough range that currently the potential for extinction is low (2001). Southern California black walnut is, however, threatened by urbanization, grazing, potentially a lack of natural reproduction, increased fire frequency in some areas and exotic understory species (pers. comm. Quinn 2002, CNPS 2001 and Quinn 1990).
urbanization. The Nature Conservancy, in cooperation with the state of
California, is giving high priority to aquiring vegetative/habitat data
on the woodland. They list it as one of California's rare and imperiled
natural communities [1,5].
Comments: Juglans californica is considered fairly threatened by the California Native Plant Society. It is threatened by urbanization, grazing, and potentially a lack of natural reproduction (CNPS 2001). Walnut forest is also highly fragmented and a declining natural community in California. Walnut forest is rare in Orange, Riverside, San Bernardino and San Diego counties (CNPS 2001). Quinn (1990) comments that the reversal of ecological changes to J. californica communities caused by overgrazing, increased fire frequency and exotic understory species is necessary in order to manage this species. Posing the largest threat is urbanization. Specifically, in 1990 one of the most extensive stands in Puente Hills, Los Angeles co. was under threat of removal of thousands of trees due to construction of a golf course (Quinn 1990). Additional threats that existed in 1990 included proposals to bulldoze J. californica trees for implementation of a housing subdivision, and an expressway. In the San Jose Hills one of the most developed stands of J. californica was demolished due to expansion of a landfill (Quinn 1990). As of 2002, plans to build the golf course were halted by Cal Poly Pomona campus protests, the expressway was not built, however, some interest does still exist, and the subdivision proposal did not come to fruition (pers. comm. Quinn 2002). Further, there are extensive stands of walnuts at the eastern portion of the Puente Hills that are currently not threatened by development, but withstand extensive cattle grazing. While development efforts in the past have been thwarted, the threat of urbanization or land development continues to exist, as currently new proposals to develop land containing J. californica are being explored(pers. comm. Quinn 2002).
include urban and rural development, overgrazing, and increased
recreational use of walnut woodlands [14,25]. In Aliso Creek, Chino
Hills State Park, cattle grazing initiated dry conditions, which were
worsened by a 5-year drought. The resulting very dry environment
hindered survival of walnut seedlings . Grazing has been the
principal economic activity in California walnut forests and woodlands
for 200 years. The species composition of the southern California
walnut woodland understory in the Puente and San Jose hills is
attributed to overgrazing by cattle .
Pathogens: Southern California walnut is highly susceptible to crown
(Phytophthora spp.) rots. Walnuts planted in soil infested with P.
citricola and flooded for 48 hours biweekly showed reduced growth and
high rates of mortality .
In the San Jose Hills, southern California walnut develop heart rot
between 20 to 30 years of age. Portions of the trunk and older limbs
subsequently become infested with termites and wood-boring beetles.
Older multistemmed trees often have some stems that are healthy, some
with heart rot, and others that are dead .
Relevance to Humans and Ecosystems
Value for rehabilitation of disturbed sites
control on road slopes with deep soil at elevations below 3,500 feet
(1,066 m). Best growth is achieved in partial shade . In Los
Angeles County, southern California walnut was planted in brush wattles
during construction of a road fill. Trees reached heights of 12 feet
(3.7 m) in 10 years .
Larger southern California walnut trees provide excellent cover for
deer, nesting birds, and rodents . Raptors such as owls use the
upper reaches of trees as roosts and nesting places. California ground
squirrels dig burrows at the bases of old trees .
Other uses and values
grown commercially . Chumash Indians ate the walnuts and used the
nutshells for dice. They used the bark for making baskets .
Southern California walnut is suitable for ornamental landscaping and is
widely planted in urban forestry projects [11,13].
Importance to Livestock and Wildlife
habitat for a number of vertebrates and invertebrates. A 2-year survey
in a southern California walnut woodland in the San Jose Hills found 29
species of diurnal birds . Many rodents, including California
ground squirrels and western gray squirrels, eat the nuts [11,25]. The
nuts are rarely eaten by deer .
Juglans californica, the California black walnut, also called the California walnut, or the Southern California black walnut, is a large shrub or small tree (up to 30 feet tall) of the Juglandaceae (walnut) family endemic to California.
J. californica is generally found in the southern California Coast Ranges, Transverse Ranges, and Peninsular Ranges, and the Central Valley. It grows as part of mixed woodlands, and also on slopes and in valleys wherever conditions are favorable. It is threatened by development and overgrazing. Some native stands remain in urban Los Angeles in the Santa Monica Mountains and Hollywood Hills. J. californica grows in riparian woodlands, either in single species stands or mixed with California's oaks (Quercus spp.) and cottonwoods (Populus fremontii).
Juglans californica can be either a large shrub with 1-5 trunks, or a small, single-trunked tree. The main trunk can fork close to the ground, making it look like two trees that have grown together, then diverged. It has thick bark, deeply channeled or furrowed at maturity. It has large, pinnately compound leaves with 11-19 lanceolate leaflets with toothed margins and no hair in the vein angles. It has a small hard nut in a shallowly grooved, thick shell that is difficult to remove.
Some authorities (e.g. the California Native Plant Society) combine this species with J. hindsii. On the other hand, a 2007 molecular analysis of the genus suggests J. californica is sister to the remaining black walnuts (Rhysocaryon). This article follows the conventions of The Jepson Manual.
- Essa, Lora. "Index of Species Information". U.S. Department of Agriculture, Forest Service,. Retrieved 2013.
- Kershner, Mathews, Nelson, and Spellenberg, National Wildlife Federation field Guide to Trees of North America, 2008, Chanticleer Press, Inc. p. 229
- Aradhya, M. K, D. Potter, F. Gao, & C. J. Simon: "Molecular phylogeny of Juglans (Juglandaceae): a biogeographic perspective: Tree Genetics & Genomes (2007)3:363-378
- Juglans californica S. Watson var. hindsii Jeps.
- Juglans hindsii Jeps. ex R. E. Sm.
- Hickman, James C., ed. The Jepson Manual: Higher Plants of California. University of California Press. 1993.
- Anderson, E. N. "Some preliminary observations on the California black walnut (Juglans californica)" in Fremontia: A Journal of the California Native Plant Society. January 2002.
Names and Taxonomy
California black walnut
southern California black walnut
Juglans californica S. Watson [19,25,37]. There are two varieties: J.
c. var. californica (southern california black walnut) and J. c. var.
hindsii Jepson (northern California black wanut). California black
walnut hybridizes readily with black walnut (J. nigra) and English
walnut (J. regia).