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Brief Summary

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Acacia is a genus of shrubs and trees, often spine-bearing, that belongs to the subfamily Mimosoideae (family: Fabaceae). The fruits are the seed-bearing pods typical of Fabaceae (formerly Leguminosae). The generic name derives from ακακία (akakia, from ακις [akis, thorn]). Until recently, ~1300 species worldwide were classified as Acacias—about 960 native to Australia, with the remainder in tropical to warm-temperate regions of Europe, Africa, southern Asia, and the Americas. However, in 2005, taxonomists divided Acacia into five separate genera: Acacia (the Australian species, with a few in tropical Asia, Madagascar and Pacific Islands); Vachellia and Senegalia (which include most species outside Australia); and Acaciella and Mariosousa (which each contain about a dozen species from the Americas). Acacia leaves are generally compound pinnate. In some species, however, the leaflets are suppressed, and the leaf-stalks (petioles) become vertically flattened into phyllodes, oriented vertically to avoid intense sunlight. Some species (such as A. glaucoptera) lack leaves or phyllodes altogether, but possess instead cladodes, modified leaf-like photosynthetic stems functioning as leaves. Various species have sap that hardens into gum. Flowers are small with five petals, almost hidden by 10 (or more) long stamens, and arranged in dense globular or cylindrical clusters; they are generally yellow or cream-colored, but can be white, purple, or red. Acacias have diverse and extensive uses, ranging from food (many species contain edible shoots and seeds, and the flowers are used to produce a delicate and non-crystallizing honey) and medicine to paint and perfume to timber and firewood. The leaves of many species bear large amounts of tannins, with wide commercial production for tanning leather. In the North America and Europe, acacia ingredients are used in soft drinks, chewing gums, candies, and mints. The sap of A. senegal and A. seyal yields gum arabic, which is used in foods, paints, inks, cosmetics, hair products, textiles, and numerous other products (Wikipedia 2011). Acacias are used to make incense, and may have been the “burning bush” of the Bible and the wood used to construct the Ark of the Tabernacle. Many acacias produce alkaloid chemicals as defensive compounds to deter insects and mammalian herbivores. Some of these alkaloids are toxic to livestock, while others have psychoactive properties. Acacias interact in various well-known mutualisms, such as one in which swollen thorn acacias of Costa Rica offer food and shelter to ants in exchange for protection from plant competitors and insect herbivores (Janzen 1966). (See National Geographic video, http://video.nationalgeographic.com/video/player/animals/bugs-animals/ants-and-termites/ant_acaciatree.html.) In another notable mutualism, acacia seeds that pass through the gut of large mammalian herbivores (such as elephants) are more likely to germinate—not because the digestive juices soften the hard seed coat, as previously thought, but because they kill the Bruchidae beetle larvae that parasitize most uneaten seeds (Attenborough 1995, Palmer et al. 2008); see ARKive video of a bruchid beetle eating seeds of A. tortilis (http://www.arkive.org/umbrella-thorn/acacia-tortilis/video-11b.html).
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Protected areas in East Africa may not be conserving iconic plants - News and events, The University of York

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A new study led by researchers from the University of York suggests protected areas in East Africa are not conserving plants such as the iconic Acacia tree.

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Description

provided by Flora of Zimbabwe
Trees or shrubs, sometimes climbing, armed with thorns (native spp.) or unarmed (introduced spp.). Leaves alternate, 2-pinnate or (in some introduced spp.) modified to phyllodes and thus appearing to have simple leaves. Pinnae opposite, subopposite, sometimes alternate. Gland usually present on the upper side of the petiole, sometimes also glands on the rhachis. Inflorescences usually axillary, racemose or paniculate; flowers in elongated spikes or spherical heads, bisexual or unisexual. Calyx gamosepalous, subtruncate or with 4-5 lobes. Corolla 4-5(-7)-lobed. Stamens numerous. Anthers glandular or eglandular. Pod very variable, dehiscent or indehiscent. An important genus, occurring commonly in a wide range of habitats. A careful distinction should be made between the stipules which in some species are modified into straight spines arising from near the leaf base and prickles, which are usually curved and are usually found near the nodes but may occur along the stems. Some acacias, especially older trees, may be unarmed, which can be very puzzling. It is recommended that a careful search is made of the tree (if possible) or of similar trees in the area. An important general rule with acacias is that those with straight spines (stipules) have flowers in spherical heads and those with curved prickles have flowers in spikes. The exceptions are: Faidherbia albida has straight spines and flowers in spikes. Acacia mellifera has curved prickles but the flower spike is so short as to appear almost spherical. Acacia schweinfurthii has curved prickles and flowers in spherical heads. Acacia tortilis has both curved prickles and straight spiny stipules and flowers in spherical heads.
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bibliographic citation
Hyde, M.A., Wursten, B.T. and Ballings, P. (2002-2014). Acacia Flora of Zimbabwe website. Accessed 28 August 2014 at http://www.zimbabweflora.co.zw/speciesdata/genus.php?genus_id=667
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Bart Wursten
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Petra Ballings
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Associated Forest Cover

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Botanists and foresters have listed more than 80 trees, shrubs, vines, herbs, ferns, club mosses, grasses, and sedges associated with koa. Trees associated with koa (20,33,48) include:


'ahakea (Bobea spp.)
'ala'a (Pouteria sandwicensis)
kalia (Elaeocarpus bifidus)
kauila (Alphitonia ponderosa)
kawa'u (Ilex anomala)
kolea (Myrsine lessertiana)
kopiko (Psychotria spp.)
loulu palm (Pritchardia spp.)
mamani (Sophora chrysophylla)
naio (Myoporum sandwicense)
'ohe'ohe (Tetraplasandra hawaiiensis)
'ohi'a (Metrosideros polymorpha)
'olapa (Cheirodendron trigynum)
olomea (Perrottetia sandwicensis)
olopua (Osmanthus sandwicensis)
pilo (Coprosma spp.)
sandalwood (Santalum spp.)

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Climate

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Hawaii is tropical in latitude, with mild and equable temperatures at low elevations (table 1). Day length is nearly uniform year-round, varying by 2 hours. The northeasterly trade winds dominate; however, "Kona" storms from the south or west during winter, and occasional tropical storms throughout the year, bring high winds and heavy rains to the islands. Hawaii's mountains, especially massive Mauna Loa and Manna Kea on Hawaii, and Haleakala on Maui, have a strong influence on the weather and provide climates ranging from the tropic to the subarctic (7).

Table 1-Mean temperature at five stations on the east flank of Mauna Kea, island of Hawaii¹

Station Elevation
(m) Mean Temperature January
(°C) August
(°C) Olaa (6) 85 21 24 Waiakea Forest 550 18 21 Waiakea Forest 915 17 19 Waiakea Forest 1220 13 16 Kulani Camp (6) 1580 4 14

(ft) (°F) (°F) Olaa 280 70 75 Waiakea Forest 1800 64 69 Waiakea Forest 3000 62 67 Waiakea Forest 4000 55 61 Kulani Camp 5190 39 5 ¹Data on file at the Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Honolulu, HI.

Rainfall varies greatly within short distances. Monthly amounts recorded over a period of years at weather stations in the koa belts show a phenomenal range. A Forest Service station at 1200 m (4,000 ft) elevation recorded a mean annual rainfall of 4300 mm (170 in) for a 14-year period, with extremes of 3450 to 5500 mm (136 to 216 in). During the driest month, only 19 mm (0.74 in) was recorded; the wettest month was 1380 mm (54.4 in).

Koa grows best in the high rainfall areas, those receiving 1900 to 5100 mm (75 to 200 in) annually. It also grows in areas that receive much less than this amount, but growth is slower and tree form is generally poorer. Cloud cover and fog commonly shroud the middle forest zone (600 to 1800 m or 2,000 to 6,000 ft) where commercial koa stands are concentrated. Frost is not uncommon during winter months above 1200 m (4,000 ft) elevation. Temperature ranges within the koa belt are small, as may be seen from data for Mauna Kea, island of Hawaii (table 1).

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Damaging Agents

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Hawaiian forestry literature is full of references to the disastrous effects of cattle, pigs, sheep, and goats on koa and other native species (1,4,5,1 7,26,35,38,40,77). Records of the Hawaii Division of Forestry and Wildlife show that more than 250,000 pigs, goats, and sheep were destroyed from 1921-46 in the forests of the island of Hawaii (10) during an eradication program. Such efforts did much to reduce the amount of browsing by these animals on koa forests. Feral cattle are particularly fond of koa root sprouts, seedlings, pods, and leaves. They straddle and trample large saplings to devour the foliage and bark. Feral goats have nearly disrupted the replacement cycle of koa on the Hawaii Volcanoes National Park (84). In recent years, park rangers have taken steps to radically reduce the size of the g oat herds within the park. A study was conducted on the recovery of vegetation on koa parklands on Maui following the exclusion of goats. After 7 years, young koa regeneration was present both within and outside the enclosure, but the koa got large only if the goats were excluded (69). Koa will recover on these parklands if goats are eliminated. A large number of feral pigs inhabit the kon rain forests, and their rooting destroys many koa seedlings. It is thought that if the pig population is permitted to increase, the koa rain forest ecosystem will deteriorate (16).

Koa attracts other kinds of animals. Black-tailed deer, introduced from Oregon to the island of Kauai in 1961, eat koa seedlings, but have little impact on the native vegetation. Less than 10 percent of the koa was browsed (94). The tree rat and the Hawaiian rat damage koa saplings by stripping off bark. One thousand koa saplings (2 to 5 years old) along an elevation transect from 770 to 1330 m (2,520 to 4,370 ft) in the Laupahoehoe area of the Hilo Forest Reserve were examined (71). Thirty percent of the trees had been wounded by rats, with wounds occurring as high as 10 m (33 ft). Bark along the main trunk and on lateral branches was subject to stripping. Terminal and lateral shoot dieback were observed where complete girdling occurred. In a study of mortality of koa saplings severely wounded by rats, damage was reported most severe in the vicinity of brush piles where nests were likely to be located.

In 1925, more than 40 species of native insects were considered enemies of koa (92), and by 1983 the number of phytophagous insects associated with koa reached 101 (87). Insect damage to koa is well documented (18,22,58,59,91). One authority believes "there are more endemic insect species attached to this koa complex (Acacia koa and related koa members) than to any other genus in the Hawaiian islands" (93)

One of the most destructive insects of koa is the koa moth (Scotorythra paludicola), a lepidopterus defoliator found on the islands of Hawaii, Maui (105), Oahu, and Kauai ( 87). Severe outbreaks occur periodically. When these insects appear in large numbers, they may completely defoliate koa stands. Following an outbreak on Maui in which 1841 ha (4,550 acres) were completely defoliated, growth was reduced 71 percent, and about one-third of the trees died within 20 months (88).

The introduced koa haole seed weevil (Araecerus levipennis) is the most prevalent insect that infests koa seeds, the next most common being Stator limbatus (85). The koa seedworm (Cryptophlebia illepida) destroys seeds and is a problem to control when seeds are collected for reforestation purposes. Eighty percent of the damage from this Tortricid occurs above 1037 m (3,400 ft) (85). Three other Tortricid species destroy koa pods or seeds (85,91). These seed moths may destroy 90 percent or more of any given seed crop in the pods (93). Stein (86) reviewed the biology and host range of koa seed insects, their parasites, and hyperparasites.

At high elevations, koa terminals are sometimes heavily attacked by the Fuller rose beetle (Pantomorus cervinus), but the attacks appear to be highly seasonal and of no serious consequence. The acacia psyllid (Psylla uncatoides), first found in Hawaii in 1966, feeds and breeds in the new growth of koa. This psyllid also has become a serious pest of the closely related koaia (Acacia koaia) (47). The black twig borer (Xylosandrus compactus) is associated with injury and mortality.

Information on diseases of koa has increased in recent years. Seedlings may be attacked by Calonectria theae, which causes a shoot blight (55) and C. crotalariae, which causes a crown rot (57). This pathogen also caused a collar rot that severely affected koa seedlings regenerating a burned-over area (2). A wilt disease, Fusarium oxysporum, was observed among koa seedlings (24). This fungus may contribute to the premature decline or death of old koa trees growing within the Hawaii Volcanoes National Park. Indications are that this fungus is seed-borne, but seed disinfection did not reduce disease incidence (24). Koa was moderately tolerant to Phytophihora cinnamomi in greenhouse tests (42).

Dieback is common in the crowns of old trees, and it was observed in more than half the sawtimber-size koa measured during the 1959-61 forest survey The root-rot fungus Armillaria mellea is associated with this dieback (44,61). Stands possibly weakened by old age, extended droughts, and grazing have succumbed to attacks by this fungus. Other diseases of koa are those caused by the sooty molds, such as Meliola koae, that cover the leaves and restrict growth.

Four rust fungi, Uromyces koae, U. digitatus, Endoracejum acaciac, and E. hawaiiense, occur on koa (25,32). Both species in the genus Uromyces, obligate parasites, cause witches' brooms and leaf blisters that deform branches and phyllodes. When infections are heavy, they can deform and debilitate both young and old trees (23,30,31).

The Hawaiian mistletoe (Korthalsella complanata) has been observed in many koa stands, and it can deform young koa. Heart rot, caused principally by Laetiporus sulphureus and Pleurotus ostreatus, is common in most mature and overmature koa (6). More than half the large koa measured in the 1959-61 forest survey were unmerchantable because of excessive rot (98).

Pole-size and small, sawtimber-size koa have thin bark and are easily damaged by fires.

Weeds are serious problems in certain areas. The banana poka (Passiflora mollissima) smothers both koa reproduction and mature trees by laying a curtain of vines over them. The German-ivy (Senecio mikanioides) is also difficult to control.

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Flowering and Fruiting

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The flowers of koa are borne over the outer part of the crown. Seedlings have been observed in flower and fruit (3,80) at 2 and 3 years of age. One of the pollinating insects found on koa flowers is the honeybee (Apis mellifera). The extent to which other insects, birds, and wind affect pollination is not well documented. Koa initiates flower development nearly year-round at the high elevation on Mauna Loa, reaching a peak during the wet season in late winter (46). On adjacent Mauna Kea, koa flowers appear from December through February, with few flowers at any other time. At lower elevations, on all of the islands, flowering usually occurs from late winter to early summer (July). Weather conditions, especially severe droughts, influence the timing and extent of flowering at any time of the year.

The inflorescence of koa is an axillary raceme of pale yellow heads averaging 8.5 mm (0.3 in) in diameter (29), one to three on a common peduncle, and composed of many hermaphroditic (bisexual) flowers. Each flower has an indefinite number of free stamens and a single elongated style. The heads are highly dichogamous, with anthers dehiscing 3 to 8 days before the stigmas are fully exserted (8).

The fruit is a legume, slow to dehisce, about 15 cm (6 in) long and 2.5 to 4 cm (1 to 1.5 in) wide. The pods contain about 12 seeds that vary from dark brown to black. They mature at different times throughout the year, depending on location and weather conditions.

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Genetics

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Morphological differences in koa have been observed on several islands. In 1920, Rock (63), named two varieties: Acacia koa var. lanaiensis (Hillebrand's A. koa-B var.) and A. koa var. hawaiiensis, after the islands on which they were found. Ecotypic variation can be found from island to island. Studies of such variation are complicated by past plantings of mixed seed lots collected throughout the islands; such mixed plantings could now be hybridizing.

Collections of the koa group, all commonly referred to as koa, were studied, and in 1979 this classification was presented by St. John (90):
Acacia koa
var. koa, grows on the six larger is-lands.
Acacia koa var. waianacensis grows only on Oahu, and most commonly in the Waianae Mountains.
Acacia koa var. latifolia; syn. A. koa var. hawaiiensis Rock, grows on the island of Hawaii in the rain forest, and at higher elevations on the more open ranch and park land. Altitudinal races of koa probably exist (52).

Two other native species related to koa are recognized. On western Kauai, one of the oldest Hawaiian Islands, a form of acacia is found that differs from koa in sepals, petals, inflorescence (63), and seed shape (37). This species, also called koa, is Acacia kauaiensis. A second species closely related to koa is koaia (A. koaia), a narrowly distributed, small, shrubby tree occupying dry, leeward sites below 1050 m (3,500 ft) on Molokai, Maui, and Hawaii (89). Acacia koaia differs from koa in the shape of the pods and phyllodes (63). The native and introduced species of Acacia found on Lanai have been described (20).

Other Acacia species related to koa are found outside of Hawaii. Mascarene acacia (A. heterophylla) is endemic to Reunion island and Mauritius island, both about 725 km (450 mi) east of Madagascar, in the Indian Ocean. It is so similar to koa that the trees were initially identified as the same species. The two were identified by another botanist as separate species, however, entirely on the basis of distance and isolation. In 1969, significant differences were found in fruit and seed size, corolla structure, and morphology of the first two leaves of the two species (95).

Tasmanian blackwood (Acacia melanoxylon), native to Australia but planted in many countries, resembles koa. It has straighter and shorter phyllodes, a narrower curved pod, a more pointed crown (63), but similar wood. Another closely related species, A. simplicifolia, grows in Samoa, New Hebrides, New Caledonia, and Fiji (20,59,60).

In 1948, one investigator determined that koa is a tetraploid with 2n = 52 and stated that all other phyllodinous acacias studied have the diploid chromosome complement (3). He reasoned "that polyploidy in Acacia koa occurred after the initiation of phylloidy. This is supported by its distribution as an endemic island extension of the Australian flora." In 1978, koa was observed to have a gametic number of 26, verifying that it is tetraploid (14). Another investigator (95) reported on the work of Lescanne, who observed that the closely related A. heterophylla was also a tetraploid, with 2n = 52.

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Growth and Yield

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Age of koa trees cannot be determined. Growth rings were not correlated with "annual rings" (102). Old relic forests still in existence were probably present at the time Captain James Cook discovered the Hawaiian Islands in 1778. The largest koa tree on record had a d.b.h. of 363 cm (143 in), total height of 43 m (140 ft), and a crown spread of 45 m (148 ft) (56).

Stocking and growth data for natural regeneration on heavily disturbed sites and one plantation on the island of Hawaii are available (table 2).



Table 2-Characteristics of koa growing in three natural stands and a plantation in Hawaii¹

Location Annual rainfall
(mm) Age
(yr) Stand stocking
(stems/ha) Dominants D.b.h. (cm) Height (m) Natural stands 1 3810 8 3460 12.7 6.0 2 5080 17 790 23.1 17.4 3 2540 15 2720 18.5 13.1 Plantation 3810 27 395 31.0 14.4 (in) (yr) (stems/acre) (in) (ft) Natural stands 1 150 8 1400 5.0 19.6 2 200 17 320 9.1 57.0 3 100 15 1100 7.3 43.0 Plantation 150 27 160 12.2 47 ¹Ching, Wayne F. 1981. Growth of koa at selected sites on the island of Hawaii. Unpublished report. HDepartment of Land and Natural Resources, Division of Forestry and Wildlife, Honolulu, HI. l0p.

The form of koa varies greatly. Most mature trees have large, open, scraggly crowns with limby, fluted boles. In the rain forests, on deep, rich soil, an occasional koa tree may surpass 34 m (110 ft) in height, but few possess clean, straight boles. On drier sites, the form of koa is even poorer, and trees are often stunted and misshapen. Precise yield figures from koa stands are not available.

Missing from the koa and ohia-koa forests in many areas are the koa-size classes that normally form the recently mature, vigorous stands. In 1913, the condition of large tracts of koa forest was graphically described by Rock (62):

"Above Kealakekua, South Kona, of the once beautiful koa forest, 90 percent of the trees are now dead, and the remaining 10 percent in a dying condition. Their huge trunks and limbs cover the ground so thickly that it is difficult to ride through the forest, if such it can be called.... It is sad, however, to see these gigantic trees succumb to the ravages of cattle and insects." Forest survey data from 1959-61 (98) indicated the condition of much of the sawtimber-size koa (trees more than 27.7 cm [10.9 in] in d.b.h.). Of 103 trees classified according to merchantability on the basis of form and defect, 36 percent were merchantable, 15 percent sound cull (with such defects as crook, excessive limbs, or poor form), and 49 percent rotten cull (excessive rot). Of the 103 trees, the average d.b.h. was 89 cm (35 in); of 31 trees, the average height was 22 m (72 ft), and the average crown diameter was 18 m (58 ft). Log grades were determined for logs in 103 koa trees. Less than two-fifths of all butt logs (first 4.9 m [16 ft]) met the specifications for either factory lumber logs or tie and timber logs. More than three-fifths were cull. Only 35 percent of the 103 trees sampled had an upper log of 2.4 m (8 ft) or more, and more than half of these logs were graded cull (98). Remeasurements in 1969-70 of the plots inventoried 10 years earlier (54) permit estimates of annual growth and mortality of koa on the island of Hawaii. Net annual growth was found to be a negative 4.52 million board feet of sawtimber and a negative 15 660 m³ (553,000 ft³) of growing stock (50). One study offers guidelines for estimating the volume of unsound wood associated with log surface defects common in koa (12)

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Reaction to Competition

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Koa is classed as intolerant of shade both in the dry forest (28) and in the rain forest, and at all ages (26). Under favorable light, moisture, and soil conditions, koa competes aggressively with other vegetation.

Koa has been classified in various ways by different investigators. One referred to koa as a pioneer species on the grassy slopes of dry forest sites (28), but another considered it a climax species (21). Koa has been considered the ultimate forest type, following the ohm forest on the ancient "an" lava flows (37). "At maturity a grove (of koa) casts a shade in which its own seedlings have difficulty in growing, and unless they fill a vacancy in the parental ranks, they must seek the outer limits of the stand" (64). Another investigator believed that koa "reproduction need not be especially frequent to maintain the forest (type)"

Koa failed when underplanted in a dense native ohia rain forest at 870 m (2,850 ft), showing poor survival (44 percent), vigor, and form (70 percent cull), but three introduced nonleguminous species from Australia performed well (100).

The effect of thinning and/or fertilizing a 12-year-old, stagnated kon stand were studied on the island of Maui. In this precommercial thinning, the number of stems was reduced 50 percent. Basal area growth rates for a 3-year period indicated that thinning increased growth significantly. Fertilizer yielded limited response; and the investigator thought that the fertilizer should be applied before crown closure (72).

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Rooting Habit

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Little is known of the root development of koa. The tree grows on the deep Hawaiian soils, but also reaches impressive size on the shallow "an" lava flows. "The root system of the mature koa is shallow and extensive, spreading out radially from the base for distances as great as 30 m (100 ft) or more" (5). "The tree has a shallow rooted system, a flat plane of roots spreading out in all directions just beneath the surface of the ground. For this reason the larger top-heavy trees are easily overturned by severe windstorms...." (37). Large koa trees were toppled during a severe earthquake on the island of Hawaii in 1973. In describing the root systems of lava-flow plants, a researcher classified koa as one of the comparatively deep-rooted woody species (48).

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Seed Production and Dissemination

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No records of the frequency of exceptionally good or poor seed years are available, but seed years do vary. Koa seed pods dehisce while on the tree or fall to the ground unopened, where they either dehisce or disintegrate. "The horny seed often remains on the tree for a year after it ripens, and when lying on the ground is known to have retained for a period for 25 years its ability to germinate" (37). Koa seeds are seldom dispersed far beyond the crown, but, occasionally, wind may carry unopened pods some distance. Seeds from koa growing in gulches may be carried downstream to lower elevations, especially during torrential rains.

Koa seeds, like those of other acacias, are among the most durable of tree seeds and need not be kept in sealed containers. They will germinate after many years of storage if kept in a cool, dry place. The seeds have hard coats that retard germination unless they are first mechanically scarified, briefly treated with sulfuric acid, or soaked in hot water. The water treatment is the most practical. The seeds are placed in nearly boiling water, after the heat source is removed, and allowed to soak for 24 hours. Seeds that fail to swell the first time may again be subjected to this pregermination treatment, often with success (99). In seven samples, the number of clean seeds ranged from a low of 5.3OO/kg to a high of 16,3OO/kg (2,400 to 7,400/lb).

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Seedling Development

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The mode of germination is epigeal (99). Light is not a requirement for germination (83). Under favorable conditions-bare mineral soil, adequate moisture, and exposure to sunlight-koa seedlings will grow readily. Soil aeration and soil temperature may influence germination (83).

Until recent years, the standard nursery practice was to sow koa seeds in wooden flats, then transplant the seedlings to tin cans (35). Now, plastic bags or tubes are used. Tube-grown seedlings are easier to plant.

Properly pretreated koa seeds should be covered with 6 to 12 mm (0.25 to 0.5 in) of soil; they begin to germinate within a week. Seedlings in bags or tubes can be grown to plantable size of 20 cm (8 in) high in 10 to 14 weeks.

Direct seeding of koa on prepared seed spots has been moderately successful (9,13) In two trials comparing broadcast sowing with direct sowing into prepared spots, stocking was four times higher on the direct seeded spots on Maui, whereas no difference in the percentage of stocked spots or of height growth was evident on the island of Hawaii.

Koa has been recommended for watershed planting on well drained areas (34,37,39) and is described as "the one native tree which can be easily handled in nursery and planting operations... suitable for the larger portion of areas in need of reforestation and particularly for the drier ridges and slopes" (35).

Other investigators, less enthusiastic about planting koa, did not recommend it (13,17), commenting as follows: "Results on older soil formations have been uniformly disappointing. Frequently, the trees die out after 15 to 20 years" (17). Plantations established on Maui during the late 1930's developed scattered, large trees of exceptionally poor form. Relatively few koa seedlings were planted after World War II. However, in the past 10 years, private land owners on the island of Hawaii, influenced by the short supply, began planting koa (104).

Seedlings usually appear soon after land is cleared for pasture or roads, or after fires. As many as 354,700 koa seedlings per hectare (143,537/acre) were counted in the vicinity of old koa trees in burned-over areas (41). Seeds escaping the flames may be induced to germinate by the heat.

Koa seedlings grow rapidly. One month after a burn, koa seedlings were at least 2.5 cm (1 in) tall; after 3 months they ranged from 10 to 28 cm (4 to 11 in) tall, averaging about 13 cm (5 in) (41). On a cleared area at 500 m (1,700 ft) elevation, 1-year-old seedlings ranged from 0.6 to 4 m (2 to 13 ft) tall and averaged 2 m (6 ft). On favorable sites, seedlings attain 9 m (30 ft) in 5 years (37). Eight months after a fire on Kauai, koa regeneration was most common near fire-killed parent trees, and maximum height growth was 4.6 m (15 ft) (103). The abundance, distribution, growth, and mortality of koa on burned-over areas on Oahu were monitored over a 2.5-year period (73). During this time, seedling density declined dramatically. The root-crown fungus Calonectria crotalariae caused more than half of this mortality. On these sites the seedlings grew about 2.3 cm (1 in) per month. Koa did poorly when planted on abandoned sugarcane land on the windward coast of the island of Hawaii. Survival at age 6 years was 78 percent, but trees averaged only 3 m (10 ft) tall, and only 62 percent were judged vigorous. Tree form varied from good to poor, with 77 percent cull (101).

The abundance and distribution of natural regeneration after logging were studied on a 200-ha (500-acre) tract heavily infested with pigs and vines on the island of Hawaii (70). Seedling density of koa was about three times as great in disturbed as in undisturbed areas. Most koa seedlings found on the ground disturbed by logging were well established, but none of those growing on undisturbed ground were large enough to have much chance of surviving the menacing pigs and cattle. Koa seedlings in disturbed areas tend to be clustered around seed trees (70). In 1922, Krabel stated: "Where cattle have been excluded for a number of years, koa groves are developing with surprising speed on exposed and barren ridges" (43).

The stimulating effect of soil scarification on seedling emergence is helpful in regenerating koa on degraded forest land where seed reserves still exist in the soil. Disking in the sparsely wooded pastures of the Hakalau Forest National Wildlife Refuge resulted in koa reproduction. Even in open areas far removed from live or skeletal remains of koa, a few seedlings emerged (15).

In the natural rain forest, koa seedlings can emerge from mineral soil and organic seedbeds, such as decaying logs and treefern trunks. Seedling growth is generally slower on old logs than on mineral soil, possibly due to low nutrient availability. However, seedlings tend to survive better on organic seedbeds because these sites are elevated and out of reach of feral pigs. In the Kilauea Forest, more than 60 percent of the mature koa initially emerged from logs or other large organic seedbeds (16). Nevertheless, rarely do koa seedlings survive in the dense rain forest unless openings have been created, as by windthrow. Gap-phase replacement seems to be the primary mechanism by which koa is maintained in natural rain forest communities (53). Serious disturbances, such as fire or hurricane-induced windthrow, typically stimulate large-scale koa reproduction.

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Soils and Topography

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Koa is found on volcanic soils of all geologic ages and degrees of development, from the young ash and "aa" lava rock soils on the island of Hawaii to the oldest soils on Oahu and Kauai. The tree grows best on moderately well drained and well drained, medium to very strongly acid soils. These recent soils are higher in plant nutrients, having been subjected to less leaching and erosion than have the soils on the older islands.

Most koa forests grow on two of the great groups in the soil order Inceptisol: Hydrandepts and Dystrandepts. Hydrandepts are found in areas of high rainfall. They are high in amorphous materials and have high cation exchange capacities, but extremely low base saturations due to the high rainfall. Although deficient in available phosphorus, sodium, potassium, calcium, and silica, they have a high content of organic matter and hydrous oxides of iron and aluminum, manganese, and titanium. Infiltration rates are rapid and erosion is slight to moderate, depending upon the degree of slope. Dystrandepts are formed under lower rainfall than the Hydrandepts. They have slightly greater base saturations than the Hydrandepts.

The next most abundant soil great group on which koa grows is the well drained Tropofolists (organic soils of the order Histosols). Other minor soils include Haplohumults and Kandihumults of the order Ultisols and Hapludox and Acrudox of the order Oxisols.

Koa grows at elevations ranging from 90 m (300 ft) on Oahu (45) to 2100 m (7,000 ft) on Hawaii (37), on flatlands and slopes. Koa has been listed as a component of the forests occupying gulch and ravine walls sloping 40 to 800 (49). The flora of Hawaii have been divided into groups occupying different zones of elevation (29):

The lowland zone, at or near sea level; open country, with isolated trees or clumps of trees. Koa rarely grows here.
The lower forest zone, upper limit 300 to 600 m (1,000 to 2,000 ft); tropical in character, woods rather open. Koa grows in scattered stands, in admixture with ohia.
The middle forest zone, upper limit 1500 to 1800 m (5,000 to 6,000 ft); within the region of clouds, where vegetation develops the greatest luxuriance. Here koa reaches its greatest development in size and number.
The upper forest zone,
upper limit as high as 2400 to 2700 m (8,000 to 9,000 ft). Koa reaches into this zone, but seldom above 2100 m (7,000 ft).

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Special Uses

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The most important use of koa timber by the Hawaiians was to build canoes. The largest of the giant war canoes extended 21 m (70 ft). Canoe hulls were made of single, giant koa logs. Koa was also used for sursurfboardsiboards, some 5.5 m (18 ft) or longer, for paddles, and for framing grasshouses. The bark provided dye to tapa, a light cloth made from the bark of wauke (Broussonetia papyrifera) (11,19).

Koa wood is now used primarily for furniture, cabinet work, and face veneers. It is widely used in woodcraft. Cabinet makers recognize a dozen or more types of koa wood, including curly or "fiddle back" koa, red koa, and yellow koa (11). One local use is for making ukuleles. At one time koa was sold on the world market as Hawaiian mahogany (62).

Large logs have a narrow, creamy-white band of sapwood. The heartwood may vary through many rich shades of red, golden brown, or brown. The heartwood seasons well without serious degrade from warping, checking, or splitting (74).

Although it has been stated that foresters in Hawaii have paid little attention to koa (83), more than 1.3 million koa seedlings were planted by the Hawaii Division of Forestry and Wildlife between 1915 and 1946 (78) for watershed protection. Koa, however, did not perform as well as many introduced species on these deteriorated sites.

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Vegetative Reproduction

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An intensive study of koa reproduction was made in 1943 (5) in an area of the Volcanoes National Park on the island of Hawaii, where annual rainfall is about 1000 mm (40 in). Koa stands appeared to regenerate almost entirely by means of root suckers on this once heavily grazed site. The researchers reported that "many vigorous suckers arise from the buried and exposed roots of a single tree. In three cases, suckers were seen 15, 27, and 29 m (50, 90, and 95 ft) away from the base of isolated koa trees. Suckers developed into healthy trees 8 to 16 cm (3 to 6 in) in diameter breast height in 5 to 6 years and were estimated to be 4 m (12 ft) tall." Koa colonies (root sprouts originating from the mother tree) in the park expanded at the rate of 0.5 to 2.5 m (1.5 to 8 ft) per year (51). In 1973, a study to determine the influence of feral goats on growth of these root suckers found that the suckers became more numerous and vigorous once the goats were excluded (84). Suckering, however, did not occur where the soil was covered with tall dense grass (83).

Koa can be propagated by rooting of cuttings under mist and shade when the material is in the immature, true-leaf stage of growth. Air layers of root suckers gave 16 percent rooting success, but rooting of root sucker cuttings under mist was highly variable, generally with a 20 percent success rate (76). Koa can be also propagated by callus cultures derived from shoot tips, but the method is slow and labor-intensive and not presently adaptable to large scale propagation (79). However, one clone, comprised of hundreds of ramets, has been produced by tissue culture of seedling shoot-tip callus (81). These tissue-cultured trees have been successfully out-planted in progeny tests (82).

Koa root sprouts are common in rain forests as well as in savanna stands. Efforts to induce suckering of roots of selected plus trees, in situ, on both wet and dry areas failed, however. Attempts to simulate the actions of pigs and cattle with treatments including knife wounding, "chewing" with pliers, pounding, and exposure had no effect. Koa root suckers in rain forests are much more common on roots in deep shade or hidden under dense grass than in roots exposed to direct sunlight (76). Stump sprouts have rarely been observed but do occur.

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Distribution

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The range of koa extends from longitude 154° to 160° W; its latitude ranges from 19° to 22° N. It is found on all six of the major islands of the Hawaiian chain: Kauai, Oahu, Molokai, Maui, Lanai, and Hawaii.


-The native range of koa.

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Brief Summary

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Koa

Leguminosae Legume family

Craig D. Whitesell

From the time of the early Hawaiians, koa (Acacia koa) has been prized for its exceptionally fine wood and is currently considered the most valuable of the common native timber species in Hawaii (29,60). Koa frequently has curly grain and striking coloration and has excellent working properties (11,37,75). It grows in nearly pure stands or in admixtures with ohia (Metrosideros polymorpha). Other tree species are sparse in these forests. A large evergreen hardwood tree endemic to the State, koa belongs to the thornless, phyllodinous group of the Acacia subgenus Heterophyllum.

Koa forests were more extensive in the past than they are today Land clearing, poor cutting practices, and destruction by animals, insects (49), and fire (26,36,67;96) have all taken a toll. The volume of koa sawtimber totaled about 187 million board feet in 1970. At that time the commercial koa forest land in the State totaled about 7500 ha (18,600 acres), and commercial ohia-koa forests about 17,500 ha (43,200 acres). The estimated growing-stock volume of commercial koa exceeded 0.7 million m³ (25 million ft³) in 1978 (50).

Koa is an important component of montane Hawaiian rain forests. It is a nitrogen-fixing species. In dense, pole-size stands, nitrogen-rich koa foliage can account for 50 to 75 percent of the leaf-litter biomass produced annually (68). On the floor of cool mesic forests, koa phyllodes decompose rapidly; mean residence time has been estimated at 0.6 year (68). The abundance and distribution of the akiapolaau, akepa, and Hawaiian creeper, three of the endangered forest birds on the island of Hawaii, are strongly associated with koa in forest communities (66). Mature koa is needed for bird habitat: endangered birds do not use young, pure stands of koa, but do use the old, mixed-species stands adjacent to young stands (65).

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Acacia

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"
Acacia fasciculifera shoot, showing phyllodes on the pinnate leaves, formed by dilation of the petiole and proximal part of the rachis[2]

Acacia, commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australia, with the first species A. nilotica described by Linnaeus.

Controversy erupted in the early 2000s when it became evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia was not closely related to the mainly African lineage that contained A. nilotica—the first and type species. This meant that the Australian lineage (by far the most prolific in number of species) would need to be renamed. Botanist Les Pedley named this group Racosperma, which was inconsistently adopted. Australian botanists proposed that this would be more disruptive than setting a different type species (A. penninervis) and allowing this large number of species to remain Acacia, resulting in the two African lineages being renamed Vachellia and Senegalia, and the two New World lineages renamed Acaciella and Mariosousa.[3] This was officially adopted, but many botanists from Africa and elsewhere disagreed that this was necessary.

A number of species have been introduced to various parts of the world, and two million hectares of commercial plantations have been established.[4] The heterogeneous group[5] varies considerably in habit, from mat-like subshrubs to canopy trees in forest.[6]

Taxonomy

The genus was first described from Africa by C. F. P. von Martius in 1829. Several hundred combinations in Acacia were published by Pedley in 2003.[1] The genus of 981[7] species, Acacia s.l., in the subfamily Mimosoideae of the pea family Fabaceae is monophyletic. All but 10 of its species are native to Australia,[7] where it constitutes the largest plant genus.[5]

Following a controversial decision to choose a new type for Acacia in 2005, the Australian component of Acacia s.l. now retains the name Acacia.[8][9] At the 2011 International Botanical Congress held in Melbourne, the decision to use the name Acacia, rather than the proposed Racosperma for this genus, was upheld.[10][11] Other Acacia s.l. taxa continue to be called Acacia by those who choose to consider the entire group as one genus.[11]

Australian species of the genus Paraserianthes s.l. are deemed its closest relatives, particularly P. lophantha.[12] The nearest relatives of Acacia and Paraserianthes s.l. in turn include the Australian and South East Asian genera Archidendron, Archidendropsis, Pararchidendron and Wallaceodendron, all of the tribe Ingeae.[13]

Etymology

The origin of "wattle" may be an Old Teutonic word meaning "to weave".[14] From around 700 A.D. watul was used in Old English to refer to the interwoven branches and sticks which formed fences, walls and roofs. Since about 1810 it refers to the Australian legumes that provide these branches.[14]

Species

One species of Acacia (sensu stricto) is native to Madagascar, one to Reunion island, 12 to Asia, and the remaining species (over 900) are native to Australasia and the Pacific Islands.[8] These species were all given combinations by Pedley when he erected the genus Racosperma, hence Acacia pulchella, for example, became Racosperma pulchellum. However these were not upheld with the retypification of Acacia.

Evolution

Acacias in Australia probably evolved their fire resistance about 20 million years ago when fossilised charcoal deposits show a large increase, indicating that fire was a factor even then. With no major mountain ranges or rivers to prevent their spread, the wattles began to spread all over the continent as it dried and fires became more common. They began to form dry, open forests with species of the genera Allocasuarina, Eucalyptus and Callitris (cypress-pines).

The southernmost species in the genus are Acacia dealbata (silver wattle), Acacia longifolia (coast wattle or Sydney golden wattle), Acacia mearnsii (black wattle), and Acacia melanoxylon (blackwood), reaching 43°30' S in Tasmania, Australia.

Fossil record

An Acacia-like 14 cm long fossil seed pod has been described from the Eocene of the Paris Basin.[15] Acacia like fossil pods under the name Leguminocarpon are known from late Oligocene deposits at different sites in Hungary. Seed pod fossils of †Acacia parschlugiana and †Acacia cyclosperma are known from Tertiary deposits in Switzerland,.[16]Acacia colchica has been described from the Miocene of West Georgia. Pliocene fossil pollen of an Acacia sp. has been described from West Georgia and Abkhazia.[17] Oldest records of fossil Acacia pollen in Australia are from the late Oligocene epoch, 25 million years ago.[18]

Distribution and habitat

They are present in all terrestrial habitats, including alpine settings, rainforests, woodlands, grasslands, coastal dunes and deserts.[6] In drier woodlands or forest they are an important component of the understory. Elsewhere they may be dominant, as in the Brigalow Belt, Myall woodlands and the eremaean Mulga woodlands.[6]

In Australia, Acacia forest is the second most common forest type after Eucalypt forest, covering 980,000 square kilometres (378,380 sq mi) or 8% of total forest area. Acacia is also the nation’s largest genus of flowering plants with almost 1,000 species found.[19]

Description

Several of its species bear vertically oriented phyllodes, which are green, broadened leaf petioles that function like leaf blades,[20] an adaptation to hot climates and droughts.[21] Some phyllodinous species have a colourful aril on the seed.[2] A few species have cladodes rather than leaves.[22]

Uses

Aboriginal Australians have traditionally harvested the seeds of some species, to be ground into flour and eaten as a paste or baked into a cake. The seeds contain as much as 25% more protein than common cereals, and they store well for long periods due to the hard seed coats.[21] In addition to utilizing the edible seed and gum, the people employed the timber for implements, weapons, fuel and musical instruments.[6] A number of species, most notably A. mangium (hickory wattle), A. mearnsii (black wattle) and A. saligna (coojong), are economically important and are widely planted globally for wood products, tannin, firewood and fodder.[8] A. melanoxylon (blackwood) and A. aneura (mulga) supply some of the most attractive timbers in the genus.[6] Black wattle bark supported the tanning industries of several countries, and may supply tannins for production of waterproof adhesives.[6]

Acacia is a common food source and host plant for butterflies of the genus Jalmenus. The imperial hairstreak, Jalmenus evagoras, feeds on at least 25 acacia species.[23]

Wattle bark collected in Australia in the 19th century was exported to Europe where it was used in the tanning process. One ton of wattle or mimosa bark contained about 150 lbs of pure tannin.[24]

In ancient Egypt, an ointment made from the ground leaves of an Acacia (senso latu) was used to treat hemorrhoids.[25] Acacia (senso latu) is repeatedly mentioned in the Book of Exodus, perhaps referring to Vachellia tortilis (previously known as Acacia raddiana), in regards to the construction of the Tabernacle.[26]

The hardened sap of various species of the acacia tree (senso latu) are known as acacia gum. Acacia gum is used as an emulsifier in food, a binder for watercolour painting, an additive to ceramic glazes, a binding in gum bichromate photography, a protective layer in the lithographic processes and as a binder to bind together fireworks.

Acacia honey is not collected from plants in the acacia family, but rather from Robinia pseudoacacia, known as black locust in North America. Honey collected from Caragana arborescens is sometimes also called (yellow) acacia honey. See also Monofloral honey.

Cultivation

Some species of acacia - notably A. baileyana, A. dealbata and A. pravissima - are cultivated as ornamental garden plants. The 1889 publication 'Useful native plants of Australia' describes various uses for eating.[27]

Toxicity

Some species of acacia contain psychoactive alkaloids, and some contain potassium fluoroacetate, a rodent poison.[28]

References

  • Pedley, L. (2002). "A conspectus of Acacia subgen. Acacia in Australia". Austrobaileya 6(2): 177–186.
  • Pedley, L. (2003). A synopsis of Racosperma C.Mart". Austrobaileya 6(3): 445–496.
  1. ^ a b Pedley, Les (2003). "A synopsis of Racosperma. C.Mart. (Leguminosae: Mimosoideae)". Austrobaileya. 6 (3): 445–496. JSTOR 41738994.
  2. ^ a b Wu, Delin; Nielsen, Ivan C. (2009). "Flora of China, 6. Tribe Acacieae" (PDF). Missouri Botanical Garden Press. Retrieved 19 November 2015.
  3. ^ Kyalangalilwa B, Boatwright JS, Daru BH, Maurin O, van der Bank M (2013). "Phylogenetic position and revised classification of Acacia s.l. (Fabaceae: Mimosoideae) in Africa, including new combinations in Vachellia and Senegalia". Botanical Journal of the Linnean Society. 172 (4): 500–523. doi:10.1111/boj.12047.
  4. ^ Midgley and Turnbull
  5. ^ a b Murphy, Daniel J. (2008). "A review of the classification of Acacia (Leguminosae, Mimosoideae)". Muelleria. 26 (1): 10–26. Retrieved 22 November 2015.
  6. ^ a b c d e f Orchard, Anthony E.; Wilson, Annette J.G. (2001). Flora of Australia. Volume 11A, Mimosaceae, Acacia part 1. Melbourne: CSIRO. pp. x–. ISBN 9780643067172.
  7. ^ a b Pedley, Les (February 2004). "Another view of Racosperma" (PDF). Acacia Study Group Newsletter (90): 3. ISSN 1035-4638. Retrieved 22 November 2015.
  8. ^ a b c Thiele, Kevin R. (February 2011). "The controversy over the retypification of Acacia Mill. with an Australian type: A pragmatic view" (PDF). Taxon. 60 (1): 194–198. doi:10.1002/tax.601017. Retrieved 15 November 2015.
  9. ^ Brummitt, R. K. (December 2010). "(292) Acacia: a solution that should be acceptable to everybody" (PDF). Taxon. 59 (6): 1925–1926. doi:10.1002/tax.596050. Retrieved 19 November 2015.
  10. ^ "The Acacia debate" (PDF). IBC2011 Congress News. Retrieved May 5, 2016.
  11. ^ a b Smith, Gideon F. & Figueiredo, Estrela (2011). "Conserving Acacia Mill. with a conserved type: What happened in Melbourne?". Taxon. 60 (5): 1504–1506. doi:10.1002/tax.605033. hdl:2263/17733.
  12. ^ Brown, Gillian K.; Daniel J. Murphy & Pauline Y. Ladiges (2011). "Relationships of the Australo-Malesian genus Paraserianthes (Mimosoideae: Leguminosae) identifies the sister group of Acacia sensu stricto and two biogeographical tracks". Cladistics. 27 (4): 380–390. doi:10.1111/j.1096-0031.2011.00349.x.
  13. ^ Brown, Gillian K.; Murphy, Daniel J.; Miller, Joseph T.; Ladiges, Pauline Y. (1 October 2008). "Acacia s.s. and its Relationship Among Tropical Legumes, Tribe Ingeae (Leguminosae: Mimosoideae)". Systematic Botany. 33 (4): 739–751. doi:10.1600/036364408786500136.
  14. ^ a b Austin, Daniel F. (2004). Florida ethnobotany Fairchild Tropical Garden, Coral Gables, Florida, Arizona-Sonora Desert Museum, Tucson, Arizona: with more than 500 species illustrated by Penelope N. Honychurch ... [et al.] Boca Raton, Florida: CRC Press. p. 58. ISBN 9780203491881.
  15. ^ Fossil Plants by Paul Kenrick & Paul Davis, Natural History Muyseum, London, 2004, ISBN 0-565-09176-X
  16. ^ Distribution of Legumes in the Tertiary of Hungary by L. Hably, Advances in Legume Systematics: Part 4, The Fossil Record, Ed. P.S. Herendeen & Dilcher, 1992, The Royal Botanic Gardens, Kew, ISBN 0947643400
  17. ^ Leguminosae species from the territory of Abkhazia by Alexandra K. Shakryl, Advances in Legume Systematics: Part 4, The Fossil Record, Ed. P.S. Herendeen & Dilcher, 1992, The Royal Botanic Gardens, Kew, ISBN 0947643400
  18. ^ The Greening of Gondwana by Mary E. White, Reed Books Pty Ltd, Australia, Reprinted issue 1988, ISBN 0730101541
  19. ^ "Acacia forest". Commonwealth of Australia. 6 February 2017. Retrieved 19 April 2017.
  20. ^ Armstrong, W. P. "Unforgettable Acacias, A Large Genus Of Trees & Shrubs". Wayne's Word. Archived from the original on 10 November 2015. Retrieved 17 November 2015.
  21. ^ a b Tan, Ria. "Acacia auriculiformis, Black Wattle". Naturia. Archived from the original on 5 May 2015. Retrieved 17 November 2015.
  22. ^ "Acacia, Thorntree". EOL. Retrieved 22 November 2015.
  23. ^ Biology of Australian butterflies. Kitching, R. L. (Roger Laurence), 1945-, CSIRO (Australia). Collingwood, VIC, Australia: CSIRO Pub. 1999. ISBN 978-0643050273. OCLC 40792921.CS1 maint: others (link)
  24. ^ The National Cyclopaedia of Useful Knowledge Vol II, (1847) Charles Knight, London, p.873.
  25. ^ Ellesmore, Windsor (2002). "Surgical History of Haemorrhoids". In Charles MV (ed.). Surgical Treatment of Haemorrhoids. London: Springer.
  26. ^ "Plants of the Bible - ODU Plant Site". Old Dominion University. 11 April 2007. Retrieved 3 October 2016.
  27. ^ J. H. Maiden (1889). Useful native plants of Australia : Including Tasmania. Turner and Henderson, Sydney.
  28. ^ Leong, L. E.; Khan, S.; Davis, C. K.; Denman, S. E.; McSweeney, C. S. (2017). "Fluoroacetate in plants - a review of its distribution, toxicity to livestock and microbial detoxification". Journal of Animal Science and Biotechnology. 8: 55. doi:10.1186/s40104-017-0180-6. PMC 5485738. PMID 28674607.

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Acacia: Brief Summary

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" Acacia fasciculifera shoot, showing phyllodes on the pinnate leaves, formed by dilation of the petiole and proximal part of the rachis

Acacia, commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australia, with the first species A. nilotica described by Linnaeus.

Controversy erupted in the early 2000s when it became evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia was not closely related to the mainly African lineage that contained A. nilotica—the first and type species. This meant that the Australian lineage (by far the most prolific in number of species) would need to be renamed. Botanist Les Pedley named this group Racosperma, which was inconsistently adopted. Australian botanists proposed that this would be more disruptive than setting a different type species (A. penninervis) and allowing this large number of species to remain Acacia, resulting in the two African lineages being renamed Vachellia and Senegalia, and the two New World lineages renamed Acaciella and Mariosousa. This was officially adopted, but many botanists from Africa and elsewhere disagreed that this was necessary.

A number of species have been introduced to various parts of the world, and two million hectares of commercial plantations have been established. The heterogeneous group varies considerably in habit, from mat-like subshrubs to canopy trees in forest.

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