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Koa redirects here. For other uses, seeKoa (disambiguation).
A young koa tree showing compound leaves and phyllodes
Acacia koais a species ofin theFabaceae. It isendemicto theHawaiian Islands,where it is the second most common tree.The highest populations are onHawaiiMauiandOahu. Its name in theHawaiian language,koa, also means brave, bold, fearless, or warrior.
Koa is a largetree, typically attaining a height of 1525m (4982ft) and a spread of 612m (2039ft).In deepvolcanic ash, a koa tree can reach a height of 30m (98ft), a circumference of 6m (20ft), and a spread of 38m (125ft).It is one of the fastest-growing Hawaiian trees, capable of reaching 69m (2030ft) in five years on a good site.
Initially,bipinnately compound leaveswith 1224 pairs ofleafletsgrow on the koa plant, much like other members of thepea family. At about 69 months of age, however, thick sickle-shaped leaves that are not compound begin to grow. These arephyllodes, blades that develop as an expansion of the leafpetiole. The vertically flattened orientation of the phyllodes allows sunlight to pass to lower levels of the tree. True leaves are entirely replaced by 725cm (2.89.8in) long, 0.52.5cm (0.200.98in) wide phyllodes on an adult tree.
Flowersof the koa tree are pale-yellowwith a diameter of 810mm (0.310.39in).Flowering may be seasonal or year round depending on the location.
Fruitproduction occurs when a koa tree is between 5 and 30 years old. The fruit arelegumes, also called pods, with a length of 7.515cm (3.05.9in) and a width of 1.52.5cm (0.590.98in). Each pod contains an average of 12seeds. The 612mm (0.240.47in) long, 47mm (0.160.28in) wide seeds are flattenedellipsoidsand range from dark brown to black in color. The pods are mature and ready for propagation after turning from green to brown or black. Seeds are covered with a hardseed coat, and this allows them to remaindormantfor up to 25 years.Scarificationis needed beforeA. koaseeds willgerminate.
Koa isendemicto theislandsofHawaiiMolokaiMauiLnaiOahu, andKauai, where it grows at elevations of 1002,300m (3307,550ft). It requires 8505,000mm (33197in) of annual rainfall.Acidicto neutralsoilspHof 4-7.4)that are either anInceptisolderived fromvolcanic ashor a well-drainedhistosolare preferred.Its ability tofix nitrogenallows it to grow in very young volcanic soils.Koa andhia lehua (Metrosideros polymorpha) dominate thecanopyofmixed mesic forests.It is also common inwet forests.
The koas trunk was used byto buildwaa)andpapa hee nalu(surfboards). Onlypaipo(bodyboards),kikoo, andalaiasurfboards were made from koa, however;olo, the longest surfboards, were made from the lighter and morebuoyantwiliwili(Erythrina sandwicensis).The reddish wood is very similar in strength and weight to that of Black Walnut (Juglans nigra), with a specific gravity of 0.55,and is sought for use inwood carvingandfurniture.Koa is also atonewood,often used in the construction ofukuleles,acoustic guitars,andWeissenborn-style Hawaiiansteel guitars.B.C. Richused koa on some of theirelectric guitarsas well,and still uses a koa-on certain models.Fender made limited edition koa wood models of theTelecasterand theStratocasterin 2006.Trey Anastasio, guitarist for the bandPhish, primarily uses a koa hollowbodyLanguedocguitar. Commercialsilvicultureof koa takes 20 to 25 years before a tree is of useful size.
Among otherPacific Islandsofvolcanic(non-continental) origin, onlyVanuatuhas nativeAcaciaspecies.A. heterophylla, from distantRunion, is very similar and has been suggested to be the closest relative of koa.Genetic sequence analysis results announced in 2014 confirmed this close relationship; the estimated time of divergence is about 1.4 million years ago.A. heterophyllasequences nest within those of the more diverseA. koa, making the latterparaphyletic.Both species are thought to be descended from an ancestral species in Australia, presumably theirsister species,Acacia melanoxylon. Dispersals most likely occurred via seed-carrying by birds such aspetrels.Both species have very similarecological niches, which differ from that ofA. melanoxylon.
A closely related species, koaia or koaie (A. koaia), is found in dry areas. It is most easily distinguished by having smaller seeds that are arranged end-to-end in the pod, rather than side-by-side. Thephyllodesare also usually straighter, though this character is variable in both species. The wood is denser, harder, and more finelygrainedthan koa wood.Koaia has been much more heavily impacted bycattleand is now rare, but can be seen onranchland inNorth Kohala.
The koa population has suffered from grazing andlogging. Manywet forestareas, where the largest koa grow, have been logged out, and it now comes largely from dead or dying trees or farms on private lands. Although formerly used foroutrigger canoes, there are few koa remaining which are both large and straight enough to do so today.In areas wherecattleare present, koa regeneration is almost completely suppressed. However, if the cattle are removed, koa are among the few native Hawaiian plants able togerminatein grassland, and can be instrumental in restoring nativeforest. It is often possible to beginreforestationin apastureby diskharrowingthe soil, as thisscarifiesseeds in the soil and encourages large numbers of koa to germinate.Experiments at theHakalau Forest National Wildlife Refugehave shown thathia lehua (Metrosideros polymorpha) survives best in pasture when planted under koa. This is because koa trees reduceradiative cooling, preventingfrostdamage tohia lehua seedlings.
Koa is the preferredhostplant for thecaterpillarsof the green Hawaiian blue (Udara blackburni), which eat the flowers and fruits.Adults drinknectarfrom the flowers. Koasapis eaten by the adult Kamehameha butterfly (Vanessa tameamea).The koa bug (Coleotichus blackburniae) uses its rostrum to suck the contents out of koa seeds.Koa is vulnerable to infection bykoa wilt.
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Acacia koawithphyllodebetween the branch and thecompound leaves
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This page was last edited on 1 April 2018, at 21:02