Acacia koa Gray
Koa is unquestionably Hawaii's
most prized tree species-culturally, ecologically and economically. Hawaiians have
always valued koa for its exceptionally beautiful and durable wood. It remains the
premier Hawaiian timber for furniture, cabinetry, interior work and woodcrafts.
Equally important, native koa forests provide unique wildlife habitat, critical watershed
recharge areas and recreational opportunities. Unfortunately, forest clearing for
agriculture, cattle grazing and feral pig activity have much diminished Hawaii's once
extensive koa forest. The scarcity of koa wood is reflected in its ever increasing
price-now high enough to economically justify helicopter logging.
Distribution
Acacia koa occurs at elevations from 180 to
6000 meters between 19 and 22 latitude on all of the major Hawaiian islands. It
prefers an annual rainfall of 1900 to 5100 mm, and well drained acid soils. However, koa
adapts to almost any of Hawaii's diverse environments indicating its potential elsewhere
in the Pacific. Koa is found on all volcanic soil types of all geologic ages.
It grows well in moderately to well-drained, medium to very strongly acid soils on both
flatland and steep slopes. On dry, shallow, poorly drained soils Koa’s growth
is slow and its form generally poor.
Occurring in both pure and mixed forest
stands, koa is most commonly associated with the native ohia (Metrosideros polymorpha). It
is also a codominant in several other major forest types including: Koa/Mamane (Sophora
chrysophylla) Montane Dry Forest and Koa/Ohia/A'e (Sapindus soponaria) Forest (Wagner et
al. 1990). Today Acacia koa stands are fragmented and concentrated in areas between
600 and 1800 meters elevation (Whitesell 1990). This distribution is largely the
result of land conversion to agriculture and ranching. Cattle avidly graze koa
seedlings, preventing regeneration.
Uses
Wood: Koa heartwood
is highly valued by furniture and crafts people throughout Hawaii, and consumers the
world-over, for its unique grain, varied color and workability. It seasons well
without serious warping or splitting. Curly-grained wood, the result of both stress
and genetics, is preferred over straight-grained wood. Wood color ranges from a
subtle yellow to a striking dark red-purple. The specific gravity of koa wood
averages .40, but with curly-grained wood can be as high as .65. Mature koa boles are
commonly forking or fluting and often suffer from heart rot. These characteristics
and wide branch angles limit its value as a large timber. Fortunately, these defects may
be corrected through silviculture.
Forage and Wildlife Habitat:
Cattle, sheep and pigs browse koa foliage aggressively, especially its juvenile leaves.
Koa is spread geographically throughout Hawaii and thus offers a variety of
wildlife habitats of diverse moisture regimes, soils and vegetative compositions. An
overlay of a koa forest area map onto a forest bird "habitat island" map
produced by Walker (1986) shows remarkable correlation.
Land Reclamation:
Most koa plantations in Hawaii have been established to provide vegetative cover on sites
degraded by decades of intense grazing. Where scattered koa already exists, seed
stored in the soil will likely germinate if the soil is scarified and grass competition
controlled.
Botany and Ecology
Acacia koa is a large, evergreen broadleaf
tree and the only Acacia native-and endemic-to Hawaii. Trees occurring in dense, wet
native forest stands typically reach heights of 25 m and stem diameters (DBH) of 150 cm,
while retaining a straight, narrow form. In the open, trees develop more spreading,
branching crowns and shorter, broader trunks. Koa bark is gray, rough, scaly and thick.
Observations indicate that koa has one main tap root - and an otherwise shallow,
spreading root system.
Koa belongs to the thorn-less, phyllodinous
group of the Acacia subgenus Heterophyllum (Whitesell 1990). Like other phyllodial
species, mature koa trees do not have true leaves. Instead they produce phyllodes,
or flattened leaf petioles. Young seedlings have bipinnate compound true leaves with
12 to 15 pairs of leaflets. Where forest light is sufficient, seedlings stop
producing true leaves while they are small less then 2m tall. True leaves are
retained longer by trees growing in dense shade.
Phyllodes are sickle-shaped and often more
than 2.5 cm wide in the middle and blunt pointed on each end. Investigations suggest
that true leaves promote more rapid early growth when moisture is adequate, whereas
phyllodes are better adapted to drought. Phyllodes transpire only 20 percent as much
as true leaves, and their stomata close four times faster after dark. Phyllodes
typically hang down vertically, a position that enhances their ability to capture light
during early morning and late afternoon hours. Seedlings are able to switch back
from phyllode to true leave production when the sunlight reaching them is reduced (Walters
and Bartholomew 1990). This adaptation allows them to survive and grow under a wide
range of light regimes.
Observations suggest koa can flower almost
any time of year, depending upon local weather conditions. The inflorescence of koa
is a pale yellow ball averaging 8.5 mm in diameter, one to three on a common stalk.
Each inflorescence is composed of many bisexual flowers. Each flower has an
indefinite number of stamens and a single elongated style. One known pollinator of
koa is the honeybee (Apis mellifera). Koa appears to be fully self-fertile
(Brewbaker 1977).
Koa pods are slow to dehisce and about 15
cm long and 2.5 to 4 cm wide. They normally contain between 6 and 12 seeds that vary
from dark brown to black. Pods reach maturity at 4 to 6 months, depending on
location and weather conditions. Insect larvae of many species typically destroy a
large proportion of the mature seeds before they dehisce.
Seed production typically begins when trees
are 5 years old. Koa bears seed often and abundantly. Seeds are seldom dispersed far
from the tree and remain viable in the soil for up to 25 years. Thus remnant koa
stands are capable of dominant regeneration under favorable conditions. Koa seeds do
not require sunlight to germinate, but seedling growth is slow in dark understories or in
thick grass. The species thus requires large forest gaps, such as those created by
storms, to successfully regenerate.
Silviculture
Propagation is most successful from seed.
One study recommends air- layering as the best vegetative propagation technique
(Skolmen 1978). Koa seeds are durable and easy to store. They germinate after
many years of storage if kept in a cool, dry place. The most effective method for
improving seed germination is mechanical scarification. However, hot water soaking works
well and is a more practical method. Boil water and remove it from the heat source.
Soak seed in the boiled water for 24 hours. Once treated, seeds are typically sown
in nursery beds. One week after germination, seedlings are transplanted into nursery
tubes or bags. Seedlings are ready for transplanting into the field when they are
approximately 20 cm tall-after 3 to 4 months in the nursery. Observations suggest
that heart rot problems may be partially caused by root damage during transplanting.
Therefore, establishment by direct seeding or encouragement of natural regeneration is
recommended. On favorable sites, planted seedlings typically grow to 9 m in 5 years
time (Judd 1920).
Koa's wide branching form is the result of
open growth. Trees with long clear boles-called "Canoe trees" by native
Hawaiians are now rare, but still found in forest gaps created by fallen trees.
Dense stocking of seedlings, which mimics the competitive environment where superior
"canoe trees" grow, encourages straight and rapid height growth. Initial
spacing of 1.2 x 1.2 meters is currently recommended. Observation indicates that
effective self-thinning will result in an adequate number of potential crop trees by age
25.
Where scattered koa cover is adequate,
plantation establishment is most easily and successfully accomplished through the
stimulation of natural regeneration. Pasture soils are scarified and competition
from grasses reduced by the application of a contact herbicide. Gaps in the
regeneration are filled with planted seedlings. Fertilizers are applied to give
seedlings an initial "boost". Plantation thinning prescriptions should be
based on desired products and management capabilities. The most important factors to
consider in picking koa crop trees is stem form and height. Research on koa
plantation management and various spacing and thinning regimes is direly needed.
Symbiosis
Acacia koa is nodulated by the slow-growing
Bradyrhizobium spp. common in tropical soils. It nodulates heavily in a variety of
soils, suggesting it is effective with a wide variety of Bradyrhizobia strains.
Pests and Diseases
Banana poka (Passiflora
mollissima) is a fast growing vine that commonly outgrows and smothers young koa trees.
Kikuyu grass (Pennisetum clandestinum), a dominant and extremely aggressive
highland grass in Hawaii, is a major deterrent to the emergence of koa seedlings on
cleared or formerly grazed lands. Successful koa plantation monoculture has
historically been difficult to achieve due to associated insect and disease problems.
Examples include the defoliating koa moth (Scotorythra paludicola) and a lethal
"koa blight" first observed in 1988 on the island of Oahu.
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*This was written by Karl Dalla Rosa,
Hawaii State Department of Land and Natural Resources, Division of Forestry and Wildlife A
Publication of the Forest, Farm, and Community Tree Network
References: Brewbaker, J.L. 1977. Final
Report, Acacia koa project. Unpublished report on file at the Institute of Pacific Islands
Forestry and University of Hawaii, Department of Horticulture, Honolulu, Hawaii.
Judd, C.S. 1920. The koa tree. Hawaii
Forester and Agriculturist 17(2):30-35.
Skolmen, R.G. 1978. Vegetative propagation
of Acacia koa Gray. In Proceedings, Second Conference in Natural Sciences, Hawaii
Volcanoes National Park, June 1-3, 1978, edited by C.W. Smith. p. 260-273.
Wagner, W.L., D.R. Herbst and S.H. Sohmer.
1990. Manual of the Flowering Plants of Hawai'i. Vol. 1. University of Hawaii Press,
Bishop Museum, Honolulu, Hawaii.
Walker, R.L. 1986. Koa and wildlife - An
enduring relationship. Unpublished paper on file at the Hawaii Division of Forestry and
Wildlife, Honolulu, Hawaii.
Walters, G.A., and D.P. Bartholomew. 1990.
Adaptation of Acacia koa leaves and phyllodes to changes in photosynthetic photon flux
density. Forest Science 36(4):1050-1060.
Whitesell, C.D. 1990. Acacia koa Gray. In
Silvics of North America; 2, Hardwoods. R.M. Burns and B.H. Honkala, Tech. Coordinators.
Agricultural Handbook No. 654. USDA Forest Service, Washington, D.C. |
