April 2006 Species Profiles for Pacific Island Agroforestry ver. 3 www.traditionaltree.org Acacia koa (koa) and Acacia koaia (koai‘a) Fabaceae (legume family) koa (Acacia koa) koai‘a, koai‘e (Acacia koaia) Craig R. Elevitch, Kim M. Wilkinson, and J. B. Friday IN BRIEF h C Distribution Endemic to the major Hawai- vit ian islands of Hawai‘i, Moloka‘i, Maui, Lāna‘i, E O‘ahu, and Kaua‘i. Size Typically attains heights of 15–25 m (50–80 ft) with a canopy spread of 6–12 m (20–40 ft). El C. photo: Habitat Native to a wide elevation range of 100–2300 m (330–7500 ft) (recommended for planting only above 610 m [2000 ft]), with an- nual rainfall of 850–5000 mm (34–200 in). Vegetation Associated with dozens of other native species. Soils Requires well drained soils. Koa stands on shallow soils are not as productive as those on deep soils and may be short-lived. Growth rate Can grow faster than 1.5 m (5 ft) per year in height for the first 5 years in favor- able environments. Main agroforestry uses Wildlife habitat, silvo- pastoral systems, ornamental. Main products Timber. Yields Projected yields of 90–180 m3/ha (6.5– 13 mbf/acre) may be possible in pure stands in 30–50 years; much less in open-grown stands in pastures or in mixed stands in natural forests. Intercropping Difficult to integrate with an- nual crops due to koa’s aggressive surface root system that competes with crops, while also susceptible to damage by human, animal, and machine traffic. Invasive potential Poses a small risk of being invasive if introduced outside of Hawai‘i. Large koa trees such as this in South Kona, Hawai‘i, have mostly disappeared due to logging and land clearing. INTRODUCTION DISTRIBUTION Acacia koa, known worldwide as koa, is the largest native tree of the Hawaiian Islands. It can reach heights in excess Native range of 35 m (115 ft), although more commonly trees reach about Koa is endemic to all major Hawaiian Islands (Hawai‘i, 15 m (50 ft) in height at maturity. Koa is very important to Moloka‘i, Maui, Lāna‘i, O‘ahu, Kaua‘i), at 100–2300 m the ecology, economy, and culture of Hawai‘i. Koa trees in (300–7500 ft) elevation. natural ecosystems provide habitat for many birds, insects, and plants, some endangered. As a nitrogen-fixing species, Current distribution koa plays an important role in forest fertility. A key tra- Scattered remnants of koa forests are still found on the ditional use of koa logs by early Hawaiians was to build main Hawaiian Islands, mostly in upland areas. The health- canoes. The resurgence of interest in Hawaiian voyaging iest and largest populations grow at elevations of 900–1800 and racing vessels using traditional materials has led to a m (2950–5900 ft). New plantings are taking place in agri- greater public awareness of the scarcity of trees suitable for cultural and former pasture areas, primarily above 610 m “canoe koa” and the importance of renewing this depleted (2000 ft). resource. Commercially, koa is Hawai‘i’s premier timber, and cur- BOTANICAL DESCRIPTION rently one of the most expensive woods in the world. It is utilized for furniture, veneer, and crafts. Most commer- cial koa is harvested from remnant individuals or stands in Preferred scientific name pasture land. Efforts in the private and public sectors are Acacia koa Gray being made to re-establish koa, either through fostering natural regeneration (through the removal of livestock and Family occasionally scarifying the ground to expose buried koa Fabaceae (legume family) seeds), or by planting seedlings. There are many unknowns regarding the cultivation and management of koa for com- Subfamily mercial timber. Private landholders sometimes choose to Mimosoideae plant koa for the ecosystem and heritage value, while rec- ognizing that as a financial investment the outcome of koa Non-preferred scientific names reforestation is highly uncertain. Acacia hawaiiensis (Rock) Degener & I. Degener Koa trees once thrived in areas as low as 90 m (300 ft) A. heterophylla Willd. var. latifolia Benth. in elevation. However, pests and diseases currently limit A. kauaiensis Hillebr. koa’s optimal range to elevations above 610 m (2000 ft). Acacia koaia is a closely related species previously classified Hawai‘i’s highly variable climate means that matching an as a subspecies of A. koa (Wagner et al. 1999) appropriate seed source to the site conditions is important to foster a viable koa planting. Koa is a large, fast-growing tree with an aggressive root system. As such, its uses for interplanting with crops are limited. It is recommended that crops be planted no closer than 10–12 m (33–40 ft) to mature trees. However, many kinds of herbaceous plants can thrive in the dappled shade cast by koa. The proliferation of this species is desirable within Hawai‘i. However, as a fast-growing ni- trogen-fixing tree with long-lived seeds and the ability to regenerate after fire, koa may have po- tential as a problematic invasive species outside its native range. Current remnant areas of koa (after Whitesell 1990). Acacia koa (koa) Common names with 12–24 pairs of leaflets. Mature trees have no true leaves, koa (worldwide) but only sickle-shaped phyllodes (flattened leaf stalks) 7–25 cm (2.8–10 in) long and 0.5–2.5 cm (0.2–1 in) wide. GENERAL DESCRIPTION Seeds Seeds are contained within a pod (legume) 15–20 cm (6–8 Size in) long, containing 6–12 seeds. Koa can grow to over 35 m (115 ft) in height but more com- monly reaches heights of 15–25 m (50–80 ft) with a canopy Rooting habit spread of 6–12 m (20–40 ft). Some populations are much Koa has an extensive, shallow, and lateral-spreading root smaller, with a shrub-like form. system that can often be seen on the soil surface, just under the leaf mulch. Roots can sucker, particularly when they Form are damaged or the tree experiences stress. The form of trees is highly variable, ranging from an upright, single trunk to low and sprawling with multiple trunks. How to distinguish from similar species/look- alikes Flowers Sickle-shaped phyllodes (“leaves”) at maturity distinguish Pale yellow, borne in axillary racemes with spherical heads it from “haole koa” (Leucaena leucocephala); larger stature, about 8.5 mm (0.33 in) in diameter. Flowering peaks in mid larger phyllodes, and paler yellow and larger flowers dis- to late winter. Flowers are produced throughout the year, tinguish it from Formosan koa (Acacia confusa); longer starting when the tree is 2–3 years of age. sickle-shaped phyllodes distinguish it from Australian blackwood (Acacia melanoxylon), which is also usually a Leaves much straighter-stemmed tree; and its much larger stat- Young seedlings have bipinnately compound true leaves ure and larger phyllodes distinguish it from its close rela- Left [scale: ⅓ size]: Young koa trees have true leaves (on left). At about 6–9 months of age, trees begin producing flattened, elongated leaf stems called phyllodes (on right) instead of true leaves. Right: Flowering branch tip. photos: C. ElEvitCh Species Profiles for Pacific Island Agroforestry (www.traditionaltree.org) et al. 1999). Acacia koa var. koa occurs on all the main islands. Koa trees with characteristics in between Acacia koa and A. koaia (koai‘a) occur on Kaua‘i (Wagner et al. 1999). Isozyme studies have shown differences between Hawai‘i island populations of koa and the populations of the other Hawaiian islands (Conkle 1996). ASSOCIATED PLANT SPECIES Koa is the second most important and wide- spread tree in Hawaiian forests after ‘ōhi‘a lehua (Metrosideros polymorpha). While koa may oc- casionally occur in pure stands, usually the spe- cies is found in mixed forests where ‘ōhi‘a lehua predominates. On mesic sites, the koa canopy may also be shared with a‘e (Sapindus saponaria) and naio (Myoporum sandwicense). A wide vari- ety of native understory trees are found in koa forests, including naio, ‘ōlapa (Cheirodendron Root suckering (on left) occurs frequently when a tree is severely stressed trigynum), kāwa‘u (Ilex anomala), kōlea (Myr- (on right) or when surface roots are damaged. photo: C. ElEvitCh sine lessertiana), kōpiko (Psychotria spp.), ‘iliahi (Santalum spp., sandalwood), olopua (Nestegis tive koai‘a (Acacia koaia). The rare koai‘a is a shorter tree sandwicensis), and pilo (Coprosma spp.). In wetter sites, tree with a bushy, often gnarled habit, primarily of drier areas ferns (Cibotium spp.) may be prominent in the understory, of Kaua‘i, Moloka‘i, Lāna‘i, Maui, and Hawai‘i (see koai‘a while other species of ferns such as palapalai (Microlepia section below). strigosa) and Dryopteris wallichiana cover the ground. The native Hawaiian raspberry or ‘ākala (Rubus hawaiensis) GENETICS and the shrub māmaki (Pipturus albidus) are also common in the understory of koa forests. At higher elevations the Variability of species forest gradually becomes dominated by māmane (Sophora chrysophylla) (Mueller-Dombois and Fosberg 1998). Today, Koa is a highly variable species in tree form, wood quality, many koa forests consist of scattered old trees in rangelands and environmental tolerances. Populations on the island of dominated by kikuyu grass (Pennisetum clandestinum). Hawai‘i tend to be larger trees with broad (2.5 cm [1 in]) phyllodes and long, straight boles, whereas populations on other islands tend to be shorter trees with narrower and more curved phyllodes. Wood colors vary from blonde to red to deep brown, sometimes showing curl or fiddleback. Growth rates, leaf shape, and bole form have been shown to be highly heritable (Sun 1996).