The Koa (Acacia koa) is the largest and most commercially important native tree species in the Hawaiian Islands, but it was more than two years after I first arrived here before I saw my first one in the wild. It didn’t help matters that I didn’t know what a Koa tree looked like. I bought several good field manuals for tree-spotting, read them all, but I still couldn’t find this tree, no matter where I looked on the island. The botanical guides, all written by knowledgeable experts, contained slightly out-of-focus photographs of large, green, but ultimately anonymous stands of trees obscured by thick, impenetrable foliage.
Typical stand of mature Koa Trees (Acacia koa).
So I did what any scientist does when confronted with his own ignorance: I cheated. I visited a place where trees already come with convenient labels attached: a botanical garden. It didn’t help. Out in the field, I kept running into trees that looked like the big fellow in the tree museum, but they had the wrong leaves! They looked more like those you’d find on a eucalyptus tree: long, narrow, and slightly curved like a Marine Corps dress sword.
A newly mature Koa Tree.
One day I got a real surprise: I found an entire stand of these trees that had TWO KINDS OF LEAVES! I went home and consulted the ultimate source of questionable but plentiful knowledge, Wikipedia. Sure enough, said the Great Wiki, at one point in their deceitful lives, Koa Trees actually DO have two kinds of what appear to be “leaves” on the same tree! All that time I’d spent crashing around in the bush looking for them, I had probably seen hundreds of Koa trees and didn’t even know it. That seemed unfair, somehow.
The phyllode (left) and true leaf of an immature Koa Tree.
Description
A large evergreen hardwood tree, Koa trees can reach heights of 15 to 35 m (50-115 ft); their branches create a canopy spread tens of meters wide. It is also one of the fastest growing native trees: young trees can reach 10 m in height in the first five years. However, there is a lot of variation in size and growth habit from one population to another. Its form can range from that of a large upright trunk to several low, sprawling shrub-like trunks. In the Hawaiian language, “koa” means brave, fearless, bold, or warrior.
Phyllodes and bipinnately-compound true leaves of an immature Koa Tree.
Leaves – Aside from its great stature, the distinctive silver-green foliage is the most notable of the Koa tree’s physical attributes. Immature seedlings display bipinnately compound true leaves with 12–24 pairs of leaflets, like other members of the Fabaceae (legumes) family. At about nine months of age, it begins to produce large (7-25 cm long and 0.5–2.5 cm wide), flat, vertically elongated sickle-shaped non-compound “leaves”. These are phyllodes, blade-like leaf-stalks that develop as an expansion of the leaf petiole. For a time, a maturing tree bears both the true leaves and these leaf-like phyllodes. Eventually, all of the true leaves are replaced by the phyllodes on the fully mature tree. The vertically flattened orientation of the phyllodes allows direct sunlight to pass easily to lower levels of the tree.
Close-up of a true leaf (left) and a phyllode from an immature Koa Tree.
True leaves promote more rapid early growth when moisture is adequate; however, during periods of drought, phyllodes are better adapted to promote growth. Phyllodes transpire about 20 percent more moisture than true leaves during times of moisture stress, and their stomata (moisture-transpiring pores on the surface) close four times faster after dark, preventing moisture loss. While fully mature trees usually bear only phyllodes, sometimes true leaves appear on the trunk or lower branches of the tree, particularly after a tree sustains wounding or other traumatic damage.
Cross-section of the trunk of a Koa sapling showing wood's grain and color.
Outer Bark – The outer bark of young trees is smooth in texture and may be covered with harmless growths of bright orange-red lichen. At 8 to 10 years of age, the bark develops a rough, fissured texture. At this point in the life cycle, its color is light to dark brown, often highlighted by numerous growths of silvery to light-green lichens.
Flowers and foliage of a mature Koa Tree.
Flowers – Flowers are usually borne over the outer part of the tree’s crown, at the tips of branches. The pale-yellow axillary racemes with spherical heads (8-10 mm in diameter) are nearly identical to those of the Koa Haole (Leucaena leucocephala). Starting at about 2-3 years of age, flowers are produced throughout the year or seasonally, depending on the location.
Fruit – Fruit production begins at about five years of age and continues for another 20 to 25 years. The fruit is a pod (7-15 cm long and 1.5-2.5 cm wide) containing 12 dark-brown to black flattened ellipsoidal seeds (5-10 mm in diameter). Each seed is protected by a very tough seed coat that enables an ungerminated seed to remain dormant yet viable for as long as 25 years. The seed coat must be thoroughly abraded (scarification) before the seed will germinate successfully.
Close-up of the fruit (seed pods) of a Koa Tree.
Genetic Variation – Although native to the Hawaiian Islands, Koa is a highly variable species in its general appearance, the nature of its wood, and its environmental tolerances. Trees found on Hawai’i Island tend to be taller and more robust with broader phyllodes, while trees of the other islands have shorter trunks and more curved phyllodes. These and other distinctive physical attributes appear to be readily inherited by subsequent generations of trees within a specific local population, as well as within island-wide populations.
Reproduction, Growth, and Regeneration
Pollination – One of the pollinating insects known to visit Koa flowers is the Honeybee (Apis mellifera). Other insects, birds, and wind may also enable the pollination of Koa trees, but the mechanisms for this process are not well documented for this tree species.
Close-up of the flower (raceme) of a mature Koa Tree.
Seed Propagation – Koa seeds are usually not dispersed very far beyond the crown of the parent tree but seeds can be carried long distances by strong winds and torrential rains. In order for Koa seeds to germinate, the tough walls of the seed pod must first be thoroughly abraded (known as “scarification”). The mode of germination is epigeal: that is, the seed expands, throws off the seed coat, and begins photosynthesis above the ground. Under favorable conditions (bare mineral-rich soil, adequate moisture, and nominal exposure to sunlight), Koa seedlings will grow readily and rapidly. Soil aeration and temperature also may influence the rate and volume of germination.
Natural Regeneration – Koa is both a rapidly growing, light-demanding pioneer tree that quickly colonizes open sites, and a long-lived tree that is part of mature forests. Koa naturally regenerates from buried “seed banks”, large numbers of seeds that have been buried in the soil for years or even decades. These germinate when the soil is exposed to sunlight and warmth by natural means, such as fire or wind-throw of large individual trees. Naturally regenerated stands of pure Koa may have as many as 20,000 seedlings per hectare. These grow quickly and the maturing trees may close canopy and form a solid stand in less than 10 years. Growth of individual trees slows thereafter as the trees compete for growing space.
“Suckering” – Koa trees also employ vegetative reproduction by producing numerous root suckers. This method of regeneration is often evident in overgrazed stands of trees. Once a pasture formerly occupied by Koa trees is withdrawn from grazing, healthy seedlings quickly develop, often tens of feet from the bases of dead or severely damaged trees (in some cases, more than 30 m away).
Koa trees adapt well to adverse environmental conditions. Stands of trees subjected to fire damage and logging usually recover quickly, sending up new seedlings within months. This is due to the ability of the seeds to germinate in almost any seedbed, from bare ground to the stumps and fallen branches of trees and other plants, as long as moisture is available in adequate quantities. However, like other trees native to the Hawaiian Islands, they do not fare well where feral pigs and goats, or range cattle overgraze. But once an area previously occupied by a stand of Koa trees is withdrawn as pasture land or otherwise protected from the predations of feral pigs, seedlings soon reappear. Koa seedlings do not grow well under the denser canopy of the rain forest unless openings in it have been created. Serious disturbances of the rain forest canopy, such as fires or hurricane-induced wind throw, commonly stimulate large-scale colonization by Koa seedlings.
Phyllodes and true leaves growing on the same twigs of an immature Koa Tree.
Distribution and Habitat
The Koa Tree is native to the mesic and wet forests of the islands of Hawai’i, Maui, Lana’i, Moloka’i, O’ahu, and Kaua’i. It grows in nearly pure stands or in mixed stands dominated by ‘Ohi’a lehua (Metrosideros polymorpha). While a wide variety of smaller native understory trees and plants are found in Koa forests, the actual amount of growth and ground coverage of these species is sparse.
Koa trees prefer flatlands and hillsides at altitudes of 100 and 2,300 m where annual rainfall is between 85 and 500 cm (33-200 in). They also grow in areas with much less than this amount of rainfall, but growth is considerably slower and the overall health of the tree may be adversely affected. This species’ nitrogen-fixing abilities allow it to grow in a wide variety of volcanic soils, regardless of the geologic age or geochemistry of the parent rocks. It grows best in soils that are acidic to neutral (pH = 4 – 7.4), principally Inceptisols derived from volcanic ash deposits and Histosols with good drainage.
The foliage (all phyllodes) of a mature Koa Tree.
Environmental Tolerances – Although this species prefers an environment with moderately high amounts of annual rainfall, it is also somewhat drought-tolerant, being able to survive under drought conditions for as long as five months. It prefers full sunlight throughout its lifecycle and is intolerant of shade. Root suckers will not produce healthy seedlings in areas covered by tall grass or impenetrable undergrowth. As mentioned above, Koa root suckers will re-sprout above ground after a fire, but the trunk, branches, and foliage are not fire resistant and are very susceptible to fire damage. Mature trees are resistant to frost damage, but young seedlings are easily killed off after a single hard freeze.
Reactions to Competition – Competition among individual Koa trees in a dense stand severely limits the growth and development of older saplings. In very densely populated stands, many trees are not growing at all, although a few individuals may emerge out of the shade of the forest canopy and so overtop their companions. The Koa’s ability to shed its lower branches completely when subjected to crowded conditions is a useful adaptation.
Mature, healthy native Hawaiian forests contain a very diverse mixture of tree species that may include only a few Koa trees per hectare. The reason for this is that individual Koa trees do not propagate in great numbers in the full shade of a forest; quite simply, under normal conditions, it is not able to grow up through the canopy of other trees. Once a Koa is overtopped by another tree, even one of its own species, it ceases to grow any taller. The Koa tree must depend on large tree-fall gaps in order to propagate in large numbers and colonize a normal forest. Stands of Koa saplings are often seen growing amongst large tree stumps of old-growth trees felled by logging or storms.
Potential for Invasiveness – Although it is not commonly planted outside its native range, the Koa is a fast-growing nitrogen-fixing tree species, so its potential for invasiveness may be high outside Hawaii. However, this species’ intolerance to shade, hard freezes, drought, and fire would not allow it to survive in a temperate climate.
Diseases and Pests
The Koa tree is susceptible to numerous fungal diseases, particularly several species of the vascular fungus Fusarium sp., a disease-causing fungus found throughout the world. Koa Wilt is a serious, often fatal disease caused by Fusarium oxysporum, a soil-borne fungus that is common in Hawaiian soils. Ambrosia beetles (such as Xyloborus sp.) transmit the fungus Fusarium solani, which causes wood staining and mainly affects seedlings. In both cases, the fungus enters through the trees’ roots and migrates upward through stems and branches, where it blocks vascular tissue and chokes off the water supply to the leaves. The leaves then wilt and turn brown.
Twig borer insects damage the branches of a Koa tree and may kill young seedlings. Livestock (particularly goats and beef cattle), as well as feral pigs, readily consume small seedlings and uproot maturing trees. and can also damage young trees by eating the leaves, stripping the bark, and trampling the extensive surface root systems. Seed predators include seed weevils (such as Araecerus levipennis) and the Koa Seedworm moth (Scotorythra paludicola) cause defoliation that can kill trees outright. When grown in grassy areas at elevations lower that its normal altitude range, the Koa is susceptible to parasitic infestation by Root-Knot Nematodes (usually of the genus Meloidogyne sp.). They exist in the soils of regions with hot climates or short winters. The larval form of the worm infects plant roots, causing the development of root-knot galls that drain the plant of nutrients. An infestation can kill a sapling outright; mature trees usually survive infestation, but are seriously debilitated and underdeveloped.
Banana Poka (Passiflora tarminiana), an aggressively invasive species of climbing vine, has overgrown and suppressed stands of Koa, particularly on the Big Island and Kaua‘i. Tropical ash (Fraxinus uhdei) occupies the same environment as the Koa and often shades out its saplings. Strawberry Guava (Psidium cattleianum), considered a noxious weed in the Hawaiian Islands, completely occupies the under story of many forests and prevents propagation and regeneration in Koa-dominated stands of trees.
Ecological Importance
Habitat Quality and Remediation – The Koa tree is a nitrogen-fixing leguminous species; for this reason, it is thought to contribute significantly to the nitrogen content of otherwise nitrogen-poor volcanic forest soils. The Koa is one of the few native Hawaiian plant species that is able to germinate in grassland and therefore can be instrumental in restoring native forest habitat. Disk harrowing the soil of former pastureland scarifies Koa seed pods already present in the soil and encourages large numbers of Koa to germinate. Koa trees reduce radiative cooling of the ground surface, preventing frost damage to its own seedlings and those of other species of native trees, such as the O’hia (Metrosideros polymorpha).
Wildlife Habitat – The Koa tree is a native Hawaiian species of critical ecological importance because it provides a suitable habitat for many native species of birds, insects, and other plants. Koa flowers provide pollen and nectar for bees. Large old trees provide important nest sites for birds. The endangered ‘Akiapola’au (Hemignathus munroi), a species of finch in the Hawaiian Honeycreeper family endemic to the Koa-dominated mesic and wet forests of the Island of Hawai’i, eats insects found only in the bark and rotting wood of Koa trees. Koa forests are also the home of the endangered ‘Akepa (Loxops coccinea) and the Hawai‘i Creeper (Oreomystis mana).
Traditional and Modern Commercial Uses
Koa is an important timber tree in Hawaii. The rich, dark colors and intricately-patterned grain of its wood is much sought after as a decorative veneer in Island homes; it is processed in California from wood shipped from Hawaii. I recently bought a solid-core wooden door with Koa veneer on both sides. When I applied a coat of clear acrylic preservative to the exterior of the door, a beautiful reddish-brown curly grain leapt out of the wood!
Koa-wood veneer applied to the body of an acoustic guitar. Note the intricate patterns of the tight grain of the wood. (photo courtesy of The 12th Fret
Ancient Hawaiians used Koa wood for canoe paddles, spears, surf boards, and decorative bowls. Today, Koa wood is the preferred medium of a flourishing crafts industry in Hawaii.
Canoe Craft – Traditionally, a single trunk of a Koa tree was used for the hull of a canoe (wa‘a), from one-person fishing canoes to the huge ocean-going voyaging. Today, canoe carving is a living tradition in Hawaii, and Koa-wood canoes are still made for and used in races.
A fair amount of milling is required during the production of Koa-wood lumber, veneer, molding, and flooring. The high value of the wood favors small-scale milling in the field with a portable mill; however, this production method typically yields smaller amounts of usable wood than do large, fixed mills.
Koa-wood furniture is the main product of Hawaii’s $30 million-a-year forest industry. In the past, harvesting large quantities for export led to rapid boom-and-bust cycles, and depleted resources of Koa trees. Alternatively, a gradual, well-planned sustained harvest of Koa trees should ensure that Hawaii’s local furniture and craft industry remains viable well into the future.
Tags: acacia, Acacia koa, epigeal, Fabaceae, hardwood trees, indigenous plants, indigenous trees, Koa, Koa tree, Koa wood, leaf morphology, leaves, legume family, legumes, native plantss, native trees, phyllodes, plant structures, plants of Hawaii, trees of Hawaii, trees of kauai, trees of Maui, types of leaves
