Wide ranging and found between 60 - 2,060 m on all the main islands except Ni`ihau and Koho`olawe (Wagner, et al. 1990: 641).
Carlquist (1980: 285) characterizes Acacia koa as "the most conspicuous trees between the low, open dry forest and the web ohia forest," noting that they usually "form a broad band on the lower slopes."
Koa is often found in dense stands in the mesic forests of the Park. Koa is the dominant tree species in only one rainforest habitat in the Park, the ola`a forest (Stone and Platt 1990: 173). Ordinarily, `ohi`a is the dominant tree species in the rainforest.
The place to see large intact koa stands is along the Mauna Loa Strip road.
The stems are relatively straight.
The trees change as they age. Younger individuals have a whitish gray, smooth bark. Older ones are darker, with the bark becoming orange-brown. The texture also gets rougher and more deeply furrowed.
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| Some koa get huge. You can get the idea from the size of the base of this tree. |  |
| Even medium sized trees are quite imposing when you can see them at the edge of a clearing. |  |
| Young leaves that are
found on saplings, and sometime growing from the larger tree's
trunk, are true leaf blades. Note
that they are bipinnate.
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| Older "leaves" are called
"phyllodes" because they are not leaf blades, but flattened
petioles. Note the characteristic sickle shape.
They have a characteristic gray-green color. |
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| Juvenile koa trees often show both types of leaves on the same individual. | |
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| Look for phyllodes on the forest floor. This is a good way to spot the presence of a tree even if its phyllodes are difficult to see high in the canopy. |  |
| The flowers are round, cream-colored or pale yellowish heads less than 0.5 inches wide. |  |
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| Seed pods are flattened and range from 3 to 12 inches
long and look somewhat like a lima bean pod.
The pods turn brown when mature. The seeds are flattish and shiny black. |
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| Black seeds are found in the pod. Small seedlings are sometimes seen on the forest floor. | |
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Phenology Notes
| Kipuka Puaulu, tree at entrance | Flowers just starting. There are also some mature pods. | 7/15/00 |
| near Kilauea Military Camp | One tree with abundant flowers. | 5/6/01 |
| Kipuka Puaulu | Tree at parking lot with a few flowers. | 5/6/01 |
| Kipuka Puaulu | Same tree without flowers. | 8/4/01 |
Koa is seen as a "keystone" species in the Mauna Loa Strip section of the park. It is host to a number of other species, including insects and birds.
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Koa is a prized wood and it has been logged for two centuries. This makes koa a rare tree in the lowland forests. High-elevation lands lost much of their koa forests through their conversion to grazing lands for cattle. The picture on the right was taken at the Hawaii Volcano Observatory and shows a light-green patch of pasture adjacent to the Park boundary. Cattle browse the seedlings, suckers and foliage. This has limited the growth and reproduction of koa in areas that are not protected. Cattle grazing was (finally) eliminated from HVNP in 1948. Goats were excluded in the early 1980s. |
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| A fire burned more than 2,000 acres
in the area around the Strip
Road in 1975. This impacted the koa woodlands, perhaps by stimulating the growth of the
trees by stimulating root sucker development. Therefore, most of the
smaller trees shown in these two pictures are about 25 years old.
Look for the presence or evidence of a large individual in the middle of stands of smaller-trunk trees. As a legume, this species can fix atmospheric nitrogen (Stone and Pratt 1990: 243). This may contribute to the palatability of the shoots and suckers. Cows and goats eat these structures if these grazers are allowed into koa stands. In the 1940s, these animals were prevalent in the Park. Cattle were removed by 1948 and goats in the 1980s. |
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| The clump of koa trees
growing at the top of the strip road is at the top of the elevation
range for this species.
Note that the trees in this "ring" are about the same size. It appears that they may all be the result of sucker growth from an older individual that was once growing in the center. |
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This doesn't mean that there are a lot of places where you can see large stands of straight-trunked, old koa. This is the species that was favored for Hawaiian canoes. The lowland koa forests were logged and the upland forests converted to cattle pastures even by the end of the 19th century (Stone and Pratt 1990: 242).
This is a fire adapted species (Stone and Pratt 1990: 243). Fire stimulates growth from the suckers (or shoots) from the shallow roots. Some of the dense koa forests on the lower portions of the strip road appear to be the result of a 1975 burn that covered some 2,000 acres (Stone and Pratt 1990: 243).
This species also includes what were once considered separate species, such as A. koaia.