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Overview

Brief Summary

Koa

    Leguminosae Legume family

    Craig D. Whitesell

        From the time of the early Hawaiians, koa (Acacia koa) has  been prized for its exceptionally fine wood and is currently  considered the most valuable of the common native timber species  in Hawaii (29,60). Koa frequently has curly grain and  striking coloration and has excellent working properties  (11,37,75). It grows in nearly pure stands or in  admixtures with ohia (Metrosideros polymorpha). Other  tree species are sparse in these forests. A large evergreen  hardwood tree endemic to the State, koa belongs to the thornless,  phyllodinous group of the Acacia subgenus Heterophyllum.

    Koa forests were more extensive in the past than they are today  Land clearing, poor cutting practices, and destruction by  animals, insects (49), and fire (26,36,67;96) have all  taken a toll. The volume of koa sawtimber totaled about 187  million board feet in 1970. At that time the commercial koa  forest land in the State totaled about 7500 ha (18,600 acres),  and commercial ohia-koa forests about 17,500 ha (43,200 acres).  The estimated growing-stock volume of commercial koa exceeded 0.7  million m³ (25 million ft³) in 1978 (50).

    Koa is an important component of montane Hawaiian rain forests. It  is a nitrogen-fixing species. In dense, pole-size stands,  nitrogen-rich koa foliage can account for 50 to 75 percent of the  leaf-litter biomass produced annually (68). On the floor  of cool mesic forests, koa phyllodes decompose rapidly; mean  residence time has been estimated at 0.6 year (68). The  abundance and distribution of the akiapolaau, akepa, and Hawaiian  creeper, three of the endangered forest birds on the island of  Hawaii, are strongly associated with koa in forest communities  (66). Mature koa is needed for bird habitat: endangered birds do  not use young, pure stands of koa, but do use the old,  mixed-species stands adjacent to young stands (65).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Acacia koa, koa, is a large evergreen flowering tree in the Fabaceae (legume family) native to the Hawaiian Islands. Koa was prized by early Hawaiians for its exceptionally fine wood. It is the second most common tree species on the islands, and the most valuable native timber species. Its wood has a curly grain and striking coloration, and was traditionally used to build canoes, which were made of single, giant koa logs; the largest were war canoes that extended 21 m (70 ft). Koa wood was also used for surfboards, paddles, framing grasshouses, and making ukuleles. It is now used primarily for furniture, cabinet work, and face veneers.

During its lifetime, A. koa undergoes a change from true leaves (consisting of compound leaves with 12 to 15 paired, bipinnate leaflets) to sickle-shaped phyllodes, leaves in which the leaflets are suppressed, and the leaf-stalks (petioles) become vertically flattened, generally oriented vertically to avoid intense sunlight. This change often occurs while plants are small, often less than 2 m (6 ft) tall. Phyllodes persist under moisture stress, transpiring about 20 percent as much as true leaves. Old trees usually bear only laurel green phyllodes, but true leaves sometimes appear on the trunk or lower branches, or after wounding.

In contrast to many acacia species, A. koa is thornless. The tree may flower and fruit starting at 2 to 3 years of age. In some locations, it flowers year-round. The insect-pollinated, pale yellow flowers are arranged in axillary racemes with many hermaphroditic (bisexual) flowers. Individual flowers average 8.5 mm (0.3 in) in diameter, with one to three on a common peduncle, each with a single elongated style and an indefinite number of free stamens. The fruit is a legume or pod, slow to dehisce (split open), about 15 cm (6 in) long and 2.5 to 4 cm (1 to 1.5 in) wide. Pods contain about 12 dark brown to black seeds, which are hard and durable. Seeds may germinate after one or two years, or persist in the seed bank for 25 years or more.

Koa grows best in high rainfall areas—those receiving 190 to 510 cm (75 to 200 in) annually. It is an important component of montane Hawaiian rain forests, where it grows in nearly pure stands or in mixtures with ohia (Metrosideros polymorpha), and is also associated with more than 80 species of trees, shrubs, vines, herbs, ferns, club mosses, grasses, and sedges. Koa forests provide habitat for numerous native forest birds, including three endangered species on the island of Hawaii: akiapolaau, Hemignathus munroi; akepa, Loxops coccineus; and Hawaiian creeper, Manucerthia mana.

Koa forests were more extensive in the past than they are today. Land clearing, poor cutting practices, and destruction by animals, insects, and fire have all taken a toll. Koa is susceptible to numerous native insects, including the koa moth (Scotorythra paludicola), a lepidopterus defoliator that caused mortality of one third of a stand of koas on Maui in an 1841 outbreak; such severe outbreaks occur periodically. In addition, the introduced koa haole seed weevil (Araecerus levipennis) and other seed moth species may destroy 90 percent or more of any given seed crop in the pods. Koa also suffers damage and mortality from browsing by pastured or feral cattle, pigs, sheep, and goats.

Craig D. Whitesell, modified by Jacqueline Courteau

  • Whitesell, C. D. Acacia koa A. Gray, Koa. In Burns, Russell M., and Barbara H. Honkala, tech. coords. 1990. Silvics of North America: Vol. 2. Hardwoods. Agriculture Handbook 654. U.S. Department of Agriculture, Forest Service, Washington, DC. Available online from http://na.fs.fed.us/spfo/pubs/silvics_manual/volume_2/acacia/koa.htm. And references therein.
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Distribution

Range Description

Acacia koa is endemic to the Hawaiian islands - Kauai, Oahu, Molokai, Lanai, Maui and Hawaii.
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National Distribution

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

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The range of koa extends from longitude 154° to  160° W; its latitude ranges from 19° to 22°  N. It is found on all six of the major islands of the Hawaiian  chain: Kauai, Oahu, Molokai, Maui, Lanai, and Hawaii.

   
  -The native range of koa.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Localities documented in Tropicos sources

Acacia koa var. hawaiiensis Rock:
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.
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© Missouri Botanical Garden, 4344 Shaw Boulevard, St. Louis, MO 63110 USA

Source: Missouri Botanical Garden

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Localities documented in Tropicos sources

Acacia kauaiensis Hillebr.:
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.
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© Missouri Botanical Garden, 4344 Shaw Boulevard, St. Louis, MO 63110 USA

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Localities documented in Tropicos sources

Acacia koa A. Gray:
Honduras (Mesoamerica)
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.
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Global Range: Kauai, Oahu, Molokai, Lanai, Maui, Hawaii.
Kauai - A dominant in much of the mesic native forest.
Oahu - Common to dominant in many mesic forests.
Molokai - A. koa was originally very rare on Molokai. It was recorded from the island in the 1800's, but no individuals are currently known.
Lanai - Very spotty. Probably only a few hundred individuals.
West Maui - Localized.
East Maui - Most of the koa forests were destroyed in the 1800's. Still a dominant in a few parts of East Maui.
Hawaii - Much reduced, but still common over large areas despite grazing and logging.

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Physical Description

Morphology

Physical Description

Perennial, Trees, Woody throughout, Nodules present, Stems erect or ascending, Stems greater than 2 m tall, Stems solid, Stems or young twigs glabrous or sparsely glabrate, Leaves alternate, Leaves petiolate, Extrafloral nectary glands on petiole, Stipules inconspicuous, absent, or caducous, Stipules deciduous, Stipules free, Leaves simple, or appearing so, Leaf or leaflet margins entire, Leaflets 1, Leaves glabrous or nearly so, Leaves reduced to phyllodia, Inflorescences racemes, Inflorescence panicles, Inflorescences globose heads, capitate or subcapitate, Inflorescence axillary, Inflorescence terminal, Bracts very small, absent or caducous, Flowers actinomorphic or somewhat irregular, Calyx 5-lobed, Calyx glabrous, Petals united, valvate, Petals white, Stamens numerous, more than 10, Stamens completely free, separate, Stamens long exserted, Filaments glabrous, Style terete, Fruit a legume, Fruit unilocular, Fruit freely dehiscent, Fruit elongate, straight, Fruit oblong or ellipsoidal, Fruit coriaceous or becoming woody, Fruit exserted from calyx, Fruit compressed between seeds, Fruit glabrous or glabrate, Fruit 3-10 seeded, Seed with elliptical line or depression, pleurogram, Seeds ovoid to rounded in outline, Seed surface smooth, Seeds olive, brown, or black.
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Dr. David Bogler

Source: USDA NRCS PLANTS Database

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Type Information

Syntype for Acacia koa A. Gray in Wilkes
Catalog Number: US 62798
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Original publication and alleged type specimen examined
Preparation: Pressed specimen
Collector(s): Wilkes Explor. Exped.
Year Collected: 1838
Locality: Maui, Hawaii, United States, Hawaiian Archipelago, Pacific Islands
  • Syntype: Gray, A. 1854. U.S. Explor. Exped. 15: 480.
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Syntype for Acacia koa A. Gray in Wilkes
Catalog Number: US 62797
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Verification Degree: Original publication and alleged type specimen examined
Preparation: Pressed specimen
Collector(s): Wilkes Explor. Exped.
Year Collected: 1838
Locality: Maui, Hawaii, United States, Hawaiian Archipelago, Pacific Islands
  • Syntype: Gray, A. 1854. U.S. Explor. Exped. 15: 480.
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Ecology

Habitat

Habitat and Ecology

Habitat and Ecology
A. koa is a large evergreen broadleaf tree (it can reach 35 m, but more commonly reaches 20-25 m in height, and some populations are much smaller, with a shrub-like form), which can grow in pure stands, but usually is found in mesic forest. 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. Occurs in a variety of habitats, has a large elevation range and is often a dominant plant in dry to wet forests at elevations ranging from 60 to 2,300 m (Wagner et al. 1990). Morphological differences in koa have been observed on several islands. Seeds are contained within a pod 15–20 cm long, containing 6–12 seeds. The species is a fast-growing tree.

Systems
  • Terrestrial
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Soils and Topography

Koa is found on volcanic soils of all geologic ages and  degrees of development, from the young ash and "aa"  lava rock soils on the island of Hawaii to the oldest soils on  Oahu and Kauai. The tree grows best on moderately well drained  and well drained, medium to very strongly acid soils. These  recent soils are higher in plant nutrients, having been subjected  to less leaching and erosion than have the soils on the older  islands.

    Most koa forests grow on two of the great groups in the soil order  Inceptisol: Hydrandepts and Dystrandepts. Hydrandepts are found  in areas of high rainfall. They are high in amorphous materials  and have high cation exchange capacities, but extremely low base  saturations due to the high rainfall. Although deficient in  available phosphorus, sodium, potassium, calcium, and silica,  they have a high content of organic matter and hydrous oxides of  iron and aluminum, manganese, and titanium. Infiltration rates  are rapid and erosion is slight to moderate, depending upon the  degree of slope. Dystrandepts are formed under lower rainfall  than the Hydrandepts. They have slightly greater base saturations  than the Hydrandepts.

    The next most abundant soil great group on which koa grows is the  well drained Tropofolists (organic soils of the order Histosols).  Other minor soils include Haplohumults and Kandihumults of the  order Ultisols and Hapludox and Acrudox of the order Oxisols.

    Koa grows at elevations ranging from 90 m (300 ft) on Oahu (45)  to 2100 m (7,000 ft) on Hawaii (37), on flatlands and  slopes. Koa has been listed as a component of the forests  occupying gulch and ravine walls sloping 40 to 800 (49). The  flora of Hawaii have been divided into groups occupying different  zones of elevation (29):

    The lowland zone, at or near sea level; open country, with  isolated trees or clumps of trees. Koa rarely grows here.
  The lower forest zone, upper limit 300 to 600 m (1,000 to  2,000 ft); tropical in character, woods rather open. Koa grows in  scattered stands, in admixture with ohia.
  The middle forest zone, upper limit 1500 to 1800 m (5,000  to 6,000 ft); within the region of clouds, where vegetation  develops the greatest luxuriance. Here koa reaches its greatest  development in size and number.
  The upper forest zone,
upper limit as high as 2400 to 2700 m  (8,000 to 9,000 ft). Koa reaches into this zone, but seldom above  2100 m (7,000 ft).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Climate

Hawaii is tropical in latitude, with mild and equable  temperatures at low elevations (table 1). Day length is nearly  uniform year-round, varying by 2 hours. The northeasterly trade  winds dominate; however, "Kona" storms from the south  or west during winter, and occasional tropical storms throughout  the year, bring high winds and heavy rains to the islands.  Hawaii's mountains, especially massive Mauna Loa and Manna Kea on  Hawaii, and Haleakala on Maui, have a strong influence on the  weather and provide climates ranging from the tropic to the  subarctic (7).

    Table 1-Mean temperature at five stations on the east  flank of Mauna Kea, island of Hawaii¹

    Station  Elevation
  (m)  Mean Temperature    January
  (°C)  August
  (°C)    Olaa (6)  85  21  24    Waiakea Forest  550  18  21    Waiakea Forest  915  17  19    Waiakea Forest  1220  13  16    Kulani Camp (6)  1580  4  14   
 
    (ft)  (°F)  (°F)        Olaa  280  70  75    Waiakea Forest  1800  64  69    Waiakea Forest  3000  62  67    Waiakea Forest  4000  55  61    Kulani Camp  5190  39  5        ¹Data on file at the Pacific Southwest  Forest and Range Experiment Station, Forest Service, U.S.  Department of Agriculture, Honolulu, HI.

    Rainfall varies greatly within short distances. Monthly amounts  recorded over a period of years at weather stations in the koa  belts show a phenomenal range. A Forest Service station at 1200 m  (4,000 ft) elevation recorded a mean annual rainfall of 4300 mm  (170 in) for a 14-year period, with extremes of 3450 to 5500 mm  (136 to 216 in). During the driest month, only 19 mm (0.74 in)  was recorded; the wettest month was 1380 mm (54.4 in).

    Koa grows best in the high rainfall areas, those receiving 1900 to  5100 mm (75 to 200 in) annually. It also grows in areas that  receive much less than this amount, but growth is slower and tree  form is generally poorer. Cloud cover and fog commonly shroud the  middle forest zone (600 to 1800 m or 2,000 to 6,000 ft) where  commercial koa stands are concentrated. Frost is not uncommon  during winter months above 1200 m (4,000 ft) elevation.  Temperature ranges within the koa belt are small, as may be seen  from data for Mauna Kea, island of Hawaii (table 1).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Associations

Associated Forest Cover

Botanists and foresters have listed more than 80 trees,  shrubs, vines, herbs, ferns, club mosses, grasses, and sedges  associated with koa. Trees associated with koa (20,33,48)  include:
 

    'ahakea (Bobea spp.)
  'ala'a (Pouteria sandwicensis)
  kalia (Elaeocarpus bifidus)
  kauila (Alphitonia ponderosa)
  kawa'u (Ilex anomala)
  kolea (Myrsine lessertiana)
  kopiko (Psychotria spp.)
  loulu palm (Pritchardia spp.)
  mamani (Sophora chrysophylla)
  naio (Myoporum sandwicense)
  'ohe'ohe (Tetraplasandra hawaiiensis)
  'ohi'a (Metrosideros polymorpha)
  'olapa (Cheirodendron trigynum)
  olomea (Perrottetia sandwicensis)
  olopua (Osmanthus sandwicensis)
  pilo (Coprosma spp.)
  sandalwood (Santalum spp.)

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Diseases and Parasites

Damaging Agents

Hawaiian forestry  literature is full of references to the disastrous effects of  cattle, pigs, sheep, and goats on koa and other native species  (1,4,5,1 7,26,35,38,40,77). Records of the Hawaii  Division of Forestry and Wildlife show that more than 250,000  pigs, goats, and sheep were destroyed from 1921-46 in the forests  of the island of Hawaii (10) during an eradication  program. Such efforts did much to reduce the amount of browsing  by these animals on koa forests. Feral cattle are particularly  fond of koa root sprouts, seedlings, pods, and leaves. They  straddle and trample large saplings to devour the foliage and  bark. Feral goats have nearly disrupted the replacement cycle of  koa on the Hawaii Volcanoes National Park (84). In recent  years, park rangers have taken steps to radically reduce the size  of the g oat herds within the park. A study was conducted on the  recovery of vegetation on koa parklands on Maui following the  exclusion of goats. After 7 years, young koa regeneration was  present both within and outside the enclosure, but the koa got  large only if the goats were excluded (69). Koa will recover on  these parklands if goats are eliminated. A large number of feral  pigs inhabit the kon rain forests, and their rooting destroys  many koa seedlings. It is thought that if the pig population is  permitted to increase, the koa rain forest ecosystem will  deteriorate (16).

    Koa attracts other kinds of animals. Black-tailed deer, introduced  from Oregon to the island of Kauai in 1961, eat koa seedlings,  but have little impact on the native vegetation. Less than 10  percent of the koa was browsed (94). The tree rat and the  Hawaiian rat damage koa saplings by stripping off bark. One  thousand koa saplings (2 to 5 years old) along an elevation  transect from 770 to 1330 m (2,520 to 4,370 ft) in the  Laupahoehoe area of the Hilo Forest Reserve were examined (71).  Thirty percent of the trees had been wounded by rats, with  wounds occurring as high as 10 m (33 ft). Bark along the main  trunk and on lateral branches was subject to stripping. Terminal  and lateral shoot dieback were observed where complete girdling  occurred. In a study of mortality of koa saplings severely  wounded by rats, damage was reported most severe in the vicinity  of brush piles where nests were likely to be located.

    In 1925, more than 40 species of native insects were considered  enemies of koa (92), and by 1983 the number of phytophagous  insects associated with koa reached 101 (87). Insect  damage to koa is well documented (18,22,58,59,91). One  authority believes "there are more endemic insect species  attached to this koa complex (Acacia koa and related koa  members) than to any other genus in the Hawaiian islands"  (93)

    One of the most destructive insects of koa is the koa moth  (Scotorythra paludicola), a lepidopterus defoliator found  on the islands of Hawaii, Maui (105), Oahu, and Kauai  ( 87). Severe outbreaks occur periodically. When these  insects appear in large numbers, they may completely defoliate  koa stands. Following an outbreak on Maui in which 1841 ha (4,550  acres) were completely defoliated, growth was reduced 71 percent,  and about one-third of the trees died within 20 months (88).

    The introduced koa haole seed weevil (Araecerus levipennisis the most prevalent insect that infests koa seeds, the next  most common being Stator limbatus (85). The koa seedworm  (Cryptophlebia illepida) destroys seeds and is a problem  to control when seeds are collected for reforestation purposes.  Eighty percent of the damage from this Tortricid occurs above  1037 m (3,400 ft) (85). Three other Tortricid species  destroy koa pods or seeds (85,91). These seed moths may  destroy 90 percent or more of any given seed crop in the pods  (93). Stein (86) reviewed the biology and host  range of koa seed insects, their parasites, and hyperparasites.

    At high elevations, koa terminals are sometimes heavily attacked  by the Fuller rose beetle (Pantomorus cervinus), but the  attacks appear to be highly seasonal and of no serious  consequence. The acacia psyllid (Psylla uncatoides), first  found in Hawaii in 1966, feeds and breeds in the new growth of  koa. This psyllid also has become a serious pest of the closely  related koaia (Acacia koaia) (47). The black twig borer  (Xylosandrus compactus) is associated with injury and  mortality.

    Information on diseases of koa has increased in recent years.  Seedlings may be attacked by Calonectria theae, which  causes a shoot blight (55) and C. crotalariae, which  causes a crown rot (57). This pathogen also caused a  collar rot that severely affected koa seedlings regenerating a  burned-over area (2). A wilt disease, Fusarium oxysporumwas observed among koa seedlings (24). This fungus  may contribute to the premature decline or death of old koa trees  growing within the Hawaii Volcanoes National Park. Indications  are that this fungus is seed-borne, but seed disinfection did not  reduce disease incidence (24). Koa was moderately  tolerant to Phytophihora cinnamomi in greenhouse tests  (42).

    Dieback is common in the crowns of old trees, and it was observed  in more than half the sawtimber-size koa measured during the  1959-61 forest survey The root-rot fungus Armillaria mellea  is associated with this dieback (44,61). Stands  possibly weakened by old age, extended droughts, and grazing have  succumbed to attacks by this fungus. Other diseases of koa are  those caused by the sooty molds, such as Meliola koae, that  cover the leaves and restrict growth.

    Four rust fungi, Uromyces koae, U. digitatus, Endoracejum  acaciac, and E. hawaiiense, occur on koa (25,32).  Both species in the genus Uromyces, obligate  parasites, cause witches' brooms and leaf blisters that deform  branches and phyllodes. When infections are heavy, they can  deform and debilitate both young and old trees (23,30,31).

    The Hawaiian mistletoe (Korthalsella complanata) has been  observed in many koa stands, and it can deform young koa. Heart  rot, caused principally by Laetiporus sulphureus and Pleurotus  ostreatus, is common in most mature and overmature koa (6).  More than half the large koa measured in the 1959-61 forest  survey were unmerchantable because of excessive rot (98).

    Pole-size and small, sawtimber-size koa have thin bark  and are easily damaged by fires.

    Weeds are serious problems in certain areas. The banana poka (Passiflora  mollissima) smothers both koa reproduction and mature trees  by laying a curtain of vines over them. The German-ivy (Senecio  mikanioides) is also difficult to control.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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General Ecology

Reaction to Competition

Koa is classed as  intolerant of shade both in the dry forest (28) and in  the rain forest, and at all ages (26). Under favorable  light, moisture, and soil conditions, koa competes aggressively  with other vegetation.

    Koa has been classified in various ways by different  investigators. One referred to koa as a pioneer species on the  grassy slopes of dry forest sites (28), but another  considered it a climax species (21). Koa has been  considered the ultimate forest type, following the ohm forest on  the ancient "an" lava flows (37). "At  maturity a grove (of koa) casts a shade in which its own  seedlings have difficulty in growing, and unless they fill a  vacancy in the parental ranks, they must seek the outer limits of  the stand" (64). Another investigator believed that koa "reproduction  need not be especially frequent to maintain the forest (type)" 

    Koa failed when underplanted in a dense native ohia rain forest at  870 m (2,850 ft), showing poor survival (44 percent), vigor, and  form (70 percent cull), but three introduced nonleguminous  species from Australia performed well (100).

    The effect of thinning and/or fertilizing a 12-year-old, stagnated  kon stand were studied on the island of Maui. In this  precommercial thinning, the number of stems was reduced 50  percent. Basal area growth rates for a 3-year period indicated  that thinning increased growth significantly. Fertilizer yielded  limited response; and the investigator thought that the  fertilizer should be applied before crown closure (72).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Rooting Habit

Little is known of the root  development of koa. The tree grows on the deep Hawaiian soils,  but also reaches impressive size on the shallow "an"  lava flows. "The root system of the mature koa is shallow  and extensive, spreading out radially from the base for distances  as great as 30 m (100 ft) or more" (5). "The  tree has a shallow rooted system, a flat plane of roots spreading  out in all directions just beneath the surface of the ground. For  this reason the larger top-heavy trees are easily overturned by  severe windstorms...." (37). Large koa trees were  toppled during a severe earthquake on the island of Hawaii in  1973. In describing the root systems of lava-flow plants, a  researcher classified koa as one of the comparatively deep-rooted  woody species (48).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Life History and Behavior

Reproduction

Vegetative Reproduction

An intensive study of koa  reproduction was made in 1943 (5) in an area of the  Volcanoes National Park on the island of Hawaii, where annual  rainfall is about 1000 mm (40 in). Koa stands appeared to  regenerate almost entirely by means of root suckers on this once  heavily grazed site. The researchers reported that "many  vigorous suckers arise from the buried and exposed roots of a  single tree. In three cases, suckers were seen 15, 27, and 29 m  (50, 90, and 95 ft) away from the base of isolated koa trees.  Suckers developed into healthy trees 8 to 16 cm (3 to 6 in) in  diameter breast height in 5 to 6 years and were estimated to be 4  m (12 ft) tall." Koa colonies (root sprouts originating from  the mother tree) in the park expanded at the rate of 0.5 to 2.5 m  (1.5 to 8 ft) per year (51). In 1973, a study to  determine the influence of feral goats on growth of these root  suckers found that the suckers became more numerous and vigorous  once the goats were excluded (84). Suckering, however,  did not occur where the soil was covered with tall dense grass  (83).

    Koa can be propagated by rooting of cuttings under mist  and shade when the material is in the immature, true-leaf stage  of growth. Air layers of root suckers gave 16 percent rooting  success, but rooting of root sucker cuttings under mist was  highly variable, generally with a 20 percent success rate (76).  Koa can be also propagated by callus cultures derived from  shoot tips, but the method is slow and labor-intensive and not  presently adaptable to large scale propagation (79). However,  one clone, comprised of hundreds of ramets, has been produced by  tissue culture of seedling shoot-tip callus (81). These  tissue-cultured trees have been successfully out-planted in  progeny tests (82).

    Koa root sprouts are common in rain forests as well as in savanna  stands. Efforts to induce suckering of roots of selected plus  trees, in situ, on both wet and dry areas failed, however.  Attempts to simulate the actions of pigs and cattle with  treatments including knife wounding, "chewing" with  pliers, pounding, and exposure had no effect. Koa root suckers in  rain forests are much more common on roots in deep shade or  hidden under dense grass than in roots exposed to direct sunlight  (76). Stump sprouts have rarely been observed but do  occur.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Seedling Development

The mode of germination is  epigeal (99). Light is not a requirement for germination (83).  Under favorable conditions-bare mineral soil, adequate  moisture, and exposure to sunlight-koa seedlings will grow  readily. Soil aeration and soil temperature may influence  germination (83).

    Until recent years, the standard nursery practice was to sow koa  seeds in wooden flats, then transplant the seedlings to tin cans  (35). Now, plastic bags or tubes are used. Tube-grown  seedlings are easier to plant.

    Properly pretreated koa seeds should be covered with 6 to 12 mm  (0.25 to 0.5 in) of soil; they begin to germinate within a week.  Seedlings in bags or tubes can be grown to plantable size of 20  cm (8 in) high in 10 to 14 weeks.

    Direct seeding of koa on prepared seed spots has been moderately  successful (9,13) In two trials comparing broadcast  sowing with direct sowing into prepared spots, stocking was four  times higher on the direct seeded spots on Maui, whereas no  difference in the percentage of stocked spots or of height growth  was evident on the island of Hawaii.

    Koa has been recommended for watershed planting on well drained  areas (34,37,39) and is described as "the one native  tree which can be easily handled in nursery and planting  operations... suitable for the larger portion of areas in need of  reforestation and particularly for the drier ridges and slopes"  (35).

    Other investigators, less enthusiastic about planting koa, did not  recommend it (13,17), commenting as follows: "Results  on older soil formations have been uniformly disappointing.  Frequently, the trees die out after 15 to 20 years" (17).  Plantations established on Maui during the late 1930's  developed scattered, large trees of exceptionally poor form.  Relatively few koa seedlings were planted after World War II.  However, in the past 10 years, private land owners on the island  of Hawaii, influenced by the short supply, began planting koa  (104).

    Seedlings usually appear soon after land is cleared for pasture or  roads, or after fires. As many as 354,700 koa seedlings per  hectare (143,537/acre) were counted in the vicinity of old koa  trees in burned-over areas (41). Seeds escaping the  flames may be induced to germinate by the heat.

    Koa seedlings grow rapidly. One month after a burn, koa seedlings  were at least 2.5 cm (1 in) tall; after 3 months they ranged from  10 to 28 cm (4 to 11 in) tall, averaging about 13 cm (5 in) (41).  On a cleared area at 500 m (1,700 ft) elevation, 1-year-old  seedlings ranged from 0.6 to 4 m (2 to 13 ft) tall and averaged 2  m (6 ft). On favorable sites, seedlings attain 9 m (30 ft) in 5  years (37). Eight months after a fire on Kauai, koa  regeneration was most common near fire-killed parent trees, and  maximum height growth was 4.6 m (15 ft) (103). The  abundance, distribution, growth, and mortality of koa on  burned-over areas on Oahu were monitored over a 2.5-year period  (73). During this time, seedling density declined  dramatically. The root-crown fungus Calonectria crotalariae  caused more than half of this mortality. On these sites the  seedlings grew about 2.3 cm (1 in) per month. Koa did poorly when  planted on abandoned sugarcane land on the windward coast of the  island of Hawaii. Survival at age 6 years was 78 percent, but  trees averaged only 3 m (10 ft) tall, and only 62 percent were  judged vigorous. Tree form varied from good to poor, with 77  percent cull (101).

    The abundance and distribution of natural regeneration after  logging were studied on a 200-ha (500-acre) tract heavily  infested with pigs and vines on the island of Hawaii (70).  Seedling density of koa was about three times as great in  disturbed as in undisturbed areas. Most koa seedlings found on  the ground disturbed by logging were well established, but none  of those growing on undisturbed ground were large enough to have  much chance of surviving the menacing pigs and cattle. Koa  seedlings in disturbed areas tend to be clustered around seed  trees (70). In 1922, Krabel stated: "Where cattle  have been excluded for a number of years, koa groves are  developing with surprising speed on exposed and barren ridges"  (43).

    The stimulating effect of soil scarification on seedling emergence  is helpful in regenerating koa on degraded forest land where seed  reserves still exist in the soil. Disking in the sparsely wooded  pastures of the Hakalau Forest National Wildlife Refuge resulted  in koa reproduction. Even in open areas far removed from live or  skeletal remains of koa, a few seedlings emerged (15).

    In the natural rain forest, koa seedlings can emerge from mineral  soil and organic seedbeds, such as decaying logs and treefern  trunks. Seedling growth is generally slower on old logs than on  mineral soil, possibly due to low nutrient availability. However,  seedlings tend to survive better on organic seedbeds because  these sites are elevated and out of reach of feral pigs. In the  Kilauea Forest, more than 60 percent of the mature koa initially  emerged from logs or other large organic seedbeds (16). Nevertheless,  rarely do koa seedlings survive in the dense rain forest unless  openings have been created, as by windthrow. Gap-phase  replacement seems to be the primary mechanism by which koa is  maintained in natural rain forest communities (53). Serious  disturbances, such as fire or hurricane-induced windthrow,  typically stimulate large-scale koa reproduction.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Seed Production and Dissemination

No records of the frequency of  exceptionally good or poor seed years are available, but seed  years do vary. Koa seed pods dehisce while on the tree or fall to  the ground unopened, where they either dehisce or disintegrate. "The  horny seed often remains on the tree for a year after it ripens,  and when lying on the ground is known to have retained for a  period for 25 years its ability to germinate" (37). Koa  seeds are seldom dispersed far beyond the crown, but,  occasionally, wind may carry unopened pods some distance. Seeds  from koa growing in gulches may be carried downstream to lower  elevations, especially during torrential rains.

    Koa seeds, like those of other acacias, are among the most durable  of tree seeds and need not be kept in sealed containers. They  will germinate after many years of storage if kept in a cool, dry  place. The seeds have hard coats that retard germination unless  they are first mechanically scarified, briefly treated with  sulfuric acid, or soaked in hot water. The water treatment is the  most practical. The seeds are placed in nearly boiling water,  after the heat source is removed, and allowed to soak for 24  hours. Seeds that fail to swell the first time may again be  subjected to this pregermination treatment, often with success  (99). In seven samples, the number of clean seeds ranged from a  low of 5.3OO/kg to a high of 16,3OO/kg (2,400 to 7,400/lb).

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Flowering and Fruiting

The flowers of koa are borne over  the outer part of the crown. Seedlings have been observed in  flower and fruit (3,80) at 2 and 3 years of age. One of  the pollinating insects found on koa flowers is the honeybee (Apis  mellifera). The extent to which other insects, birds, and  wind affect pollination is not well documented. Koa initiates  flower development nearly year-round at the high elevation on  Mauna Loa, reaching a peak during the wet season in late winter  (46). On adjacent Mauna Kea, koa flowers appear from December  through February, with few flowers at any other time. At lower  elevations, on all of the islands, flowering usually occurs from  late winter to early summer (July). Weather conditions,  especially severe droughts, influence the timing and extent of  flowering at any time of the year.

    The inflorescence of koa is an axillary raceme of pale yellow  heads averaging 8.5 mm (0.3 in) in diameter (29), one to  three on a common peduncle, and composed of many hermaphroditic  (bisexual) flowers. Each flower has an indefinite number of free  stamens and a single elongated style. The heads are highly  dichogamous, with anthers dehiscing 3 to 8 days before the  stigmas are fully exserted (8).

    The fruit is a legume, slow to dehisce, about 15 cm (6 in)  long and 2.5 to 4 cm (1 to 1.5 in) wide. The pods contain about  12 seeds that vary from dark brown to black. They mature at  different times throughout the year, depending on location and  weather conditions.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Growth

Growth and Yield

Age of koa trees cannot be  determined. Growth rings were not correlated with "annual  rings" (102). Old relic forests still in existence  were probably present at the time Captain James Cook discovered  the Hawaiian Islands in 1778. The largest koa tree on record had  a d.b.h. of 363 cm (143 in), total height of 43 m (140 ft), and a  crown spread of 45 m (148 ft) (56).

    Stocking and growth data for natural regeneration on  heavily disturbed sites and one plantation on the island of  Hawaii are available (table 2).

   

Table 2-Characteristics of koa growing in three natural  stands and a plantation in Hawaii¹

        Location  Annual  rainfall
  (mm)  Age
  (yr)  Stand  stocking
  (stems/ha)  Dominants    D.b.h. (cm)  Height (m)        Natural stands      1  3810  8  3460  12.7  6.0    2  5080  17  790  23.1  17.4    3  2540  15  2720  18.5  13.1    Plantation  3810  27  395  31.0  14.4      (in)  (yr)  (stems/acre)  (in)  (ft)    Natural stands      1  150  8  1400  5.0  19.6    2  200  17  320  9.1  57.0    3  100  15  1100  7.3  43.0    Plantation  150  27  160  12.2  47    ¹Ching,  Wayne F. 1981. Growth of koa at selected sites on the island  of Hawaii. Unpublished report. HDepartment of Land and  Natural Resources, Division of Forestry and Wildlife,  Honolulu, HI. l0p.       

    The form of koa varies greatly. Most mature trees have large,  open, scraggly crowns with limby, fluted boles. In the rain  forests, on deep, rich soil, an occasional koa tree may surpass  34 m (110 ft) in height, but few possess clean, straight boles.  On drier sites, the form of koa is even poorer, and trees are  often stunted and misshapen. Precise yield figures from koa  stands are not available.

    Missing from the koa and ohia-koa forests in many areas are the  koa-size classes that normally form the recently mature, vigorous  stands. In 1913, the condition of large tracts of koa forest was  graphically described by Rock (62):

  "Above Kealakekua, South Kona, of the once beautiful  koa forest, 90 percent of the trees are now dead, and the  remaining 10 percent in a dying condition. Their huge trunks and  limbs cover the ground so thickly that it is difficult to ride  through the forest, if such it can be called.... It is sad,  however, to see these gigantic trees succumb to the ravages of  cattle and insects."    Forest survey data from 1959-61 (98) indicated the  condition of much of the sawtimber-size koa (trees more than 27.7  cm [10.9 in] in d.b.h.). Of 103 trees classified according to  merchantability on the basis of form and defect, 36 percent were  merchantable, 15 percent sound cull (with such defects as crook,  excessive limbs, or poor form), and 49 percent rotten cull  (excessive rot). Of the 103 trees, the average d.b.h. was 89 cm  (35 in); of 31 trees, the average height was 22 m (72 ft), and  the average crown diameter was 18 m (58 ft). Log grades were  determined for logs in 103 koa trees. Less than two-fifths of all  butt logs (first 4.9 m [16 ft]) met the specifications for either  factory lumber logs or tie and timber logs. More than  three-fifths were cull. Only 35 percent of the 103 trees sampled  had an upper log of 2.4 m (8 ft) or more, and more than half of  these logs were graded cull (98). Remeasurements in  1969-70 of the plots inventoried 10 years earlier (54) permit  estimates of annual growth and mortality of koa on the island of  Hawaii. Net annual growth was found to be a negative 4.52 million  board feet of sawtimber and a negative 15 660 m³ (553,000 ft³)  of growing stock (50). One study offers guidelines for  estimating the volume of unsound wood associated with log surface  defects common in koa (12)

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Molecular Biology and Genetics

Genetics

Morphological differences in koa have been observed on  several islands. In 1920, Rock (63), named two varieties:  Acacia koa var. lanaiensis (Hillebrand's A.  koa-B var.) and A. koa var. hawaiiensis, after  the islands on which they were found. Ecotypic variation can be  found from island to island. Studies of such variation are  complicated by past plantings of mixed seed lots collected  throughout the islands; such mixed plantings could now be  hybridizing.

    Collections of the koa group, all commonly referred to as koa,  were studied, and in 1979 this classification was presented by  St. John (90):
  Acacia koa var. koa,
grows on the six larger  is-lands.
  Acacia koa var. waianacensis grows only on Oahu,  and most commonly in the Waianae Mountains.
  Acacia koa var. latifolia; syn. A. koa var.  hawaiiensis Rock, grows on the island of Hawaii in the  rain forest, and at higher elevations on the more open ranch and  park land. Altitudinal races of koa probably exist (52).

    Two other native species related to koa are recognized. On  western Kauai, one of the oldest Hawaiian Islands, a form of  acacia is found that differs from koa in sepals, petals,  inflorescence (63), and seed shape (37). This  species, also called koa, is Acacia kauaiensis. A second  species closely related to koa is koaia (A. koaia), a  narrowly distributed, small, shrubby tree occupying dry, leeward  sites below 1050 m (3,500 ft) on Molokai, Maui, and Hawaii (89).  Acacia koaia differs from koa in the shape of the pods and  phyllodes (63). The native and introduced species of  Acacia found on Lanai have been described (20).

    Other Acacia species related to koa are found outside of  Hawaii. Mascarene acacia (A. heterophylla) is endemic to  Reunion island and Mauritius island, both about 725 km (450 mi)  east of Madagascar, in the Indian Ocean. It is so similar to koa  that the trees were initially identified as the same species. The  two were identified by another botanist as separate species,  however, entirely on the basis of distance and isolation. In  1969, significant differences were found in fruit and seed size,  corolla structure, and morphology of the first two leaves of the  two species (95).

    Tasmanian blackwood (Acacia melanoxylon), native to  Australia but planted in many countries, resembles koa. It has  straighter and shorter phyllodes, a narrower curved pod, a more  pointed crown (63), but similar wood. Another closely  related species, A. simplicifolia, grows in Samoa, New  Hebrides, New Caledonia, and Fiji (20,59,60).

    In 1948, one investigator determined that koa is a  tetraploid with 2n = 52 and stated that all other phyllodinous  acacias studied have the diploid chromosome complement (3).  He reasoned "that polyploidy in Acacia koa occurred  after the initiation of phylloidy. This is supported by its  distribution as an endemic island extension of the Australian  flora." In 1978, koa was observed to have a gametic number  of 26, verifying that it is tetraploid (14). Another investigator  (95) reported on the work of Lescanne, who observed that  the closely related A. heterophylla was also a  tetraploid, with 2n = 52.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2012

Assessor/s
Contu, S.

Reviewer/s
Hilton-Taylor, C.

Contributor/s

Justification
Acacia koa is currently known to be the dominant canopy endemic tree occurring on six of the Hawaiian islands. The species occurs in mesic forest in a wide range of habitats and elevations (60-2,300 m), and is therefore currently rated as Least Concern. Due to the restricted distribution range and to the fact that processes, such as timber extraction or invasion of alien species, might cause a population decline, A. koa should be monitored over a longer period of time to make sure of the health and status of the populations, along with in situ conservation measures to ensure that the supopulations number and range do not decrease in the future.
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National NatureServe Conservation Status

United States

Rounded National Status Rank: N4 - Apparently Secure

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NatureServe Conservation Status

Rounded Global Status Rank: G4 - Apparently Secure

Reasons: Widespread and often a dominant; occurs in a variety of habitats and in a large elevation range (Wagner, et. al. 1999).


Kauai, Oahu, Molokai, Lanai, Maui, Hawaii.
Kauai - A dominant in much of the mesic native forest.
Oahu - Common to dominant in many mesic forests.
Molokai - A. koa was originally very rare on Molokai. It was recorded from the island in the 1800's, but no individuals are currently known.
Lanai - Very spotty. Probably only a few hundred individuals.
West Maui - Localized.
East Maui - Most of the koa forests were destroyed in the 1800's. Still a dominant in a few parts of East Maui.
Hawaii - Much reduced, but still common over large areas despite grazing and logging.

In some areas alien insects destroy almost all of the A. koa seeds.
Mostly in mesic forests.

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Population

Population
A. koa is known to be widespread within its range and often the dominant species.

Population Trend
Stable
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Threats

Major Threats
A. koa forests were more extensive in the past but land clearing, destruction by animals (animals such as horses, cattle, goats, and pigs are also a very common source of high seedling losses and damage to trees less than 10 years old), insects (in some areas alien insects destroy almost all of the A. koa seeds), alien invasive plants and fire have all contributed to the reduction of its range. The species in the past could be found at 60 or 90 m of elevation, but pests and diseases currently limit koa’s optimal range to elevations above 610 m. On the other hand with long-lived seeds and the ability to regenerate after fire, koa may have potential as a problematic invasive species outside its native range. The species is used for timber, but at present there is not evidence of a population decline due to the species utilization; population dynamic study should be carried out to better understand the species trend.
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Comments: In some areas alien insects destroy almost all of the A. koa seeds.
Mostly in mesic forests.

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Management

Conservation Actions

Conservation Actions
A. koa trees were planted on the islands (more than 1.3 million koa seedlings were planted by the Hawaii Division of Forestry and Wildlife between 1915 and 1946 for watershed protection; in the Hakalau Forest National Wildlife Refuge etc.), because it is an important endemic species for reforestation of degraded lands and is of critical ecological importance because it provides habitat for numerous endemic birds and insects. The abundance and distribution of the Akiapolaau (Hemignathus munroi; Endangered IUCN ver 3.1), Akepa (Loxops coccineus; Endangered IUCN ver 3.1), and Hawaiian Creeper (Oreomystis mana; Endangered IUCN ver 3.1), three of the endangered forest birds on the island of Hawaii, are strongly associated with Koa in forest communities. A. koa has been listed as Apparently Secure (G4) from NatureServe (2009), which is equivalent to IUCN category Least Concern (LC).
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Relevance to Humans and Ecosystems

Benefits

Special Uses

The most important use of koa timber by the Hawaiians was  to build canoes. The largest of the giant war canoes extended 21  m (70 ft). Canoe hulls were made of single, giant koa logs. Koa  was also used for sursurfboardsiboards, some 5.5 m (18 ft) or  longer, for paddles, and for framing grasshouses. The bark  provided dye to tapa, a light cloth made from the bark of wauke  (Broussonetia papyrifera) (11,19).

    Koa wood is now used primarily for furniture, cabinet work, and  face veneers. It is widely used in woodcraft. Cabinet makers  recognize a dozen or more types of koa wood, including curly or "fiddle  back" koa, red koa, and yellow koa (11). One local  use is for making ukuleles. At one time koa was sold on the world  market as Hawaiian mahogany (62).

    Large logs have a narrow, creamy-white band of sapwood. The  heartwood may vary through many rich shades of red, golden brown,  or brown. The heartwood seasons well without serious degrade from  warping, checking, or splitting (74).

    Although it has been stated that foresters in Hawaii have paid  little attention to koa (83), more than 1.3 million koa  seedlings were planted by the Hawaii Division of Forestry and  Wildlife between 1915 and 1946 (78) for watershed  protection. Koa, however, did not perform as well as many  introduced species on these deteriorated sites.

  • Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods.   Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965).   U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp.   http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm External link.
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Craig D. Whitesell

Source: Silvics of North America

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Wikipedia

Acacia koa

Acacia koa is a species of flowering tree in the pea family, Fabaceae. It is endemic to the Hawaiian Islands,[1] where it is the second most common tree.[2] The highest populations are on Hawaiʻi, Maui and Oʻahu. Its name in the Hawaiian language, koa, also means brave, bold, fearless, or warrior.[3]

Description[edit]

Upper branches of a koa tree, showing the bark, sickle-shaped phyllodes, greenish rounded flower heads, and seedpods

Koa is a large tree, typically attaining a height of 15–25 m (49–82 ft) and a spread of 6–12 m (20–39 ft).[4] In deep volcanic ash, a koa tree can reach a height of 30 m (98 ft), a circumference of 6 m (20 ft), and a spread of 38 m (125 ft).[5] It is one of the fastest-growing Hawaiian trees, capable of reaching 6–9 m (20–30 ft) in five years on a good site.[6]

Leaves[edit]

Initially, bipinnately compound leaves with 12–24 pairs of leaflets grow on the koa plant, much like other members of the pea family. At about 6–9 months of age, however, thick sickle-shaped "leaves" that are not compound begin to grow. These are phyllodes, blades that develop as an expansion of the leaf petiole. The vertically flattened orientation of the phyllodes allows sunlight to pass to lower levels of the tree. True leaves are entirely replaced by 7–25 cm (2.8–9.8 in) long, 0.5–2.5 cm (0.20–0.98 in) wide phyllodes on an adult tree.[4]

Flowers[edit]

Flowers of the koa tree are pale-yellow spherical racemes with a diameter of 8–10 mm (0.31–0.39 in).[7] Flowering may be seasonal or year round depending on the location.[4]

Fruit[edit]

Fruit production occurs when a koa tree is between 5 and 30 years old. The fruit are legumes, also called pods, with a length of 7.5–15 cm (3.0–5.9 in) and a width of 1.5–2.5 cm (0.59–0.98 in). Each pod contains an average of 12 seeds. The 6–12 mm (0.24–0.47 in) long, 4–7 mm (0.16–0.28 in) wide seeds are flattened ellipsoids and 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 hard seed coat, and this allows them to remain dormant for up to 25 years. Scarification is needed before A. koa seeds will germinate.[7]

Habitat[edit]

Koa is endemic to the islands of Hawaiʻi, Molokaʻi, Maui, Lānaʻi, Oʻahu, and Kauaʻi, where it grows at elevations of 100–2,300 m (330–7,550 ft). It requires 850–5,000 mm (33–197 in) of annual rainfall. Acidic to neutral soils (pH of 4-7.4)[4] that are either an Inceptisol derived from volcanic ash or a well-drained histosol are preferred.[8] Its ability to fix nitrogen allows it to grow in very young volcanic soils.[2] Koa and ʻōhiʻa lehua (Metrosideros polymorpha) dominate the canopy of mixed mesic forests.[9] It is also common in wet forests.[10]

Uses[edit]

The American country singer Taylor Swift with a Taylor acoustic guitar made of Acacia koa wood

The koa's trunk was used by ancient Hawaiians to build waʻa (dugout outrigger canoes)[11] and papa heʻe nalu (surfboards). Only paipo (bodyboards), kikoʻo, and alaia surfboards were made from koa, however; olo, the longest surfboards, were made from the lighter and more buoyant wiliwili (Erythrina sandwicensis).[12] The reddish wood is very similar in strength and weight to that of Black Walnut (Juglans nigra), with a specific gravity of 0.55,[7] and is sought for use in wood carving and furniture.[4] Koa is also a tonewood,[13] often used in the construction of ukuleles,[14] acoustic guitars,[15] and Weissenborn-style Hawaiian steel guitars.[16] B.C. Rich used koa on some of their electric guitars as well,[17] and still uses a koa-veneered topwood on certain models.[18] Fender made Limited Edition Koa wood models of the Telecaster and the Stratocaster in 2006. Trey Anastasio, guitarist for the band Phish, primarily uses a Koa hollowbody Languedoc guitar. Commercial silviculture of koa is difficult because it takes 20 to 25 years before a tree is of useful size.[19]

Relation to other species[edit]

The relationships of koa are not clear. Among other Pacific Islands of volcanic (non-continental) origin, only Vanuatu has native Acacia species. A. heterophylla, from distant Réunion, is very similar and has been suggested to be the closest relative of koa, but this is far from certain.[20]

Koaiʻa seedpod, showing the end-to-end arrangement of seeds.

A closely related species, koaiʻa or koaiʻe (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. The phyllodes are also usually straighter, though this character is variable in both species. The wood is denser, harder, and more finely grained than koa wood.[4] Koaiʻa has been much more heavily impacted by cattle and is now rare, but can be seen on ranch land in North Kohala.

Conservation[edit]

The koa population has suffered from grazing and logging. Many wet forest areas, 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 for outrigger canoes, there are few koa remaining which are both large and straight enough to do so today.[4] In areas where cattle are present, koa regeneration is almost completely suppressed. However, if the cattle are removed, koa are among the few native Hawaiian plants able to germinate in grassland, and can be instrumental in restoring native forest. It is often possible to begin reforestation in a pasture by disk harrowing the soil, as this scarifies seeds in the soil and encourages large numbers of koa to germinate.[8] Experiments at the Hakalau Forest National Wildlife Refuge have shown that ʻōhiʻa lehua (Metrosideros polymorpha) survives best in pasture when planted under koa. This is because koa trees reduce radiative cooling, preventing frost damage to ʻōhiʻa lehua seedlings.[21]

Ecology[edit]

Koa is the preferred host plant for the caterpillars of the Green Hawaiian Blue (Udara blackburni), which eat the flowers and fruits.[22] Adults drink nectar from the flowers. Koa sap is eaten by the adult Kamehameha Butterfly (Vanessa tameamea).[23] The Koa Bug (Coleotichus blackburniae) uses its rostrum to suck the contents out of koa seeds.[24]

Gallery[edit]

References[edit]

  1. ^ a b "Acacia koa A. Gray". Germplasm Resources Information Network (United States Department of Agriculture). 2007-04-30. Retrieved 2010-05-03. 
  2. ^ a b Idol, Travis (2008-04-16). "Environmental Controls Over Acacia koa Productivity". Travis Idol's Research Page. University of Hawaiʻi at Mānoa. Retrieved 2009-01-31. 
  3. ^ Kepler, Angela Kay (1998). Hawaiian Heritage Plants. University of Hawaii Press. p. 106. ISBN 978-0-8248-1994-1. 
  4. ^ a b c d e f g Elevitch, Craig R.; Kim M. Wilkinson; J. B. Friday; C. Baron Porter (April 2006). Acacia koa (koa) and Acacia koaia (koaiʻa) (PDF). The Traditional Tree Initiative. 
  5. ^ Welsbacher, Anne (2003). Life in a Rain Forest. Twenty-First Century Books. p. 30. ISBN 978-0-8225-4685-6. 
  6. ^ Wilkinson, Kim M.; Craig R. Elevitch (2003). Growing Koa: A Hawaiian Legacy Tree. Hōlualoa, Hawai'i: Permanent Agriculture Resources. p. 13. ISBN 978-0-9702544-2-9. 
  7. ^ a b c Allen, James A. (2003-01-01). "Acacia koa A. Gray" (PDF). Tropical Tree Seed Manual. Reforestation, Nurseries & Genetics Resources. Retrieved 2009-02-23. 
  8. ^ a b Whitesell, Craig D. "Koa". Silvics of North America Volume 2: Hardwoods. United States Department of Agriculture. Archived from the original on 20 January 2009. Retrieved 2009-02-17. 
  9. ^ Ziegler, Alan C. (2002). Hawaiian Natural History, Ecology, and Evolution. University of Hawaii Press. pp. 175–176. ISBN 978-0-8248-2190-6. 
  10. ^ Sohmer, S. H.; R. Gustafson (1987). Plants and Flowers of Hawaiʻi. University of Hawaii Press. pp. 45–52. ISBN 978-0-8248-1096-2. 
  11. ^ Bryan, William Alanson (1915). Natural History of Hawaii, Being an Account of the Hawaiian People, the Geology and Geography of the Islands, and the Native and Introduced Plants and Animals of the Group. Hawaiian Gazette Co, Ltd. p. 339. ISBN 1-4446-7820-5. 
  12. ^ Marcus, Ben; Juliana Morais; Jeff Divine; Gary Linden (2007). The Surfboard: Art, Style, Stoke. MBI Publishing Company. pp. 17–19. ISBN 978-0-7603-2753-1. 
  13. ^ Beberman, Norman L. "Koa: Beautiful Looking, Beautiful Sounding Tonewood". GuitarNation.com. Archived from the original on 7 February 2009. Retrieved 2009-02-23. 
  14. ^ Total Ukulele: D-Tuning Method for Beginners. Mel Bay Publications. 2002. p. 93. ISBN 978-3-8024-0446-7. 
  15. ^ Gerken, Teja (2000). Acoustic Guitar Owner's Manual: The Complete Guide. String Letter Publishing. p. 10. ISBN 978-1-890490-21-8. 
  16. ^ Sheperd, John (2003). "Guitars". Continuum Encyclopedia of Popular Music of the World. Volume II: Performance and Production. Continuum International Publishing Group. p. 283. ISBN 978-0-8264-6322-7. 
  17. ^ Achard, Ken (1989). The History and Development of the American Guitar. Bold Strummer Ltd. p. 163. ISBN 978-0-933224-18-6. 
  18. ^ Kirkland, Eric (December 2006). "Rich and Famous". Guitar World: 154. 
  19. ^ "Silviculture: Diversification of Rural Economy". Leeward Haleakalā Watershed Restoration Partnership. Retrieved 2009-02-01. 
  20. ^ Whitesell, Craig D (1964). Silvical Characteristics of Koa (Acacia koa Gray) (PDF). United States Forestry Service. 
  21. ^ Friday, J. B.; Darrell A. Herbert (April 2006). Metrosideros polymorpha ('ōhi'a lehua) (PDF). The Traditional Tree Initiative. p. 21. 
  22. ^ Scott, James A. (1992). The Butterflies of North America: A Natural History and Field Guide. Stanford University Press. p. 399. ISBN 978-0-8047-2013-7. 
  23. ^ Scott, Susan (1991). Plants and Animals of Hawaii. Bess Press. p. 78. ISBN 978-0-935848-93-9. 
  24. ^ http://www.uhh.hawaii.edu/affiliates/prism/documents/KoaBugLesson.pdf
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