Stripping of Acacia koa Bark by Rats on Hawaii and MauP

Koa (Acacia koa) is the most valuable native timber species in Hawaii. Bark stripping of young trees by rats, a common but unstudied phenomenon, may affect survival, growth, and quality of koa. Up to 54% of the trees sampled in 4to 6-year-old stands in the Laupahoehoe and Waiakea areas on Hawaii were wounded by rats; only 5% of trees sampled in a l-year-old stand on Borge Ridge, Maui, were wounded. Wounds were generally long and narrow. Complete girdling was not observed, and direct mortality seemed low. However, indirect effects of damage-deformation of stems, infection by pathogens, and premature death-require further study. Because only young trees seem susceptible to bark stripping, rodent control may be desirable during the first 5 years of koa stand growth. Waiakea, Island ofHawaii The study area in the Waiakea Forest Reserve is on the east flank of Mauna Loa, 80 KOA (Acacia koa), A LEGUMINOUS TREE, is the most valuable native timber species in Hawaii (Whitesell 1964) and may playa major role in the nitrogen cycle of the koa-'6hi'a (Metrosideros polymorpha) ecosystem. It is also an important habitat component for several native birds, including four endangered species, 'akiapola'au (Hemignathus wilsoni), 'akepa (Loxops coccineus) , Hawaii creeper (Loxops maculatus mana), and Hawaiian crow (Corvus tropicus) (Scott et aI., in press; Giffin et aI., submitted). Bark stripping of young trees by rodents, probably the black rat (Rattus rattus) or the Polynesian rat (Rattus exulans) or both, may affect survival, growth, and quality of koa. But such damage is virtually undocumented, even though it is commonly observed (c. Wakida, H. Horiuchi, and E. Pung, pers. comm.). This paper reports on the incidence ofbarkstripping and the size, location, and severity of wounds in young koa stands in forests at Laupahoehoe and Waiakea, island ofHawaii, 1 Manuscript accepted 25 June 1983. 2 U.S. Department of Agriculture, Forest Service, Institute of Pacific Islands Forestry, Pacific Southwest Forest and Range Experiment Station, 1151 Punchbowl Street, Room 323, Honolulu, Hawaii 96813. 3 Present address: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station, Redwood Sciences Laboratory, 1700 Bayview Drive, Arcata, California 95521. and Makawao, island of Maui. To aid interpretation of the data, it describes the structure of each stand in terms of koa diameter size class distribution and stem density. This information should prove useful to forest managers in establishing healthy koa stands.