Acacia koa for up to 25 years. Scarification is needed before "A. koa" seeds will germinate. Koa is endemic to the islands of Hawaii, Molokai, Maui, Lānai, Oahu, and Kauai, where it grows at elevations of . It requires of annual rainfall. Acidic to neutral soils (pH of 4-7.4) that are either an Inceptisol derived from volcanic ash or a well-drained histosol are preferred. Its ability to fix nitrogen allows it to grow in very young volcanic soils. Koa and ōhia lehua ("Metrosideros polymorpha") dominate the canopy of mixed mesic forests. It is also common in wet forests. The koa's trunk was
Hawaiian tropical rainforests milo ("Thespesia populnea") for the past 1,000–2,000 years. The Polynesian-introduced noni ("Morinda citrifolia"), pia ("Tacca leontopetaloides"), and kī ("Cordyline fruticosa") are also common in this zone. Other native species include pololei ("Ophioglossum concinnum"), ākia ("Wikstroemia" spp.), loulu fan palms ("Pritchardia" spp.), ōhia lehua ("Metrosideros polymorpha"), and lama ("Diospyros sandwicensis"). Mixed mesic forests, at on the windward slopes of the large islands in addition to the summit of Mount Lānaihale on Lānai, receive of rainfall annually and thus may not be true rainforests. The forest canopy, dominated by koa ("Acacia koa") and ōhia lehua ("Metrosideros polymorpha"), is somewhat open, but tree