The powerful volcanoes that formed the high islands of the Hawaiian archipelago block northeasterly tradewinds, creating wet, windward rain forests and much drier, leeward forests. Dryland forests in Hawai'i receive only about 20 inches of rain a year. However, the trees in these forests intercept fog and increase ground moisture levels, thereby enabling these seemingly inhospitable habitats to support a diverse assemblage of plants and animals.
Dryland forests of the Hawaiian Islands, like those worldwide, have been heavily impacted by humans both directly and indirectly. Less than 10% of Hawai'i's original dryland forest habitat remains. These forests have been severely impacted by urban development, ranching and agriculture, and invasive species. In particular, browsing animals and alien grasses have caused significant damage. Feral ungulates, including goats, sheep, cattle, and pigs, consume sensitive plants. Alien grasses have become dominant in the understory in many dryland habitats. In addition, these introduced grasses are fire-adapted and have increased the incidence of wildfire in these ecosystems. Native Hawaiian plants did not evolve with frequent fires or mammalian herbivores and typically do not survive well under these pressures.