Alien Grass Invasion and Fire In the Seasonal Submontane Zone of Hawai'i

Abstract

Island ecosystems are notably susceptible to biological invasions (Elton 1958), and the Hawaiian islands in particular have been colonized by many introduced species (Loope and Mueller-Dombois 1989). Introduced plants now dominate extensive areas of the Hawaiian Islands, and 86 species of alien plants are presently considered to pose serious threats to Hawaiian communities and ecosystems (Smith 1985). Among the most important invasive plants are several species of tropical and subtropical grasses that use the C4 photosynthetic pathway. These grasses now dominate extensive areas of dry and seasonally dry habitats in Hawai'i. They may compete with native species, and they have also been shown to alter hydrological properties in the areas they invade (MuellerDombois 1973). Most importantly, alien grasses can introduce fire into areas where it was previously rare or absent (Smith 1985), thereby altering the structure and functioning of previously native-dominated ecosystems. Many of these grasses evolved in fire-affected areas and have mechanisms for surviving and recovering rapidly from fire (Vogl 1975, Christensen 1985), while most native species in Hawai'i have little background with fire (Mueller-Dombois 1981) and hence few or no such mechanisms. Consequently, grass invasion could initiate a grass/fire cycle whereby invading grasses promote fire, which in turn favors alien grasses over native species. Such a scenario has been suggested in a number of areas, including Latin America, western North America, Australia, and Hawai'i (Parsons 1972, Smith 1985, Christensen and Burrows 1986, Mack 1986, MacDonald and Frame 1988). In most of these cases, land clearing by humans initiates colonization by alien grasses, and the grass/fire cycle then leads to their persistence. In Hawai'i and perhaps other areas, however, grass invasion occurs without any direct human intervention. Where such invasions initiate a grass/fire cy-