Effects of long-term ungulate exclusion and recent alien species control on the preservation and restoration of a Hawaiian tropical dry forest

Although the destruction of tropical rain forests receives much attention, tropical dry forests are in general far more threatened and endangered. Eli minating grazing ungulates is often considered a key first step toward prot ecting these ecosystems, but few studies have investigated the long-term ef fects of this technique. We examined the effects of ungulate exclusion from a 2.3-ha native dry-forest preserve on the island of Hawaii by comparing i ts present flora to the flora of an adjacent area subjected to continuous g razing since the preserve was fenced over 40 years ago. Relative to this ad jacent area, the fenced preserve contained a more diverse flora with substa ntially greater coverage of native overstory and understory species. Until recently, however, regeneration of native canopy trees within the preserve appears to have been thwarted by a dominant herbaceous cover of alien fount ain grass (Pennisetum setaceum) and predation by alien rodent species. Our results indicate that although ungulate exclusion may be a necessary and cr itical first step, it is not sufficient to adequately preserve and maintain Hawaii's remaining tropical dry forest remnants. Our recent efforts to con trol the dominant alien species within the fenced preserve suggest that thi s practice may facilitate both the regeneration of native species and the c olonization and potential invasion of new alien plants. Comparisons of seed lings of the dominant native canopy tree Diospyros sandwicensis growing in sites both dominated by and free of fountain grass suggested that fountain grass inhibits Diospyros seedling growth and photosynthesis but may increas e survival if seedlings are protected from ungulates.