EFFECTS OF LIGHT, ALIEN GRASS, AND NATIVE SPECIES ADDITIONS ON HAWAIIAN DRY FOREST RESTORATION

Alien species invasions have already caused substantial ecological and economic damage and will likely have even greater negative consequences in the future. Thus, it is imperative to improve our basic ecological understanding of these invasions and enhance our ability to reverse or mitigate their often devastating effects. Invasions by fire-promoting alien grasses have played a particularly important role in the destruction of tropical dry forests and are a major reason why these ecosystems are now among the most endangered in the world. We investigated how light availability (full sun and 50% shade), alien grass control (bulldoze, herbicide, plastic mulch, and trim treatments), and native species additions (outplanting and direct-seeding) affected the establishment of native plants and the suppression of a dominant invasive bunchgrass (fountain grass, Pennisetum setaceum) within a highly degraded fenced dry forest remnant on the island of Hawaii. The percent cover of native species increased in all light, grass control, and species addition treatments throughout the 20 mo of the experiment, and was greatest in the shade, bulldoze, and outplant treatments. Although fountain grass cover also increased over time in all grass control treatments, the three more aggressive techniques all significantly reduced grass cover relative to the more moderate trim treatment. In addition, there was a significant overall negative correlation between the final cover of fountain grass and native species, suggesting that these native species may successfully compete with fountain grass for water and/or nutrients. Outplant survival and the number of individuals established from direct-seeding differed significantly among the grass control treatments, and in each case, the response was highly species specific. Photosynthetic rates of established outplanted individuals and fountain grass did not differ significantly across most experimental environments, indicating that the local dominance of fountain grass may not be due to superior physiological attributes. The results of this experiment highlight the importance of investigating species- and treatment-specific responses before attempting larger-scale restoration projects, particularly when using rare and endangered species. This study also suggests that relatively simple techniques may be used to simultaneously establish populations of vigorous understory native species and suppress alien grasses at relatively large spatial scales, and that remnant or newly created favorable microsites may be exploited to facilitate the establishment of rarer native overstory species.