Ecosystem and Population Consequences of Alien Grass Invasion of Seasonally Dry Hawaiian Woodlands

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Invasions by introduced grasses are a common phenomenon worldwide, with the potential for dramatic community and ecosystem-level effects. This study will investigate changes in ecosystem properties induced by alien grass invasion in a nitrogen-limited, seasonally dry woodland in Hawai'i and a natural savanna lands in Venezuela. Hawaiian ecosystems are ideal for such tests because of the relative simplicity of the communities and the ease with which invasions can be documented. Two grass species have a distinct pattern of invasion into theses woodlands: Schizachyrium condensatum invades undisturbed woodland and promotes fire. After fire, Melinis minutiflora largely replaces both Schizachyrium and native woodland species resulting in a low diversity, more readily burned grassland. Nutrient availability changes predictably with fire, yielding, a clear pattern of vegetation change that likely coincides with a predictable change in soil nutrients. The researchers propose to investigate the processes driving vegetation and soil nutrient changes and to examine the influence of fire and the post-fire presence of alien grasses on nutrient limitation. Specifically, they will test the hypothesis that grass invasion induces largescale changes in nutrient cycling and that these changes feedback to influence population-level processes involving both native and introduced species. The Venezuela study will permit tests of the generality of any conclusions.