Several forested islands (i.e., forest fragments) were created when Lake Richard B. Russell was created by damming the Savannah River to provide hydroelectric power in 1984. Contrary to the predictions of Island Biogeography Theory, we have found that woody plant species richness is higher on islands compared to mainland sites. Islands also differed in their woody species composition from mainland sites with a suite of pioneer and invasive species identified as significant indicators of islands. Both exotic and native lianas and shrubs were six to ten times more abundant in these fragments. Higher species richness on islands and differences in species composition may be a product of pronounced edge effects on islands in contrast to mainland sites, which were located in protected coves to minimize the influence of multiple edges. Greater differences in forest structure, resource availability, and environmental variables between edges and interior sites (i.e., edge effects) on islands compared to mainland sites could promote greater site heterogeneity with some areas favoring the establishment and proliferation of high-light demanding species and others favoring shade-tolerant species. We sampled a suite of environmental measurements (i.e. light levels, soil pH, and nutrients) along two transects from the edge to interior for each of five reservoir islands and five mainland sites to determine if edge effects may be contributing to elevated richness on islands and differences in species composition between islands and mainland sites.
Results/Conclusions
Soil pH was not significantly different between island and mainland sites and showed no discernable pattern from the edge to interior in either site type (i.e. island vs. mainland). While zinc was elevated at interior island and mainland sites, all other soil nutrients showed little pattern in relation to distance from the edge or site type. Canopy openness was significantly higher on islands compared to mainland sites and also decreased significantly from the edge to the interior on islands but not on mainland sites. The probability of encountering four of the 12 island indicator species (Gelsemium sempervirens, Vaccinium arboreum, Pinus echinata, and Crataegus spp.) was significantly and positively related to canopy openness. Conversely, the remaining eight island indicator species showed either a negative or no association with canopy openness, indicating that factors other than light are contributing to their establishment and proliferation. Island fragments in this study are experiencing marked edge effects, leading to a continuum of light levels where different species may specialize.
