The theory of island biogeography and related work regarding forest fragmentation predict that woody plant species richness increases as reservoir island size increases. The forest community of smaller islands is also predicted to contain a subset of the flora on larger islands. In addition, islands located closer together should be more compositionally similar than those that occur at further distances from one another due to spatial autocorrelation. We tested these predictions by examining forested islands of different sizes in two reservoirs along the Savannah River system between South Carolina and Georgia: J. Strom Thurmond Lake and Richard B. Russell Lake. Both reservoirs were created by damming rivers to provide hydroelectric power. Lake Russell is located immediately northwest of Lake Thurmond and was created 30 years later (1984 vs. 1954). We sampled small (0.08-0.35 ha), medium (0.56-0.88 ha), and large (1.15-2.47 ha) islands and adjacent mainland sites. Our study examined whether mainland sites differed in species composition, richness, and diversity of woody species when compared to islands; whether islands located closer together had more similar species composition and whether species richness and diversity increased with increasing island size.

Results/Conclusions

Species richness and diversity did not differ significantly between mainland sites and islands in either reservoir, although mainland sites exhibited slightly lower diversity and species richness than islands. Invasion of exotic species and other species that thrive in edge habitats accounted for this higher richness on islands. Medium islands showed a trend toward higher diversity and richness than either small or large islands, but overall small, medium, and large islands had similar levels of diversity and species richness. Mantel tests demonstrated a small but significant relationship between geographic location and species composition such that islands that were closer together exhibited more similar species composition. Island and mainland sites also differed significantly in their species composition. While species composition largely followed that described for the oak-hickory forest association, Juniperus virginiana, Nyssa sylvatica, and Pinus taeda were particularly abundant on islands compared to mainland sites. Most notably, both exotic and native lianas and shrubs were six to ten times more abundant on islands than mainland sites. These results indicate that continued fragmentation of the piedmont forest may lead to an increase in both exotic and native liana and shrub species in the understory. This change in composition may have negative impacts on regeneration of tree species that occur in the native oak-hickory forest association.