The objective of this project was to examine how degree of habitat fragmentation affects the susceptibility of temperate forests to invasion by exotic plants. We used a natural experiment of continuous forest that was fragmented into islands by creation of reservoirs for hydroelectric power. The reservoirs used for this study, Richard B. Russell and J. Strom Thurmond, are located along the South Carolina-Georgia border and contain numerous islands that were hilltops prior to dam construction and flooding. We examined how environmental conditions, forest structure, and the abundance and composition of invasive plants differ among islands of different size and among edge and interior parts of islands (i.e. edge effects). We surveyed the woody vegetation and measured several environmental and forest structural parameters in plots at increasing distance from the water’s edge on 24 islands and 5 mainland areas for each reservoir. Islands were divided into three sizes: small (0.08–0.35 ha), medium (0.56–0.88 ha), and large (1.15–2.47 ha).
Results/Conclusions
The most common exotic species on the islands was Lonicera japonica, which was found on 13 islands. Albizia julibrissin was found on eight islands, Elaeagnus angustifolia on five, and most other invasive species were found only on one island. Neither the number of invasive individuals nor the number of invasive species differed among islands of different size. Edge effects were apparent on islands in terms of the litter mass, height of the vegetation, and soil pH. Litter mass, forest height, and soil pH were greater closer to the center of the island than at the edges, whereas tree and liana basal area and number of individuals did not differ. Islands of different size did not differ significantly in these relationships, although there was a trend for larger islands to be less prone to edge effects (smaller differences between edge and interior plots) in productivity (as measured by litter mass) and vegetation structure (as measured by maximum vegetation height). Tree height and litter mass were greater on medium and large islands than on small islands, but medium-sized islands unexpectedly supported the greatest tree height and litter mass as well as the lowest pH. Using experimental outplantings, we will determine whether invasive plants have greater potential for establishing and growing along edges than interior areas of these reservoir islands. We are ultimately interested in which invasive species become established and proliferate in forest fragments and how these invasions may change ecosystem functioning.
