Print PageIt is often reported that many exotic, invasive plant species tend to proliferate on edges of forest fragments compared to interior forest sites. A pressing question regarding this type of invasion is whether these species appear to be relegated to edges and disturbed sites simply because their propagules have not dispersed to interior forests. Alternatively, it may be that these species are found in edges and disturbed forests because they are unable to survive and reproduce in mature forests. Creation of the Richard B. Russell reservoir along the Savannah River corridor has given us a unique opportunity to experimentally evaluate survival and growth of two common woody invaders, Lonicera japonica and Albiza julibrissin, in two habitats: small forest fragments (former hilltops that are now reservoir islands) and relatively undisturbed forest (mainland). To address the question of whether these two species can invade relatively undisturbed forest, we out-planted five seedlings of each species on edges and interiors of nine islands and five mainland sites in January, 2009. We measured survival, growth, and herbivory biannually for 2.5 years. We predicted that survival and growth rates for L. japonica and A. julibrissin would be highest on fragment edges and lowest in mainland forest interiors.
Results/Conclusions
We found that the two species respond very differently to fragmentation. Lonicera japonica survival rates were as predicted with the highest rates of survival on fragment edges, followed by mainland forest edges. However, L. japonica still had survival rates over 35% in fragment and mainland forest interiors. Relative growth rates for L. japonica were negative for the entire period of study in all treatments except for fragment interiors. Negative relative growth rates for L. japonica reflect the harsh conditions associated with fragment edges along with elevated herbivory in fragments and mainland forest edges observed during this study. Interestingly, A. julibrissin had high rates of survival and growth on mainland forest interior and fragment edges. Over 30% of A. julibrissin individuals survived in mainland forest interior. This may be a reflection of the comparatively low light levels on fragment interiors along with elevated herbivory on mainland forest edges. Future studies will investigate whether L. japonica and A. julibrissin have the ability to germinate in mainland forest interiors. Overall, if dispersal and germination barriers are overcome, it appears that both of these exotic species have the capacity to invade relatively undisturbed forest.
