Factors affecting community composition of forest regeneration in deforested, abandoned land in Panama

We tested alternative hypotheses concerning factors affecting early forest succession and community composition in deforested and abandoned areas invaded by an exotic grass, Saccharum spontaneum, in Panama. We hypothesized three barriers to natural regeneration: (1) Saccharum competition, (2) seed dispersal limitations, and (3) fire. We measured natural free and shrub regeneration in a factorial experiment combining distances from adjacent forest. mowing treatments of the Saccharum, and a prescribed burn. To determine the applicability of the general model of neotropical succession and the nucleation model Of succession to species composition of forest regeneration in these anthropogenic grasslands in Panama the effect of time since fire and distance to remnant vegetation (isolated trees, shrubs, and large monocots) was measured. Fire significantly affected species composition and decreased species richness because most species had either their resprouting ability or seed germination inhibited by fire: the few species that had regeneration enhanced by fire dominated early successional communities. Sites differed in time since fire, ranging from 1 to 4 yr; the interaction of site and distance from the forest significantly affected community composition and the prevalence of species with different dispersal mechanisms and shade tolerance. At recently burned sites. light-dependent wind-dispersed species predominated; most were found near the forest edge. As time since fire increased, significantly more shade-tolerant, larger-animal-dispersed species were recorded, and the proximity to and species identity of remnant vegetation became more important in affecting species composition of the natural regeneration; no significant effect of distance from the forest was found at sites that were unburned for three or more years. Our results support both successional models; the temporal sequence of species composition corresponded to later stages of the general model, while the spatial distribution of species followed the nucleation model. Our results highlight the importance of effective seed dispersal in structuring successional species composition and distribution and in regaining, lost diversity resulting from frequent fires ill the Saccharum.