How species assemble into communities can best be explored by direct observation over time combined with experimental manipulations. Pioneer species can impact successional trajectories through their influence on interspecific competition for soil and light resources. In each of three different experiments on recent landslides in a Puerto Rican tropical forest, we removed one type of pioneer: early successional woody plants, thicket-forming scrambling ferns or tree ferns. After seven years, we harvested aboveground biomass and asked if these pioneers excluded other guilds of colonizing plants, if they altered species richness, vegetation cover and biomass or soil properties, and if their removal promoted colonization by late successional woody plants.
Results/Conclusions
Removal of woody pioneers promoted dense growth and higher species richness of herbaceous, understory plants on some landslides but did not promote late successional woody plants. These results suggest that in the long term, pioneer woody plants might indirectly facilitate establishment of late successional woody plants by suppressing the herbaceous understory. Removal of scrambling ferns facilitated the establishment of early successional woody plants and also increased species richness and cover of forbs, grasses, lycophytes and fine litter. Removal of tree ferns had little effect on plant communities and none of the removal treatments substantially altered soil conditions. The strongest effect was found by the plants with the smallest stature and densest growth form (the scrambling ferns). High spatial and temporal heterogeneity within and among landslides was demonstrated by markedly different species composition or biomass and highly variable rates of species change across landslides. Understory plants such as scrambling ferns, forbs, grasses and lycophytes appear to have a critical impact on initial plant community assembly with possible long-term implications for forest succession. However, generalizations about plant guilds must be tempered by site-specific variability and the diverse set of potential successional trajectories that reflect the remarkably high spatial heterogeneity found on landslides.
