Long-term research impacts on seedling community structure and composition in a permanent forest plot

Long-term ecological research projects have become cornerstones for the study of forest dynamics worldwide. The intense, large-scale research efforts necessary to monitor ecological processes may alter natural processes and be a source of error in analyses. This study evaluated whether trampling due to concentrated researcher presence has altered the structure and composition of the seedling layer in the 50 ha permanent sample plot on Barro Colorado Island (BCI), Panama. Since 1980, major research projects in the plot have included complete tree censuses every 5 years, weekly seed trap collection, and the more recent annual censuses of 20,000 1 m2 seedling quadrats. We compared data from these pre-existing seedling quadrats with data from 600 newly established seedling quadrats in an area of much lower research intensity adjacent to the 50 ha plot and tested for differences in seedling density, height-class distributions, species richness and composition. Although we expected to find evidence of researcher impacts on the seedling layer, we found no significant differences in seedling community structure or composition inside and outside of the BCI 50 ha plot. We conclude that there is no evidence that research efforts within the BCI plot have thus far resulted in significant changes in the seedling layer. The extent of research impacts is likely to differ under varying environmental conditions and research protocols. Continued efforts should be made to quantify the impacts of research methodology at long-term research sites in order to detect site-specific or long-term changes.