Unraveling the roles of dispersal limitation and a dominant herbivore on seedling diversity and dynamics in temperate forests

Background/Question/Methods

A central goal of community ecology is to disentangle the relative importance of mechanisms that shape community diversity and dynamics. Ecological mechanisms may be viewed through the lens of three fundamental processes: dispersal, ecological drift, and niche selection. These processes may have an especially strong influence on seedling communities, where spatial patterns created by dispersal may be modified by the strength of niche selection along abiotic or biotic gradients. Here, we examine the interplay between dispersal limitation and niche selection by a dominant herbivore, white-tailed deer, during the assembly of temperate forest seedling communities. In an oak-hickory forest near St. Louis, Missouri, we censused all woody stems > 1 cm diameter in seven 0.5-ha sites, measured dispersal using seed traps, and monitored the density and composition of woody seedlings. To examine effects of deer on seedling communities, each site was divided into a paired deer-exclosure and unfenced control treatment. We are testing three main hypotheses: (1) dispersal limitation constrains local seedling diversity and increases variation in species composition among sites; (2) deer impose a biotic selection pressure on seedling diversity and composition; and (3) deer modify the effects of dispersal limitation on seedling community structure.

Results/Conclusions

Our preliminary results suggest that dispersal limitation may have a stronger influence than deer on seedling diversity and composition. Seeds of all species occurred in fewer than 30% of the seed traps, suggesting strong dispersal limitation. Moreover, the most common species, white oak (Quercus alba), occurred in only 35% of the seed traps, despite above average seed production in the first year of the experiment. In contrast, deer exclusion had no influence on local diversity or species composition during the initial phase of the experiment. As our study continues, we will examine the potential role of deer as a biotic selection pressure, including their effects on density dependent mortality of common and rare plant species. This large-scale experiment will help unravel the relative importance of dispersal and dominant herbivores as ecological drivers of seedling diversity and dynamics in temperate forests.