Temporal Dynamics and Patterns of Diversity in Old-growth Forest Tree Communities

The study of biodiversity has long been a core component of community ecology research and may be broken into three major themes: (1) describing patterns of diversity, (2) determining drivers of biodiversity, and (3) understanding consequences of diversity. The body of work presented in this dissertation addresses questions pertaining to these themes using long-term data collected in old-growth (primary) forests of both North America and Asia. In Big Everidge Hollow (BEH), a 52-hectare stand of old-growth forest in central Appalachia, I examined how relative abundances and spatial distributions of dominant tree species have changed over a 30-year period and tested for significant shifts in overall community composition. I also employed phylogenetic and functional diversity measures to better understand how environmental factors, particularly topography and soil characteristics, play a role in the assembly of tree communities. Data from the Fushan Forest Dynamics Plot (FDP), a subtropical broadleaved forest in Taiwan, were used to test whether taxonomic and functional diversity play a role in maintaining temporal stability of carbon storage (aboveground biomass and coarse woody productivity). Overall, I found that tree communities in BEH have been undergoing a shift in species dominance from oak (Quercus spp.) to maple (Acer spp.) over the past thirty years that is consistent with trends described in numerous other forests in eastern North America. Local tree communities in BEH are spatially structured across gradients of both topography and soil characteristics such that assemblages of low functional and phylogenetic diversity occur in areas of greater environmental stress (low soil moisture and nutrients, especially at high elevation and on south-facing slopes). The Fushan FDP data revealed weak yet significant biodiversity-stability relationships, indicating that as biodiversity increases, levels of ecosystem function (carbon storage, in this case) tend to be more consistent over time. As disturbance due to human activities continues to exert greater influence on natural ecosystems across the globe, the need to document changes in biodiversity becomes increasingly important. Employment of phylogenetic and functional diversity metrics can provide greater insight on the ecological consequences of shifting species dominance, especially in areas considered biodiversity hotspots that serve as reference ecosystems for restoration of degraded habitats and as baseline models for predicting future change. The work presented here aims to provide the necessary control treatment for understanding baseline patterns and drivers of diversity in comparison to anthropogenically-driven changes in forests across the globe