Many forests of eastern North American are undergoing a species dominance shift in which maples (Acer spp.) are increasingly important while oak (Quercus spp.) regeneration has declined. The elimination of fire has been postulated as the cause of this dynamic; however, a variety of interacting drivers are known to influence dynamics in practically all ecosystems and a broader framework may be useful. In this project we used community assembly as a conceptual framework and PATH analysis focused on functional diversity indices to assess long-term dynamics in an old-growth Appalachian forest. In 1979, 80 permanent plots were established in Lilley Cornett Woods using a stratified random sampling scheme and these plots of have been surveyed regularly since that time. This project has included a wide variety of environmental measurements including soil sampling, hemispherical canopy imaging, and tree-ring based reconstructions of gap dynamics and fire. We used 30 years of vegetation data from this study to assess the factors that structure ecological communities in this forest with particular focus on how environmental filters may influence community assembly.
Results/Conclusions
Gap dynamics were detected in this site with a relatively consistent temporal distribution dating to the earliest years in our chronology. Fires were also an important process in this forest for the last 350 years; however, we found long lags between fires in the prior to 1850 and more frequent fires post 1880. Maple regeneration is widespread and recruitment into mid-canopy strata is ongoing. Oak regeneration patterns appear to have shifted through time with widespread recruitment in the past which has become increasingly limited to the driest portions of the watershed. Community-scale dominance patterns in the watershed were relatively stable through time with oak, mesophytic and beech communities continuing to be distinguishable throughout the study. Our PATH models indicated strong environmental filtering: stressful areas (high elevation, dry sites) were occupied by communities of low functional diversity, which suggests a selective effect for species with traits adapted to such harsh conditions. The effects of topography on functional diversity were often indirect and moderated through soil moisture and fertility. Soil moisture was a key component of our models having either strong direct effects on functional diversity or indirect effects via soil fertility. Our results provide a comprehensive view of the interplay among functional trait assemblages, topography, edaphic conditions and disturbance that suggests a complex suite of environmental filters drive long-term patterns in temperate forest community assembly.