Neighborhood interactions and local edaphic gradients have a weak influence on long-term vegetation dynamics in an old-growth forest community

The Eastern Deciduous Forest (EDF) region is undergoing a shift in species composition, the most dramatic trend being an increase in mesic taxa such as maple (Acer spp.) and American beech (Fagus grandifolia), and a subsequent decline in xeric species such as oaks (Quercus spp.). While several hypotheses have been proposed to explain these trends, most focus on allogenic processes such as disturbance and climate change, and little attention has been given to local species interactions. To help bridge this gap, we first quantify long-term changes in community composition of an old-growth forest using census data collected over a fifteen-year interval. Within two 50 × 70 m plots, all stems ⩾2.5 cm DBH were tagged and measured in 1996, and their spatial coordinates were mapped. Seedlings were measured in smaller subplots, and soil cores were sampled at these locations. Hemispherical photographs were then taken to quantify the light environment, and these procedures were replicated in 2011. As expected, we find an increase in the dominance of both American beech and sugar maple (Acer saccharum), and a marked decline for most others, particularly white oak (Quercus alba). Neighborhood models incorporating the density, basal area, and composition (i.e., conspecific versus heterospecific) of stems surrounding focal individual trees reveal that local density-dependent interactions were indeed weak for both American beech and sugar maple. At the same time, however, all other species experienced similarly weak effects. Likewise, soil resource gradients had little influence on long-term demographics, contrary to our expectations. Together, our results indicate that the changes observed across the Eastern Deciduous Forest region may indeed be most strongly impacted by allogenic disturbance processes, although we still encourage further research on the potential influence of local species interactions at mediating these dynamics, and expect neighborhood models to become an important tool in understanding the causes of compositional shifts in the EDF region.