COS 248-2 - Effects of drought, disturbance, and biotic neighborhood on experimental tree seedling performance

Abstract: Forest biodiversity is likely maintained by a complex suite of interacting drivers that vary in importance across both space and time. Contributing factors include disturbance, interannual variation in abiotic variables like precipitation, and biotic neighborhood effects (including conspecific inhibition). To help resolve ongoing uncertainties and probe potential interactions, we investigated tree seedling performance in a temperate mid-Atlantic forest ecosystem. We planted seedlings of five native tree species in mapped study plots, half of which had been previously subjected to disturbance, and then monitored seedling survival, height growth, and foliar condition. The second full year of data collection encompassed a drought, enabling comparison between intervals varying in water availability. Seedling performance was analyzed as a function of canopy cover and biotic neighborhood (conspecific and heterospecific abundance), including interactions, with separate generalized linear mixed models fit for each interval. All species exhibited: a) pronounced declines in height growth during the drought year, b) detrimental effects of adult conspecifics, and c) beneficial effects of canopy openness. However, despite these consistencies, there was considerable variation across species in terms of the relevant predictors for each response variable in each interval. For instance, we found evidence that drought can strengthen or reveal conspecific inhibition in some instances, while weakening it or obscuring it in others. In addition, our analyses suggest that some forms of conspecific inhibition may manifest only under particular canopy conditions. Overall, our work reveals a complex forest ecosystem that is simultaneously and interactively governed by biotic neighborhood structure (e.g., conspecific abundance), local habitat conditions (e.g., canopy cover), and interannual variability (e.g., drought).