Competition can drive differential patterns of survival and growth, and therefore successional dynamics among species in plant communities. The extent to which species-specific interactions matter to community structure and how stable those interactions are across environments and over ontogenetic development remains controversial. There is rich literature suggesting that trade-offs in competitive effects or response, or in competitive abilities over space or time, are essential for species coexistence and the maintenance of diversity. Yet there are very few empirical tests of these theories using robust, multi-species datasets of long-lived, woody plants. Using plot and tree data from the Forest Inventory and Analysis (FIA) program, we constructed individual-based and spatially-explicit neighborhood growth models for the 50 most common tree species in the 31 eastern U.S. states. The research addresses 3 fundamental questions: How do species differ in their competitive effects and responses? Are hierarchies of competitive effect or response consistent across different species groups or throughout a tree’s lifespan? Do tradeoffs occur between effect and response or are they positively correlated within species?
Results/Conclusions
Competitive effects were size and distance-dependent, and varied greatly among species, although highly shade-intolerant species had the smallest effects as neighbors, while oak species had consistently large competitive effects as neighbors. On average, the effects of intraspecific neighbors were greater than interspecific neighbors, particularly for juveniles. The magnitude of growth responses to competition ranged four-fold across the species studied. Shade-tolerant species were less sensitive to competition than shade-intolerant species if they were adult trees with inferior crown position, otherwise shade tolerance had no impact on competitive responses. Overall, the rankings of species competitive effects and responses were highly consistent across different species mixtures, with the exception of response rankings among juvenile trees, and there was a significant, positive relationship between a species competitive effect and response for adult targets equal to or greater in size than their neighbors. Together, these findings suggest that superior competitors are better at both exploiting resources (effect) and avoiding exploitation by others (response), and that they maintain superiority regardless of the species they compete with. This could lead to the exclusion of inferior competitors from communities, if it weren’t for the fact that species rankings of competitive effects and responses changed from juvenile to adult trees for all but 1 species analyzed. This suggests that competitive rank reversals with ontogeny provide a potentially important mechanism for species coexistence.