Co-occurring tree species change rank in seedling performance with resources varied experimentally

Parallel hypotheses are advanced: (1) the distribution of resources in mesic upland temperate forests is patchy at a scale that is fine for adult trees and coarse grained for seedlings and (2) co-occurring tree species differ in abilities to exploit resources, resulting in differences in species rank order, by absolute growth rates of seedlings, among different resource combinations. Spatial variation in growth rate ranking among species may translate, through competition, into patterns of relative species abundances of seedlings that could persist in the community composition of adult trees. Here I report the results of factorial greenhouse experiments that address the second hypothesis. Seedlings of six co-occurring tree species of temperate eastern North American upland forests showed strong interspecific differences in growth rates and patterns of biomass allocation in response to variation in light and mineral nutrients. The ranks of four species in growth measures associated with competitive ability, absolute rates of change in stem height, total leaf area, and root biomass, varied significantly among nutrients treatments. Seedlings of American chestnut, Castanea dentata, ranked highest in traits associated with competitive ability over the broadest range of combinations of resource levels.