The effects of soil fertility and scale on competition in ponderosa pine

The intensity of competition between neighboring trees depends on local stand structure, and the influence of stand structure may vary across gradients in soil resource supplies. We used model selection techniques to look for variation in the nature and intensity of interactions between trees along a gradient of soil nitrogen supply in a 9-ha stand of old-growth ponderosa pine (Pinus ponderosa) in Colorado, USA. We used spatially explicit competition indexes to describe the interactions between trees and developed individual tree growth models to look at how soil nitrogen (N) supply affects competition. The growth of focal trees showed an asymmetric influence of neighbors up to 14-m distance. The predictive ability of our growth models more than doubled (to an r2 = 0.69) as the size of the neighborhood used to calculate the competition indexes increased from a 2-m to a 14-m radius. The supply of soil nitrogen modified competition, with increasing N enhancing competition from neighbors. Neighborhood structure and soil resource supplies jointly influenced the growth of individual trees, but at different scales. Tree interactions are both spatially and temporally complex and may be studied most usefully with explicit evaluation of gradients in resource availability.