Changes in competition intensity, herbivory and stress along a soil depth gradient in an old field.

Abstract

Four herbaceous plant communities were described along a natural soil depth gradient in an old field. Biomass and canopy height increased with soil depth. Species richness showed a quadratic relationship, whereas the proportion of native species increased with soil depth. The limiting resource was found to be nutrients, not water, even though the year was abnormally dry. I hypothesized that competition was the main factor structuring these communities. The relationship between competition intensity and soil depth was determined through experimentation using two phytometer species, Liatris spicata and Lythrum salicaria. Competition intensity decreased with soil depth across the four communities, which contradicts the Competitor/Stress-Tolerator/Ruderal Hypothesis (CSR) of Grime and Tilman's Resource Ratio hypothesis. Competition in this old field was primarily below ground. Herbivory from large herbivores did not change along the gradient, whereas stress was found to be greatest at both ends of the gradient.