Plant Functional Community Assembly in Grasslands and Wetlands II : Effects of Stress

Abstract

We sought to investigate how stress (low moisture and mineral nutrients) influences how functional community assembly differs from random drift assembly. Plant strategy theory suggests that stressful habitats should be dominated by short species with adaptations for conserving resources. The “stress-dominance hypothesis” suggests that stressful habitats should have low functional diversity, and both have been confirmed in European grasslands. We sampled plants at 18 locations along well-known moisture gradients from dry prairies to wet sedge meadows. At each location, three nested sample plots were established using three grain sizes (0.1 m^2, 1.0 m^2, 10 m^2). Four functional traits (two size traits and two leaf economics traits) were measured in every instance of each plant species. We used Monte Carlo simulations to estimate the standardized effect size of abundance-weighted mean and functional diversity using four species pool scales: regional (all species), within wet and dry habitats, within specific vegetation types, and within each stand. Results for weighted mean traits were consistent with strategy theory. Results for functional diversity were consistent with the stress-dominance hypothesis in only one of four cases. In addition, the results were strongly dependent on the spatial extent of the species pool and sample grain size.