Effects of community- and neighborhood-scale spatial patterns on semi-arid perennial grassland community dynamics.

Although plant spatial patterns strongly influence community-structuring processes, few empirical studies have addressed pattern effects on perennial community dynamics. We tested the effects of community- and neighborhood-scale patterns in experimental semi-arid grassland communities comprising the stronger competitor crested wheatgrass (Agropyron cristatum) and the weaker competitor Snake River wheatgrass (Elymus wawawaiensis). Treatments consisted of community-scale patterns (Poisson random, regular, and aggregated) and neighborhood-scale patterns (Poisson random, small, and large aggregations) applied to 6.25-m(2) plots, with aggregations generated through simulated realizations of Neyman-Scott cluster processes. Two years of data were collected on aboveground biomass of both species, and variability in light (photosynthetically active radiation; PAR) was also quantified. We found that plant performance was strongly affected by community-scale spatial patterns and time, with additional effects of neighborhood-scale pattern in certain treatments. Mean biomass and relative growth rates of both species were highest in plots with community-scale regularity and random neighborhoods, suggesting a strong effect of pattern on competition that was magnified for the weaker competitor E. wawawaiensis, especially in the second year. There were also significant effects of treatment and time on variability of PAR, supporting past research on the importance of canopy patterns for light distribution near the soil surface. We observed more variable light environments in plots with community-scale aggregation, and variability also increased in the second year. Our research provides new information on the effects of plant patterns on community dynamics, with particular relevance for semi-arid perennial grasslands.