Linking Plant Spatial Patterns and Ecological Processes in Grazed Great Basin Plant Communities

Abstract

Observational studies of plant spatial patterns are common, but are often criticized for lacking a temporal component and for their inability to disentangle the effect of multiple community-structuring processes on plant spatial patterns. We addressed these criticisms in an observational study of Great Basin shrub-steppe communities that have been converted to a managed grazing system of planted crested wheatgrass (Agropyron cristatum [L.] Gaertn.) stands. We hypothesized that intraspecific interference and livestock grazing were important community-structuring processes that would leave unique spatiotemporal signatures. We used a survey-grade global positioning system to quantify crested wheatgrass spatial patterns along a chronosequence of stands that differed only in time since planting (9–57 yr), as well as in a 57-yr-old grazing exclosure to examine pattern formation in the absence of grazing. Three replicate survey plots were established in each stand, and a total of 6 197 grasses were marked with a spatial error of # 2 cm. The data were analyzed using L-statistics in program R, and hypothesis testing was conducted using Monte Carlo simulation procedures. We detected fine-scale regularity, frequently considered a sign of interference via resource competition, in all stands including the exclosure. Coarser-scale aggregation, which we attributed to the effects of prolonged grazing disturbance, was only detected in the oldest grazed stand. Our results suggest that interference acts over finer spatial and temporal scales than grazing in structuring these stands, reinforcing the importance of interference in semiarid communities. Analysis of exclosure data suggests that, in the absence of grazing, crested wheatgrass stands organize into a statistically regular pattern when primarily influenced by interference. In the presence of prolonged grazing, crested wheatgrass stands become more heterogeneous over time, likely a result of seedling mortality via disturbance by cattle.