Spatial signature of environmental heterogeneity, dispersal, and competition in successional grasslands

The fate of individual plants is strongly influenced by their spatial context (e.g., their location relative to competitors or environmental gradients). Recent theoretical developments indicate that spatial structure in plant communities arises as an outcome of environmental heterogeneity, intra- and interspecific competition, and localized dispersal. Furthermore, each of these forces is predicted to leave a signature on the spatial and temporal pattern within the community. We track the development of spatial structure in successional grasslands in southern California for five years, using a series of large-scale field experiments. We compare the observed changes in spatial structure to the predictions of a spatially explicit model of plant competition in heterogeneous landscapes. We conclude that environmental heterogeneity led to the development of aggregation in the first year at scales of 1-2 in. Over time, aggregation was detectable at distances greater than 4 m, and species become segregated as a result of competition and localized dispersal. Large-scale disturbances (fire) homogenized the community by decreasing the level of aggregation and segregation. In contrast, disturbance by pocket gophers and ground squirrels increased local-scale heterogeneity by creating annual-dominated aggregates. The speed with which spatial structure develops in seemingly homogeneous environments illustrates the importance of incorporating spatial context in empirical studies and models of plant community dynamics.