Regional and decadal patterns of native and exotic plant coexistence in California grasslands.

Coexistence through a variety of mechanisms is possible for species with differential responses to environmental conditions. Understanding the role of environmental heterogeneity in mediating coexistence of species of different provenance (i.e., native vs. exotic) has important implications for theory and management. We used two California grassland data sets, one spanning seven years at three reserves along a 500-km latitudinal gradient and one spanning 47 years at 11 sites within a single 1000-ha reserve, to determine how environmental heterogeneity in space and time contributes to variability in provenance group abundance and diversity, and whether native and exotic species respond similarly to spatial and temporal variability. We found that temporal environmental heterogeneity is the primary determinant of provenance group abundance, while spatial and temporal environmental heterogeneity both contribute to community diversity. Spatial and temporal heterogeneity must therefore be considered simultaneously when examining community dynamics and species coexistence. Provenance was a poor general predictor of species response; native and exotic species exhibit similar spatiotemporal patterns in some cases but not others. Plant persistence may depend more upon the abiotic environment than competition from the other provenance group as native and exotic diversity were generally positively correlated. Furthermore, mesoscale (10(2)-10(3) m) spatial heterogeneity may be a greater mediator of provenance group coexistence than temporal heterogeneity or spatial heterogeneity at other scales.