Long-term dynamics and hotspots of change in a desert grassland plant community.

Natural and anthropogenic disturbances are key drivers of vegetation dynamics. The hierarchical-response framework proposes that directional change in communities is driven by chronic resource alterations resulting from global environmental change in the absence of disturbance. Because vegetation is spatially heterogeneous, some local areas within a larger community may be more dynamic than others. Thus, the average rate of change may mask dynamic hotspots and local areas where vegetation remains stable. We used long-term data from two line-intercept transects in undisturbed desert grassland to quantify large-scale community dynamics, small-scale local dynamics, and boundary dynamics of grass patches in the absence of disturbance. We found that directional change in species composition was evident but that the overall rate of change varied spatially. Cover of both dominant grasses, Bouteloua eriopoda and Bouteloua gracilis, increased over the full transects, but most change occurred in localized hotspots. Patch boundaries of the dominant grasses exhibited both stability and local dynamics. Overall, the increasing abundance of B. eriopoda may predispose this grassland to shrub encroachment, whereas locally stable areas may prove resistant to state transition. More generally, global environmental change may be a pervasive driver of vegetation dynamics through localized hotspots of temporal change and spatially varying changes in patch boundaries in the absence of disturbance.