Pyric herbivory, scales of heterogeneity and drought

1. Understanding how extreme drought alters spatial patterns and temporal stability in grassland biomass will become increasingly important by the end of the century when climate model forecasts suggest drought events will occur more frequently. In grassland landscapes where grazing is driven by fire (termed pyric herbivory), temporal stability in above-ground plant biomass at landscape scales typically coincides with greater spatial variability across local communities (time-since-fire patches), whereas variability within local communities is associated with lower temporal stability. 2. We assess the robustness of this relationship during one of the most severe droughts on modern record in the southern Great Plains. We use a long-term pyric herbivory experiment to test whether extreme drought (1) changes patterns of spatial variability in above-ground plant biomass among or within local communities, (2) changes the temporal stability in above-ground plant biomass and the relationship between spatial variability and temporal variability and (3) changes the underlying feedback mechanisms associated with pyric herbivory. 3. Ordination revealed a shift from spatially distinct patches before drought to a convergence of patches during drought in terms of above-ground plant biomass and crude protein. Permutation tests revealed that within-patch variability in above-ground plant biomass significantly increased during drought compared to pre-drought levels. Temporal stability in above-ground plant biomass decreased during drought and indeed corresponded with reduced variability in biomass among local communities and increased variability within local communities compared to pre-drought levels. 4. The results from this study strongly suggest that both positive and negative feedback loops responsible for pyric herbivory were maintained by scale-switching during periods of extreme drought and further reinforce the importance of scale in the study of pattern-process relationships.