Data from: 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 aboveground 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 aboveground plant biomass among or within local communities, 2) changes the temporal stability in aboveground plant biomass and the relationship between spatial variability and temporal variability, and 3) changes the underlying feedback mechanisms associated with pyric herbivory. Ordination revealed a shift from spatially distinct patches before drought to a convergence of patches during drought in terms of aboveground plant biomass and crude protein. 3. Permutation tests revealed that within-patch variability in aboveground plant biomass significantly increased during drought compared to pre-drought levels. Temporal stability in aboveground 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.1. 理解极端干旱如何改变草地生物量的空间格局与时间稳定性，将在本世纪末愈发重要——彼时气候模型预测显示干旱事件发生频率将显著提升。在以火驱动放牧的草地景观（即火调控放牧（pyric herbivory））中，景观尺度下的地上植物生物量时间稳定性，通常与本地群落（火烧后恢复斑块）间更高的空间变异性相契合；而本地群落内部的变异性则与更低的时间稳定性相关联。 2. 本研究在北美大平原南部有现代记录以来最严重的干旱事件之一期间，评估上述关系的稳健性。我们依托一项长期火调控放牧实验，检验极端干旱是否会：1）改变本地群落间及群落内部地上植物生物量的空间变异格局；2）改变地上植物生物量的时间稳定性，以及空间变异性与时间变异性之间的关联；3）改变与火调控放牧相关的潜在反馈机制。排序分析结果显示，就地上植物生物量与粗蛋白含量而言，干旱前空间分化明显的斑块在干旱期间发生了趋同。 3. 置换检验结果表明，与干旱前水平相比，干旱期间斑块内部的地上植物生物量变异性显著升高。干旱期间地上植物生物量的时间稳定性有所下降，且这一变化确实与本地群落间生物量变异性降低、以及斑块内部生物量变异性升高（相较于干旱前水平）相契合。 4. 本研究结果充分表明，支撑火调控放牧的正负反馈环路，在极端干旱期间可通过尺度转换得以维持，并进一步凸显了尺度在格局-过程关系研究中的重要性。