Drought Resistance and Recovery in Drylands

Abstract:

Projected global change will increase the level of land use and environmental stressors such as drought and grazing in the near future, particularly in drylands. Still, combined effects of drought and grazing on plant production are poorly understood, thus hampering establishment of adequate projections and mitigation strategies.

We used a large, cross-continental database on long-term studies from drylands to quantify ecosystem responses to drought and grazing with the ultimate goal to increase functional understanding of these responses. Two key aspects of ecosystem stability, resistance to and recovery after a drought, were evaluated based on standardized and normalized aboveground net primary production (ANPP) data. Drought events and intensities were classified via the standardized precipitation index (SPI). We tested effects of drought intensity (SPI class), grazing regime (grazed, ungrazed), biome (grassland, shrubland, savanna) or dominant life history of the herbaceous layer (annual, perennial) to assess the relative importance of these factors for ecosystem stability, and to identify predictable relationships between drought intensity and ecosystem resistance and recovery. We found that ecosystem stability was better explained by dominant life history of the herbaceous layer than by biome. Increasing drought intensity (quasi-) linearly reduced ecosystem resistance. Even though annual and perennial systems had a similar response rate to increasing drought intensity, they differed in their general magnitude of resistance, with annual systems being ca. 27% less resistant. Vice versa, systems with an herbaceous layer dominated by annuals had substantially higher post-drought recovery, particularly when grazed. Combined effects of drought and grazing were not merely additive but modulated by dominant life history of the herbaceous layer.

Our study established predictable relationships between drought intensity and drought-related relative losses in ANPP, and suggest that systems with an herbaceous layer dominated by annuals are more prone to ecosystems degradation under future global change regimes.