Drought timing and degradation status determine the grassland sensitivity to simulated drought events

The intensity and frequency of drought are constantly increasing, threatening the ecosystem functions of grasslands. Although drought can generally limit vegetation growth, the effect of drought timings and grassland degradation status remains unclear. We selected three grasslands with different levels of degradation (extremely, moderately and slightly degraded) in northern China and examined the effects of 30-day drought events during different timings (early, middle, and late growing seasons) on soil water content (SWC), vegetation coverages, and aboveground net primary productivity (ANPP). We found that by reducing SWC by approximately 22–75 %, drought events led to significant declines in seasonal vegetation coverage, but weaker effects on ANPP. Among different drought timings, vegetation coverages showed the minimum sensitivity to early-season droughts with positive legacy effects and the maximum sensitivity to mid-season droughts with negative legacy effects. Late season drought can lead to negative legacy effects on next spring, but positive legacy effects on next summer. Vegetation compositions in terms of the proportions of degradation indicator plants play an important role in regulating seasonal drought sensitivities in degraded grasslands. Our findings underscore that, to better understand the performance of grassland ecosystems during drought events, we must consider the impact of drought timing and grassland degradation status.