Global change–induced extreme droughts are increasing in grasslands worldwide, and drought legacies may greatly affect the responses of grassland ecosystems to these changes. However, it remains poorly understood whether and how severe droughts have a positive legacy effect on grassland productivity. By combining a 4-year precipitation manipulation experiment with a 40-year observational study in a semiarid grassland, we showed that extreme droughts could create strong positive legacies on community productivity and that such legacies could last for multiple years. The mechanism behind this was the coupled effect of the drought-induced increase in annuals and the favorable precipitation pattern that facilitated the flourishing of annuals in subsequent years. This study provides experimental and observational evidence for positive drought legacies and reveals their underlying mechanisms. Our findings suggest that positive drought legacies should be incorporated into Earth system models to better predict the impact of extreme droughts on grassland ecosystems.

Changes in supplies of resources will modify plant functional traits. However, few experimental studies have addressed the effects of nitrogen and water variations, either singly or in combination, on functional traits. A 2-year field experiment was conducted to test the effects of nitrogen and water addition on leaf longevity and other functional traits of the two dominant (Agropyron cristatum and Stipa krylovii) and three most common species (Cleistogenes squarrosa, Melilotoides ruthenica and Potentilla tanacetifolia) in a temperate steppe in northern China. Additional nitrogen and water increased leaf nitrogen content and net photosynthetic rate, and changed other measured functional traits. Leaf longevity decreased significantly with both nitrogen addition (-6 days in 2007 and -5·4 days in 2008; both P < 0·001) and watering (-13 days in 2007 and -9·9 days in 2008; both P < 0·001), and significant differences in leaf longevity were also found among species. Nitrogen and water interacted to affect leaf longevity and other functional traits. Soil water content explained approx. 70 % of the shifts in leaf longevity. Biomass at both species and community level increased under water and nitrogen addition because of the increase in leaf biomass production per individual plant. The results suggest that additional nitrogen and water supplies reduce plant leaf longevity. Soil water availability might play a fundamental role in determining leaf longevity and other leaf functional traits, and its effects can be modified by soil nitrogen availability in semi-arid areas. The different responses of species to resource alterations may cause different global change ramifications under future climate change scenarios.