Legacy effects of extreme drought and wetness events on mountain grassland ecosystems and their elevation dependence

Extreme climate currently has significant impacts on global carbon cycling, ecosystem services, and biodiversity, and these impacts will continue in the future. The effects of extreme drought on terrestrial ecosystems have been extensively researched. Mountain regions are undergoing more pronounced climate change compared to the global average. However, the differential responses of mountain grassland ecosystems to extreme drought and wetness remain unclear. Therefore, this study focuses on grasslands in the Qilian Mountains (QLMs) and utilizes the normalized difference vegetation index (NDVI) and the standardized precipitation evapotranspiration index (SPEI) to compare and analyze the legacy effects of extreme drought and extreme wetness events on grassland ecosystems. The results indicate that between 2000 and 2018, extreme drought generally has one-year negative legacy effects on grassland ecosystems, particularly in alpine meadow. Conversely, extreme wetness predominantly exhibits positive legacy effects on grassland growth. The acceleration of growth caused by extreme wetness (wet compensation effect) can compensate for 143% of the drought legacy effect. This compensatory effect is particularly evident in temperate typical meadow, highlighting the asymmetrical response of grassland vegetation to extreme drought and wetness events. Furthermore, the legacy effects of extreme drought and wetness events are spatially heterogeneous and demonstrate clear elevation dependence. For alpine meadow and alpine steppe, the most significant positive and negative legacy effects occur at high and low elevations, while mid-elevations (3300–3800 m) experience the smallest legacy effects. These findings provide crucial guidance for understanding the impacts of extreme climate events on the structure and functioning of mountain grassland ecosystems.