Changes in peak greenness timing and senescence duration codetermine the responses of leaf senescence date to drought over Mongolian grassland

Leaf senescence is a key trait for vegetation in temperate regions to overcome winter temperatures, and extreme drought events have profoundly affected its timing. The effects of drought on autumn phenology have been investigated by correlating leaf senescence date (LSD) to multiple drought indices. However, leaf senescence is a dynamic process starting from peak greenness timing (PGT) and lasting several months (senescence duration, SD). How PGT and SD changes affected the LSD's response to drought is still poorly understood. Here, we investigated LSD's response to drought in the Mongolian grassland based on PGT and SD. We first extracted the LSD, PGT, and SD from the MODIS Enhanced Vegetation Index (EVI) during 2001–2020 and determined the late-season drought events (LDE) using the Standardized Precipitation Evapotranspiration Index. Subsequently, we quantified the response of LSD to LDE as the difference between drought and normal years (ΔLSD), and then quantified the contributions of changes in PGT and SD on ΔLSD. We also analyzed the effects of drought on late-season productivity. The results showed that LSD may advance or delay under LDE, but the earlier LSD (in 68.4 % of areas) was mainly distributed in warmer and drier regions. Earlier LSD under drought was more controlled by earlier PGT, while later LSD under drought was more controlled by extended SD. Changes in late-season productivity under LDE were more correlated with changes in annual maximum EVI (EVImax) than ΔLSD. These results enhanced our understanding of how LSD of temperate grassland responds to drought events and emphasized the importance of investigating the response of autumn phenology to drought from a process-based perspective.