Spatiotemporal Heterogeneity of Autumn Phenology Responses to Preseason Drought and Temperature by Grasslands in Cold and Arid Areas of China

Autumn phenology plays a key role in carbon and nutrient cycling in the terrestrial ecosystem. However, the indicator is still poorly understood and the climatic drivers of autumn phenology remain unclear. In this study, we applied four methods to extract the end of growing season (EOS) dates based on three vegetation index datasets from the Moderate Resolution Imaging Spectroradiometer (MODIS) database between 2001 and 2020 for temperate and alpine grasslands in China. We determined the temporal EOS correlations with temperature and drought. Overall, we observed a trend toward later EOS dates in 64.37% of the pixels in the study area, with a mean rate of 0.1 days/yr. In most areas, the warming preseason temperature was positively correlated with EOS date and showed a “freeze-to-death” effect. Drought played a dominant, positive role in relatively arid regions and showed a “thirst-to-death” effect. The wetter and colder areas were prone to “freeze-to-death” effects. In the alpine grasslands, except for a small part of the northeast, the “freeze-to-death” effect was more dominant, whereas in temperate grassland, it was jointly dominated by the “freeze-to-death” effect and the “thirst-to-death” effect. The positive effect of drought on EOS date occurred earlier than that for temperature in alpine grasslands, while opposite relationships were observed for temperate grasslands. Interestingly, reduced drought at high latitudes and altitudes might also lead to earlier EOS dates, especially in autumn. In contrast, a warming temperature led to an earlier EOS in arid areas. Our results confirmed that drought had an important impact on phenological processes in autumn and suggested there should be increased attention paid to drought responses.