The increasing sensitivity of photosynthesis to land surface temperature over China's grasslands

China's grasslands represent a vital yet vulnerable component of the global terrestrial ecosystem. As climate warming intensifies, the grassland ecosystems face an increasing risk of thermal stress. However, the time delay (lag) in photosynthetic response to land surface temperature (LST) and associated sensitivity changes remain insufficiently investigated. Here, we used 8-day resolution solar-induced chlorophyll fluorescence (SIF) and LST data from 2001 to 2024 to identify the optimal lag period over regions where SIF significantly negatively responds to LST. Our results show that the optimal lag period exhibits clear spatial heterogeneity, with 0-, 8-, 16-, 24-, and 32-day lag periods jointly accounting for 93% of the area. Based on the five main lag period patterns, we found the overall sensitivity of SIF to LST increased, despite some fluctuations. Specifically, across the 0-day, 8-day, 16-day, and 24-day lag regions, more than 70% of pixels showed increasing sensitivity. Significant increasing trends were observed in the 0-day, 8-day, and 16-day lag regions. In addition, we found the spatial heterogeneity of the optimal lag period was closely associated with regional climatic and ecological characteristics. Further analyses identified water availability, canopy structure, and grazing intensity as key drivers of the increasing sensitivity, with atmospheric dryness playing a dominant role. Our findings provide crucial theoretical support for developing targeted management and climate adaptation strategies for China's grasslands. • The optimal lag period of photosynthetic response to temperature varies spatially. • Photosynthesis shows highly immediate responses to land surface temperature over the Tibetan Plateau. • The sensitivity of photosynthesis to land surface temperature is increasing over China's grasslands. • Rising atmospheric dryness dominates the increasing sensitivity.