Influences of climatic variation and human activities on vegetation photosynthesis dynamics in Southwest China

Photosynthesis is a direct route for carbon sequestration in vegetation, and is influenced by climatic variation (CV) and human activities (HA). Therefore, a quantitative assessment of their influence on vegetation photosynthesis dynamics is pivotal for formulating effective carbon neutrality strategies. Herein, based on the solar-induced chlorophyll fluorescence index (SIF), which reflects the vegetation photosynthesis intensity, and TerraClimate meteorological data, we refined the residual trend approach by incorporating more climatic variables and their time effects on vegetation to assess the influences of CV and HA on SIF dynamics in Southwest China. Our results revealed that an increasing rate of vegetation SIF across Southwest China of 0.0312 Wm^-2μm^-1sr^-1/10a (p < 0.001) from 2000 to 2019, and over 90% of the region exhibited an increase in SIF. The influence of CV on SIF dynamics had time effects, including time-cumulative and time-lag effects. Nevertheless, these effects varied by climatic variables and vegetation types. The variable importance in projection demonstrated that temperature was the primary factor influencing SIF dynamics, followed by precipitation, potential evapotranspiration, downward surface solar radiation, vapor pressure deficit, and wind speed. Furthermore, both CV and HA collectively enhanced the vegetation photosynthesis intensity in the region, HA was the main driver of the SIF increase, contributing 0.0239 Wm^-2μm^-1sr^-1/10a, while CV accounted for 0.0073 Wm^-2μm^-1sr^-1/10a. Overall, we refined the previous residual trend approach and provided a new way for quantitatively assessing the influences of CV and HA on vegetation photosynthetic intensity.