High air temperature exacerbates stomatal closure and afternoon photosynthesis depression in a subtropical forest

Evergreen broadleaved forests (EBFs) contribute more than one-third of global terrestrial gross primary productivity (GPP) and sustain the critical carbon sink by longer photosynthetic period. Evidence suggests EBFs are approaching their photosynthetic thermal optima, raising critical concerns about their photosynthetic resilience thresholds to increasing heat stress. Here we combine half-hourly eddy covariance flux and tower-based solar-induced chlorophyll fluorescence （SIF）to construct the near-real time photosynthesis spectral system for a subtropical forest ecosystem, experiencing unprecedent heat stress, to explore the thermal response of diurnal GPP and SIF. We estimated the averaged optimum temperature for ecosystem-level photosynthesis and SIF were 30.61 °C and 32.15 °C, and the high temperature above the thermal threshold reduced the canopy photosynthesis by 7.25%. Diurnally, the high air temperature led to a significant afternoon photosynthesis decline and thereby an afternoon depression, with the diurnal peak photosynthesis shifting toward earlier morning hours. The afternoon depression of photosynthesis was mainly attributed to high air temperature, accompanied by stomatal closure. Overall, this study underscored heat-induced afternoon photosynthesis drops, highlighting the vulnerability and resilience of vegetation photosynthesis over the EBFs under future hot extremes.