Short-term responses of urban forest carbon dynamics to combined heatwave and drought in subtropical China

Understanding how urban forest carbon exchange responds to short-term heatwaves and droughts is critical for its effective management, especially given the increase in frequency and compounded extreme events. The impacts of individual heatwaves or droughts on urban forest carbon fluxes have been studied, however, their combined effects and the roles of key environmental factors remain unclear. This study employs daily-scale eddy covariance data collected over 102 days in the summer of 2022 to examine urban forest carbon fluxes-gross primary productivity (GPP), ecosystem respiration (Re), and net ecosystem exchange (NEE)-responses to extreme heatwave-drought events. A random forest model combined with SHAP analysis was employed to evaluate carbon flux differences and identify key environmental drivers. The results show that concurrent heatwave-drought conditions led to the largest carbon sink capacity reduction, decreasing NEE by 66 % (95 % CI: 50 %-82 %, p < 0.001) compared to normal conditions observed within the same period, driven by extreme temperatures and soil moisture deficits. Heatwaves with sufficient water availability resulted in the strongest physiological activity, with GPP and Re increasing by approximately 17 % (95 % CI: 2 %-33 %, p < 0.01) and 47 % (95 % CI: 23 %-74 %, p < 0.01), respectively, relative to normal periods; however, the trade-off between the two processes reduced the overall carbon sink compared to normal periods. Short-term droughts during nonheatwave periods had negligible effects, with carbon sequestration remaining stable. Solar radiation and soil moisture were the most influential environmental factors, varying in importance across conditions. These findings demonstrate the high sensitivity of urban forest carbon dynamics to the combined effects of heat and drought. Even though limited by preliminary evidence from a single growing season, this still highlights the need for targeted management interventions to protect the functionality of ecosystems in the face of increasing climate extremes.