Macro- and Micro-Climate-Controlled Impacts of Heatwave With Drought on the Autumn Phenology of a Subtropical Forest Ecosystem

Heatwave and flash droughts are increasingly diminishing global vegetation productivity. Phenology, a key indicator of vegetation ecosystem responses to climate change, is crucial for regulating terrestrial carbon and water cycles. How heatwave with flash drought would influence phenology, particularly the autumn phenology, remains under-explored. Here we studied the heatwave with flash drought in summer 2022 over a subtropical forest ecosystem in China, and investigated its effects on autumn phenology from multiple aspects. We quantified land surface phenology using the MCD12Q2 derived End of Season (EOS) and compared the EOS in 2022 with that in normal years between 2000 and 2021. We found EOS was 8 days earlier than normal year, which indicated forests dropping leaf earlier to avoid excessive water loss and to increase survival rate. But their sensitivity to the event varied by macro-climatic conditions, terrain features, and forest attributes. Higher summer temperature advanced the EOS by approximately 1.49 days per 1°C (R² = 0.82). Microclimatic differences determined by terrain also impacted the EOS, with EOS advancing by 1 day for every 100 m decrease in elevation and 1 day for every 3.5° decrease in slope, respectively (R² = 0.94 and 0.95). Forests with different structural and composition features also responded differently to the extreme events. Coniferous forests were the least affected, while mature and taller forests showed greater resilience with less EOS advancement. These findings underscore the complex interactions between climatic events and ecosystem responses, highlighting the need for targeted conservation strategies and biodiversity protection.