Understory leaf phenology exhibits greater sensitivity than overstory to extreme drought in a subtropical forest

Increasing drought stress threatens subtropical forests, which are renowned for their complex vertical canopy stratification. However, the differential responses of leaf phenology to drought across vertical strata (i.e., overstory and understory) remain unclear. In this study, we established a near-surface remote sensing system integrating unmanned aerial vehicles and ground-fixed cameras. Through a 70% throughfall exclusion experiment, we amassed over 430,000 images to investigate drought responses in overstory and understory leaf phenology. Our results reveal significantly greater sensitivity of understory leaf phenology to extreme drought compared to the overstory. Drought exerted no statistically significant effect on overstory leaf phenology during either leaf development or senescence phases. In contrast, understory leaf senescence phenology advanced markedly under drought conditions, with 11.75 and 15.76 days for the start and end of the leaf-falling event, respectively. For the understory layer, our analysis detected that pre-season temperature primarily regulated leaf development phenology, while soil moisture dominated variability in leaf senescence phenology. Furthermore, we demonstrated that divergent water-use efficiency modulates stratum-specific phenological responses to drought. High water-use efficiency in overstory tree and seedling conferred greater drought resistance, whereas low water-use efficiency in understory shrubs increased susceptibility. These findings highlight the necessity for coordinated multi-stratum monitoring of forest responses to climate change and underscore the pivotal role of water availability in shaping understory phenological patterns in subtropical forests.