Nonlinear autumn phenology responses to compound drought and heatwave events: results from a manipulative experiment

Climate change-induced shifts in plant phenology have substantially impacted terrestrial ecosystem structure and function. While the effects of drought and heatwaves on leaf senescence have been studied, the response of leaf senescence to compound drought and heatwave events remains poorly understood, especially due to a lack of experimental evidence. In this study, we investigated the responses of leaf senescence to varying durations (13, 28, and 43 days) of compound drought and heatwave stress in saplings of three temperate deciduous tree species. We found that prolonged drought and heatwave conditions delayed leaf senescence by 20.2 in Koelreuteria paniculata and 22.4 days Hibiscus syriacus, respectively, potentially as a compensation for stress-induced reductions in growth. However, leaf senescence in the lowly tolerant Acer palmatum shifted from delayed to advanced, indicating a nonlinear response. Total photosynthesis, relative height increment, and basal diameter growth decreased in all three species, with the strongest reductions in Acer palmatum, followed by Hibiscus syriacus and Koelreuteria paniculata. Our findings demonstrate delayed effects of environmental stress on leaf senescence and highlight species-specific variation in response to compound drought-heatwave events, providing insights into how plants respond to climate change.