Leaf shedding of Pan-Asian tropical evergreen forests depends on the synchrony of seasonal variations of rainfall and incoming solar radiation

Abstract

Despite a perennial evergreen canopy, tropical evergreen forests continuously shed old leaves and grow new leaves at different time across different tropical regions. To examine how water and light availability control the leaf phenology of tropical evergreen forests, we used monthly litterfall mass data from 30 sites in Pan-Asian tropics and solar-induced chlorophyll fluorescence (SIF) as leaf shedding and a proxy for canopy photosynthetic activity and productivity, respectively. Two types of climate were identified depending on the positive and negative corrections between seasonal precipitation (Pre) and shortwave radiation (SW), named as synchronous and asynchronous climates, respectively. In regions where Pre and SW are positively correlated, litterfall and SIF are parabolically greater in the wet season (named as synchronous-unimodal regime), or show bimodal litterfall peaks in the dry and wet seasons (named as synchronous-bimodal regime). In regions where Pre and SW are negatively correlated, litterfall is parabolically greater in the dry-sunny season (named as asynchronous-unimodal regime), or show multiple peaks at the wet season, coincides with relatively high SIF (named as asynchronous-multimodal regime). The adaptive strategies of leaf phenology could be generated as a response to water and light availability balancing the benefits and costs from the plant perspective. On the one hand, plant prioritizes light acquisition by exchanging old, senescent leaves for new, efficient leaves to optimize photosynthesis with sufficient water availability. On the other hand, plant sheds old leaves to minimize maintenance costs and to avoid hydraulic failure under water stress during dry season. Our findings are crucial for assessing both the impacts of future climate change on tropical evergreen forests and the feedbacks of tropical evergreen forests on the terrestrial carbon cycle. This study also suggests that the ongoing changes in tropical rainfall seasonality relative to radiation seasonality may significantly alter canopy leaf phenology under future climate conditions.