Leaf development and demography explain photosynthetic seasonality in Amazon evergreen forests

In evergreen tropical forests, the extent, magnitude, and controls on photosynthetic seasonality are poorly resolved and inadequately represented in Earth system models. Combining camera observations with ecosystem carbon dioxide fluxes at forests across rainfall gradients in Amazônia, we show that aggregate canopy phenology, not seasonality of climate drivers, is the primary cause of photosynthetic seasonality in these forests. Specifically, synchronization of new leaf growth with dry season litterfall shifts canopy composition toward younger, more light-use efficient leaves, explaining large seasonal increases (~27%) in ecosystem photosynthesis. Coordinated leaf development and demography thus reconcile seemingly disparate observations at different scales and indicate that accounting for leaf-level phenology is critical for accurately simulating ecosystem-scale responses to climate change.

在常绿热带森林中，光合季节动态的范围、强度及其调控机制尚未得到清晰解析，且在地球系统模型（Earth system models）中也未能得到充分表征。本研究结合亚马逊流域不同降雨梯度下森林的相机观测数据与生态系统二氧化碳通量数据，揭示出：驱动这些森林光合季节动态的首要因素是冠层整体物候，而非气候驱动因子的季节变化。具体而言，新叶生长与旱季凋落物的同步性，使得冠层组成向更年轻、光能利用效率更高的叶片倾斜，这解释了生态系统光合速率约27%的季节性增幅。叶片发育与周转的协同调控，由此调和了不同尺度下看似相悖的观测结果，同时表明：若要准确模拟生态系统对气候变化的响应，亟需纳入叶片尺度的物候过程。