Adaptive strategies of leaf shedding and consequences for photosynthesis in humid tropical forests

Abstract:

Abstract: Rainfall can absorb photosynthetically active radiation, usually introducing a negative seasonal correlation between moisture and light availability. But this seasonal covariation can be altered to a positive one caused by overland regional features, including orography and continentality. Despite growing in prevailing wet environments, humid tropical forests—the Earth’s most productive biome—also experience seasonal constraints of moisture and light availability during recurrent dry and wet seasons. However, the geographic distribution of seasonal covariation of moisture and light availability and their contributions to the long-term adaptation of these forests are poorly constrained. Using monthly litterfall records from 70 humid tropical forests and satellite photosynthetic proxies, we show two seasonal patterns of moisture and light availability and three regimes of leaf phenology and productivity seasonality predominate across humid tropical forests. Moisture and light availability tend to be strongly negatively or strongly positively correlated. Litterfall and productivity increase in the wetter, sunnier seasons where the moisture-light correlation is positive. Litterfall increases and productivity decreases in the drier, sunnier seasons where the moisture-light correlation is negative excepting Amazonia where productivity also increases in the drier, sunnier seasons. These results suggest the timing of leaf turnover optimizes light use and productivity in Amazonia and where the moisture-light correlation is positive and avoids water stress elsewhere in the humid tropics. Our findings suggest that ongoing changes in tropical rainfall seasonality may push tropical forests out of their adaptation envelope, while little is known about the plasticity of their phenological traits.

Keywords: Humid tropical forests, leaf shedding strategy, litterfall, water stress, canopy rejuvenation