Can Carbon Economy Explain Leaf Dynamic Seasonality in a Tropical Seasonal Rainforest?

AbstractSeasonal leaf dynamics in tropical rainforests directly affect canopy photosynthetic capacity and carbon assimilation. Although hypotheses such as water stress, solar light, and herbivore pressure have been proposed to elucidate the underlying mechanisms, controversies remain because of the complexity of this phenomenon. In this study, we tested the "carbon-gain (C-gain) benefit" hypothesis, which proposes that seasonal leaf dynamics optimize C-gain by matching photosynthetic capacity with favorable environmental conditions, using 13-year community-level data from a tropical seasonal rainforest in southwestern China. We evaluated the seasonality of leaf shedding, leaf flushing, leaf age structure, and leaf longevity using modified models. According to the C-gain hypothesis, we would expect leaf dynamics to be synchronized with environmental conditions to maximize photosynthetic carbon assimilation, resulting in significantly higher C-gain compared with scenarios without seasonal optimization. However, we found that seasonal variation in leaf amount and age structure had negligible effects on annual C-gain. When considering the contribution of leaf seasonality to annual canopy photosynthesis, the difference between annual carbon gain calculated with fixed and varied maximum photosynthetic capacity was statistically insignificant. Our results thus did not support the C-gain benefit hypothesis. Instead, the robust seasonal patterns, coupled with minimal carbon benefit, lend support to the "intrinsic rhythm" hypothesis, wherein leaf dynamics are governed more by internal constraints than by direct optimization for carbon gain. These findings highlight the need to reconsider the ultimate drivers of tropical forest phenology.