Asymmetric photosynthetic responses to hydrothermal variations between the two halves of the year in the Amazon rainforest

The Amazon rainforest, which stores approximately 120 billion tons of carbon and contributes around 16% of global terrestrial photosynthetic productivity, plays a pivotal role in global carbon cycling. Unlike temperate and boreal forests, tropical forests exhibit a bimodal photosynthetic pattern, characterized by distinct peaks in the first and second halves of the year. However, the intra-annual differences in photosynthetic responses to hydrothermal variations between these two periods in the Amazon rainforest remain largely unexplored. Here, utilizing satellite-derived photosynthetic proxies alongside ground-based flux tower observations from 2001 to 2020, we investigated the differences in photosynthetic responses to hydrothermal variations between the first and second halves of the year in the Amazon rainforest. Our observations revealed weaker temperature limitations but stronger precipitation limitations on photosynthesis in the second half of the year compared to the first half. Temperature constraints on photosynthesis have progressively weakened in both periods, while precipitation limitations have intensified, particularly in the latter half. Although the optimal temperature for photosynthesis is higher in the second half of the year, it is reached earlier, resulting in a sharper decline in photosynthetic productivity over the past two decades. Our findings reveal a shift from temperature to precipitation limitation in the Amazon, underscoring intra-annual asymmetry in vulnerability to intensifying heatwaves and droughts and calling for its explicit integration into management strategies and predictive models.