Light-driven acclimation sustains Amazon dry-season photosynthesis

Resolving the nature and drivers of photosynthetic seasonality in the Amazon is central to understanding tropical forest dynamics. A key uncertainty centers on whether this seasonality is driven by leaf phenology, meteorology, or acclimation – reflecting seasonal physiological adjustments to prevailing conditions. We integrate eddy covariance measurements, multi-sensor satellite retrievals, and eco-evolutionary optimization (EEO)-based modeling to elucidate the drivers. Optical, lidar, and microwave observations consistently reveal an apparent structural green-up during the dry season, predominantly in the understory (< 5m). However, radiative transfer and EEO modeling suggest that such green-up plays a minor role in driving dry-season increase in photosynthesis. The dry-season photosynthesis enhancement is instead driven by greater light availability but can only be sustained through acclimation-driven increases in photosynthetic capacity, which is most evident in the upper canopy (> 5m). Further model-data analyses show that increased radiation triggers stomatal opening, elevates vapor pressure deficit, diminishes soil moisture, and reduces intercellular CO2 concentrations, which together lead to higher photosynthesis and evapotranspiration but lower water and light use efficiencies. Our findings highlight the need for acclimation-aware models to capture Amazon’s responses to changing light and water conditions.