Drought-induced shifts toward photoprotection reduce Amazon photosynthesis

Amazonian forests have experienced increasingly frequent and intense droughts in recent decades, often associated with El Niño–Southern Oscillation (ENSO). These droughts have triggered complex forest responses—from increased tree mortality, reduced carbon uptake, and structural changes to increased canopy productivity—that cannot be explained by climate variability alone. While drought-related changes in light availability and physiological responses are likely to vary along the canopy profile, a key question is how drought-driven changes in canopy conditions, especially across vertical gradients, impact canopy production. To investigate this, we combined tree climbing techniques and pulse amplitude modulated (PAM) fluorometry to quantify how leaves partition absorbed photon energy across seasons and canopy strata in central Amazonian forests during typical wet and dry seasons, and throughout the 2023–2024 ENSO drought. By conducting extensive in‑canopy sampling , we show that photosynthetic efficiency and photoprotective responses differ significantly across canopy strata during drought. We found that the typical seasonal dry period had little impact on the fates of photons absorbed by leaf light-harvesting centers for a given microenvironment, consistent with multi-scale observations of sustained or high dry season canopy function in the central Amazon. In contrast, during the ENSO drought we found reduced photochemical yield in all canopy strata, with increased photoprotective heat dissipation. We also observed nonlinear relationships between photosynthetic linear electron flow between photosystems II and I and leaf fluorescence, mainly driven by the joint dynamics of PSII open reaction centers (qL) and non-photochemical quenching (NPQ). Finally, we found in situ leaf-level evidence that, in contrast to dry season resilience, drought reduces photosynthesis of large trees, driving shifts in energy partitioning from photosynthesis to photoprotective dissipation. However, yields to leaf fluorescence remained stable during drought, suggesting that extreme drought systematically alters the linkage between fluorescence and carbon assimilation. These results show that drought‐resilience mechanisms strongly modulate photosynthesis and suggest that productivity estimates based on remotely sensed sun‑induced fluorescence (SIF) alone are likely to underestimate drought responses in Amazonian forests.