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Tropical forests substantially influence the terrestrial carbon sink. Their contributions to the forest carbon sink may increase due to the stimulation of photosynthesis by rising atmospheric CO₂ (C_a); however, the magnitude of this effect is poorly quantified for tropical canopy trees.

We measured the ratio of two deuterium isotopomers of glucose derived from tree rings to estimate how photosynthetic efficiency (photorespiration-to-photosynthesis ratio) has responded to C_a rise at a centennial scale. Wood samples were obtained from Toona ciliata trees from three climatically distinct forests in Asia and Australia. We applied Bayesian mixed effect models to test how the isotopomer ratio changes with C_a, tree diameter (as a proxy for crown exposure), temperature, and precipitation.

Across all sites, long-term C_a rise increased photosynthetic efficiency, likely due to increased photosynthesis and the concurrent suppression of photorespiration. Increasing tree size reduced photosynthetic efficiency, likely due to reduced leaf internal CO₂ at higher irradiance and stronger hydraulic limitation. Associations of photosynthetic efficiency with temperature and precipitation were inconclusive.

Our study reveals a centennial-scale association between photosynthetic efficiency and increasing C_a in canopy trees and provides a new and independent line of evidence for C_a-induced stimulation of photosynthetic efficiency in tropical forests.