Physiological and structural trends rather than photosynthetic optimum temperature explain the recent increase of terrestrial carbon uptake

Gross primary productivity (GPP) is the largest terrestrial carbon flux and is highly sensitive to global warming. Despite global warming, relationships of the optimum temperature of GPP and the maximum GPP rate remain uncertain. We investigated the drivers of maximum GPP trends during 2000-2019 using global observations of ground-based eddy-covariance and satellite-based sun-induced chlorophyll fluorescence. Although maximum GPP increased worldwide, its optimum temperature increased only in tropical and temperate regions, but remained unchanged globally, and in arid and cold regions. Thermal acclimation via shifting optimum temperature was constrained by atmospheric and soil dryness, explaining less than a fifth of the global rise in maximum GPP. In contrast, increasing maximum GPP trends were more strongly driven by stomatal regulation improving water-use efficiency (as determined by the stomatal slope at the ecosystem scale, G1) and canopy development (as determined by the leaf area index). These results challenge the expectation that thermal acclimation is essential for terrestrial carbon uptake, and reveal that dynamic plant physiological and structural trends are critical for improving carbon cycle predictions at the ecosystem to global scales.