Climatic Constraints and Thermal Adjustment Shape the Apparent Response of Ecosystem Photosynthetic Optimum Temperature to Growth Temperature

The apparent optimum air temperature at which ecosystem gross primary productivity (GPP) peaks () exhibits a globally observed positive correlation with local growth temperature, represented by growing‐season‐averaged daily maximum temperature (). Despite recognition of this spatial covariation, the drivers underlying this variability remain unclear. Combining in situ and satellite‐derived GPP measurements, we find that growth temperature is a stronger large‐scale correlate of than plant functional type (PFT) identity, while the magnitude of the ‐to‐ sensitivity varies among PFTs. This strong dependence on growth temperature is reproduced by a simple, mechanistic photosynthesis model and an ensemble of Dynamic Global Vegetation Models. We then conducted factorial simulations to test two overarching mechanisms that may underlie the emergent – relationship: biochemical thermal adjustments (acclimation and adaptation) of photosynthetic capacity and suboptimal meteorological conditions. The results show that the observed relationship cannot be attributed to either mechanism alone. Biochemical thermal adjustment contributes substantially to the drawdown of apparent in boreal and some temperate regions, while suboptimal environmental conditions, including co‐occurring high vapor pressure deficit, limited light, and infrequent occurrence of physiologically favorable temperatures, also constrain the diagnosed optimum, with their relative contributions varying markedly among regions. These findings caution against interpreting the apparent –growth‐temperature relationship as a direct measure of intrinsic physiological acclimation or long‐term ecosystem resilience.