Rapid climate acclimation (not traits or phylogeny) drives variation in photosynthesis temperature response

Understanding variation in plant assimilation-temperature (AT) responses is essential for improving forecasts of climate change feedbacks and their impacts on the biosphere. Previous studies have focused on acclimation to weather or adaptation to climate of origin, but relationships between AT response parameters and leaf functional traits or phylogeny have received little attention. To evaluate the influence of climate, traits, and phylogeny on AT response, we used the new Fast Assimilation-Temperature Response (FAsTeR) gas exchange method to measure 243 AT response curves in 102 species from 96 families grown in a common garden. We also quantified climate variables, saturating light intensity, and key leaf functional traits. Local environmental conditions were the strongest predictors of AT response parameters. The optimal temperature for photosynthesis responded positively to recent air temperature and light exposure (partial r² = 0.27 and 0.53, respectively), and was best predicted by mean air temperature on the day of measurement; other AT parameters exhibited weak or no relationships with recent air temperature (all partial r² < 0.1). AT response parameters showed no phylogenetic structure and only modest variation with leaf functional traits or climate of origin (all partial r² < 0.07). Plant AT responses were primarily driven by acclimation to local climate variables, rather than adaptation to climate of origin. Thermal acclimation of photosynthesis occurred on much shorter timescales than expected (≤ 1 day). These findings underscore the need to account for rapid acclimation in Earth system models and climate change forecasts.