Large differences in photorespiration and its temperature response among temperate trees

Summary Photorespiration significantly influences terrestrial carbon fluxes, yet empirical measurements of its variability across tree species and temperature conditions remain limited, constraining predictions of vegetation and climate models. We quantified leaf-level photorespiration rates ( R p ) for seven temperate broadleaf tree species in northern Europe, measuring in situ in Uppsala, Sweden during peak summer. R p was estimated by contrasting net photosynthetic CO 2 assimilation under ambient and O 2 -free conditions at three leaf temperatures (25, 30, and 35°C), encompassing typical and heatwave scenarios. Results reveal pronounced interspecific variation in R p and increasing photorespiration with temperature, consistent with the temperature sensitivity of Rubisco’s oxygenase function. Notably, thermal responses varied substantially among species: some exhibited pronounced Rp increases at higher temperatures, while others maintained stable or even reduced rates, suggesting differences in metabolic and thermal resilience. The photorespiration-to-photosynthesis ratio increased with temperature, surpassing unity at 35°C for some species and indicating reduced photosynthetic efficiency during heat stress. Unexpectedly, the correlation between photosynthesis and photorespiration strengthened at elevated temperatures. Our results reveal high variability and temperature sensitivity in photorespiration among temperate trees. Compared to crop-based model parameters, tree values are substantially higher, with important implications for dynamic vegetation model predictions.