Nonlinear Thermal Responses Outweigh Water Limitation in the Attenuated Effect of Climatic Warming on Photosynthesis in Northern Ecosystems

Abstract Plant photosynthesis is strongly limited by temperature at high northern latitudes. The temperature sensitivity of plant photosynthesis in scenarios of future climatic warming, however, is highly uncertain. We used sun‐induced fluorescence—a satellite proxy for plant photosynthesis—to analyze the spatiotemporal response of photosynthesis to temperature at high northern latitudes. We detected a widespread decline in the sensitivity of photosynthesis to temperature during the last two decades: 3.25 versus 2.19%/°C, in the 2000s and 2010s, respectively. We used methods of machine learning and temporal composition to characterize the contribution of nonlinear thermal responses and water limitation. Both methods consistently identified the nonlinear thermal responses as the main driver of the attenuated positive warming effect. We highlighted the traditionally temperature‐limited ecosystems at high northern latitudes may be approaching the temperature tipping point, beyond which the warming effect on plant photosynthesis would transition from positive to negative.