Climatic warming strengthening water constraint on seasonal peak photosynthesis in northern ecosystems

Abstract The seasonal peak of vegetation photosynthesis is a key indicator of terrestrial ecosystem productivity, bearing significant impacts on atmospheric CO 2 concentrations. Recent remote sensing observations have indicated an escalating trend in peak photosynthesis, yet the sustained trajectory of this increase under warming remains uncertain. Notably, water availability and temperature emerge as paramount limiting factors governing vegetation photosynthesis. To probe these constraints, here we employ solar-induced chlorophyll fluorescence as a surrogate for photosynthesis, focusing on the examination of temperature and water-related restrictions on peak photosynthesis for northern ecosystems. Our findings unveil a discernible expansion of water-constrained zones in northern ecosystems over the past two decades. A general relationship is identified between mean annual temperature and precipitation that effectively distinguishes temperature-constrained and water-constrained ecosystems, which can be used to predict future changes in the constraint status in various climate change scenarios. Our results show that water-constrained regions will further expand by 4.51% and 11.13% by 2100 under the SSP245 and SSP585 scenarios.