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Boreal regions are undergoing rapid climate change but our understanding of the long-term consequences for forest processes is hampered by limited knowledge of how trees acclimate to rising atmospheric CO2 concentrations and temperature. This study used the detailed canopy flux model MAESTRA to simulate the effects of elevated CO2 (eCO2) and warming on net photosynthesis (An) and transpiration (E) of mature boreal Norway spruce, investigating how these effects are influenced by the observed acclimation of photosynthetic capacity, respiration, stomatal behavior, and phenology. Without any type of acclimation, eCO2 increased shoot and crown An during the non-frost growing season by 23-44%, while warming only had a minor effect (±2%). Photosynthetic downregulation greatly decreased the positive effect under eCO2. Under warming, both stomatal and phenological acclimation had substantial …