Carbon fluxes acclimate more strongly to elevated growth temperatures than to elevated CO2 concentrations in a northern conifer

Increasing temperatures and atmospheric CO₂ concentrations will affect tree carbon fluxes, generating potential feedbacks between forests and the global climate system. We studied how elevated temperatures and CO₂ impacted leaf carbon dynamics in Norway spruce (Picea abies), a dominant northern forest species, to improve predictions of future photosynthetic and respiratory fluxes from high‐latitude conifers. Seedlings were grown under ambient (AC, c. 435 μmol mol^?1) or elevated (EC, 750 μmol mol^?1) CO₂ concentrations at ambient, +4 °C, or +8 °C growing temperatures. Photosynthetic rates (A_sat) were high in +4 °C/EC seedlings and lowest in +8 °C spruce, implying that moderate, but not extreme, climate change may stimulate carbon uptake. A_sat, dark respiration (R_dark), and light respiration (R_light) rates acclimated to temperature, but not CO₂: the thermal optimum of A_sat increased, and R_dark and R_light were suppressed under warming. In all treatments, the Q₁₀ of R_light (the relative increase in respiration for a 10 °C increase in leaf temperature) was 35% higher than the Q₁₀ of R_dark, so the ratio of R_light to R_dark increased with rising leaf temperature. However, across all treatments and a range of 10–40 °C leaf temperatures, a consistent relationship between R_light and R_dark was found, which could be used to model R_light in future climates. Acclimation reduced daily modeled respiratory losses from warm‐grown seedlings by 22–56%. When R_light was modeled as a constant fraction of R_dark, modeled daily respiratory losses were 11–65% greater than when using measured values of R_light. Our findings highlight the impact of acclimation to future climates on predictions of carbon uptake and losses in northern trees, in particular the need to model daytime respiratory losses from direct measurements of R_light or appropriate relationships with R_dark.