Our results indicate that the photosynthesis of seedlings of two white spruce seed sources from southern and northern origins had a lower thermal optimum of photosynthesis and no ability to acclimate to warmer temperature. In contrast, the seedlings showed a clear acclimation of dark respiration by the downshift of the basal rate of Rd. However, dark respiration acclimation was insufficient to counterbalance the low photosynthetic rate in warmer plantation sites. In addition, the temperature response of photosynthesis was limited by Rubisco capacity, which suggests an effect of Rubisco activase or a lack of adjustment of nitrogen allocation. The results highlight the need for more research on thermal responses of photosynthesis and its biochemical limitations, with particular emphasis on Rubisco activase and on understanding of the main cues of photosynthesis phenology in spruces and other boreal forest trees. Together with monitoring at a more mature stage, this should help evaluate the effect of predicted autumnal warming on the global trend of photosynthesis. Overall, our results on growth and thermal acclimation-related traits for photosynthesis and dark respiration suggest that white spruce populations from southern Québec are already above their thermal thresholds and will remain so under predicted climate warming.