White spruce (Picea glauca) is one of the ecologically and commercially most important conifer species of the boreal forest of Canada (Beaulieu et al., 2009). It is the subject of large reforestation efforts and several intensive breeding programmes in Canada (Mullin et al., 2011). It has been shown not to be optimally adapted to local climate conditions in relation to recent temperature warming (e.g. Andalo et al., 2005). But to our knowledge, no one has investigated the temperature response of both photosynthesis and respiration to determine the thermal acclimation capacity and potential adaptive differences among seed sources from geographically distant regions, which could affect the productivity of forest ecosystems and assisted migration strategies. The objectives of this study were (1) to evaluate the thermal acclimation of photosynthesis and respiration of two geographically distant white spruce seed sources in response to short-term variation in climatic conditions during a growing season and to long-term variation in growing conditions along a regional thermal gradient of 5.5 °C, and (2) to assess the involvement of morphological, biochemical and biophysical processes in the temperature response of photosynthesis.