Boreal tree species, which are well adapted to cold temperature, were often assumed to be limited in their capacity to adapt to warm conditions (Girardin et al., 2016). We showed that the qualitative aspect of thermal acclimation of photosynthesis, such as the ability to shift the thermal optimum (Topt) of An, was lacking. Topt was similar along the climatic gradient and was close to the mean July temperature at southern plantation sites. Our results do not agree with those of previous studies on other species of Picea conducted in controlled conditions, which showed a shift in Topt when the growing temperature was increased by 10 °C (Way and Sage, 2008a; Zhang et al., 2015). Despite the variation in mean temperature (MAT and MJT), a similar range of temperatures (minimum and maximum) along the climatic gradient tested here may explain the unchanging Topt of photosynthesis observed in our study. Increased or constant photosynthetic rate at Topt towards a warmer environment has been widely used as a quantitative proxy of thermal acclimation of photosynthesis (Way and Yamori, 2014). In accordance with results of previous studies on other Picea species (Way and Sage, 2008a; Zhang et al., 2015), we showed the inability of the two seed sources to maintain photosynthetic performance (Aopt) in warmer sites. In addition, the relationship between Aopt and the growing temperature along the gradient followed a quadratic shape, which may suggest adaptation to a narrow climate niche. However, this pattern may also result from complex interaction between soil and climatic conditions (temperature and precipitation) along the climatic gradient tested here rather than temperature per se (Reich and Oleksyn, 2004; Chi et al., 2013; Scafaro et al., 2016). In our study, variation in leaf nitrogen content appeared to play a minor role in photosynthetic adjustments.