Thermal acclimation of CO2 exchange has been mostly investigated in controlled conditions using static day–night temperature treatments or from seasonal variation in CO2 exchange in response to the seasonal courses of temperature (Hikosaka et al., 2006; Way and Yamori, 2014; Yamori et al., 2014). Although these studies helped increase our understanding of the processes involved in thermal acclimation, their results cannot realistically be used to infer thermal responses in natural conditions, and consequently to better evaluate the quantitative aspects of tree responses to global warming. The main reason for this is the lack of representation of diurnal and daily temperature variations during the growing season, which are especially important in boreal regions. In addition, it is unclear whether or not thermal acclimation of An to temporal variation in temperature during the growing season and spatial variation in temperature along a climate gradient may result from similar physiological adjustments. In fact, the predominant role of photoperiod in the regulation of the seasonal pattern of photosynthetic rate, also known as the phenology of photosynthesis, is still largely controversial for tree species (Busch et al., 2007; Bauerle et al., 2012; Stinziano and Way, 2017).