We observed thermal acclimation of both needle respiration and photosynthetic rate in response to seasonal variation in temperature (i.e. reduction in Rd10 at higher temperatures and increase in Topt and Aopt with increasing temperature). Aopt reached a maximum value in June and September, with a clear linear relationship with temperature as expressed in 5-d mean temperature windows. It is still unclear whether photosynthesis phenology relies on temperature or photoperiod cues (Busch et al., 2007; Bauerle et al., 2012; Stinziano and Way, 2017). Unfortunately, our results cannot help clarify this issue. In fact, the decrease in An from June to August may be related to a decline in photosynthesis following bud set or to the increase in mean temperature. On the other hand, the increase in An from August to September cannot be linked to photoperiod. The similar values of Topt observed from June to August is in accordance to the pattern along the latitudinal gradient. The strong adjustment in Topt to lower temperatures in May and October, which may relate to a change in the activation energy of Vcmax, represents good evidence for higher acclimation of An to cold temperature in white spruce. The latter may explain the higher performance of seed sources from Québec in northern regions of Canada, such as Alberta (Lu et al., 2014).