Photoperiod does not affect thermal acclimation of shoot-scale gas exchange but is important for shoot development in cuttings of Norway spruce ( <i>Picea abies</i> (L.) H. Karst)

The growth of boreal trees is expected to benefit from increasing global temperatures through enhanced photosynthetic rates and longer growing seasons. However, since photoperiod is independent of climate change, it may limit the expected growth benefits from a longer growing season and could thus constrain boreal trees' physiological responses to warming. We carried out a growth chamber experiment on 2-year-old Norway spruce (Picea abies) cuttings from two latitudinal origins to investigate the interaction between day length (20/4 h vs 14/10 h light/dark) and enhanced temperatures (25/20 °C vs 15/10 °C day/night) on height growth, bud development and shoot-scale gas exchange. Height growth was greater under longer day length while bud development occurred faster both under longer day length and higher growth temperature. Growth temperature did not have a significant effect on the light-saturated photosynthetic rate but higher growth temperature resulted in lower dark respiration rate. Cuttings in the low-growth temperature treatment exhibited higher apparent quantum yields indicating that lower growth temperature benefited net carbon uptake under low light availability, such as the conditions experienced by seedlings growing in the forest understory. Day length did not influence the thermal acclimation of shoot-scale gas exchange. The two populations from different origins did not differ in the measured parameters, except for a higher dark respiration rate in the high latitude cuttings. Overall, while day length did not affect the thermal acclimation of photosynthetic processes, it appears to constrain height growth and bud development, thereby reducing the potential performance benefit of a warming-induced lengthening of the growing season.