Photosynthetic capacity in middle-aged larch and spruce acclimates independently to experimental warming and elevated...

Photosynthetic acclimation to both warming and elevated CO₂ of boreal trees remains a key uncertainty in modelling the response of photosynthesis to future climates. We investigated the impact of increased growth temperature and elevated CO₂ on photosynthetic capacity (V_cmax and J_max) in mature trees of two North American boreal conifers, tamarack and black spruce. We show that V_cmax and J_max at a standard temperature of 25°C did not change with warming, while V_cmax and J_max at their thermal optima (T_opt) and growth temperature (T_g) increased. Moreover, V_cmax and J_max at either 25°C, T_opt or T_g decreased with elevated CO₂. The J_max/V_cmax ratio decreased with warming when assessed at both T_opt and T_g but did not significantly vary at 25°C. The J_max/V_cmax increased with elevated CO₂ at either reference temperature. We found no significant interaction between warming and elevated CO₂ on all traits. If this lack of interaction between warming and elevated CO₂ on the V_cmax, J_max and J_max/V_cmax ratio is a general trend, it would have significant implications for improving photosynthesis representation in vegetation models. However, future research is required to investigate the widespread nature of this response in a larger number of species and biomes.