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

Photosynthetic acclimation to both warming and elevated CO 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 CO on photosynthetic capacity (V and J) in mature trees of two North American boreal conifers, tamarack and black spruce. We show that V and J at a standard temperature of 25°C did not change with warming, while V and J at their thermal optima (T) and growth temperature (T) increased. Moreover, V and J at either 25°C, T or T decreased with elevated CO. The J/V ratio decreased with warming when assessed at both T and T but did not significantly vary at 25°C. The J/V increased with elevated CO at either reference temperature. We found no significant interaction between warming and elevated CO on all traits. If this lack of interaction between warming and elevated CO on the V, J and J/V 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.