Warming shifts the thermal optimum of net photosynthesis (*T*_optA) to higher temperatures. This study, using a whole-ecosystem warming controlled experiment under ambient or elevated CO₂ levels, shows that *T*_optA of mature boreal conifers increased with warming, but not at the pace of warming itself (*T*_optA increased by only 0.26–0.35 °C per 1 °C of warming). Net photosynthetic rates at mean growth temperature increased with warming in elevated CO₂ spruce, but remained constant in other treatments. Despite insufficient *T*_optA shifts to fully keep pace with warming, these boreal conifers thermally acclimate photosynthesis to maintain carbon uptake.

Mirindi Eric Dusenge, Jeffrey M. Warren, Peter B. Reich, Eric J. Ward, Bridget K. Murphy, Artur Stefanski, Raimundo Bermudez, Marisol Cruz, David A. McLennan, Anthony W. King, Rebecca A. Montgomery, Paul J. Hanson, Danielle A. Way