AbstractWarming shifts the thermal optimum of net photosynthesis (T_optA) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO₂) conditions. It is unclear whether shifts in T_optA of field-grown trees will keep pace with the temperatures predicted for the 21^st century under elevated atmospheric CO₂ concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO₂ levels, we show that T_optA of mature boreal conifers increased with warming. However, shifts in T_optA did not keep pace with warming as T_optA only increased by 0.26–0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO₂ spruce, while remaining constant in ambient CO₂ spruce and in both ambient CO₂ and elevated CO₂ tamarack with warming. Although shifts in T_optA of these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures.