Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures.

Warming shifts the thermal optimum of net photosynthesis (T) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO) conditions. It is unclear whether shifts in T of field-grown trees will keep pace with the temperatures predicted for the 21 century under elevated atmospheric CO concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO levels, we show that T of mature boreal conifers increased with warming. However, shifts in T did not keep pace with warming as T 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 CO spruce, while remaining constant in ambient CO spruce and in both ambient CO and elevated CO tamarack with warming. Although shifts in T 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.