In situ warming causes thermal divergence in photosynthetic capacity between two boreal conifers classified as the same plant functional type

The boreal forest in North America plays a major role in global vegetation–atmosphere interactions and is expected to experience significant warming by the end of this century; therefore, accurately predicting carbon dynamics in this region is increasingly important. Carbon dynamics are often predicted using terrestrial biosphere models, which rely on parameters such as the maximum rate of Rubisco carboxylation to simulate carbon uptake. Terrestrial biosphere models generally group species into broad plant functional types that may oversimplify species-specific responses to warming. To address this limitation, we conducted a field warming experiment using custom-built phytotrons to evaluate the photosynthetic responses of two-year-old balsam fir ( Abies balsamea (L.)) and white spruce ( Picea glauca (Moench)) seedings—two boreal conifers commonly grouped in the same plant functional type—under four treatments: control (no phytotron), ambient (phytotron with no warming), and +3 °C and +6 °C above ambient. Under the +6 °C treatment, stomatal conductance, transpiration, net assimilation, and maximum rate of Rubisco carboxylation in white spruce were significantly greater than those in balsam fir, while no major differences were observed between these species under the ambient and +3 °C treatments. Interspecific variation in gas exchange and thermal acclimation likely drove the observed divergence in photosynthetic capacity under the warmest treatment. These findings support a growing body of research indicating that grouping physiologically distinct species into broad plant functional types may misrepresent the effects of climate change on forest carbon dynamics in terrestrial biosphere models.