Increases in CO2 could raise air temperatures in northern latitudes by up to 9 °C by the end of the century. Boreal forests in these regions play a large role in the global carbon cycle, and the responses of boreal tree species to climate drivers will thus have considerable impacts on the trajectory of future CO2 increases. We assessed how two dominant North American boreal tree species responded to a range of future climate conditions to assess how carbon fluxes were altered by high CO2 and warming. Field-grown black spruce (Picea mariana) and tamarack (Larix laricina) trees were exposed to a range of growth temperatures of 0, 2.25, 4.5, 6.75 and 9 °C warming above ambient in the SPRUCE experiment. Data from SPRUCE was compared to results from seedlings of the same two species grown in glasshouses under either ambient (400 ppm) or elevated CO2 concentrations (750 ppm) and ambient temperatures, or moderate (+4 °C), or extreme (+8 °C) warming. We measured growth and survival, as well as temperature responses of net photosynthesis and dark respiration to determine acclimation to the climate treatments.
Results/Conclusions
In the mature trees, warming increased leaf N and stomatal conductance in tamarack, thereby offsetting direct temperature-induced decreases in Anet. In spruce, increased leaf N in the warming treatments compensated for the suppression of Anet caused by elevated temperatures, but stomatal conductance was similar across the treatments. Respiration acclimated more strongly in tamarack than spruce. Growth rates of the two species reflected their ability to maintain their carbon balance: tamarack grew equally well across all the treatments, but spruce growth declined with warming. In the seedlings, growth temperature also had a strong effect on carbon fluxes, but there were no significant effects of growth CO2. Maximum photosynthetic rates were reduced by warming in both species, but this effect was ameliorated by high stomatal conductance in tamarack seedlings. Respiration acclimated in warm-grown seedlings of both species. Our results show a consistent signal in both seedlings and mature trees, indicating that climate warming is likely to reduce carbon fluxes in these boreal conifers. Differences in stomatal behavior between these two species underlie their contrasting photosynthetic responses to climate change factors, with important implications for tree growth and water use efficiency.