Since the Industrial Revolution, atmospheric CO2 concentrations have been increasing rapidly, causing a rise in global surface temperatures. Climate warming is expected to be most pronounced at high latitudes, such that boreal forests will encounter a sharp temperature increase over the coming century. We examined how future climate scenarios will alter photosynthesis and respiration in two dominant boreal tree species, Picea mariana (black spruce) and Larix laricina (tamarack). Trees were grown from seed in three temperature treatments (ambient, ambient +4 °C, and ambient +8 °C) at either 400 ppm or 750 ppm CO2, to simulate future CO2 and temperature conditions. Net CO2 assimilation and respiration rates were measured on trees from each treatment at leaf temperatures from 10 °C to 40°C at both a common CO2 and at their respective growth CO2 concentrations.
The thermal optimum of net CO2 assimilation increased in both species under warming, but more in spruce than in tamarack. Maximum photosynthetic rates were suppressed in warm-grown plants, although this effect was more pronounced in spruce than in tamarack. Trees grown at high CO2 had similar photosynthesis and respiration rates as seedlings from ambient CO2 when measured at a common CO2 concentration, although photosynthesis was stimulated in elevated CO2 measurement conditions. There was no interactive effect of growth CO2 and temperature on photosynthesis. Respiration acclimated to increased growth temperatures, with a greater degree of acclimation in spruce than in tamarack, and there was no effect of CO2 on respiration rates in either species. Spruce showed decreased growth at the warmest treatment, while growth in tamarack was stimulated at +4 °C, but suppressed at +8 °C, where we also saw considerable mortality of tamarack. High CO2 increased growth in both species and moderated the mortality of tamarack at +8 °C. Our results show that: 1) the effects of warming on boreal seedling carbon fluxes are greater than the effects of high CO2; and 2) threshold effects may become apparent as temperatures increase, particularly regarding tree growth and survival.