Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature

Biochemical models of photosynthesis suggest that rising temperatures will increase rates of net carbon dioxide assimilation and enhance plant responses to increasing atmospheric concentrations of CO2. We tested this hypothesis by evaluating acclimation and ontogenetic drift in net photosynthesis in seedlings of five boreal tree species grown at 370 and 580 μmol mol−1 CO2 in combination with day/night temperatures of 18/12, 21/15, 24/18, 27/21, and 30/24 °C. Leaf-area-based rates of net photosynthesis increased between 13 and 36% among species in plants grown and measured in elevated CO2 compared to ambient CO2. These CO2-induced increases in net photosynthesis were greater for slower-growing Picea mariana (Mill.) B.S.P., Pinus banksiana Lamb., and Larix laricina (Du Roi) K. Koch than for faster-growing Populus tremuloides Michx. and Betula papyrifera Marsh., paralleling longer-term growth differences between CO2 treatments. Measures at common CO2 concentrations revealed that net photosynthesis was down-regulated in plants grown at elevated CO2. In situ leaf gas exchange rates varied minimally across temperature treatments and, contrary to predictions, increasing growth temperatures did not enhance the response of net photosynthesis to elevated CO2 in four of the five species. Overall, the species exhibited declines in specific leaf area and leaf nitrogen concentration, and increases in total nonstructural carbohydrates in response to CO2 enrichment. Consequently, the elevated CO2 treatment enhanced rates of net photosynthesis much more when expressed on a leaf area basis (25%) than when expressed on a leaf mass basis (10%). In all species, rates of leaf net CO2 exchange exhibited modest declines with increasing plant size through ontogeny. Among the conifers, enhancements of photosynthetic rates in elevated CO2 were sustained through time across a wide range of plant sizes. In contrast, for Populus tremuloides and B. papyrifera, mass-based photosynthetic rates did not differ between CO2 treatments. Overall, net photosynthetic rates were highly correlated with relative growth rate as it varied among species and treatment combinations through time. We conclude that interspecific variation may be a more important determinant of photosynthetic response to CO2 than temperature