SEEDLINGS OF 5 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 pla nt responses to increasing atmospheric concentrations of CO2. We teste d this hypothesis by evaluating acclimation and ontogenetic drift in n et photosynthesis in seedlings of five boreal tree species grown at 37 0 and 580 mu mol mol(-1) CO2 in combination with day/night temperature s of 18/12, 21/15, 24/18, 27/21, and 30/24 degrees 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 sl ower-growing Picea mariana (Mill.) B.S.P., Pinus banksiana Lamb., and Larix laricina (Du Roi) K. Koch than for faster-growing Populus tremul oides Michx. and Betula papyrifera Marsh., paralleling longer-term gro wth differences between CO2 treatments. Measures at common CO2 concent rations revealed that net photosynthesis was down-regulated in plants grown at elevated CO2. In situ leaf gas exchange rates varied minimall y across temperature treatments and, contrary to predictions, increasi ng growth temperatures did not enhance the response of net photosynthe sis to elevated CO2 in four of the five species. Overall, the species exhibited declines in specific leaf area and leaf nitrogen concentrati on, and increases in total nonstructural carbohydrates in response to CO2 enrichment. Consequently, the elevated CO2 treatment enhanced rate s 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 increas ing plant size through ontogeny. Among the conifers, enhancements of p hotosynthetic 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 correla ted with relative growth rate as it varied among species and treatment combinations through time. We conclude that interspecific variation m ay be a more important determinant of photosynthetic response to CO2 t han temperature.