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

M. G. Tjoelker (1), J. Oleksyn (1, 2) and P. B. Reich (1)

1. Department of Forest Resources, University of Minnesota, 1530 Cleveland Ave. N, St. Paul, MN 55108, USA / 2. Polish Academy of Sciences, Institute of Dendrology, Parkowa 5, PL-62-035 Kórnik, Poland / Received November 26, 1997

Summary

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.

Keywords: Betula papyrifera, carbon fixation, elevated carbon dioxide, Larix laricina, ontogenetic drift, ontogeny, Picea mariana, Pinus banksiana, Populus tremuloides.

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