GAS EXCHANGE, LEAF NITROGEN, AND GROWTH EFFICIENCY OFPOPULUS TREMULOIDESIN A CO2-ENRICHED ATMOSPHERE

Predicting forest responses to rising atmospheric CO2 will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric CO2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO2 and soil-N availability on the flow of C and N in the plant–soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvestable yield. We grew six P. tremuloides genotypes at ambient (35 Pa) or elevated (70 Pa) CO2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45°35′ N, 84°42′ W). In the second year of growth, net CO2 assimilation rate was significantly higher in elevated-CO2 compared to ambient-CO2 plants in both soil-N treatments, and we found little evidence for photosynthetic acclimation to high CO2. In the third year, however, elevated-CO2 plants in low-N soil had reduced photosynthetic capacity compared to ambient-CO2, low-N plants. Plants in high-N soil showed the opposite response, with elevated-CO2 plants having higher photosynthetic capacity than ambient-CO2 plants. Net CO2 assimilation rate was linearly related to leaf N concentration (log:log scale), with identical slopes but different intercepts in the two CO2 treatments, indicating differences in photosynthetic N-use efficiency. Elevated CO2 increased tissue dark respiration in high-N soil (+22%) but had no significant effect in low-N soil (+9%). There were no CO2 effects on stomatal conductance. At the final harvest, stem biomass and total leaf area increased significantly due to CO2 enrichment in high-N but not in low-N soil. Treatment effects on wood production were largely attributable to changes in leaf area, with no significant effects on growth efficiency. We conclude that harvest intervals for P. tremuloides on fertile sites will shorten with rising atmospheric CO2, but that tree size at canopy closure may be unaffected.