Gas exchange, leaf nitrogen, and growth efficiency of Populus tremuloides in a CO2-enriched atmosphere

Predicting forest responses to rising atmospheric CO2 will require an under standing of key feedbacks in the cycling of carbon and nitrogen between pla nts and soil microorganisms, We conducted a study for 2.5 growing seasons w ith Populus tremuloides grown under experimental atmospheric CO2 and soil-N -availability treatments. Our objective was to integrate the combined influ ence of atmospheric CO2 and soil-N availability on the flow of C and N in t he plant-soil system and to relate these processes to the performance of th is widespread and economically important tree species. Here we consider tre atment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvest able 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 degrees 35' N, 84 degrees 42' W). In the second year of growth, net CO2 assimilation rate was significantly higher in elevated-CO2 compared to ambient-CO2 plants in bot h soil-N treatments, and we found little evidence for photosynthetic acclim ation 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 pl ants, Plants in high-N sail 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 trea tments, indicating differences in photosynthetic N-use efficiency, Elevated CO2 increased tissue dark respiration in high-N soil (+22%) but had no sig nificant effect in low-N soil(+9%). There were no CO2 effects on stomatal c onductance. At the final harvest, stem biomass and total leaf area increase d significantly due to CO2 enrichment in high-N but not in low-N soil. Trea tment effects on wood production were largely attributable to changes in le af area, with no significant effects on growth efficiency, We conclude that harvest intervals for P. tremuloides on fertile sites will shorten with ri sing atmospheric CO2, but that tree size at canopy closure may be unaffecte d.