Gas Exchange, Leaf Nitrogen, and Growth Efficiency of Populus tremuloides in a CO 2 -Enriched Atmosphere

Abstract

Abstract. Predicting forest responses to rising atmospheric CO 2 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 CO 2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO 2 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) CO 2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45Њ35Ј N, 84Њ42Ј W). In the second year of growth, net CO 2 assimilation rate was significantly higher in elevated-CO 2 compared to ambient-CO 2 plants in both soil-N treatments, and we found little evidence for photosynthetic acclimation to high CO 2 . In the third year, however, elevated-CO 2 plants in low-N soil had reduced photosynthetic capacity compared to ambient-CO 2 , low-N plants. Plants in high-N soil showed the opposite response, with elevated-CO 2 plants having higher photosynthetic capacity than ambient-CO 2 plants. Net CO 2 assimilation rate was linearly related to leaf N concentration (log : log scale), with identical slopes but different intercepts in the two CO 2 treatments, indicating differences in photosynthetic N-use efficiency. Elevated CO 2 increased tissue dark respiration in high-N soil (ϩ22%) but had no significant effect in low-N soil (ϩ9%). There were no CO 2 effects on stomatal conductance. At the final harvest, stem biomass and total leaf area increased significantly due to CO 2 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 CO 2 , but that tree size at canopy closure may be unaffected.

Atmospheric CO 2 , Soil-N Availability, and Allocation of Biomass and Nitrogen by Populus tremuloides

“…Similarly, leaf area increased under elevated CO 2 by 28% in high-N soil, but elevated CO 2 did not influence leaf area in low-N soil . Although leaf-level C assimilation increased to a much smaller extent from low-to high-N soil (e.g., 22%, Curtis et al 2000), greater C acquisition in high-N soil was facilitated by a much larger increase in leaf area (197%). Thus, greater rates of leaf-level C assimilation and leaf-area accumulation resulting from increases in atmospheric CO 2 and soil-N availability directly corresponded to differences in total biomass among our atmospheric-CO 2 -soil-N-availability treatments.…”

“…Leaf litter produced following the 1994 and 1995 growing seasons fell on the soil surface, where it resided during the remaining portion of our experiment. Additional details regarding plant growth conditions can be found in Curtis et al (2000).…”

“…Further details regarding our CO 2 -exposure system and its performance can be found in Curtis et al (2000).…”

“…Nitrogen mineralization rates in our experiment fall well within the range that this species encounters under field conditions. A complete summary of soil physical, chemical, and biological properties is presented in Curtis et al (2000) and Zak et al (2000).…”

Phenological and Temperature Controls on the Temporal Non-Structural Carbohydrate Dynamics of Populus grandidentata and Quercus rubra

“…Dried extract was resuspended with 3 ml deionized water and 40 mg polyvinylpolypyrrolidone (PVPP) and thoroughly vortexed. PVPP was spun down using a centrifuge and a 0.5 ml aliquot removed and assayed enzymatically according to a colorimetric assay adapted from Jones et al [34] and modified for use on aspen tissues [35]. Soluble sugar recovery was >95%.…”