Seasonal responses of leaf gas exchange to elevated carbon dioxide in Populusgrandidentata

Rising atmospheric carbon dioxide concentrations may have important consequences for forest ecosystems. We studied above- and below-ground growth and leaf gas exchange responses of Populusgrandidentata Michx. to elevated CO 2 under natural forest conditions over the course of a growing season. Recently emerged P. grandidentata seedlings were grown in native, nutrient-poor soils at ambient and twice ambient (707 μbar (1 bar = 100 kPa)) CO 2 partial pressure for 70 days in open-top chambers in northern lower Michigan. Total leaf area and shoot and root dry weight all increased in high CO 2 grown plants. Photosynthetic light and CO 2 response characteristics were measured 28, 45, and 68 days after exposure to elevated CO 2 . In ambient grown plants, light saturated assimilation rates increased from day 28 to day 45 and then declined at day 68 (15 September). This late-season decline, typical of senescing Populus leaves, was due both to a decrease in the initial slope of the net CO 2 assimilation versus intercellular CO 2 partial pressure relationship and to decreased CO 2 saturated assimilation rates. Specific leaf nitrogen (mg N•(cm 2 leaf area) −1 ) did not change during this period, although leaf carbon content and leaf weight (mg•cm −2 ) both increased. In ambient grown plants stomatal conductance also declined at day 68. In contrast, plants grown at elevated CO 2 showed no late-season decline in photosynthetic capacity or changes in leaf weight, suggesting a delay in senescence with long-term exposure to high CO 2 . High CO 2 grown plants also maintained photosynthetic sensitivity to increasing C i throughout the exposure period, while ambient CO 2 grown plants were insensitive to C i above 400 μbar on day 68. These results indicate the potential for direct CO 2 fertilization of P. grandidentata in the field and provide evidence for a new mechanism by which elevated atmospheric CO 2 could influence seasonal carbon gain.