Poster Session #49: Elevated CO2 II.
Thursday, August 8. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC


Photosynthetic carbon isotope discrimination in tree canopies under CO2 and O3 enrichment.

Kubiske, Mark*,1, McDonald, Evan1, 1 USDA Forest Service, Rhinelander, WI

ABSTRACT- Our goal was to examine the effects of elevated CO2 and O3 treatments and canopy position on photosynthetic C isotope fractionation by leaves of different temperate hardwood tree species. In September, 2001, we collected recently mature leaf tissues from aspen (Populus tremuloides), birch (Betula papyrifera), and maple (Acer saccharum) trees in the aspen FACE experiment in Rhinelander, Wisconsin, for measurements of 13C discrimination (). did not differ among species, but leaves from the elevated CO2 treatments (+CO2 and +CO2+O3) had significantly higher values compared to leaves exposed to ambient CO2 concentrations. These differences were also evident in estimates of Ci/Ca, which were significantly higher with elevated compared to ambient CO2. In addition, elevated CO2 significantly affected the relationship between and leaf N concentration such that lower canopy leaves with low N had a relatively greater increase in than their upper canopy counterparts. Surprisingly, elevated O3 had no effect on or Ci/Ca. Results are consistent with gas exchange measurements indicating a strong interaction between elevated CO2 and canopy microclimate that decreases photosynthetic capacity at shaded canopy positions. Responses of to elevated O3 are inconsistent with gas exchange measurements, however, and are discussed in terms of hypothetical changes in the activities of subsidiary carboxylases.

KEY WORDS: elevated CO2, elevated O3, 13C discrimination, photosynthesis