CARBON-ISOTOPE DISCRIMINATION DURING PHOTOSYNTHESIS AND THE ISOTOPE RATIO OF RESPIRED CO2 IN BOREAL FOREST ECOSYSTEMS

Our objective was to measure the carbon isotope ratio of CO2 released by respiration (delta(r)) within forest canopies at different times du ring the growing season and to use this information to estimate forest ecosystem carbon isotope discrimination. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the s outhern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem-Atmosphere Study (BOREAS). The delta(r) values, calculated from measurements of c hange in the concentration and carbon isotope ratio of atmospheric CO2 in air samples collected at night, ranged from -28.1 parts per thousa nd to -25.9 parts per thousand with an average (+/- s.d.) of -26.8 par ts per thousand +/- 0.5 parts per thousand. There was good correlation between calculated delta(r) values and measurements of (1) the carbon isotope ratio of CO2 released directly from the soil and (2) the delt a(13)C values of foliage collected from the dominant tree species at e ach site. Carbon isotope discrimination during photosynthetic gas exch ange (Delta(A)) by each forest ecosystem was estimated as the differen ce between the carbon isotope ratio of atmospheric CO2 at the top of t he canopy (delta(a)) and the isotopic composition of respired CO2: Del ta(A) = delta(a) - delta(r) All three of the major forest types had si milar values of Delta(A), with an average (+/- s.d.) of 19.1 parts per thousand +/- 0.5 parts per thousand. However, a seasonal change in fo rest discrimination was observed for aspen forests in both the norther n and southern study areas, with an increase in Delta(A) occurring bet ween the middle and end of the growing season. In contrast, the evergr een conifer canopies exhibited relatively constant discrimination valu es throughout the active growing season.