Carbon isotope dynamics in Abies amabilis stands in the Cascades

Carbon isotope ratios (delta(13)C) of canopy air and carbon isotope discrimination at the ecosystem level were studied in three montane Pacific silver fir (Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes) stands, an old-growth and two younger stands. Spatial and temporal variations of canopy CO2 concentrations ([CO2]) and their delta(13)C were strongly related to stand structure. Within the old-growth stand, both daytime canopy [CO2] and canopy delta(13)C stayed close to those of the troposphere, either indicating low overall photosynthetic rates or high turbulent mixing. Pronounced periods of photosynthetic drawdown below baseline [CO2] accompanied by more enriched canopy delta(13)C were observed for the two younger and denser stands. Canopy [CO2] profiles seemed closely related to changes in soil conditions. Soil respiration rates were positively related to soil temperature, but negatively to soil moisture. delta(13)C of soil-respired CO2 stayed relatively constant at -24.55 +/- 0.20 parts per thousand during the growing season. Significant relationships existed between canopy delta(13)C and 1/[CO2] in all three stands. Using the intercepts of these regressions, we calculated an average delta(13)C for ecosystem respiration of -26.4 +/- 0.1 parts per thousand. Ecosystem carbon isotope discrimination (Delta(e)), an integrating measure for carbon exchange between the troposphere and the entire: ecosystem, stayed relatively constant through time, a, showed no significant stand structure effect (leaf area index, density) and averaged 18.9 parts per thousand for the old-growth and 19.2 +/- 0.2 parts per thousand for the two younger stands.