Buchmann, N., T.M. Hinckley, and J.R. Ehleringer. 1998. Carbon isotope dynamics in Abies amabilis stands in the Cascades. Can. J. For. Res. 28:808-819.

Carbon isotope ratios (d13C) 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 d13C were strongly related to stand structure. Within the old-growth stand, both daytime canopy [CO2] and canopy d13C 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 d13C 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. d13C of soil-respired CO2 stayed relatively constant at -24.55±0.20% during the growing season. Significant relationships existed between canopy d13C and 1/[CO2] in all three stands. Using the intecepts of these regressions, we calculated an average d13C for ecosystem respiration of -26.4±0.1%. Ecosystem carbon istope discrimination (De), an integrating measure for carbon exchange between the troposphere and the entire ecosystem, stayed relatively constant through time. De showed no significant stand structure effect (leaf area index, density) and averaged 18.9% for the old-growth and 19.2±0.2% for the two younger stands.