Abstract
Carbon isotope ratios ( delta 13C) 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 13C were strongly related to stand structure. Within the old-growth stand, both daytime canopy [CO2] and canopy delta 13C 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 13C 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 13C of soil-respired CO2 stayed relatively constant at -24.55 ± 0.20%oduring the growing ...
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