6C.6
Fluxes of the stable carbon isotope 13C above a spruce forest measured by hyperbolic relaxed eddy accumulation method
Bodo Wichura, University of Bayreuth, Bayreuth, Germany; and N. Buchmann and T. Foken
Eddy covariance measurements of carbon dioxide fluxes provide information about the net C balance of an ecosystem, i.e. photosynthetic and respiratory CO2 fluxes. Both processes affect the isotopic composition of carbon dioxide, CO2, in turbulently exchanged air. Thus, flux measurements for stable carbon isotopes give insight in processes within the ecosystem. We combined the eddy covariance method for CO2 fluxes with a new technique, the hyperbolic relaxed eddy accumulation method (HREA), to measure turbulent fluxes of the stable carbon isotope 13C. By analyzing the mass balance equations for d13C and CO2, we derived a method for the evaluation of combined 13C and CO2 flux measurements and gained process information about the net ecosystem CO2 exchange of a spruce forest in Bavaria, Germany.
d13C values of CO2, measured by HREA for up- and downdraft CO2 samples above the canopy, ranged between -8.29 and -7.51 ‰ (vs. PDB). The differences in d13C values between up- and downdrafts varied between -0.16 to 0.17 ‰. At noon, the d13C values for updrafts were more positive, due to fractionation during net ecosystem photosynthetic assimilation. During the morning and the afternoon, the d13C values of updraft CO2 were more negative due to net ecosystem respiration. d13C values of downdraft CO2 were more negative in the morning than during daytime. Downdraft d13C showed only a slight daily trend, due to high turbulent mixing of the boundary layer air. Both, the daily variation and the magnitude of the 13C fluxes did depend on environmental constraints of ecosystem activity as well as on turbulent exchange processes. Our results for a temperate spruce forest showed a lower ecosystem isotopic fractionation compared to that of a deciduous temperate forest in Tennessee (Bowling et al., 1999). Both results show the feasibility of 13C flux measurements by the HREA technique above forest ecosystems.
Session 6C, Vegetative Canopies
Friday, 11 August 2000, 2:15 PM-4:45 PM
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