Fluxes of the stable carbon isotope 13C above a spruce forest measured by hyperbolic relaxed eddy accumulation method

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.