PhD Thesis
Untersuchung zum Kohlendioxid-Austausch über einem Fichtenwaldbestand auf der Grundlage von Hyperbolic-Relaxed-Eddy-Accumulation Messungen für das stabile Kohlenstoffisotop 13C und von Wavelletanalysen
Bodo Wichura (09/1998-02/2009)
Support: Thomas Foken
Exchange processes and mechanisms of carbon dioxide (CO2) as well as an approach to separate CO2-flux components (as net photosynthesis and respiration) from net ecosystem exchange were investigated on the basis of combined measurements of turbulent fluxes of the stable carbon 13C isotope and of carbon dioxide (CO2) for a forest ecosystem. 13C-isotope ratios for up- and downdraft events and turbulent 13CO2-iso-fluxes were measured by the use of a Hyperbolic-Relaxed-Eddy-Accumulation (HREA) measuring system in the course of two measurement campaigns in 1999 and 2000 at a the spruce forest site Waldstein/Weidenbrunnen. Differences between 0,168 ‰ to -0,143 ‰ (1999) and 0,391 ‰ to -0,132 ‰ (2000) were determined for isotope ratios ( ) in up- and downdraft events. Results of 13CO2-iso-fluxes varied between 1082 mol‰m 2s-1 to 170 mol‰m-2s-1 for the measurements in 2000. The 13CO2-iso-flux measurements were implemented by a HREA-measurement system. The HREA-measuring method, 13CO2-HREA-measurements and measurement results are specified and discussed. The discussion of the 13CO2-HREA-measuring results, taking into account the results of CO2-fluxes and CO2-profiles, shows that the exchange of carbon dioxide above a spruce forest stems from a superposition of different processes and mechanisms. The change in the atmospheric and/or metabolic conditions influences both the turbulent exchange between forest canopy and atmosphere (coupling/decoupling of the turbulent exchange of CO2 between canopy and atmosphere, in-canopy CO2-storage) and the metabolism of the ecosystem (assimilation, respiration). It results in different consequences for CO2 exchange processes, depending on the boundary conditions. Mechanisms of CO2-exchange between the ecosystem and the atmosphere are studied by the use of wavelet-analysis. The development and the existence of coherent structures are demonstrated. The median of wavelengths between successive coherent eddies has been estimated as 110 m for the time series of vertical wind velocity during the measurement campaign 2000. On the other hand, for the time series of CO2, the median was 832 m. This result indicates that exchange processes for momentum and for CO2 are temporarily decoupled from each other. Furthermore, results demonstrate that the turbulent CO2 exchange between the spruce forest and the atmosphere show properties of a mixing layer only temporarily. The results of wavelet analysis provide information for a consistent interpretation of CO2- and 13CO2-iso-flux measurements. Four mechanisms of CO2 exchange between the ecosystem and the atmosphere are identified by comparison of results from 13CO2-HREA measurements, of CO2-fluxes and of CO2-profiles as well as by findings of wavelet analysis: CO2 is exchanged outside (1), (3), (4), or within (2) a mixing layer. Momentum and CO2 are exchanged in different (1), (3) or comparable (2), (4) time-/length scales. By analyzing mass balance equations for 13C and CO2 the basic principle for the evaluation of combined 13C and CO2 flux measurements is derived. This methodological approach provides a basis for the separation of net assimilation and net respiration out from net ecosystem exchange of CO2 at ecosystem level by the use of simultaneous 13CO2-iso-flux and CO2-flux measurements.