- Investigation of high nighttime CO2-fluxes at the Wetzstein spruce forest site in Thuringia, Germany (2008)
- Nighttime fluxes of CO2 are uncommonly high at the Wetzstein spruce site, in Thuringia, Germany (Anthoni et al., 2004). As a result, the annual sums of CO2- flux obtained for this site are not comparable with the ones obtained at similar spruce ecosystems (Rebmann, 2004; Grünwald and Bernhofer, 2007). The site is located on a hill and subject to strong winds and high levels of turbulence for continuous periods. The unusual high fluxes of CO2 are predominantly associated with these situations. As reported by Anthoni et al. (2004), measurements of soil respiration performed with automatic chambers in 2003 did not explain such high ecosystem respiration. Also, recent results of soil respiration have revealed that the ecosystem respiration Reco should be comparable to the value obtained for the similar spruce site of Waldstein/Weidenbrunnen (Rebmann, 2004). According to the modeling work of Finnigan and Belcher (2004), the flow over low hills as the Wetzstein site disturbs the wind field at the sub-canopy space across the ridge. As a consequence, advective fluxes of scalars as CO2 are generated at the upwind and lee sides of the hill. Two results from the model were confirmed for the first time in an experiment, namely (1) higher wind speeds below the canopy at the slope, when compared with the same level at the crest, and (2) the flows above and below the canopy in opposite direction at the lee side of the hill. Besides the agreement of the measured wind field with the model results, the existence of horizontal advection was confirmed by Feigenwinter et al. (2007), using data from the advection experiment ADVEX. However, the sign of the advective flux was positive on average, meaning that CO2 was flushed out of the control volume enclosed by the additional towers. This result was not directly related to the high nighttime fluxes of CO2, as these situations were rarely observed during the experiment. However, it has shown that sub-canopy flows exist at the site and should be further investigated. The analysis of several cospectra of the vertical flux of CO2 have shown that the frequencies, or time scales, associated with the high nocturnal fluxes are mainly the turbulent ones, typically observed for nighttime turbulent regimes. The high turbulent activity observed during neutral situations leads then to enhanced turbulent fluxes of CO2. High nocturnal fluxes of CO2 were related to specific micrometeorological conditions, namely high values of friction velocity, southwesterly and northeasterly winds, and neutral stratification. These conclusions were used in a gap-filling approach as annual sums of CO2 were calculated (Rebmann et al., in preparation). As a result of this new approach the site presented a higher uptake of CO2 by the vegetation. Additionally, the newly found results have shown that the uptake of CO2 has been increasing since 2003, caused by factors as the improved growth rate induced by a thinning operation (after snow damage) in 2002, and the recovery from the drought of 2003. The tendency observed in the annual sums of CO2-flux indicates that the site may be in a transition period after the disturbances described above. The proposed removal of unusually high values of CO2-fluxes (with objective criteria) for the determination of annual sums should be also applied in future analysis, in order to capture changes in uptake rates. In this case the CO2 balance for the Wetzstein site would be more comparable to similar ecosystems as Waldstein/Weidenbrunnen (Rebmann, 2004) and Tharandt (Bernhofer et al., 2003; Grünwald and Bernhofer, 2007).