Treatment and assessment of the CO2-exchange at a complex forest site in Thuringia, Germany

Eddy covariance measurements were carried out at the Wetzstein site in Thuringia, Germany sinceDecember 2001. Soon after the start of the measurements night-time fluxes well above average CO 2 -fluxes  measured  in  temperate  forest  ecosystems  were  detected  which  could  not  be  explained  bybiological processes but were valid with respect to standard quality criteria. The Wetzstein site is part ofthe CarboEurope-IP flux-network and the CO 2 -exchange of this spruce forest is of general ecologicalinterest as the site is typical for central European spruce forest ecosystems at mountainous elevation.Additional investigations were made in order to identify the causes for the large difference between theflux balance and the inventory based NEP. Specific weather patterns and micrometeorological situationswere identified during which a decoupling of the flows above and below the canopy leads to additionalCO 2 -effluxes at the tower site which are not part of the net ecosystem exchange (NEE) at night. Rejectingdata from these periods and gap-filling thereafter results in yearly sums of NEE, GPP and TER which are inbetter agreement with the biometric measurements at the tower site and comparable to other spruceforest sites. In this process ecosystem respiration was determined not only from extrapolation of night-time data but also from flux partitioning based on day-time data using the hyperbolic light responsefunction. It can be shown that flux measurements at this complex site need to be treated in a modifiedprocedure compared to what is generally performed, namely extrapolating ecosystem respiration fromnight-time data. Using multiple data sources and applying a careful filtering of the data, confidence in theestimates of the carbon balance components increased.