As it was proposed by Baldocchi et al. (1996), direct and long term carbon dioxide flux measurmements are needed to clarify the role of the different ecosystems located under different climatic conditions in the global carbon cycle. Current advances in micrometeorology allow us to conduct long term uninterrupted turbulence measurements (AmeriFlux, EUROFLUX, AsiaFlux and others, see the FLUXNET website, 2001, e.g. Wofsy et al., 1993; Vermetten et al., 1994; Valentini et al., 1995, 1996, 2000; Grace et al., 1995, 1996; Black et al., 1996; Baldocchi et al., 1996, 1997; Grelle and Lindroth, 1996; Goulden et al., 1996; Greco and Baldocchi, 1996; Hensen et al., 1997; Grelle, 1997; Aurela et al., 1998; Hollinger et al., 1998; Lindroth et al., 1998; Saigusa et al., 1998; Anthoni et al., 1999; Yamamoto et al., 1999; Malhi et al., 1999; Yi et al., 2000; Baldocchi et al., 2000; Aubinet et al., 2000; Markkanen et al., 2001) even during conditions when the classical assumptions for turbulent flux measurements are not fulfilled (McMillen, 1988; Grelle and Lindroth, 1996; Moncrieff et al., 1997).
Therefore, a measuring system for the flux estimation by means of the eddy covariance (EC) method was also installed on the tower. The system was put into operation in April, 1997.
The height for the turbulence sensors was chosen to satisfy the few requirements: the measurement should be representative to a larger region (regional scale), so the sensors are supposed to be mounted as high as possible, but still low enough to be inside the surface layer during unstable conditions, which is approximated as the lower 10% of the height of the convective boundary layer (100-150 m). Because of further technical reasons the height was chosen to be 82 m.
The region and the method would let us determine fluxes representative to a single species (e.g. agricultural crops) using data selection based on source area models (Schmid, 1994, 1997), but the strategy applied is to determine net ecosystem exchange which is representative to the whole region with mixed vegetation. Thus the ensemble fluxes will reveal the carbon exchange of a region with mixed vegetation and therefore the net value gives us a chance to extend the validity of the calculations to a larger region, of course with critical evaluation.