Two years of continuous CO2 eddy-flux measurements over a Danish beech forest

As part of the EUROFLUX network a long-term monitoring station for fluxes of CO2 and water vapour has been established in an 80-year old beech forest in Denmark. The station has been in continuous operation since June 1996 and will be so at least to the end of 2002. A primary goal of EUROFLUX is to combine flux measurements on a continuous multi-year time basis with ecological processes interpretation and modeling. The station consists of a 57 m high mast with conventional meteorological profile instrumentation and one level of eddy-flux measurements. Ancillary measurements such as soil respiration, soil moisture, soil temperature, leaf surface temperature and leaf area index are also made. Results from the first 2 years of measurements are described. The observed diurnal and seasonal variation in the fluxes are discussed and the monthly and annual sums of ecosystem exchange are contrasted between the 2 years. The first year (1 June 1996–31 May 1997) had a net uptake of 223 g C m−2 whereas the net uptake during the second year (1 June 1997–31 May 1998) was only 144 g C m−2. It was found that the difference was mainly the result of higher respiration in the second year caused by a very warm summer of 1997 and an average soil and air temperature, respectively, 1.2 and 1.6°C higher than the first year. The ecosystem photosynthetic assimilation was slightly higher during the second year, mainly caused by increased incoming radiation.