Net ecosystem CO2 exchange of mixed forest in Belgium over 5 years

In this paper, we present and discuss the annual net ecosystem exchange (NEE) results from 5 years (1997–2001) of continuous eddy covariance measurements of CO2 flux above a mixed temperate forest. The forest was a 70-year-old coniferous (Scots pine)—deciduous mixture, with slow growth rate and a leaf area index (LAI) of about 3, and was part of the European CARBOEUROFLUX research network. Effects of the data pre-treatment and the gap filling method on annual NEE estimates were analyzed. The u*-correction increased the annual NEE by +61 g C m−2 per year on average. The maximum difference in annual NEE estimates from different gap filling methods amounted up to 130 g C m−2 per year in a year with a large gap in the CO2 flux series. The estimated average annual NEE over the 5 years was + 110 g C m−2 per year (ranging from −9 to 255 g C m−2 per year) when using the most defensible gap filling strategy. We also analyzed the inter-annual variability of carbon balance, which was found to be mainly dependent on the length of the growing season and on the annual temperature. The observation that this forest acted as a CO2 source contrasts with previous results from most other temperate forests.