Footprint-adjusted net ecosystem CO2 exchange and carbon balance components of a temperate forest

We combined eddy covariance measurements of CO2 exchange with a suite of ecological methods to construct the carbon balance of a mixed coniferous–deciduous forest in northern Belgium. The CO2 flux measurements were footprint-corrected to eliminate all fluxes originating from outside of the study site, and the ecological measurements were up-scaled by weighting the main vegetation types by their relative contribution to the footprint of the CO2 flux measurements. The footprint-corrected annual net ecosystem exchange (NEE) was much lower than previously published u*-corrected NEE. Annual NEE ranged from −1.1 to 1.1 t(C) ha−1 year−1, and the forest ecosystem was a moderate CO2 sink with a mean annual rate of −0.3 t(C) ha−1 year−1 over the investigated period (1997–2002). In 2001–2002, the mean NEE was −1.0 t(C) ha−1 year−1. However, despite this net CO2 sink, the forest was losing carbon because carbon export via wood harvesting amounted to 1.2 t(C) ha−1 year−1. Also in 2001–2002, gross primary productivity (GPP) calculated from the eddy covariance data was estimated to be 10.4 t(C) ha−1 year−1. Thus, of the photosynthetically absorbed CO2, 90% was offset by respiration by plants and heterotrophs. The net primary production (NPP) in the effectively contributing forest ecosystem amounted to 5.5–5.8 t(C) ha−1 year−1. Therefore, the NPP/GPP ratio was slightly higher than the previously assumed fixed ratio of 0.47. These results highlight the importance of including management-related carbon fluxes and of applying footprint corrections in carbon-balance studies.