Discriminating net ecosystem exchange between different vegetation plots in a heterogeneous forest

A model describing the half-hourly evolution of the net ecosystem exchange of a heterogeneous forest was developed. It viewed the forest as a patchwork of three homogeneous vegetation plots whose contribution varied with wind direction. The model was calibrated on eight (1997–2004) years of measurements made at the Vielsalm experimental site in Belgium. The first 6 years were used for model calibration, the last two for validation. The model predicted the eddy flux measured by the system with a degree of performance comparable with those of other models running on the same time scale on homogeneous canopies. The model also allowed the three ecosystem behaviours to be differentiated: the beech characterised by higher carbon sequestration efficiency during the growth period; but also by a shorter growth period, the Douglas fir and the spruce/silver fir characterised by a longer growth period, with the efficiency of the former higher than the latter. The evolution with wind direction of the beech forest contribution (i.e., the relative contribution of the beech plot to the total measured flux) was also obtained and was found to be in very good agreement with footprint predictions on average. However, on a half-hourly scale the agreement between observed and predicted beech forest contributions was not so good. In particular, it was found that the predictions made by footprint models of the variations due to longitudinal footprint changes were not observed by the experimental system.