CO2 UPTAKE BY A STAND OF DOUGLAS-FIR - FLUX MEASUREMENTS COMPARED WITH MODEL-CALCULATIONS

Fluxes of CO2 were calculated by the gradient method from concentratio n differences, measured in the surface roughness layer above a Douglas fir stand in the Netherlands during a full year (1989). The annual co urse of the CO2 flux density clearly showed the influence of temperatu re and incoming radiation on stand assimilation. Monthly mean fluxes w ere directed towards the stand throughout the year and ranged from 40 kg CO2 ha(-1) day(-1) in winter to 220 kg CO2 ha(-1) day(-1) in summer . Results for 1989 were compared with CO2 uptake by the stand, as esti mated by a forest growth model, after correction for the soil respirat ion flux calculated by the model. For the summer months the simulated CO2 uptake agreed with the measurements within the range of uncertaint y. During winter-time, however, the model predicted a net loss of CO2 to the atmosphere, whereas the measurements still indicated uptake of CO2 by the ecosystem, in spite of the lower temperature and radiation levels. The difference in the winter months may partially be explained by the uncertainty in the annual course of the soil respiration flux estimated by the model. More probably, however, advection from adjacen t deciduous stands and the use of the gradient method beyond its theor etical restraints, may have led to large systematic errors in the flux es during winter-time.