Short-term environmental controls on carbon dioxide flux in a boreal coniferous forest: model computation compared with measurements by eddy covariance

CO2 eddy flux measurements were coupled with detailed microclimate and soil measurements made in an undisturbed boreal pine forest in the late growing season of 1998 in order to understand the effects of short-term variation in weather on daily canopy CO2 flux. To test whether our present knowledge of leaf-level processes is sufficient to understand stand-level CO2 exchange, a parameterised forest carbon flux model was presented in which the scaling of CO2 exchange from leaf to canopy is achieved by integrating mechanistic models for physiological processes (photosynthesis, stomatal conductance and respirations of the ecosystem components) and micrometeorological ones (radiative transfer, turbulent transfer and surface energy exchanges). Testing of the model against 25 day flux measurements showed that the predicted day-time forest ecosystem CO2 fluxes matched well with those measured by the eddy covariance method but gave a poor estimate for the night-time CO2 flux. Based on 30 min average eddy flux data during the 25 day period, the mean net forest system CO2 uptake was 5.76 μmol CO2 m−2 ground s−1. The maximum rate was 15 μmol m−2 s−1, and the mean and maximum night-time rates were 2.2 and 4.8 μmol m−2 s−1, respectively. Maximum CO2 storage in the air column was 7.3 μmol m−2 s−1 in the early morning and 3.6 μmol m−2 s−1 during the night. The daily total net CO2 exchange for the forest ecosystem varied from 0.06 to 0.47 mol m−2 s−1 in the day-time and from 0.04 to 0.13 mol m−2 s−1 in the night-time. Day-time CO2 uptake in the forest ecosystem was significantly coupled with changes in photosynthetic photon flux density and modified significantly by air temperature and the canopy-air vapour pressure deficit, but it was less dependent on soil water content on most days during the growing season. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and the transfer of radiation through a canopy. The variance of about 60% in the rate of CO2 exchange at night could be explained by changes in air temperature and other factors related to the dynamics of turbulence mixing.