Measuring CO2 flux adequately at night is much more difficult than during daytime because of frequent light wind conditions. Indeed, micrometeorological techniques based on turbulence are underestimating CO2 flux during calm nights. This was observed for example over an old black spruce forest during the growing season of the BOReal Ecosystem Atmosphere Study (BOREAS) in 1994, as well as over agricultural fields. Vertical mixing is often strong during daytime periods, while the nighttime values are much lower and highly variable between nights. We found that very little CO2 flux was detected for forest canopy at the level of the instruments (12 m above displacement distance), when the standard deviation of the vertical wind velocity (sW) was below 0.1 m s-1. This implies that the canopy respiration rate was not measured adequately by the eddy-covariance technique during those periods. The CO2 flux was highly linearly correlated to the nighttime averaged sW, which demonstrates the underestimation of the CO2 flux measured by eddy-covariance when sW is low. To determine the carbon budget of the forest stand, the respiration rate should be estimated either by using the nighttime CO2 eddy fluxes during windy conditions (sW > 0.4 m s-1) or possibly by measuring the CO2 storage term during calm nights. The CO2 storage term is equal to the vertical integration of the CO2 concentration change over time, from the forest floor up to the instrument height. However, the storage term is often measured at only one site, which under light wind conditions represents a small area of the vegetation. Indeed, when there is little mixing within the canopy, the change in the concentration profile is most affected by local sources, and the footprint of each sampling point increases with height. Moreover, for nights having sporadic turbulence, adding the storage term measured at one site to the eddy flux cannot provide realistic nighttime respiration estimates. These factors tend to discredit the approach of adding a storage term based on one profile to the eddy flux measured at night. It seems to be