Northern mid-latitude forests make a major contribution to terrestrial carbon uptake. Our ability to predict future changes of northern forest carbon uptake is contingent upon how well we can identify the relative importance of environmental factors in controlling current changes. By analyzing the 1992-2004 record of CO2 flux at a deciduous forest in New England, we found that forest carbon uptake experienced the relatively steady stage in the middle growing season. The steady-stage gross ecosystem exchange (GEE) of CO2 varied from 11.9 to 20.1 μmol m-2 S-1, accounting for 55% - 73% of annual amount over 13 years. We also found that cloudiness, combined with the effect of antecede accumulated precipitation from late January to late March, dominated interannual fluctuations of the stead-stage GEE among other environmental factors. Together. they led to 81% interannual variance of steady-stage forest carbon uptake.