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Measurements of the fluxes of latent heat λE, sensible heat H, and CO2 were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand-scale, and leaf-scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H + λE)/(Rn - G - B - S) = 0.97. The midday Bowen ratio was typically in the range 1.0–2.5, with an average value of ∼1.9 in the first Intensive Field Campaign (IFCl) and 1.3–1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half-hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching −9 μmol m−2 s−1 in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4 m s−1) reached 6 μmol m−2s−1. The PPFD-saturated biotic CO2 assimilation reached 20 μmol; m−2 s−1 and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO2 in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*<0.4 m s−1. There was a net gain of CO2 on most of the 120 days, but on 31 days of high temperature or low PPFD there was a net carbon loss. High PPFD promoted influx of CO2 by the foliage, whereas high temperatures reduced net CO2 influx through high respiration rates by the roots and soil microorganisms, leading to lower net uptake at high PPFD. Over the 120 day period, 95 g m−2 of C were stored (an average of 0.8 g m−2 d−1), and 237 mm of water evaporated (an average of 2 mm d−1).