Seasonal patterns of the energy balance of a young jack pine site near Thompson, Manitoba, in the summer of 1994 are reported. The experiment was part of the Boreal Ecosystem-Atmosphere Study (BOREAS), and it ran from May 24 to September 19. The average tree density was 4.4 trees m-2, but there was substantial spatial variation associated with three primary vegetation patterns: dense cover of short trees, sparse cover of tall trees, and a mixture of short and tall trees. The frequency distribution of tree heights was bimodal with peaks at 1 and 2 m and a range from 0.3 to 5.7 m. The average tree height was 2.3 m. The daily average photosynthetically active radiation albedo was conservative, varying from 0.054 under clear-sky conditions to 0.051 under cloudy skies. The shortwave albedo was 0.136 under clear-sky conditions and decreased by 1% following rain; the presence of smoke over the site increased it by 1%. Heat storage in the soil, trees, and air was an important component of the energy balance throughout the season with soil heat flux comprising the bulk of the total storage. On the average, for the whole experimental period, the sensible heat flux was approximately twice the latent heat flux (Bowen ratio ≈2). The average daily energy balance closure varied from 85% before day-of-year (DOY) 200 to 95% after DOY 200. As a result of problems with the measurement of the net CO2 flux above the canopy under stable nighttime conditions, all nighttime values were modeled. The typical diurnal pattern of net CO2 flux shows maximum uptake by the surface in the morning and a gradual decrease through the afternoon. For the whole field season of 118 days, the site fixed 224 g m-2 of carbon. ¿ 1997 American Geophysical Union