Climatological measurements, including carbon dioxide flux density, were made from May to September 1994 and from May to November 1996 at a young jack pine (Pinus banksiana Lamb.) forest near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). The study periods were warmer and drier than the 24 year climate normals, and in general, 1996 was warmer but wetter than 1994. Volumetric soil moisture above a depth of 8 cm in 1994 was often less than 5%. Albedos for solar radiation and photosynthetic photon flux density (PPFD) were 0.13 and 0.05, respectively, in both years. The Bowen ratio for days of year 144–262 was 1.7 in 1994 and 1.5 in 1996, with maximum daily Bowen ratios from 4 to 6. The range in CO2 flux densities was the same in both years, with maximum uptake values of −0.55 mg m−2 s−1 in midsummer. Average diurnal patterns of CO2exchange for 1994 were similar to those for 1996, characterized by maximum uptake in the morning and a gradual decrease through the afternoon. Rectangular hyperbolic response curves showing CO2 uptake in relation to incident PPFD show that changes between the morning and the afternoon, and because of environmental controls, were as great or greater than changes between designated 3 and 4 week measurement periods. Over a period of 119 days the young jack pine forest accumulated approximately −200 g C m−2 in 1994 and −267 g C m−2 in 1996. Reproduction of CO2 exchange trends will be important for modeling efforts involving northern ecosystems.