Four large cuvettes were used to measure whole-branch CO2 and H2O vapour exchange at the Boreal Ecosystem-Atmosphere Study, Southern Study Area, Old Black Spruce site. Measurements started before the spring thaw and continued until after the winter freeze-up. Daytime photosynthesis, nighttime respiration, transpiration rate, and branch conductances were zero at the start and the end of the measurement period. Maximum photosynthetic uptake rates were ca. 6 µmol·m-2·s-1 (0.02 µmol·g-1·s-1). Maximum nighttime respiration was ca. 0.8 µmol·g-1·s-1. Maximum transpiration rates were ca. 2 mmol·m-2·s-1 (7.1 µmol·g-1·s-1). Maximum branch conductance was ca. 35 mmol·m-2·s-1 (0.123 mmol·g-1·s-1). A simple model combining light and temperature responses accounted for 80-90% of the day-to-day variation in daytime carbon uptake. Total annual daytime carbon uptake ranged from 151 to 271 g·m-2 (530-868 mg·g-1). Total nighttime loss ranged from 12.7 to 15.5 g C·m-2 (22.4-27.3 mg C·g-1). Total transpiration ranged from 20.4 to 35.0 kg H2O·m-2 (70.2-112.2 g H2O·g-1).