Seasonal Gas Exchange Of Black Spruce Using An Automatic Branch Bag System

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).