Growing season water balance at a boreal jack pine forest

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer-term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as Q(E), latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2-4 mm s(-1)) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single-variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with Q(E) for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced Q(E) relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen-covered surface in the overall water vapor exchange; subcanopy Q(E) was 26% of the total.