The Dynamics of Transpiration to Evapotranspiration Ratio under Wet and Dry Canopy Conditions in a Humid Boreal Forest

Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration (ET), as a component of evapotranspiration (E), behaves differently under wet and dry canopy conditions, yet very few studies have focused on the dynamics of transpiration to evapotranspiration ratio (ET/E) under transient canopy wetness states. This study presents field measurements of ET/E at the Montmorency Forest, Quebec, Canada: a balsam fir boreal forest that receives similar to 1600 mm of precipitation annually (continental subarctic climate; Koppen classification subtype Dfc). Half-hourly observations of E and ET were obtained over two growing seasons using eddy-covariance and sap flow (Granier's constant thermal dissipation) methods, respectively, under wet and dry canopy conditions. A series of calibration experiments were performed for sap flow, resulting in species-specific calibration coefficients that increased estimates of sap flux density by 34%+/- 8%, compared to Granier's original coefficients. The uncertainties associated with the scaling of sap flow measurements to stand ET, especially circumferential and spatial variations, were also quantified. From 30 wetting-drying events recorded during the measurement period in summer 2018, variations in ET/E were analyzed under different stages of canopy wetness. A combination of low evaporative demand and the presence of water on the canopy from the rainfall led to small ET/E. During two growing seasons, the average ET/E ranged from 35%+/- 2% to 47%+/- 3%. The change in total precipitation was not the main driver of seasonal ET/E variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals.