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

Abstract

Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration ( E T ), 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 ( E T / E ) under transient canopy wetness states. This study presents field measurements of E T / E at the Montmorency Forest, Québec, Canada: a balsam fir boreal forest that receives ∼ 1600 mm of precipitation annually (continental subarctic climate; Köppen classification subtype Dfc). Half-hourly observations of E and E T 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 E T , especially circumferential and spatial variations, were also quantified. From 30 wetting–drying events recorded during the measurement period in summer 2018, variations in E T / 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 E T / E . During two growing seasons, the average E T / E ranged from 35 % ± 2 % to 47 % ± 3 % . The change in total precipitation was not the main driver of seasonal E T / E variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals.

Quantifying Groundwater Recharge Dynamics and Unsaturated Zone Processes in Snow‐Dominated Catchments via On‐Site Dissolved Gas Analysis

“…(2020a) and Hadiwijaya et al. (2020) and the stable water isotope analyses of this study revealed that sublimation and plant transpiration were negligible in winter and during the snowmelt period, therefore, loss of snow to sublimation was assumed negligible for the calculation of . To calculate , d snow was transformed into snow water equivalent according to Sturm et al.…”

“…The different ET components were not measured separately. However, research by Isabelle et al (2020a) and Hadiwijaya et al (2020) and the stable water isotope analyses of this study revealed that sublimation and plant transpiration were negligible in winter and during the snowmelt period, therefore, loss of snow to sublimation was assumed negligible for the calculation of snow ΔS . To calculate snow ΔS , d snow was transformed into snow water equivalent according to Sturm et al (2010) using the average snow density ( snow  ) observed underneath the flux tower (i.e., 0.335 g/cm 3 ; Parajuli et al (2020a)).…”

Observations of canopy storage capacity and wet canopy evaporation in a humid boreal forest

“…At a particularly wet site of eastern Canada (annual precipitation ≈1600 mm), Isabelle et al (2020) reported that annual E appeared to peak at 550 mm, suggesting that this value could represent the balance between high water availability and low atmospheric demand for water vapour associated with frequent rainfall events. At the same site, Hadiwijaya et al (2020) showed that on a seasonal scale, the E T to E ratio did not exceed 0.47 and that the canopy was frequently wet, again suggesting the importance of E C . At drier sites of the boreal forest (annual precipitation between 450 and 670 mm), intercepted rainfall by the canopy ( I c ) was found to account for 9%–55% of the annual precipitation (Grelle et al, 1997; Lankreijer et al, 1999; Morris et al, 2003; Pomeroy et al, 1999), while E C could represent 23%–25% of the total E (Grelle et al, 1997; Pomeroy et al, 1999).…”

Does Data Availability Constrain Temperature-Index Snow Models? A Case Study in a Humid Boreal Forest

“…)), and black spruce (Picea mariana (Mill.)) [54]. Due to past logging operations, there are several patches of forest clearings accompanied by spatially varying stand structures ( Figure 1).…”

“…Due to past logging operations, there are several patches of forest clearings accompanied by spatially varying stand structures ( Figure 1). For detailed descriptions of the study area, please refer to Parajuli et al [51], Isabelle et al [52,55] and Hadiwijaya et al [54]. turbulent and radiative fluxes between the canopy and the atmosphere.…”