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

• Main Authors: Bram Hadiwijaya, Steeve Pepin, Pierre-Erik Isabelle, Daniel F. Nadeau
• Format: Article
• Language: English
• Published: MDPI AG 2020-02-01
• Series: Forests
• Subjects: boreal forest; eddy-covariance; evapotranspiration; sap flow; transpiration; leaf wetness; interception
• Online Access: https://www.mdpi.com/1999-4907/11/2/237

Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration (<inline-formula> <math display="inline"> <semantics> <msub> <mi>E</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula>), as a component of evapotranspiration (<i>E</i>), behaves differently under wet and dry canopy conditions, yet very few studies have focused on the dynamics of transpiration to evapotranspiration ratio (<inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula>) under transient canopy wetness states. This study presents field measurements of <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula> at the Montmorency Forest, Qu&#233;bec, Canada: a balsam fir boreal forest that receives <inline-formula> <math display="inline"> <semantics> <mrow> <mo>&#8764;</mo> <mn>1600</mn> </mrow> </semantics> </math> </inline-formula> mm of precipitation annually (continental subarctic climate; K&#246;ppen classification subtype Dfc). Half-hourly observations of <i>E</i> and <inline-formula> <math display="inline"> <semantics> <msub> <mi>E</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula> were obtained over two growing seasons using eddy-covariance and sap flow (Granier&#8217;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 <inline-formula> <math display="inline"> <semantics> <mrow> <mn>34</mn> <mo>%</mo> <mo>&#177;</mo> <mn>8</mn> <mo>%</mo> </mrow> </semantics> </math> </inline-formula>, compared to Granier&#8217;s original coefficients. The uncertainties associated with the scaling of sap flow measurements to stand <inline-formula> <math display="inline"> <semantics> <msub> <mi>E</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula>, especially circumferential and spatial variations, were also quantified. From 30 wetting&#8722;drying events recorded during the measurement period in summer 2018, variations in <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula> 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 <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula>. During two growing seasons, the average <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula> ranged from <inline-formula> <math display="inline"> <semantics> <mrow> <mn>35</mn> <mo>%</mo> <mo>&#177;</mo> <mn>2</mn> <mo>%</mo> </mrow> </semantics> </math> </inline-formula> to <inline-formula> <math display="inline"> <semantics> <mrow> <mn>47</mn> <mo>%</mo> <mo>&#177;</mo> <mn>3</mn> <mo>%</mo> </mrow> </semantics> </math> </inline-formula>. The change in total precipitation was not the main driver of seasonal <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>E</mi> <mi>T</mi> </msub> <mo>/</mo> <mi>E</mi> </mrow> </semantics> </math> </inline-formula> variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals.