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

Bram Hadiwijaya,Pierre-Erik Isabelle,Steeve Pepin,Daniel F Nadeau

doi:10.3390/f11020237

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.

Highlights

Boreal forests occupy around a third of the world’s forest biomes [1] and represent the second largest vegetated area behind tropical forests [2]
Note that this is the first time that calibration coefficients for the thermal dissipation method are being reported for balsam fir trees
Our calibration curve resulted in flux density (Fd) values that were 5%–23% higher than those reported by Bosch et al [39] and Peters et al [36] for Pinus elliottii, Pinus palustris, and Picea abies at K values ranging between 0 and 0.4

Summary

Introduction

Boreal forests occupy around a third of the world’s forest biomes [1] and represent the second largest vegetated area behind tropical forests [2]. The boreal forest regulates water fluxes over a vast area and impacts global climatology and hydrology [3,4]. The boreal biome is expected to experience a large increase in temperatures [7,8] and a modest increase in precipitation [7]. Evapotranspiration is more sensitive to changes in precipitation than temperature [9], a possible shift in the geographical distribution of conifer tree species due to climate change could alter the composition of boreal forests (e.g., more deciduous tree species) and result in greater evapotranspiration in the summertime [2].

Objectives

Methods

Results

Discussion

Conclusion