Differential transpiration by three boreal tree species in response to increased evaporative demand after variable retention harvesting

We compared the change in microclimate and tree water relations between a boreal mixedwood, variable retention (VR), partially harvested stand and an adjacent, unharvested control stand. A nearly three-fold increase in potential evapotranspiration (ETP) at the crown level in the VR site was approximately proportional to a 2.8-fold increase in wind speed (u), along with subsidiary increases in net radiation (Q*) and vapour pressure deficit (D) after harvesting. Soil volumetric moisture content (θv) also increased, while there were negligible changes in air temperature (Ta) and relative humidity (RH) after partial harvesting. Whole-tree sap flow response to cutting was measured in white spruce (Picea glauca), balsam poplar (Populus balsamifera) and paper birch (Betula papyrifera) with thermal-dissipation sap flow sensors. After partial harvesting, transpiration per unit leaf area (Ql) in all three species began earlier in the morning and extended later in the day in VR trees than control trees. Mean maximum sap flow rates per leaf area (Ql-max) during mid-day for P. glauca in the VR site were about 2.5 times greater than in the control trees, while B. papyrifera Ql-max was approximately 1.6 times higher in the VR site. For P. balsamifera, however, Ql-max was only marginally greater in the VR site than in the control. This suggests stomatal closure by the P. balsamifera and B. papyrifera residual trees likely occurred to prevent excessive xylem cavitation. Xylem pressure potential (Ψx) measurements of twigs also indicated water stress in both P. balsamifera and B. papyrifera in the VR stand, but not for P. glauca. However, similarity of sapwood hydraulic conductivity (KΨ) between the two sites, for all three species, showed that xylem cavitation of the main stems was insignificant following VR harvesting. Decoupling coefficients (Ω) indicated that all three species were more coupled to the atmosphere in the VR site than in the control. Species differences in susceptibility to atmospheric moisture-stress induced cavitation, combined with differences in stress-coping mechanisms and physiology, appeared to influence the response in transpiration to increased ETP following VR harvesting. Species susceptibility to atmospheric moisture-stress due to increased ETP following partial harvesting can be ranked as: P. balsamifera > B. papyrifera > P. glauca.