Stress and mortality of four boreal tree species following variable retention harvesting

Variable retention (VR) harvesting involves the preservation of live and dead trees within the boundaries of the harvested area, to emulate the distribution of trees following wildfire or other natural disturbances. The living remnant forest structures are believed to provide the regenerating stand with a biological legacy from the previous stand to achieve a broad array of forest management objectives, most importantly, maintenance of biological diversity and ecosystem integrity. Recent observations of VR cutovers have shown that retention trees often die shortly after the harvest, raising concerns that some of the objectives of VR could be compromised. Thus, the purpose of this research was to quantify the mortality rates of white spruce ( Picea glauca (Moench) Voss), trembling aspen (Populus tremuloides Michx.), balsam poplar (Populus balsamifera L.) and paper birch (Betula papyrifera Marsh.) residuals in boreal mixedwood VR cutovers. Mortality rates in VR plots were 2.5- to 4-fold greater than in control plots for all species. I hypothesized that atmospheric water stress, due to the abrupt change in microclimate following harvest, was the causal reason for this mortality. Accordingly, this study also examined the microclimate before and immediately after VR harvesting, and investigated the differential species responses in transpiration rates (Q 1). Nearly 3-fold increases in potential evapotranspiration (ETP) in the VR site produced greater Q1 for spruce (2.5-times) and birch (1.6-times), but only marginal increases in Q1 for poplar. This suggested stomatal closure by the poplar and birch residuals, likely to prevent xylem cavitation and diminished hydraulic conductivity. Total water potential (Psi) of twigs also indicated water stress in poplar and birch in the VR site. Measurements of the carbon isotope ratios (delta13C) in stem wood from aspen before and after VR provided additional evidence of atmospheric moisture stress in residual trees. The delta13C was also found to be greater in trees from drier regions, indicating that atmospheric water stress in residuals, potentially leading to dieback and mortality, is likely to be a greater issue in drier regions. Species differences in susceptibility to cavitation, combined with differences in stress-coping mechanisms and physiology, appeared to influence the response in Ql and delta 13C to elevated ETP following VR harvesting.