© 2007 Heron Publishing—Victoria, Canada
Carbon isotope discrimination and water stress in trembling aspen following variable retention harvesting
Kevin D. Bladon (1), Uldis Silins (1), Simon M. Landhäusser (1), Christian Messier (2) and Victor J. Lieffers (1, 3)
1. Centre for Enhanced Forest Management, Department of Renewable Resources, 442 Earth Sciences Building, University of Alberta,
Edmonton, AB T6G 2E3, Canada / 2. Groupe de recherche en écologie forestiére (GREFI), Département des sciences biologiques, Université du Québec à Montréal,
P.O. Box 8888, Centre-ville Branch, Montréal, QC H3C 3P8, Canada / 3. Corresponding author (victor.lieffers@ualberta.ca) / Received May 8, 2006; accepted October 27, 2006; published online April 2, 2007
Summary
Variable retention harvesting (VRH) has been proposed as a silvicultural practice to maintain biodiversity and ecosystem integrity.
No previous study has examined tree carbon isotope discrimination to provide insights into water stress that could lead to
dieback and mortality of trees following VRH. We measured and compared the carbon isotope ratios (δ13C) in stem wood of trembling aspen (Populus tremuloides Michx.) before and after VRH. Eight trees were sampled from isolated residual, edge and control (interior of unharvested
stand) positions from each of seven plots in three regions (Calling Lake and Drayton Valley, Alberta and Lac Duparquet, Québec).
After VRH, the general trend in mean δ13C was residual > edge > control trees. Although this trend is indicative of water stress in residual trees, it also suggests
that edge trees received some sheltering effect, reducing their stress compared with that of residuals. A strong inverse relationship
was found between the δ13C values and the mean annual precipitation in each region. The trend in mean δ13C signature was Calling Lake > Drayton Valley > Lac Duparquet trees. These results suggest that residual or edge trees in
drier regions are more likely to suffer water stress following VRH. We also observed a trend of greater δ13C in stout trees compared with slender trees, both before and after VRH. The evidence of greater water stress in stout trees
likely occurred because of a positive relationship between stem diameter and crown volume per basal area. Our results provide
evidence that water stress could be the driving mechanism leading to dieback and mortality of residual trees shortly after
VRH. Additionally, the results from edge trees indicate that leaving hardwood residuals in larger patches or more sheltered
landscape positions could reduce the water stress to which these trees are subjected, thereby reducing dieback and mortality.
Keywords:
atmospheric water stress, dieback, mortality, Populus tremuloides, regional differences, slenderness coefficient.
