Juvenile stand responses and potential outcomes of conifer release efforts on Alberta's spruce–aspen mixedwood sites

Abstract

Twelve Alberta forest regeneration blocks, situated on representative white spruce (Picea glauca (Moench) Voss) - trembling aspen (Populus tremuloides Michx.) boreal mixedwood sites, planted to white spruce, and operationally released with glyphosate herbicide, were surveyed in the fall of 2002. Stand structure and composition were quantified and compared for treated and untreated portions of each block. The Mixedwood Growth Model (MGM, Department of Renewable Resources, University of Alberta) was used to project these stands over a 100-year horizon and to model the outcomes of several additional silvicultural treatments that could be applied to these blocks. A single release treatment provided 17% and 43% gains in planted white spruce height and stem diameter, respectively, an average of five years after treatment. Treatment shifted stands from being deciduous-dominated, with only 12% conifer basal area, to more than 75% conifer basal area, increasing conifer volumes per hectare nearly three-fold, but retaining conifer-deciduous mixture. Model projections suggest that these stands will produce similar total volumes over an 80-year rotation and that conifer release essentially trades deciduous volume for conifer volume, the degree of release dictating the extent to which this trade-off takes place. A single conifer release treatment led to an average simulated mature stand that contains 21% deciduous basal area, likely meeting mixedwood rather than conifer regeneration criteria. Model simulations of additional silvicultural interventions in these stands suggested that a variety of options exist to satisfy a range of stand or landscape management objectives for spruce-aspen mixedwoods, all within a relatively fixed volume production envelope. A clearer understanding of how early stand conditions translate into stand and landscape management objectives seems prerequisite to solving management conflicts on boreal mixedwood sites. Key words: boreal mixedwoods, vegetation management, conifer release, Mixedwood Growth Model, white spruce, trembling aspen