In Canada’s boreal forest region, there is increasing demand for practical regeneration strategies that will recreate mixed stands of white spruce ( Picea glauca (Moench) Voss) and trembling aspen ( Populus tremuloides Michx.). In 2002, we implemented an experiment in both west-central Alberta and northeastern Ontario to better understand the effects of herbaceous and woody vegetation control on crop tree survival and growth, within the context of prescription development for the regeneration of a single-cohort, intimate mixture of spruce and aspen. After five growing seasons, good spruce growth, health, and survival were observed with 2 m radial treatments consisting of herbaceous and woody (i.e., complete) vegetation control centred on trees planted at 5 m spacing. These spruce were 4%–64% taller and 68%–178% larger in stem diameter than untended trees, leading to 167%–1166% gains in stem volume, and were at least equivalent to the same stock grown at 2.5 m spacing and provided with complete, continuous relief from competition. Removing only the woody vegetation within treated radii stimulated herbaceous competition, resulting in reduced survival and growth of spruce and reduced height of surrounding aspen. Early results suggest that spot treatments that provide 2–4 years of relief from herbaceous and woody competition may offer a practical strategy for growing spruce with aspen.
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 Douze blocs de régénération forestière de l'Alberta, situés dans des stations représentatives des peuplements mélangés nordiques d'épinette blanche (Picea glauca (Moench) Voss) et de peuplier faux-tremble (Populus tremuloides Michx.), plantés avec de l'épinette blanche et dégagés de façon opérationnelle avec de l'herbicide à base de glyphosate, ont été mesurés au cours de l'automne 2002. La structure et la composition du peuplement ont été mesurées et comparées en fonction des portions traitées et non traitées de chaque bloc. Le Mixedwood Growth Model (MGM, Département des Ressources renouvelables de l'Université de l'Alberta) a été utilisé pour projeter ces peuplements sur une période de 100 ans et pour modéliser les retombées de certains traitements sylvicoles supplémentaires qui pourraient être appliquées à ces blocs. Un traitement de dégagement unique a résulté en des gains de 17 % et de 43 % au niveau respectivement de la hauteur et du diamètre des épinettes blanch...
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