Abstract
Glyphosate herbicide is widely used to control bluejoint reedgrass (Calamagrostis canadensis (Michx.) Beauv.), trembling aspen (Populus tremuloides Michx.), and other competing species in regenerating white spruce (Picea glauca (Moench) Voss) plantations in Alberta, Canada. In 2004, we initiated a study to examine the effects of the aerial application of glyphosate herbicide on plant community diversity and tree growth near Calling Lake, Alberta. Four treatments were applied: (a) no treatment (control); (b) herbicide application in the first growing season after harvesting; (c) herbicide application in the third growing season after harvesting; and (d) herbicide application in the second and fourth growing seasons after harvesting (two treatments). After 11 growing seasons, species richness was not significantly affected by treatment, while Shannon and Simpson index values were highest in areas treated with herbicide in the first growing season. Herbicide treatment did not have a significant effect on the cover of bluejoint reedgrass after 11 growing seasons, but did significantly reduce trembling aspen and paper birch cover and height. Application of glyphosate in the second and fourth growing seasons resulted in the greatest reductions to aspen cover and height, as well as significant increases in spruce diameter at age 11. Simulations with the Mixedwood Growth Model indicate that all tested herbicide treatments will reduce aspen volume while increasing spruce volume at age 90, with the largest impacts evident where two treatments were applied.
Highlights
Management of competing vegetation is widely practiced in regenerating Canadian forests because it has been repeatedly shown to improve survival and growth of planted and naturally regenerated coniferous trees [1]
Our study showed that one glyphosate application in the third growing season after harvesting reduced aspen density by 89% and deciduous tree cover by 81% compared to untreated areas, while application in the first year following timber harvesting resulted in a 52% reduction in aspen densities and a 25% reduction in deciduous tree cover relative to untreated areas
Our study further indicates that later herbicide treatments or two herbicide applications does not result in significant changes in diversity compared to untreated areas
Summary
Management of competing vegetation is widely practiced in regenerating Canadian forests because it has been repeatedly shown to improve survival and growth of planted and naturally regenerated coniferous trees [1]. Mechanical treatments, herbicide application, cutting or pulling, and livestock grazing are all commonly used to provide vegetation control in Canadian forests [1]. Aerial herbicide treatments are often the least expensive option [2] and herbicide treatments generally have been shown to provide more effective reductions in competition and longer duration of control than alternatives [3]. Glyphosate-based herbicides in particular are used widely in many regions of Canada because they provide effective control of a range of shrubs, forbs, and grasses, and can be applied over regenerating and residual conifers at moderate to low rates without causing damage. Use of a broad-spectrum herbicide such as glyphosate that has the potential to kill or suppress many plant species has given rise to concern about potential impacts on plant community diversity and composition in treated forests, as well as possible larger scale impacts to other organisms, the ecological services provided by the forest, and long-term forest productivity [4].
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