Effects of timing of glyphosate application on jack pine, black spruce, and white spruce plantations in northern Manitoba

Deferring conifer release treatments is sometimes necessary, but its effects on crop tree performance are not well understood. We investigated the effects of deferring glyphosate applications on jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana [Mill.] BSP), and white spruce (Picea glauca [Moench.] Voss.) plantations established on a dry site with moderate competition and a moist site with heavier competition in The Pas, northern Manitoba. At each site, experimental treatments included no herbicide and one glyphosate (formulated as Vision (R)) at 1.42 kg acid equivalent glyphosate ha(-1) application between one and five years after planting with three replications. Survival, root collar diameter, and height of the three conifers were repeatedly measured for ten consecutive years following planting. Survival of the three conifers was not affected by glyphosate application. For all the three species, root collar diameter and stand volume of crop trees increased significantly in glyphosate-treated plots, compared to control plots. Varying the timing of glyphosate application between year I and 5 resulted in a similar conifer dimensions. Glyphosate application significantly improved height growth of black spruce and marginally white spruce, but not jack pine. On these sites, glyphosate reduced the cover of woody species, but grass, forbs and shrubs rapidly captured the newly available growing space. A two variable regression analysis revealed that grass cover negatively affected stand volume of all three conifer species. Cover of tau shrub, forbs, and interaction of cover and height of tall shrubs were the next most important variables to explain jack pine, black spruce, and white spruce volume growth, respectively.