Juvenile response to conifer release alternatives on aspen-white spruce boreal mixedwood sites.Part I: Stand structure and composition

Abstract

Stand structure and composition for planted white spruce (Picea glauca (Moench) Voss) and other naturally regenerating commercial species were compared seven years after the testing of five conifer release alternatives on three boreal mixedwood sites. No release resulted in aspen (Populus tremuloides Michx.) -dominated stands with 89% stocking and the highest basal areas (BAs, 5.1 m2/ha) and stem volume indices (SVIs, 10.7 m3/ha) observed. Release by manual or machine cutting increased planted spruce BA and SVI by 67 and 38%, respectively. However, this treatment also caused significant root and stump suckering of aspen, more than doubling stem densities and increasing stocking by 12% over untreated areas. Although cutting reduced the height of aspen from 6 m (untreated) to 2–3 m, equal to or just taller than planted spruce, it is likely that future growth will result in deciduous-dominated mixedwoods. Broadcast foliar application with Release® herbicide temporarily reduced the size of aspen, without causing the increased regeneration observed following cutting. This produced a more varied stand structure that promoted the stature of planted spruce, doubling dominant spruce stocking, BAs, and SVIs, and leading to a more balanced mixedwood. Broadcast release with Vision® herbicide produced conifer-dominated stands with few deciduous stems; these areas contained the lowest observed BAs (1.7 m2/ha) and SVIs (1.9 m3/ha). Relatively low planting densities (1350 sph), coupled with near complete deciduous removal in these plots, created very open-grown conditions that threaten overall productivity and stem quality of the spruce. The five approaches tested are capable of producing a range of stand conditions found in a healthy boreal mixedwood landscape. Key words: boreal mixedwoods, white spruce, trembling aspen, vegetation management, fibre production