Prolonged herbicide-induced vegetation changes in a regenerating boreal aspen clearcut

A soil-active herbicide (hexazinone) was applied (0, 2, and 4 kg/ha of active ingredient) in a 3-year-old regenerating boreal Populus tremuloides Michx. (aspen) clearcut to determine its effect on the compositional and structural development of the vegetation. Woody stem densities and plant foliar cover were evaluated prior to and 2, 6, and 17 years after treatment. Herbicide treatment at the 2 and 4 kg/ha rates reduced tree and total woody stem densities relative to the 0 kg/ha level. The 4 kg/ha level reduced stem densities by 27% 17 years after treatment. The primary reductions occurred in Amelanchier alnifolia (Nutt.) Nutt. ex M. Roemer (saskatoon) and Rosa acicularis Lindl. (wild rose); whereas Corylus cornuta Marsh. (beaked hazelnut) and Viburnum edule (Michx.) Raf. (low-bush cranberry) stem densities increased. Notable herbicide-caused foliar cover reductions at the 4 kg/ha level occurred in Eurybia conspicua (Lindl.) Nesom. (showy aster), Mertensia paniculata (Ait.) G. Don. (tall mertensia), Rubus pubescens Raf. (dewberry), and Spiraea betulifolia Pallas (spiraea), but Aralia nudicaulis L. (sarsaparilla), Cornus canadensis L. (bunchberry), and Symphyotrichum ciliolatum (Lindl.) A.&D. Lve (Lindley's aster) increased. Less distinctive but similar changes occurred in the 2 kg/ha treatment. Total plant cover, species richness, and species dominance concentration were similar among treatments. Eight distinctive forest understory-types were recognized among treatments in Year 17. Between the 0 and 4 kg/ha treatments, five understory-types differed in their frequency of occurrence. Hexazinone did not improve the survival of silviculturally planted Picea glauca (Moench) Voss (white spruce) seedlings relative to untreated sites, but the 4 kg/ha treatment level did increase Pinus contorta Dougl. ex Loud. (lodgepole pine) survival from 12 to 34%. Surviving seedlings had significantly greater height and basal diameter growth than those at the 0 kg/ha sites, particularly the 4 kg/ha treatment.