Sheep vegetation management: effects of grazing on tree competition in regenerating cutblocks

Effective vegetation management is crucial for the establishment and growth of tree seedlings in regenerating cutblocks. Glyphosate-based herbicides are the most widely used vegetation management practice in forestry in Alberta and worldwide. However, environmental and social concerns regarding herbicides have motivated the search for alternative vegetation control methods. This study evaluated the use of sheep grazing for vegetation management in cutblocks planted with white spruce and lodgepole pine in Alberta. Grazing trials were conducted in Forest Management Agreement areas in 2023 and 2024, and data collected on vegetation biomass, tree competition, and tree condition in paired plots (grazed and fenced) were used to analyze the effect of grazing treatments. Results of this study indicate that grazing significantly reduced competition; mean competition index values in grazed plots were consistently lower than control treatments. Analyzing competition from specific vegetation groups showed that grazing may not be effective for all vegetation types, as competition index means for shrubs and deciduous trees were similar between treatments. Vegetation biomass was also significantly lower for grazed treatments in all blocks, further supporting findings of reduced competition. The amount of biomass removed by sheep was proportional to the initial vegetation in a cutblock, suggesting that vegetation biomass after grazing depends on initial site conditions. Across the study, approximately 4.53% of seedlings were trampled by sheep, but no browsing was observed on white spruce or lodgepole pine seedlings. These findings suggest that sheep grazing can effectively reduce vegetation competition in regenerating forests, particularly for forbs and grasses, though long-term monitoring of tree response will further our understanding of grazing effects on seedling growth and survival.