Katherine Johnson, Univeristy of Alberta
In south-eastern British Columbia, suppression of wildfire has facilitated forest ingrowth in open forests and grasslands. However, several weed species are known to increase with burning. Recent increases in the infestations of invasive species such as the broadleaf forbs St. John’s Wort (Hypericum perforatum) and ashy cinquefoil (Potentilla inclinata) and the annual invasive grass downy brome (Bromus tectorum) have put ecosystem restoration projects involving prescribed burning at risk. Information on the interaction between prescribed burning and herbicide application are needed to assess the feasibility of prescribed burning in these environments.
The study examined burning and herbicide application at two locations in the East Kootenay Trench, Kikomun Creek Provincial Park (KCPP) and Yaq ̓itʔa·knuqⱡi’it (YFN). Three site types were assessed, with KCPP sites classified as either pinegrass site type or needlegrass site type, and YFN assessed as a separate site type. In July 2023 KCPP was treated with two broadleaf herbicides (HerbicideR and HerbicideC). In July 2024 YFN was treated with one broadleaf herbicide (HerbicideR) and in September 2024 with one pre-emergent herbicide (HerbicideP). A prescribed burn took place in KCPP in April 2024 and at YFN in April 2025. Foliar cover of species was assessed using nested frames.
Impacts on native species were variable between sites when comparing herbicide treatments to control at a p=0.05 confidence level. Herbicide application variably affected both broadleaf invasive species and native species at all sites. At a P level of 0.05, shrubs declined under HerbicideR in one of two site types where they were present. Broadleaf invasive species declined under HerbicideR and HerbicideC in two of three site types where they were present. Annual invasive grasses did not decline under any herbicide treatment. Herbicides negatively affected native forbs at one of three site types. Burning reduced shrub cover at both sites where shrubs were present. Burning reduced bunchgrasses at both site types where they were present but did not affect other desirable native vegetation. There was no significant interaction between burning and herbicide application for any vegetation group. Results suggest that a combination of prescribed burning and herbicide application shows the best potential for controlling forest ingrowth and invasive species in these rangelands while maintaining a healthy native ecosystem, with HerbicideR showing the greatest impacts.