EARLY RESPONSES OF NORTHERN BOREAL VEGETATION TO POWER LINE RIGHT-OF-WAY MANAGEMENT TECHNIQUES INCLUDING THE ACUTE TOXICITY OF IMAZAPYR AND TRICLOPYR TO NON-TARGET PLANTS

Integrated Vegetation Management (IVM) techniques on power line rights-of-way (ROWs) have successfully reduced environmental and economic costs of vegetation management by selecting techniques that facilitate the establishment of stable, low-growing plant communities. To test whether IVM principles can be applied to ROWs in northern Canada, I investigated the impacts of eight management strategies on plant communities in Yukon, Canada. Because forestry herbicide applications are not common in Yukon, I also examined the acute toxicity of imazapyr and triclopyr to non-target plants in standardized greenhouse tests with common boreal herbs. For treatments, triclopyr and imazapyr were each applied by three methods: broadcast spray, cut stump and point injection. Additional treatments were mechanical mowing or cutting target species and seeding native grasses. Vegetation cover surveys were completed before treatments and repeated after one year along with visual herbicide damage assessments. ROW plant communities were significantly altered by management methods one year after treatment, but clear directional changes were not yet evident. Herbicide treatments were more effective at target species control than mechanical methods. All treatments caused a minor reduction in non-target species cover. Imazapyr applications caused more damage to non-target species than triclopyr. Other treatment impacts were life form (e.g. shrub, forb, etc.) or species-specific. Vegetative vigour tests and seedling emergence and seedling growth tests in five ROW soils were used to assess toxicity of both herbicides to Achillea millefolium and Chamerion angustifolium. Test results supported field findings: imazapyr was more toxic than triclopyr. Foliar Inhibition Concentration (IC)50 estimates were 0.7 and 1.2% of the maximum imazapyr application rate vs. 31% for A. millefolium and triclopyr (C. angustifolium’s could not be calculated). Soil applied triclopyr caused IC50 estimates of 2-20 µg g-1 and imazapyr IC50 estimates were <2 µg g-1. Generally, each species was similarly sensitive to each herbicide and each herbicide was similarly toxic in each soil. A. millefolium performed well as a test organism in both tests. The differences in life form/species responses to treatments strongly suggest that shifts in plant community development have been initiated. Imazapyr’s high phytotoxicity and persistence in soil indicates the herbicide is not a suitable product for northern ROWs if maintaining non-target vegetation is a management priority. An additional study on triclopyr dissipation in plant tissue found >50% of residues remained after 30 days and indicates further research into triclopyr dissipation and risks to wildlife in northern ecosystems is needed