Invasive plants threaten Canada's National Parks, which have a mandate for conservation of native vegetation. In Jasper National Park, Russian thistle (Salsola tragus L.), an introduced annual forb, has invaded Montane grasslands that constitute critical winter range for bighorn sheep (Ovis canadensis Shaw) and other ungulates. A two year study assessed five strategies for controlling Russian thistle: (1) removal of ungulate grazing; (2) spraying with metsulfuron-methyl; (3) manual pulling of Russian thistle plants; (4) broadcast seeding of native plant species; and (5) integrated weed management combining grazing exclusion, herbicide, and seeding. All treatments that included herbicide reduced Russian thistle, but also removed native forbs. Manual removal of thistle plants reduced the cover and density of the weed, but not final biomass, and may be impractical to implement on large areas. Grazing exclusion resulted in a large decline in Russian thistle density and biomass after two years and coincided with recovery of other forbs, suggesting this weed species is responding to competition and could, therefore, be reduced through management of ungulate grazing pressure.
Nomenclature: bighorn sheep, Metsulfuron-methyl, Ovis canadensis Shaw, Russian thistle, Salsola tragus L.
