Unexpected responses in ecologically based weed management

Spotted knapweed (Centaurea stoebe L.) infestations have threatened western rangelands and grasslands since the late 1800s when it was introduced to North America in ship ballast and alfalfa seed. Traditional invasive plant management has relied on eradicating the plants with herbicide only, rather than addressing the fundamental ecological principles that allow these invasions to establish and re-establish. Field trials were established in the Marquart West pasture at the Laurie Guichon memorial grassland in the Lundbom Commonage approximately 10 km east of Merritt, British Columbia (BC), Canada. Our study used a split-plot experiment design to test combinations of herbicide, wood ash soil amendment and grass seeding to control knapweed and restore desirable grasses in a highly, knapweed invaded rangeland. Herbicide was the main plot, wood ash (0 Mg ha-1, 1 Mg ha-1and 10 Mg ha-1) were the sub-plots and seeded species were Pseudoroegneria spicata, Thinopyrum intermedium and Agropyron cristatum in sub-sub-plots.Results after two years determined that herbicide and ash interactions significantly increased Bromus tectorum, Poa pratensis and Koeleria macrantha. Herbicide significantly decreased spotted knapweed cover, and there were no significant differences in any treatments where herbicide was not present. Herbicide application significantly increased soil nitrate but those results diminished after time. Herbicide treated plots with ash had significantly less plant available nitrogen than the herbicide and no ash plot, suggesting ash did lead to immobilization of nitrate. Seeded grass species did not have any significant establishment during this study; we did see a noticeable increase in P. pratensis and K. macranthawhich were not seeded. This increase was significant only in the herbicide/high ash treatment which suggests that suppression of knapweed was necessary and that wood ash has fertilization properties. Herbicide was the only treatment that significantly decreased spotted knapweed but it lead to secondary invasion of B. tectorum. One explanation is the increased soil nitrogen from the herbicide treated knapweed as annual grasses are able to rapidly take up soil nitrogen. This study provided important insight into managing highly invaded semi-arid rangeland ecosystems.