Exotic weed control treatments for conservation of fescue grassland in Montana Original Research Article

An analytical framework is described to evaluate exotic weed control treatments applied to native vegetation for conservation purposes. The analyses were used to assess the following responses to herbicide applications for control of an exotic forb Centaurea maculosa in fescue grassland: (1) efficacy on the target weed species, (2) similarity to reference stands defining the potential natural communities, (3) resistance and resilience of the nontarget vegetation, and (4) community structure in terms of life form proportions. Vegetation data were collected from replicated plots one year before spraying and for the first three years after spraying at two sites in western Montana. Low rates of selected herbicides effectively suppressed the target weed for three years after treatment. Grasses responded positively to release from exotic forb competition, while total nontarget forb standing crops were similar between treated plots and untreated control plots three years after spraying. The site with lower pretreatment cover of exotic species and higher pretreatment composition of native species had the largest positive response to spraying in terms of similarity to the potential natural vegetation. Although some treatments caused depressions after one year, all treatments had the same or higher per cent of pretreatment species present compared to the untreated control plots in the third year after spraying. Herbicide-sensitive taxa were identified by multivariate analysis of canopy cover data. Implications of the results for conservation management of high priority grassland sites are discussed.