EXOTIC WEED-CONTROL TREATMENTS FOR CONSERVATION OF FESCUE GRASSLAND IN MONTANA

An analytical framework is described to evaluate exotic weed control t reatments applied to native vegetation for conservation purposes. The analyses were used to assess the following responses to herbicide appl ications for control of an exotic forb Centaurea maculosa in fescue gr assland: (1) efficacy on the target weed species, (2) similarity to re ference stands defining the potential natural communities, (3) resista nce and resilience of the nontarget vegetation, and (4) community stru cture in terms of life form proportions. Vegetation data were collecte d from replicated plots one year before spraying and for the first thr ee years after spraying at two sites in western Montana. Low rates of selected herbicides effectively suppressed the target weed for three y ears after treatment. Grasses responded positively to release from exo tic Forb competition, while total nontarget forb standing crops were s imilar between treated plots and untreated control plots three years a fter spraying. The site with lower pretreatment cover of exotic specie s and higher pretreatment composition of native species had the larges t positive response to spraying in terms of similarity to the potentia l natural vegetation. Although some treatment!; caused depressions aft er one year:, all treatments had the same or higher per cent of pretre atment 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 resul ts for conservation management of high priority grassland sites are di scussed. (C) 1998 Elsevier Science Ltd. All rights reserved.