PLANT COMMUNITY DIVERSITY AND GROWTH FORM RESPONSES TO HERBICIDE APPLICATIONS FOR CONTROL OF CENTAUREA-MACULOSA

1. A field experiment compared the effects of herbicide treatments (pi cloram, clopyralid and clopyralid + 2,4-D, each at two timings, at the recommended rates for control of the exotic forb Centaurea maculosa) on the structure and species diversity of native plant communities dur ing an 8-year period in western Montana, USA. 2. Floristic composition of replicated treatment plots was sampled before spraying and for 3 y ears after the initial herbicide applications at two grassland and two early seral forest sites. Following the third year post-spray measure ments, half the treatment plots were randomly selected to be resprayed and community sampling was continued for two more years. Diversity wa s quantified as species richness and Shannon Diversity Index. 3. Stand ing crops by growth form were estimated by double sampling at the conc lusion of the experiment. 4. Herbicide treatments had high efficacy on the target weed, shifting the plant communities back to a grass-domin ated structure. 5. Depressions in plant community diversity were small and transitory. In the third year after the initial applications, the re were no significant differences among treatments and some herbicide -treated plots had begun to surpass the untreated plots in community d iversity measures. With most treatments, respraying 3-4 years after th e initial applications did not reduce plant diversity compared to untr eated levels. 6. Late season applications, made after most herbicide-s usceptible forbs had entered summer drought-induced dormancy, minimize d impacts on plant community diversity. 7. The behaviour of herbicide residues in the soil is described in relation to community-level effec ts. 8. Implications of the results for exotic weed management in conse rvation settings are discussed.