Plant community diversity and growth form responses to herbicide applications for control of Centaurea maculosa.
A field experiment conducted from 1988-95 in western Montana, USA, compared the effects of herbicide treatments (picloram, clopyralid and clopyralid + 2,4-D, each at two timings and the recommended rates for control of the exotic forb Centaurea maculosa), on the structure and species diversity of native plant communities. The floristic composition of replicated treatment plots was sampled before spraying and for 3 years after the initial herbicide applications at two grassland and two early seral forest sites. Following the third year post-spray measurements, half the treatment plots were randomly selected to be resprayed and community sampling was continued for two more years. Diversity was quantified as species richness and Shannon diversity index. Standing crops by growth form were estimated by double sampling at the conclusion of the experiment. Herbicide treatments had a high efficacy on the target weed, shifting the plant communities back to a grass-dominated structure. Depressions in plant community diversity were small and transitory. In the third year after the initial applications, there were no significant differences among treatments, and some herbicide-treated plots had begun to surpass the untreated plots in community diversity measures. With most treatments, respraying 3-4 years after the initial applications did not reduce plant diversity compared to untreated levels. Late season applications, made after most herbicide-susceptible forbs had entered summer drought-induced dormancy, minimized impacts on plant community diversity. The behaviour of herbicide residues in the soil is described in relation to community-level effects. Implications of the results for exotic weed management in conservation settings are discussed.