Plant community diversity and growth form responses to herbicide applications for control of Centaurea maculosa

Published source details Rice P.M., Toney J.C., Bedunah D.J. & Carlson C.E. (1997) Plant community diversity and growth form responses to herbicide applications for control of Centaurea maculosa. Journal of Applied Ecology, 34, 1397-1412.
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Summary

Western USA bunchgrass steppe has been invaded by a number of exotic perennial forbs including spotted knapweed Centaurea maculosa (native to Eurasia). It is a highly invasive species that threatens native plant communities. A field experiment compared the effects of herbicide treatments for control of C. maculosa (picloram, clopyralid and clopyralid + 2,4-D), applied early or late in the growing season, on native grassland communities during an 8-year period in Montana, northwest USA.

Study sites: Four grassland sites in Missoula County with low to moderate knapweed infestation were selected:

Site 1 (46°57’ N, 114°08’ W) – rough fescue Festuca scabrella/bluebunch wheatgrass Agropyron spicatum grassland. Knapweed cover before herbicide spraying was 21%.

Site 2 (46°56’ N, 114°08’ W) – Idaho fescue Festuca idahoensis/bluebunch wheatgrass Agropyron spicatum grassland. Knapweed cover before herbicide spraying was 7%.

Sites 3 and 4 University of Montana Lubrecht Experimental Forest (46°53’ N, 113°30’ W) – adjacent sites in a clear cut area logged in the 1960s. After logging Richardson’s needlegrass Stipa richardsonii and Festuca scabrella became dominant. Knapweed cover before herbicide spraying was 10% (site 3) and 3% (site 4).

Experimental design: At each site, six treatment plots and a control plot (each 9 x 27 m) replicated three times were established. Vegetation in each was sampled along five randomly located transects with five evenly spaced microplots (25 x 51 cm) in which canopy cover for each vascular plant species was recorded. Pretreatment data was collected in summer (‘warm season’ July-September) 1988 and spring (‘cool season’ May-June) 1989.

Herbicide treatments: Herbicide (picloram, clopyralid or clopyralid + 2,4-D) was applied at the recommended rates for spotted knapweed control to the selected plots (by backpack sprayer) in 1989 after pre-treatment sampling.

There were two timings of application: ‘early’ (knapweed in rosette to early bolt stage); and ‘late’ (early to mid-flowering stage when cool season plants had entered summer dormancy). Post-spray data was collected during 1990-95. After sampling in 1992 half of the plots were randomly selected for repeat spraying and plots were again sampled for species vegetation cover in 1994 and 1995. Standing crop biomass was recorded at the conclusion of the experiment in 1995.

All herbicide treatments provided good control of spotted knapweed in the three years following application. Significant recovery was evident in the once only sprayed plots after 5 years. Respraying maintained knapweed suppression except for the late clopyralid + 2,4-D treatment which was least effective.

The plant communities shifted back to the desired native grass-dominated structure. Depressions in plant diversity were only small and transitory; late season applications, made after most herbicide-susceptible forbs had entered summer dormancy, minimized impacts. In the third year after the initial spraying, there were no significant differences among treatments and some herbicide-treated plots had begun to surpass the untreated plots in plant community diversity.

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