Tordon was the most persistent of the three herbi- 

 cides and was leached at the highest concentrations 

 to the greatest depths. The maximum concentration 

 of Tordon below 25 cm did not exceed 26 ppb at day 

 30 and did not exceed trace amounts below 25 cm 

 at 1 year. Tordon was not detected below 5 cm at 

 2 years, and only trace amounts were measured in 

 the surface (0 to 5 cm) soil increment. Clopyralid 

 was not detected below 25 cm and at 1 year could 

 not be detected below 5 cm depth (table 7). 



Initial plant injury and mortality predominately 

 results from absorption of the herbicide by leaves 

 and stem. Longer term injury and mortality results 

 from roots taking up herbicide residuals leached into 

 the soil. Limited leaching to depth and rapid decay 

 restrict the 1-year postspray residual herbicide ef- 

 fects to suppression of seedling establishment in the 

 0- to 5-cm surface soil layer and, with Tordon, some 

 additional suppression of shallow rooted perennials. 

 Perennials rooted below 25 cm received only a lim- 

 ited exposure to the herbicide residuals. 



Suppression of the competitively dominant spotted 

 knapweed releases resources to support growth of 

 other plant species. The grasses, and some forbs 

 and shrubs, that are inherently tolerant or not ex- 

 posed to this group of herbicides are able to respond 

 during the year of spraying. Herbicide-susceptible 

 forbs and shrubs respond to the limiting resource re- 

 lease in subsequent growing seasons as the herbi- 

 cide residuals decline. Resistant individuals expand 

 and new plants, including spotted knapweed, estab- 

 lish from various propagules. 



REFERENCES 



Baker, L. O.; Fay, P.; Jackson, M. J. 1979. Spotted 



knapweed and control. Coop. Ext. Serv. Folder 206. 



Bozeman, MT: Montana State University. 2 p. 

 Bucher, R. F. 1984. The potential cost of spotted 



knapweed to Montana range users. Coop. Ext. 



Serv. Bull. 1316. Bozeman, MT: Montana State 



University. 18 p. 

 Daubenmire, R. 1959. Canopy coverage method of 



vegetation analysis. Northwest Science. 33: 43-66. 



Forcella, F.; Harvey, S. J. 1983. Eurasian weed in- 

 festation in western Montana in relation to veg- 

 etation and disturbance. Madrono. 30: 102-109. 



Harm, W. J.; Jensen, M. E. 1987. Ecodata sampling 

 methods. In: Tripp, J., ed. Ecosystem classification 

 handbook. [Missoula, MT]: U.S. Department of 

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 Office: chapter 4, FSH 12/87 R-1 SUPP 1. 



Harris, P.; Cranston, R. 1979. An economic evalua- 

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Keane, R. E.; Jensen, M. E.; Hann, W. J. 1990. 

 ECODATA and ECOPAC: analytic tools for inte- 

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Lacey, C. 1989. Knapweed management: a decade of 

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 April 4-5; Bozeman, MT. Ext. Bull. 45. Bozeman, 

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Losensky, B. J. 1987. An evaluation of noxious 

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 with recommendations for implementing a weed 

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Mueggler, W. F.; Stewart, W. L. 1980. Grassland 

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Pfister, R. D.; Kovalchik, B. L.; Amo, S. F.; Presby, 

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Tyser, R. W.; Key, C. W. 1988. Spotted knapweed in 

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Winer, B. J. 1971. Statistical principles in experi- 

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