Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2007
Publication Date: 5/1/2008
Citation: Mangold, J.M., Sheley, R.L. 2008. Controlling performance of bluebunch wheatgrass and spotted knapweed using nitrogen and sucrose amendments. Western North American Naturalist. 68(2)pp. 129-137. Interpretive Summary: Spotted knapweed continues to spread on rangeland throughout the western U.S. The organization, structure, and function of plant communities and the economic stability of rural communities may be altered when invasive plants like spotted knapweed dominate. The objective of this study was to test whether we could alter the performance of bluebunch wheatgrass and spotted knapweed by amending the soil with nitrogen (N) (to increase plant available N) or sucrose (to decrease plant available N). We grew mixtures of bluebunch wheatgrass and spotted knapweed at various densities at two sites in southwestern Montana. We concluded that the performance of spotted knapweed was increased when plant available N was high while bluebunch wheatgrass remained unaffected. Lowering plant available N had no effect on species performance. We suggest managers should limit the frequency and intensity of activities that increase plant available N to prevent favoring spotted knapweed.
Technical Abstract: Range and wild lands are being invaded by nonindigenous plants, resulting in an unprecedented, rapid change in plant community composition across the United States. Successional management predicts that species performance may be modified by resource availability. The objective of this study was to determine whether species performance could be altered by modifying plant available N within an Idaho fescue (Festuca idahoensis)/bluebunch wheatgrass (Pseudoroegneria spicata) plant association. We planted bluebunch wheatgrass, annual sunflower (Helianthus annuus), and spotted knapweed (Centaurea maculosa) in an addition series at two sites in southwestern Montana. Each plot in the addition series matrix was divided into thirds and N was applied to a subplot, sucrose was applied to a second subplot, and the remaining subplot was not amended and considered a control. Annual sunflower did not persist through the duration of the study. Regression models for predicting bluebunch wheatgrass and spotted knapweed biomass only accounted for about 30% of the variation, suggesting other processes in addition to competition were responsible for explaining relative plant performance. Soil amendments had no effect on regression models. Nitrogen amendment increased spotted knapweed absolute biomass by 27% and 34% at the two sites, but bluebunch wheatgrass absolute biomass was not affected. Sucrose treatments only lowered plant available N at the more productive site, and had no effect on species absolute biomass. We concluded that the performance of spotted knapweed was increased when plant available N was high while bluebunch wheatgrass remained unaffected. Lowering plant available N had no effect on species performance. Although successional management theory predicted that altering resource availability could be a way to influence species performance, this study suggests this may not be as straightforward as predicted.