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Competitive Interactions Between Spotted Knapweed (Centaurea maculosa) and Native
Plants in the Eastern Upper Peninsula
Earlynn K. McClure
Department of Biology
Lake Superior State University
November 28, 2000
Abstract. Spotted knapweed is an invasive plant species that takes over fields, roadsides,
beaches, and disturbed land. The growth of spotted knapweed reduces the forage value
of these habitats. Spotted knapweed can outcompete native plants because it can
withstand periods without water and it releases a chemical that inhibits the growth of
other plants. The goal of the study was to assess competitive interactions between
spotted knapweed and native plants in selected habitats in the Eastern Upper Peninsula.
The objective of the study was to determine whether total species richness, total density,
and species composition was different inside and outside of spotted knapweed patches.
Spotted knapweed patches were located along roadsides and beaches. Stem density was
counted in two 20 cm by 20 cm quadrats in the patches and in adjacent areas without
spotted knapweed growth. A paired t-test indicated no difference in species richness
inside or outside the patches. Total stem density was greater inside the patches (p =
0.04). Species composition varied inside versus outside the spotted knapweed patches.
These results do not indicate that the spotted knapweed is establishing monocultures on
these particular sites, but species removal studies maybe necessary to determine longterm effects of spotted knapweed on native flora.
Selected References
Eddleman, L.E., and J.T. Romo. 1988. Spotted knapweed germination response to
stratification, temperature, and water stress. Canadian Journal of Botany 66:653-657.
Rice, P.M., J.C. Toney, D.J. Bedunah, and C.E. Carlson. 1997. Elk winter forage
enhancement by herbicide control of spotted knapweed. Wildlife Society Bulletin
25:627-633.