Relationships between Centaurea maculosa and indigenous plant assemblages

Ecological impacts of invasive plants include displacement of indigenous sp ecies and declines in species richness and diversity. The objective of this study was to characterize the functional relationship between plant commun ity composition and Centaurea maculosa Lam. (spotted knapweed) within a Fes tuca idahoensis / Pseudoroegneria spicatum habitat type in Montana. Density , cover, and biomass of all species were collected along a gradient of spot ted knapweed cover ranging from 0% to about 100%. Step-down regression was used to determine the relationship among C. maculosa, indigenous species, s pecies richness, and Shannon-Weaver's diversity index. Regressions showed t hat indigenous perennial grass cover, species richness, and species diversi ty were inversely related to C. maculosa cover. There was no relationship b etween C. maculosa and indigenous forbs. While this study does not imply a causal relationship, the literature suggests that C. maculosa displaces ind igenous species and/or invades areas of reduced indigenous plant cover, low diversity, or low species richness. Knowing levels of indigenous perennial grass cover will help managers predict the outcome of weed management on r angelands that are vulnerable to weed infestation.