| Abstract: | Ecological impacts of invasive plants include displacement of indigenous species and declines in species richness and diversity. The objective of this study was to characterize the functional relationship between plant community composition and Centaurea maculosa Lam. (spotted knapweed) within a Festuca idahoensis / Pseudoroegneria spicatum habitat type in Montana. Density, cover, and biomass of all species were collected along a gradient of spotted knapweed cover ranging from 0% to about 100%. Step-down regression was used to determine the relationship among C. maculosa , indigenous species, species richness, and Shannon-Weavers diversity index. Regressions showed that indigenous perennial grass cover, species richness, and species diversity were inversely related to C. maculosa cover. There was no relationship between C. maculosa and indigenous forbs. While this study does not imply a causal relationship, the literature suggests that C. maculosa displaces indigenous 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 rangelands that are vulnerable to weed infestation. |
