The role of the native soil community in the invasion ecology of spotted (Centaurea maculosa auct. non Lam.) and diffuse (Centaurea diffusa Lam.) knapweed

Diffuse (Centaurea diffusa Lam.) and spotted knapweed (Centaurea maculosa auct. non Lam.) are damaging exotic plant invaders of North American rangelands. Although existing weed management techniques can successfully reduce negative impacts of knapweeds, these plants continue to spread in an unpredictable manner. The successful spread of knapweeds into native plant communities may be driven, in part, by interactions between these plants and native soil communities. This study was conducted to evaluate the relative benefit of native soil communities to two native plants and two knapweeds and to investigate the growth of these plants in soil from knapweed infestations and from adjacent native rangelands. Individual plants of bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A. Löve), yarrow (Achillea millefolium L.), spotted and diffuse knapweed were grown in a greenhouse in field collected soil. The relative benefit of the native soil community was determined by comparing emergence and production of native plants and exotic knapweeds in autoclaved versus unautoclaved native rangeland soil. Emergence and production were also determined in the greenhouse for native plants and exotic knapweeds grown in soil collected from the core and perimeter of knapweed infestations and in soil from the adjacent, uninvaded native rangeland. The native soil community had a negative effect on the growth of bluebunch wheatgrass (P. spicata (Pursh) A. Löve) and diffuse knapweed, but a positive effect on spotted knapweed emergence. The interactions between yarrow (A. millefolium L.) and the native soil community were variable. The native soil community appears to be more beneficial to spotted knapweed than to the other plants studied, including diffuse knapweed. Therefore, it appears that two closely related knapweeds have very different interactions with soil biota and perhaps different strategies for invasion. Soil from diffuse and spotted knapweed infestations did not prevent growth of two native plants. Diffuse knapweed growth was not promoted by soil from within a diffuse knapweed infestation, but soil from the core of a spotted knapweed infestation did increase emergence of spotted knapweed.