Invasion ecology of knapweed (Centaurea spp. L.): competition, establishment and interactions with soil biota

Abstract

Knapweeds (Centaurea spp. L.) are damaging exotic plant invaders of North American rangelands. A two-part study was conducted to explore components of diffuse (C. diffusa Lam.) and spotted knapweed (C. maculosa auct. non Lam.) invasion ecology. A field experiment was conducted to study the resistance of a late-seral native foothills rangeland to invasion by diffuse knapweed and conditions that promote invasion (Part I). Greenhouse experiments were conducted to evaluate the relative benefit of native soil communities to two native plants and two knapweeds and to study plant growth in soil from knapweed infestations and adjacent native rangelands (Part II). Diffuse knapweed was planted in a late-seral, native rangeland under treatment combinations that altered; 1) gap size, 2) competition, and 3) knapweed seed incorporation. Diffuse knapweed seeds were sown in fall 2001 and spring 2002. Greenhouse experiments were conducted with individual plants of two native species and two knapweed species grown in various soil treatments (core and perimeter of knapweed infestations, intact and autoclaved native soil). In the field study (Part I), seed incorporation and adjacent live vegetation increased diffuse knapweed emergence, compared to unincorporated seed and dead adjacent vegetation. Emergence of incorporated seed increased with increasing gap size. Emergence approached 35%, but most knapweed plants died by late summer, 2002. In the greenhouse (Part II), the native soil community had negative effect on the growth of bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A. Löve), yarrow (Achillea millefolium L.) and diffuse knapweed, but a positive effect on spotted knapweed emergence. Plant growth was sometimes better in native soil than soil from knapweed infestations. However, growth of the native plants was not precluded, and knapweed growth was not promoted, in soils from knapweed infestations. The native rangeland studied was not absolutely resistant to diffuse knapweed invasion. Disturbances that ensure diffuse knapweed seed incorporation and weaken native vegetation increase susceptibility to invasion. Greenhouse studies indicated that the native soil community is more beneficial to spotted knapweed than to the other plants studied, and that soil from diffuse and spotted knapweed infestations does not prevent growth of two native plants or promote knapweed growth.