Submitted to: Restoration Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2005
Publication Date: 9/20/2005
Citation: Pokorny, M.L., Sheley, R.L., Zabinski, C.A., Engel, R.E., Svejcar, A.J., Borkowski, J.J. 2005. Plant functional group diversity as a mechanism for invasion resistance. Restoration Ecology. 13(3):1-12. Interpretive Summary: Undesireable weedy species have invaded millions of acres of rangeland in the western U.S. Many of the treatments designed to control these weedy species are effective only if the weeds are replaced by desireable species that keep the weeds from reinvading. This study was designed to determine how different "functional groups" of native rangeland plants influenced invasion by spotted knapweed. The results demonstrate the importance of a diverse plant community in reducing reinvasion, and forbs as a group were especially important. This information will prove useful in designing strategies to restore weed-infested rangelands.
Technical Abstract: A commonly cited mechanism for invasion resistance is that diverse plant assemblages use resources more completely through maximum niche occupation. Our research investigates the ability of plant functional groups in resisting invasion by a nonindigenous species, Centaurea maculosa (spotted knapweed). We quantified resource use by indigenous functional groups; grasses, shallow-rooted forbs, spikemoss, and the invader C. maculosa. The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized complete block design replicated four times at two sites. Centaurea maculosa density, counted in 2001, depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation (50 plants/m2) or all forbs (18 plants/m2) were removed from plots. Knapweed densities were the lowest (2-7 plants/m2) in plots where nothing, shallow-rooted forbs, deep-rooted forbs, grass, or spikemoss were removed. Functional group biomass was collected and analyzed for N, P, K, and S. ANOVA indicated percent tissue concentration and nutrient uptake differed among functional groups. Centaurea maculosa tissue nutrient percentage was most similar to shallow-rooted forbs. Similarly, forb functional groups were most similar in nutrient uptake levels to C. maculosa. Our study suggests that establishing and maintaining a diversity of plant functional groups within the community enhances resistance to invasion. Forbs are particularly important to resisting invasion because indigenous and nonindigenous forbs are functionally similar.