A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Spotted knapweed (Centaurea 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 each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration 3 biomass) by indigenous functional groups: grasses, shallow-rooted forbs, deep-rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallowrooted forbs, deep-rooted forbs, grasses, or spikemoss were removed. Functional group biomass was also collected and analyzed for nitrogen, phosphorus, potassium, and sulphur. Based on covariate analyses, postremoval indigenous plot biomass did not relate to invasion by C. maculosa. Analysis of variance indicated that C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of functionally similar groups as an invader may be particularly important in invasion resistance.

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The importance of species richness for repelling exotic plant invasions varies from ecosystem to ecosystem. Thus, in order to prioritize conservation objectives, it is critical to identify those ecosystems where decreasing richness will most greatly magnify invasion risks. Our goal was to determine if invasion risks greatly increase in response to common reductions in grassland species richness. We imposed treatments that mimic management-induced reductions in grassland species richness (i.e., removal of shallow- and/or deep-rooted forbs and/or grasses and/or cryptogam layers). Then we introduced and monitored the performance of a notorious invasive species (i.e., Centaurea maculosa). We found that, on a per-gram-of-biomass basis, each resident plant group similarly suppressed invader growth. Hence, with respect to preventing C. maculosa invasions, maintaining overall productivity is probably more important than maintaining the productivity of particular plant groups or species. But at the sites we studied, all plant groups may be needed to maintain overall productivity because removing forbs decreased overall productivity in two of three years. Alternatively, removing forbs increased productivity in another year, and this led us to posit that removing forbs may inflate the temporal productivity variance as opposed to greatly affecting time-averaged productivity. In either case, overall productivity responses to single plant group removals were inconsistent and fairly modest, and only when all plant groups were removed did C. maculosa growth increase substantially over a no-removal treatment. As such, it seems that intense disturbances (e.g., prolonged drought, overgrazing) that deplete multiple plant groups may often be a prerequisite for C. maculosa invasion.

High plant functional group diversity has been hypothesized to reduce resource concentrations based on the assumption that species from one functional group acquire resources similarly to one another, while species from other functional groups acquire resources dissimilarly. To determine if functional groups use soil nutrients different from one another, we investigated the impact of removing individual functional groups on soil inorganic nitrogen (NO { 3 and NH z 4) concentrations in the Idaho fescue (Festuca idahoensis Elmer)/bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Lö ve) habitat type in Montana. Treatments were imposed by removing 1) all plant species (total plant removal), 2) shallow-rooted (, 15 cm) forbs, 3) deeprooted (. 15 cm) forbs, 4) all forbs (total forb removal), 5) grasses, and 6) spikemoss, compared to intact control plots. Inorganic nitrogen was measured at 2 soil depths (0-15 cm and 16-40 cm) in the spring, summer, and fall 1 year after treatment imposition. The removal of individual functional groups generally increased soil NO { 3 and NH z 4 concentrations. Total plant removal increased NO { 3 concentrations more than removing individual functional groups. Grass removal generally increased soil NO { 3 concentrations in the 0-15-cm depth more than other functional groups removal. Whether the grass or total forb removal treatment resulted in greater soil NH z 4 concentrations in the 0-15-cm depth depended on season. These results suggest that functional groups vary in their soil nutrient acquisition patterns and that increased functional diversity decreases soil nutrient concentrations. Therefore, maintaining or improving functional diversity may be a method to more fully utilize soil nutrients because functional groups can differ in their spatial and temporal acquisition of resources. Resumen La alta diversidad del grupo funcional de plantas ha sido sugerida para reducir las concentraciones de recursos, en base al supuesto de que las especies forman un grupo funcional para adquirir recursos comunes para cada uno de los miembros del grupo, mientras que las especies de otros grupos funcionales adquieren recursos diferentes. Para determinar si los grupos funcionales usan los nutrientes en forma diferente, investigamos el impacto de la remoción de grupos funcionales individuales sobre las concentraciones de nitrógeno inorgánico (NO { 3 y NH z 4) en el suelo del hábitat ''Idaho fescue'' (Festuca idahoensis Elmer)/''Bluebunch wheatgrass'' (Pseudoroegneria spicata [Pursh] A. Lö ve) en Montana. Los tratamientos se implementaron removiendo 1) todas las especies (remoción total de plantas), 2) hierbas con raíz superficial (, 15 cm), 3) hierbas con raíz profunda (. 15 cm), 4) todas las hierbas, 5) zacates, y 6) musgos, y se compararon con parcelas intactas que fueron el control. El nitrógeno inorgánico se midió a dos profundidades del suelo (0-15 cm y 16-40 cm) en primavera, verano y otoñ o un añ o despue´s de aplicar los tratamientos. La remoción de grupos funcionales individuales general...

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