Submitted to: Biological Invasions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/1/2005
Publication Date: 9/1/2006
Citation: Mangold, J.M., Sheley, R.L., Engel, R. 2006. Can r*s predict invasion in semi-arid grasslands. Biological Invasions 8:1343-1354. Interpretive Summary: We tested the applicability of a plant competition model in semi-arid rangeland and for the management of invasive plants. The model states that the outcome of competition between species can be predicted by quantifying the minimum amount of a limiting resource that a species requires to persist in the plant community. We determined the minimum nitrogen (N) requirement for annual sunflower (native, early successional species), bluebunch wheatgrass (native, late successional species), and spotted knapweed (nonnative, invasive species) by growing monocultures of the three species for three successive growth periods and measuring soil N concentration when species' biomass declined. Annual sunflower and spotted knapweed were found to have similar N requirements for biomass maintenance, while that for bluebunch wheatgrass was an order of magnitude lower. We tested whether the outcome of competition between species could be predicted by the estimated N requirements by growing the species in two- and three-species mixtures for three growth periods. Species' estimated N requirements successfully predicted the outcome of competition between annual sunflower and bluebunch wheatgrass, but not between the native species and spotted knapweed. We concluded that mechanisms other than competition for N may be influencing competitive relationships between the native species and the nonnative invasive species.
Technical Abstract: We tested the applicability of R* theory in semi-arid rangeland and for the management of nonindigenous invasions. R* theory is a plant competition model that states that the outcome of competition can be predicted by a species' R*, or the amount of a resource that a species requires to maintain a birth rate equal to its mortality rate, hence a population growth rate of zero. The theory predicts that a population with a lower R* for the most limiting resource will competitively displace a population with a higher R* under equilibrium conditions. In a greenhouse, annual sunflower (Helianthus annuus L.), bluebunch wheatgrass (Agropyron spicatum Pursh), and spotted knapweed (Centaurea maculosa Lam.) were grown in monoculture and 2- and 3-species mixtures for three growth periods. At the end of each growth period, above ground biomass by species and soil plant available nitrogen were sampled. Decreasing biomass coupled with decreasing soil plant available nitrogen was used to quantify R*s for the three species. R*s for annual sunflower, bluebunch wheatgrass, and spotted knapweed were estimated to be 0.6 ppm NO3-, less than 0.05 ppm NO3-, and 0.4-0.6 ppm NO3-, respectively. Estimated R*s successfully predicted the outcome of competition between annual sunflower and bluebunch wheatgrass, but not between spotted knapweed and the other two species. We concluded that R* theory may be useful for predicting the outcome of competition between indigenous species on semi-arid rangeland. However, the theory seemed less applicable where nonindigenous, invasive species are concerned, and inclusion of other mechanisms may be necessary.