Submitted to: Society for Ecological Restoration Abstracts
Publication Type: Abstract only
Publication Acceptance Date: 9/1/2003
Publication Date: 11/19/2003
Citation: Krueger-Mangold, J., Sheley, R.L., Svejcar, A.J., Engel, R. 2003. Using r* to quantify competition between two native species on semi-arid, western rangeland. 15th Annual International Conference of the Society for Ecological Restoration, November 19-23, 2003, Austin, Texas. WWW.SER.org/pdf/2003_abstracts_A-L.pdf. Interpretive Summary: It may be possible to develop a unit (R*) that can be used to compare the competitive ability of species by determining the lowest level of nitrogen in which they can maintain a population. This unit could also be used to predict the outcome of community dynamics, compare species for restoration, and assess the likelihood of invasion of areas based on nitrogen status. Our study showed that the R* for sunflower, a species generally present early after a disturbance, was higher than that of bluebunch wheatgrass, a species considered competitive that is present in many stable states. This provides initial evidence that R* may be used to predict and manipulate plant change during restoration.
Technical Abstract: R* has been proposed as a quantification of plant species' competitive ability. R* is the lowest level of a limited soil resource a population of plants requires to maintain itself. The species with the lowest R* dominates when soil resources are in limited supply. The objective of our research was to quantify R* for nitrogen (N) for two species native to western rangeland and use the R*s to predict the outcome of competition when the two species were grown in mixtures. We grew monocultures and mixtures of Helianthus annuus (annual sunflower) and Agropyron spicatum (bluebunch wheatgrass) in 15 cm x 15 cm x 38 cm pots filled with a field soil/sand mixture. Monocultures contained six plants per pot and mixtures contained three plants per pot of both species. Aboveground biomass was clipped, weighed, and analyzed for N content after three months. At the same time, soil was sampled and analyzed for nitrate (NO3-) and ammonium (NH4+). The study was repeated through three growth cycles to reduce available N to each species' R*. Soil NO3- was reduced to 0.6 mg N/kg soil in monocultures and mixtures of the two species. Biomass per annual sunflower plant decreased by about 90% while biomass per bluebunch wheatgrass plant decreased by about 30% when grown in monoculture. In mixtures, biomass per annual sunflower plant decreased by 90% while biomass per bluebunch wheatgrass plant increased by about 10%. According to soil NO3- and biomass values, we concluded that 0.6 mg N/kg soil was the R* for N for annual sunflower while the R* for N for bluebunch wheatgrass was lower. We believe R* and N availability may be used to predict and manipulate successional trajectories on restoration projects on semi-arid rangeland.