Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2008
Publication Date: July 1, 2008
Citation: Vasquez, E.A., Sheley, R.L., Svejcar, A.J. 2008. Nitrogen enhances the competitive ability of cheatgrass(bromus tectorum) relative to native grasses. Journal of Invasive Plant Science and Management. 1:287-295. Interpretive Summary: In native plant communities, nutrient availability to plants tends to decrease as succession proceeds in arid and semi-arid systems. Invasion by exotic plant species often follows changes in soil processes, particularly nitrogen (N) and carbon (C) cycling. However, past and recent discoveries about nutrient cycling, plant available nitrogen, and invasion suggests conflicting insights on how soil nutrients influences invasive plants. Contrary to previous investigations, our study did not support the hypothesis that native late-seral species will produce more biomass and be more competitive than annual grasses at low N levels. However, cheatgrass did produce more biomass and be more competitive than late-seral species at high N levels suggesting that growth rate is that mechanism of interference. Furthermore, our results suggests that there may be an optimum level of plant available N where niche overlap is minimized. Soil nutrient availability may be one of the drivers of weed invasion, and a factor that we must understand more completely if we are to develop successful long-term ecologically based invasive plant management strategies. Further research of this type is warranted.
Technical Abstract: Invasion by cheatgrass and the associated high fire frequency can displace native plant communities. Previous native plant restoration projects have met with limited success. Manipulating soil resources may be one component of influencing the success of restoration efforts. We conducted two greenhouse studies at the Eastern Oregon Agricultural Research Center in Burns, OR to test the hypotheses that (1) late-seral species will produce more biomass and be more competitive than annual grasses at low soil nitrogen concentrations, and (2) annual grasses will produce more biomass and be more competitive than late-seral species at high soil N concentrations. The first study compared the competitive relationship of bluebunch wheatgrass, Idaho fescue and needle and threadgrass with that of cheatgrass while exposed to three treatment soil concentrations of inorganic nitrogen. Densities of cheatgrass and the three native bunch grasses were arranged in growth tubes to provide randomized complete blocks of four addition-series matrices and plants were grown in growth tubes. Our second study compared the absolute growth rate of the four plant species grown in isolation in a randomized complete block design for 109 days under the same soil N treatments as the interference study. The predicted growth of an isolated individual increased with increasing soil N concentrations for both cheatgrass and the three native perennials (p < 0.0001). Biomass of cheatgrass and its competitive ability increased with increasing soil N concentrations (p < 0.0001) compared to the native plant species. The absolute growth of cheatgrass grown in isolation also increased with increasing N levels (p < 0.05). Ecological implications of this study suggest that soil N increases the competitive ability of cheatgrass by creating a positive feedback both in plant size and N uptake. We speculate that nutrient levels can be modified to favor desired plant communities and that further efforts in this type of research is warranted. Nomenclature: bluebunch wheatgrass, PSSP6; Idaho fescue, Festuca idahoensis Elmer FEID; needle and thread, Hesperostipa comata (Trin. and Rupr.) Barkworth HECO26; cheatgrass, Bromus tectorum L BRTE . Key words: Invasive plants, plant available nitrogen, competition, interference.