| Invasions by nonnative plant species such as cheatgrass ( Bromus tectorum) are a major concern in many ecosystems worldwide. When invasive nonnative species dominate a new ecosystem, they can alter biodiversity, species composition, nutrient cycles, disturbance regimes, and other ecosystem functions and processes. In 2003, cheatgrass rapidly spread through the Mt. Trumbull Ecosystem Restoration Project in the Uinkaret Mountains of northwest Arizona. In several areas, cheatgrass became the dominant herbaceous species, although native vegetation continued to dominate a substantial portion of the landscape. The three studies I present here examine the roles of disturbance, propagule pressure, competition, and resource availability on cheatgrass -- native plant dynamics. The first study examines the susceptibility of remnant native vegetation to cheatgrass invasion, and persistence of the cheatgrass invasion in the presence of elevated disturbance through biomass removal and/or elevated propagule pressure through seed additions. Both cheatgrass- and native-dominated areas were persistent for three years after treatment. The second study monitored changes in plant species richness, composition, and distribution in invaded and non-invaded areas. The two community types only shared 52 -- 59% of plant species one year after invasion. By the fifth year, the invaded and non-invaded areas only shared 32 -- 41% of plant species. Furthermore, the invaded plots contained more normative species than the uninvaded plots. By 2007, nonnative species accounted for 30% of species richness in the invaded community. The third study was a greenhouse experiment testing cheatgrass competition against two native perennial grass species at different levels of competition, with nitrogen and phosphorus additions, and at high and low water availability. Competition with only a single mature perennial grass individual significantly reduced cheatgrass growth and seed production regardless of nutrient and water availability. The greenhouse results, combined with the field studies suggest that the maintenance of a robust native perennial grass community can be important in a plant community's ability to resist invasion by nonnative annual grass species. |
