Effects of Nutrient Manipulations and Grass Removal on Cover, Species Composition, and Invasibility of a Novel Grassland in Colorado

Cover and richness of a 5-year revegetation effort were studied with ,respect to small-scale disturbance and nutrient manipulations. The site, originally a relict tallgrass prairie mined for gravel, was replanted to native grasses using a seed mixture of tall-, mixed-, and short-grass species. Following one wet and three relatively dry years, a community emerged, dominated by species common in saline soils not found along the Colorado Front Range. A single species, Alkali sacaton (Sporobolus airoides), composed nearly 50% of relative vegetation cover in control plots exhibiting a negative relationship between cover and richness. Seeded species composed approximately 92% of vegetation cover. The remaining 8% was composed of weeds from nearby areas, seed bank survivors, or mix contaminants. Three years of soil nutrient amendments, which lowered plant-available nitrogen and phosphorus, significantly increased relative cover of seeded species to 97.5%. Fertilizer additions of phosphate enhanced abundance of introduced annual grasses (Bromus spp.) but did not significantly alter cover in control plots. Unmanipulated 4-m2 plots contained an average of 4.7 planted species and 3.9 nonplanted species during the 5-year period, whereas plots that received grass herbicide averaged 5.4 nonplanted species. Species richness ranged from an average 6.9 species in low-nutrient, undisturbed plots to 10.9 species in the relatively high-nutrient, disturbed plots. The use of stockpiled soils, applied sparingly, in conjunction with a native seed mix containing species uncommon to the preexisting community generated a species-depauperate, novel plant community that appears resistant to invasion by ruderal species.