Establishment of self-sustaining perennial bunchgrass populations is critical to successfully restoring sagebrush steppe ecosystems degraded by invasive annual grasses. Basic understanding of bunchgrass population dynamics has been limited due to a lack of long-term studies, and an important area of uncertainty is to what degree bunchgrasses persist in one location as compared to colonizing new locations from seed. To address this, we took advantage of a livestock forage conditioning trial established in a Wyoming big sagebrush-bunchgrass community at the Northern Great Basin Experimental Station in 1989. This study consisted of nine 313-m2 blocks, each divided into a grid of 841 0.37 m2 cells, each with 1 plant of a single species at its center, including the introduced perennial grass, crested wheatgrass, seven native sagebrush steppe perennial grasses, and bare ground. After the final forage assessment in 1998, the trial site has been ungrazed. In 2011, the grid coordinates of each block were re-established, and all grasses identified to species and total number of individuals counted in each cell.
Regardless of population dynamic (increase, decrease, or stasis), 85-90% of plants recorded in 2011 occurred in novel cells. Crested wheatgrass had the greatest population increase, being better able to maintain original occupancy, as well as having a higher probability of establishing multi-plant cells both within and outside of its original cells. Both within original and colonized cells, increasing crested wheatgrass density had a marked, negative effect on the abundance of native bunchgrasses, following a sigmoidal relationship (R2 = 0.87; p = 0.009), suggesting there is a threshold density of crested wheatgrass beyond which it is difficult for native grasses to establish. This high recruitment and colonization capacity of crested wheatgrass may also explain its success in revegetation efforts and effectiveness in halting the spread of invasive annual grasses. In addition, this study suggests sagebrush steppe bunchgrasses are not very long-lived, and that establishment of plants from seed, not vegetative growth, is highly important in perennial grass population dynamics in sagebrush steppe.