Restoration of exotic annual grass-invaded rangelands is needed to improve ecosystem function and services. Increasing plant species richness is generally believed to increase resistance to invasion and increase desired vegetation. However, the effects of species richness and individual plant life forms in seed mixes used to restore rangelands invaded by exotic annual grasses have not been investigated. We evaluated the effects of seeding different life forms and increasing species richness in seed mixes seeded after exotic annual grass control to restore desirable vegetation (perennial herbaceous vegetation) and limit exotic annual grasses at two sites in southeastern Oregon. We also investigated the effects of seeding two commonly used perennial grasses individually and together on plant community characteristics. Large perennial grasses, the dominant herbaceous plant life form, were the most important group to seed for increasing perennial herbaceous vegetation cover and density. We did not find evidence that greater seed mix species richness increased perennial herbaceous vegetation or decreased exotic annual grass dominance more than seeding only the dominant species. None of the seed mixes had a significant effect on exotic annual grass cover or density, but the lack of a measured effect may have been caused by low annual grass propagule pressure in the first couple of years after annual grass control and an unusually wet-cool spring in the third year post-seeding. Although our results suggest that seeding only the dominant plant life form will likely maximize plant community productivity and resistance to invasion in exotic annual grass-invaded northern Great Basin arid rangelands, seeding a species rich seed mix may have benefits to higher tropic levels and community stability. Clearly the dominant species are the most prudent to include in seed mixes to restore exotic annual grass-invaded plant communities, especially with finite resources and an increasingly large area in need of restoration.
Nomenclature: Imazapic, medusahead, Taeniatherum caput-medusae (L.) Nevski.
Management Implications: The minimal to no effect of seeding species, other than large perennial grasses, on total perennial herbaceous vegetation suggests that including them in restoration seed mixes will have limited impact on plant community resistance to exotic annual grass invasion in Wyoming big sagebrush rangelands. Our results suggest that seeding the dominant species is more critical to restoration success than seeding a mixture of species. In the Wyoming big sagebrush ecosystem, the most important herbaceous species to seed after disturbance is the large perennial grass group. These results probably do not apply to ecosystems with larger windows of time when conditions are adequate for plant growth. In these ecosystems, a more species rich seed mix containing species with varying temporal resource acquisition patterns is probably needed to effectively use available resources to limit invasibility. We recognize that seeding a mixture of species may have implications to productivity and invasibility over time because of species complementarity and that establishing a diverse community potentially has benefits to higher tropic levels and community stability. A mixture of species may also be more important in ecosystems with multiple invasive species. Species richness is important, but its importance may have been overly stated in some ecosystems. We suggest restoration practitioners can use the information in this study to help weight the cost-benefits of including additional plant species in restoration efforts.
