Herbicides and seeding of native species are often integrated to restore invasive plant-infested rangeland. However, revegetation often fails due to competition between seedlings of invasive plants and native grasses. This project examined how native grass seeding date may be modified to give native grasses a size advantage, and thus induce a priority effect, over invasive plants that emerge later. Bluebunch wheatgrass (Pseudoroegneria spicata) was assessed in conjunction with the forb-annual grass complex: spotted knapweed (Centaurea stoebe) and cheatgrass (Bromus tectorum) at two sites in southwestern Montana. We determined how seeding date affected survival and abundance of bluebunch wheatgrass at the end of one growing season, as well as its effect on competition between bluebunch wheatgrass and the invasive forb-annual grass complex. Bluebunch wheatgrass was hand-broadcasted into 1m2 plots at a rate of 667 seeds/m2 over eight seeding dates; one in fall (November 2015) and seven in spring (weekly, 1 April to 12 May 2016). Seedling emergence was sampled weekly from April through June 2016, and allometric measurements were collected for bluebunch wheatgrass in August 2016 to compare mean values by seeding dates. Spotted knapweed and cheatgrass were seeded in September 2016.
Results/Conclusions
Fall cohorts resulted in fewer bluebunch wheatgrass plants with greater overall growth, while spring cohorts resulted in more plants with lower overall growth. Fall cohorts were taller than spring cohorts, averaging ~17 cm compared to 8 – 13 cm, and had more seed heads than spring cohorts. Both sites had the largest plants for the fall cohort, with 32 – 43 tillers/plant compared to spring cohorts with 15 – 25 tillers/plant. Although spring cohorts resulted in smaller plants overall, early April cohorts had the highest density with over 100 seedlings/m2. No differences in seedling survival were found among seeding cohorts with survival ranging from 50-75%. Density and biomass of spotted knapweed and cheatgrass will be measured in summer 2017 to understand the effect of seeding date on competition between bluebunch wheatgrass and invasive species. In severe cases, revegetation is necessary to reintroduce desired species to rangeland degraded by invasive plants. Efforts can be challenging due to desired species being less abundant in the seed bank, competition between seeded species and invasive plants, and herbicide applications that may cause injury to seedlings of desired species. By improving rangeland conditions through revegetation, we ensure ecological and economic services are restored and maintained.