Effectiveness of low soil-disturbance treatments for improving native plant establishment in stable crested wheatgrass stands

Authors: MORRIS, CHRISTO - Powder Basin Watershed Council; MORRIS, LESLEY - Oregon State University; Monaco, Thomas

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2018
Publication Date: 12/1/2018
Citation: Morris, C., Morris, L.R., Monaco, T.A. 2018. Effectiveness of low soil-disturbance treatments for improving native plant establishment in stable crested wheatgrass stands. Rangeland Ecology and Management. 72(2019)/237-248. https://doi.org/10.1016/j.rama.2018.10.009.
DOI: https://doi.org/10.1016/j.rama.2018.10.009

Interpretive Summary: Past seedings of crested wheatgrass (Agropyron cristatum [L.] Gaertn. & A. desertorum [Fisch. ex Link] Schult.) have the potential to persist as near-monospecific stands, thereby necessitating active intervention to initiate greater species diversity and structural complexity of vegetation. However, the success of control treatments and native species seedings is limited by high resiliency of crested wheatgrass and the influx of exotic annual weeds associated with mechanical soil disturbance. We designed a long-term study to evaluate the efficacy of low-disturbance herbicide and seed-reduction treatments applied synergistically or alone and either once or twice prior to seeding native species. Both crested wheatgrass and seeded species responses depended primarily on study site and treatment year rather than on consecutive treatment applications or synergism between them. In general, consecutive herbicide applications reduced crested wheatgrass density more than single applications, but this effect was consistent for only one of the seeding years. Crested wheatgrass showed considerable resilience after herbicide applications, which was more pronounced for density than for cover, and for one of the seeding years and study sites. Although crested wheatgrass density was significantly reduced for over three years, it rebounded to values equal to pre-treament values, indicating that these low-disturbance treatments do not offer better control than previous methods used in the Great Basin. In contrast, seed removal did not reduce crested wheatgrass abundance; however, our results showed that in some cases combining herbicide application with seed removal significantly increased densities of seeded species relative to herbicide alone, especially for a study site with a more northern aspect. While our results indicate that low-disturbance treatments avoid the pitfalls of exotic annual-weed influx, synergistic treatment combinations slated for near-monoculture stands must be applied for three or more years to lessen crested wheatgrass resilience and achieve greater persistence of seeded native species.

Technical Abstract: Past seedings of crested wheatgrass (Agropyron cristatum [L.] Gaertn. & A. desertorum [Fisch. ex Link] Schult.) have the potential to persist as near-monospecific stands, thereby necessitating active intervention to initiate greater species diversity and structual complexity of vegetation. However, the success of control treatments and native species seedings is limited by high resiliency of crested wheatgrass and the influx of exotic annual weeds associatd with mechanical soil disturbance. We designed a long-term study to evaluate the efficacy of low-disturbance herbicide and seed-reduction treatments applied synergistically or alone and either once or twice prior to seeding native species. Both crested wheatgrass and seeded species responses depended primarily on study site and treatment year rather than on consecutive treatment applications or synergism between them. In general, consecutive herbicide applications reduced crested wheatgrass density more than single applications, but this effect was consistent for only one of the seeding years. Crested wheatgrass showed considerable resilience after herbicide applications, which was more pronounced for density than for cover, and for one of the seeding years and study sites. Although crested wheatgrass density was significantly reduced for over three years, it rebounded to values equal to pre-treatment values, indicating that these low soil-disturbance treatments do not offer better control than previous methods used in the Great Basin. In contrast, seed removal did not reduce crested wheatgrass abundance; however, our results showed that in some cases combining herbicide application with seed removal significantly increased densities of seeded species relative to herbicide alone, especially for a study site with a more northern aspect. While our results indicate that low-disturbance treatments avoid the pitfalls of exotic annual-weed influx, synergistic treatment combinations slated for near-monoculture stands must be applied for three or more years to lessen crested wheatgrass resilience and achieve greater persistence of seeded native species.