The continued threat of an invasive, annual brome (Bromus) species in the western United States has created the need for integrated approaches to postfire restoration. Additionally, the high germination rate, high seed production, and seed bank carryover of annual bromes points to the need to assay soil seed banks as part of monitoring programs. We sampled the soil seed bank to help assess the effectiveness of treatments utilizing the herbicide Plateau® (imazapic) and a perennial native seed mix to control annual Bromus species and enhance perennial native plant establishment following a wildfire in Zion National Park, Utah. This study is one of few that have monitored the effects of imazapic and native seeding on a soil seed bank community and the only one that we know of that has done so in a pinyon–juniper woodland. The study made use of untreated, replicated controls, which is not common for seed bank studies. One year posttreatment, Bromus was significantly reduced in plots sprayed with herbicide. By the second year posttreatment, the effects of imazapic were less evident and convergence with the controls was evident. Emergence of seeded species was low for the duration of the study. Dry conditions and possible interactions with imazapic probably contributed to the lack of emergence of seeded native species. The perennial grass sand dropseed outperformed the other species included in the seed mix. We also examined how the treatments affected the soil seed bank community as a whole. We found evidence that the herbicide was reducing several native annual forbs and one nonnative annual forb. However, overall effects on the community were not significant. The results of our study were similar to what others have found in that imazapic is effective in providing a short-term reduction in Bromus density, although it can impact emergence of nontarget species.
Nomenclature: Imazapic, brome, Bromus, sand dropseed, Sporobolus cryptandrus Torr.
Management Implications: Invasive annual bromes threaten native diversity across vast areas of the western United States. A variety of techniques have been employed to control these species, yet populations continue to persist and expand. Recently, some success had been achieved through use of the herbicide imazapic. This study sampled the soil seed banks to evaluate the ability of imazapic and a native seed application to reduce brome occurrence and promote native species reestablishment following a wildfire at Zion National Park, Utah. This study is one of the few that has monitored the effects of imazapic and native seeding on a soil seed bank community. Target brome species were significantly reduced during the first year following application, but not during the second and third years. Several nontarget species were also reduced, but overall, the effect on the entire soil seed bank community was not significant. Our results also suggest that imazapic may have had an adverse effect on the emergence of at least two of the seeded native species. However, in general the seeded native species made only a minimal contribution to the soil seed bank and had a minimal effect on reducing brome species, although the increase in sand dropseed during the last year of study suggests that it may become a more substantial component of the seed bank in the next few years. Further research needs to be conducted both on the susceptibility of native species and on the timing of seeding additions in relation to imazapic applications. Finding additional site-adapted natives that can quickly replenish fire-impoverished seed banks would also be beneficial. Finally, the increase in nonnative species in the last year of the study suggests that further steps need to be taken to insure native establishment during the first year of imazapic application.