Rough Surface and High-Forb Seed Mix Promote Ecological Restoration of Simulated Well Pads

Because of disturbance and exotic plant invasions, ecological restoration is necessary for maintaining functional big sagebrush ecosystems in western North America. Downy brome control is often necessary in restoring this ecosystem type; however, many brome control measures hinder ecological restoration by limiting the types of plants which can be established. Microtopography manipulation may aid weed control by entrapping undesirable seeds. We undertook a field experiment at four sites in the Piceance Basin of western Colorado, USA to test the effects of microtopography (rough with brush mulch or flat with straw mulch), seed mix (high-forb or balanced), and herbicide (140 g ai ha⁻¹ imazapic ammonium salt or none) on downy brome control and perennial plant establishment following disturbance. Three years post-treatment, downy brome had become established at two of the four sites, one each with high (GVM) and low (MTN) downy brome seed rain. At GVM, the rough/brush treatment augmented the effectiveness of imazapic, reducing downy brome biomass six-fold. At MTN, the rough/brush surface reduced downy brome biomass 10-fold in the absence of imazapic. Across all four sites, forb and shrub biomass were higher with the high-forb mix, and there was no effect of seed mix on downy brome or annual forb biomass. Restoring a full complement of plant functional groups in big sagebrush ecosystems may be aided by increasing forbs in seed mixes, and manipulating soil microtopography.Nomenclature: imazapic ammonium salt, big sagebrush, Artemisia tridentata Nutt. ARTR2, downy brome, Bromus tectorum L. BROTE.Management Implications: Big sagebrush ecosystems in western North America are threatened by many factors, including downy brome invasion, conifer encroachment, and disturbance. Restoring downy brome-invaded ecosystems is difficult because many effective downy brome control measures, such as seeding competitive grasses or applying herbicides, impair the growth of desirable forbs and shrubs. Previous work has shown that downy brome seeds disperse more readily when the soil surface is flat or when vegetation is removed, such as following a fire or after the creation of an oil and gas well pad. In this study, we examined whether creating obstructions to seed dispersal could improve restoration. We tested three factors: (1) a rough soil surface, comprised of pothole-sized holes and mounds with brush mulch vs. a flat soil surface with straw mulch, (2) a high-forb seed mix, containing nearly 75% forbs by seed number, vs. a mix containing roughly equal numbers of forb, grass, and shrub seeds, and (3) imazapic herbicide at 140 g ai ha⁻¹ vs. no herbicide. Downy brome established at two of the four sites, and at both of these, the rough surface helped to control downy brome. The rough surface combined with imazapic was most effective at a site that had some downy brome prior to disturbance, while the rough surface alone was effective at another site which was not invaded prior to disturbance. It is possible that the rough surface makes downy brome less competitive because the seeds get trapped in the holes, which have higher soil moisture, as downy brome is typically less competitive in wetter environments. The high-forb seed mix resulted in higher forb and shrub establishment, and lower grass establishment, than the balanced mix. Even so, grass cover was still higher than forb cover at most sites, and was higher than in the undisturbed communities. There was no difference in downy brome or weedy annual forbs due to seed mix, and the high forb cover produced by the high-forb mix is beneficial for sage-grouse habitat restoration. Seeding mostly forbs and shrubs at a high rate, 1600 seeds m⁻², should be considered in areas where erosion is not a concern. The imazapic treatment did control downy brome, but it also had a negative impact on grasses, forbs, and shrubs. The lack of perennial plant establishment led to higher annual forb cover in imazapic plots three years post-treatment. Extending the plant-back interval, applying at a lower rate, and restricting application to areas with downy brome presence prior to disturbance are recommended.