Seed Bank Depletion: The Key to Long-Term Downy Brome (Bromus tectorum L.) Management

Invasive winter annual grasses such as downy brome (Bromus tectorum L.) are a threat to native ecosystems throughout the United States. Downy brome is able to exploit moisture and nutrients throughout the fall and early spring before native plants break dormancy. This results in decreased native species abundance and development of monotypic downy brome stands. Short-term downy brome management has been shown to be effective; however, the soil seed reserve has often been overlooked, although it's the mechanism responsible for rapid reestablishment. This field study was conducted at two sites in Colorado to evaluate the longevity of the downy brome soil seed reserve and its implications on long-term downy brome control. Glyphosate plus adjuvant applications were made for 0, 1, 2, 3, 4, or 5 consecutive years. Downy brome and perennial grass biomass harvests were conducted yearly to determine changes in species composition. In addition, soil cores were collected to evaluate the yearly variation and depletion of the downy brome soil seed bank in response to consecutive glyphosate applications. We found that 1 − 3 yr of consecutive glyphosate treatments were insufficient to deplete the downy brome soil seed bank. Downy brome biomass and the soil seed bank recovered within 1 − 2 yr after glyphosate treatments were terminated; however, 4 and 5 consecutive yr of glyphosate applications were sufficient to control downy brome through depletion of the soil seed bank. Managing downy brome for 4 − 5 consecutive yr resulted in a 4- to 9-fold increase in perennial grass biomass. These data suggest that long-term management of downy brome is dependent on eliminating the soil seed bank using a multiyear approach.