The role of a pre-emergent herbicide to suppress non-native annuals and facilitate the recovery of a burned Mojave Desert shrubland

Background/Question/Methods Recurring wildfires fueled by non-native annual grasses deplete soil seed reserves of native desert plants and diminish opportunities for native plants to re-establish. Control of non-native annual brome grasses and forbs and replenishment of native seeds is vital to recovering severely degraded desert shrublands, but post-fire treatments often focus solely on restoring native plants and overlook reducing dominant non-native species. In addition, the success and failure of post-fire treatments are rarely evaluated as a function of effective precipitation. Seeding treatments were implemented in 2006 – helicopter seeding, seeding with incorporation of the seed mixture into the soil, and no seeding treatment – across eight northeast Mojave Desert sites that burned during 2005 and 2006. Within each treatment, we established replicated 0.01 ha plots with pre-emergent herbicide or no herbicide in fall 2008 to evaluate reduction of non-native annuals and re-vegetation success. Peak shoot production of annual grasses and forbs was estimated and perennial plant density and cover were measured during spring 2009 and 2010. Soil cores were collected from each plot after the first spring to determine the density of seeds in the seed reserves.

Results/Conclusions Herbicide reduced production of non-native annual grasses by 85% during the first spring following application. Production of non-native and native annual forbs was also reduced (92% and 67%, respectively). Soil seed reserves reflected the lower production of brome grasses (69% reduction in seed densities) and non-native annual forbs (71% reduction), but native annual seed reserves were not affected by herbicide. Herbicide effects persisted during the second spring for non-native annual grasses and forbs (40% and 44% reduction in shoot production, respectively) and were associated with a doubling of native annual forb production. Seed reserves of native perennial grasses, which made up 91% of the seed mixture, were unaffected by herbicide, and four years after seeding treatments, seed densities remained almost three times higher in both seeding treatments compared with the unseeded treatment. Sites that received the highest cumulative rainfall had 1.6 times the seeded perennial plant densities and 3% greater live canopy cover than sites with lower rainfall. Even though the densities of seeded species were reduced by herbicide during the second spring, the live perennial canopy cover was not different between the treatments. Our results demonstrate that management of the soil seed reserves through the combination of seeding and pre-emergent herbicide is a promising strategy for facilitating recovery of burned desert habitats.