Soil Resources Influence Vegetation and Response to Fire and Fire-Surrogate Treatments in Sagebrush-Steppe Ecosystems

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the Society for Range Management and can be found at: http://www.bioone.org/loi/rama.Current paradigm suggests that spatial and temporal competition for resources limit an exotic invader, cheatgrass (Bromus\ud tectorum L.), which once established, alters fire regimes and can result in annual grass dominance in sagebrush steppe.\ud Prescribed fire and fire surrogate treatments (mowing, tebuthiuron, and imazapic) are used to reduce woody fuels and increase\ud resistance to exotic annuals, but may alter resource availability and inadvertently favor invasive species.We used four study sites\ud within the Sagebrush Steppe Treatment Evaluation Project (SageSTEP) to evaluate 1) how vegetation and soil resources were\ud affected by treatment, and 2) how soil resources influenced native herbaceous perennial and exotic annual grass cover before\ud and following treatment. Treatments increased resin exchangeable NH₄⁺, NO₃⁻, H₂PO₄⁻, and K⁺, with the largest increases\ud caused by prescribed fire and prolonged by application of imazapic. Burning with imazapic application also increased the\ud number of wet growing degree days. Tebuthiuron and imazapic reduced exotic annual grass cover, but imazapic also reduced\ud herbaceous perennial cover when used with prescribed fire. Native perennial herbaceous species cover was higher where mean\ud annual precipitation and soil water resources were relatively high. Exotic annual grass cover was higher where resin\ud exchangeable H₂PO₄⁻ was high and gaps between perennial plants were large. Prescribed fire, mowing, and tebuthiuron were\ud successful at increasing perennial herbaceous cover, but the results were often ephemeral and inconsistent among sites.\ud Locations with sandy soil, low mean annual precipitation, or low soil water holding capacity were more likely to experience\ud increased exotic annual grass cover after treatment, and treatments that result in slow release of resources are needed on these\ud sites. This is one of few studies that correlate abiotic variables to native and exotic species cover across a broad geographic\ud setting, and that demonstrates how soil resources potentially influence the outcome of management treatments