Drivers of Bromus tectorum Abundance in the Western North American Sagebrush Steppe
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Bromus tectorum can transform ecosystems causing negative impacts on the ecological and economic values of sagebrush steppe of the western USA. Although our knowledge of the drivers of the regional distribution of B. tectorum has improved, we have yet to determine the relative importance of climate and local factors causing B. tectorum abundance and impact. To address this, we sampled 555 sites distributed geographically and ecologically throughout the sagebrush steppe. We recorded the canopy cover of B. tectorum, as well as local substrate and vegetation characteristics. Boosted regression tree modeling revealed that climate strongly limits the transformative ability of B. tectorum to a portion of the sagebrush steppe with dry summers (that is, July precipitation <10 mm and the driest annual quarter associated with a mean temperature >15°C) and low native grass canopy cover. This portion includes the Bonneville, Columbia, Lahontan, and lower Snake River basins. These areas are likely to require extreme efforts to reverse B. tectorum transformation. Our predictions, using future climate conditions, suggest that the transformative ability of B. tectorum may not expand geographically and could remain within the same climatically suitable basins. We found B. tectorum in locally disturbed areas within or adjacent to all of our sample sites, but not necessarily within sagebrush steppe vegetation. Conversion of the sagebrush steppe by B. tectorum, therefore, is more likely to occur outside the confines of its current climatically optimal region because of site-specific disturbances, including invasive species control efforts and sagebrush steppe mismanagement, rather than climate change.
Keywordscheatgrass climate disturbance fire native grass abundance sagebrush biome
This study gained momentum during 2009–2011 field studies in the Idaho National Laboratory. We are grateful to Roger Blew of Gonzales-Stoller Surveillance, LLC, for facilitating the funding and logistics of that interval of our field studies (GSS-0300-1,100,008). BDM and KTT were supported by NSF-WildFIRE PIRE, OISE 09667472. We also acknowledge Curt Freese, World Wildlife Fund, Randall Scott, Northern Arizona University, Arnold Tiehm, University of Nevada Reno, and Martin Wojciechowski, Arizona State University, for facilitating part of the fieldwork conducted during the course of this study. Antonia Hedrick, Bureau of Land Management, Boise, Idaho, first pointed out to us the dramatic landscape differences between the lower and upper Snake River Plains of southern Idaho. We thank the editors and two anonymous reviewers whose comments and suggestions greatly improved the presentation of this manuscript.
Conflict of Interest
The authors declare that they have no conflict of interest.
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