Drivers of Bromus tectorum Abundance in the Western North American Sagebrush Steppe

Abstract

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.

Refining the cheatgrass–fire cycle in the Great Basin: Precipitation timing and fine fuel composition predict wildfire trends

“…In many arid and semi‐arid ecosystems, precipitation is the main determinant of annual plant productivity (Holmgren et al., ). As expected, cheatgrass cover increased in wet years in our focal study area, a finding consistent with many other studies from the western US (e.g., Bradley, ; Brummer et al., ). Above‐average annual precipitation can result in exceptionally tall cheatgrass that forms dense, continuous cover in the Great Basin (Bradley & Mustard, ).…”

“…Alternatively, our seasonal temperature variables derived from monthly mean values may have been too coarse to predict biological phenomena. However, other studies also have found minimum influence of temperature on cheatgrass cover, with the exception of relatively weak effects of warmer winters and warmer summers (Bradley, ; Brummer et al., ).…”

“…When summer wildfires burn through areas infested with cheatgrass, the exposed soil creates an ideal environment for cheatgrass seed to germinate upon the arrival of fall and winter precipitation. The rapid vegetative growth of cheatgrass suppresses growth and recovery of native perennial plants, particularly in parts of the Great Basin where precipitation and native perennial bunchgrass cover are low (Brummer et al., ). Hence, the propensity of cheatgrass to dominate following fire is highest in warmer, drier locations (Taylor, Brummer, Rew, Lavin, & Maxwell, ) as well as where prefire biological soil crust cover is low and native perennial grasses and forbs are depleted (Chambers et al., ; Shinneman & Baker, ).…”

Recovery of biological soil crust richness and cover 12–16 years after wildfires in Idaho, USA

“…The interaction between B. tectorum and wildfires has been the subject of several studies in the western USA (for example, Chambers et al, 2014), but less is known about similar sites without this exotic annual grass. Since our study suggests that these ecosystems may differ from the paradigm in areas dominated by B. tectorum, other sites not dominated by B. tectorum, which include a substantial area of the western USA (Brummer et al, 2016), may warrant further study.…”

“…Alternatively, our sites have a climate that is cold and wet in the winter, and they support high cover of perennial graminoids and shrubs, which may mean fuels are not as strongly limiting for wildfires in our study region as they are in warmer and drier regions. Other studies have suggested that this combination of climate makes our study area, and many other regions of sagebrush steppe in the western USA, less suitable for one of the dominant annual invasive grasses, Bromus tectorum (Brummer et al, 2016).…”