Abstract
AbstractTwo Eurasian invasive annual brome grasses, cheatgrass (Bromus tectorum) and Japanese brome (Bromus japonicus), are well known for their impact in steppe ecosystems of the western United States where these grasses have altered fire regimes, reduced native plant diversity and abundance, and degraded wildlife habitat. Annual bromes are also abundant in the grasslands of the Northern Great Plains (NGP), but their impact and ecology are not as well studied. It is unclear whether the lessons learned from the steppe will translate to the mixed‐grass prairie where native plant species are adapted to frequent fires and grazing. Developing a successful annual brome management strategy for National Park Service units and other NGP grasslands requires better understanding of (1) the impact of annual bromes on grassland condition; (2) the dynamics of these species through space and time; and (3) the relative importance of environmental factors within and outside managers' control for these spatiotemporal dynamics. Here, we use vegetation monitoring data collected from 1998 to 2015 in 295 sites to relate spatiotemporal variability of annual brome grasses to grassland composition, weather, physical environmental characteristics, and ecological processes (grazing and fire). Concern about the impact of these species in NGP grasslands is warranted, as we found a decline in native species richness with increasing annual brome cover. Annual brome cover generally increased over the time of monitoring but also displayed a 3‐ to 5‐yr cycle of reduction and resurgence. Relative cover of annual bromes in the monitored areas was best predicted by park unit, weather, extant plant community, slope grade, soil composition, and fire history. We found no evidence that grazing reduced annual brome cover, but this may be due to the relatively low grazing pressure in our study. By understanding the consequences and patterns of annual brome invasion, we will be better able to preserve and restore these grassland landscapes for future generations.
Highlights
During the last century, much of the prairie within the Northern Great Plains (NGP) has been plowed for cropland, planted with non-native grasses to maximize livestock production, or otherwise developed, making it one of the most threatened ecosystems in the United States
Study area Data were collected from seven national park units in the NGP varying in size from ~337 to 40,000 ha (Fig. 1): Fort Laramie National Historic Site (NHS) and Devils Tower National Monument (NM) in Wyoming; Scotts Bluff NM and Agate Fossil Beds NM in Nebraska; and Wind Cave National Park (NP), Jewel Cave NM, and Badlands NP in South Dakota
Annual exotic brome grasses are widespread in NGP grasslands, including the national park units in the region
Summary
Much of the prairie within the Northern Great Plains (NGP) has been plowed for cropland, planted with non-native grasses to maximize livestock production, or otherwise developed, making it one of the most threatened ecosystems in the United States. Cheatgrass and Japanese brome are Eurasian, annual grasses that have been a part of the NGP landscape for more than a century, but their invasion in the region has accelerated since 1950 (Schachner et al 2008). The two species are functionally similar; both are cool-s eason grasses that germinate in the fall, winter, or early spring and begin growth earlier in the spring than many of the native perennials. This gives them a head start in competing for moisture and nutrients with native species, thereby reducing native production and nutritional content of forage (Haferkamp et al 1997, 2001b). More information on the dynamics of annual brome invasion in the NGP is needed to protect and preserve the remaining native prairies
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