40
Fig. 2.13 ( a ) Typical B. tectorum invasion of roadsides in the shortgrass steppe ecosystem, Weld
County, Colorado. Photo by CS Brown. ( b ) Conservation Reserve Program land 3 years after seed-
ing with native species. Increased beige vegetation on left side of fence is B. tectorum. Photo by M
Vandever. ( c ) Little or no Bromus at Agate Fossil Beds National Monument. Photo by US
Geological Survey. ( d ) Aerial cover at this site in Scotts Bluff National Monument is 60 %
B. tectorum and 10 % B. arvensis. Photo by US National Park Service
were decreased annual and summer precipitation. This suggests that the normal
pattern of summer precipitation in the Western Great Plains creates conditions unfa-
vorable to Bromus and drought could improve their success. However, it does not
appear that drought alone (see Van Dyne 1975 ; Munson and Lauenroth 2009 ; Moran
et al. 2014 ) or in combination with grazing (Heitschmidt et al. 1999 , 2005 ; Eneboe
et al. 2002 ) has reduced resistance of these grasslands to invasion by Bromus.
Grazing of ungulates has always been important in the Great Plains, which was
once inhabited by 2–4 million bison ( Bison bison ) and other grazing and browsing
ungulates (Van Dyne 1975 ). As a result, this ecoregion is resilient to grazing
(Vermeire et al. 2008 ). The abundances of Bromus and other weedy plants can be
reduced (Fig. 2.13b ) (Milchunas et al. 1992 , 2011 ; Harmoney 2007 ; Vermeire et al.
2008 ) or remain unaffected by grazing, even in combination with drought
(Heitschmidt et al. 1999 , 2005 ).
Fire is a natural disturbance in the Western Great Plains and has either no effect
on or reduces aboveground productivity (Scheintaub et al. 2009 ). Herbaceous pro-
ductivity of shortgrass steppe (Ford and Johnson 2006 ; Augustine and Milchunas
M.L. Brooks et al.