Exotic Brome-Grasses in Arid and Semiarid Ecosystems of the Western US

39

Monument (Fig. 2.13c ), or elsewhere in the Northern Great Plains where Bromus
cover rarely exceeded 10 % Bromus cover (Fig. 2.14 ).
Resistance to invasion by exotic annual Bromus species of southern mixed-grass
prairie may be as high as in shortgrass steppe (Fig. 2.12 ). Although B. catharticus ,
B. arvensis , and B. tectorum occur in Oklahoma (Bidwell et al. 2004 ), none of the
fact sheets for managing invasive species published by the Oklahoma Cooperative
Extension Service are about exotic annual grasses or mention them as being prob-
lems ( http://www.oces.okstate.edu/extension-fact-sheets , accessed 6 Jan 2015). The
invasive species of concern are all shrubs and subshrubs (Cummings et al. 2007 ;
Bidwell et al. 2009 ).
Drought has been a common feature of the Western Great Plains climate
(Coupland 1992 ), where potential evaporation exceeds precipitation (Sims and
Risser 2000 ). Many long-term studies found weather to be a more important infl u-
ence on vegetation response than grazing or burning treatments (Teague and
Dowhower 2003 ; Teague et al. 2004 , 2010 ; Heitschmidt et al. 2005 ; White and
Loftin 2000 ; Vermeire et al. 2008 , 2014 ). Modeling efforts by Bradford and
Lauenroth ( 2006 ) indicate that climate is a stronger infl uence on invasion of B. tec-
torum than soil characteristics. Bradley ( 2009 ) found that two of the strongest pre-
dictors of improved habitat suitability for B. tectorum in the Great Basin region

High

Low

Resilience

High

Low

Resistance

a

b

historical regime

altered regime

B. tectorum

B. japonicus

fewer, larger storms

Northern

Mixed-grass

Prairie

Southern

Mixed-grass

Prairie

Shortgrass

Steppe

longer droughts fewer, larger storms

Lower

Productivity

Hotter-Drier Colder-Wetter

Higher

Productivity

Fig. 2.12 Hypothetical ( a ) resistance to common Bromus species and ( b ) resilience to historical
and altered fi re regimes for the Western Great Plains ecoregion (primary altered regime character-
istic labeled in gray )

2 Exotic Annual Bromus Invasions: Comparisons Among Species and Ecoregions...