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

32

2.4.1 Invasion Potential of Cold Deserts by Bromus

Salt desert vegetation types typically occur at relatively low elevations and have the
warmest (mesic) and driest (aridic) soil temperature and moisture regimes in the
Cold Deserts. Vegetation types that characterize the salt desert are typically domi-
nated by members of the Chenopodiaceae such as Atriplex L. spp. and Sarcobatus
Nees spp. (West 1983a , b ). Resistance to B. tectorum and presumably B. rubens ,
which has received less study in this vegetation type, is low to moderate (Fig.
2.10a ). Under the warmest and driest regimes, establishment of B. tectorum is lim-
ited by frequent low and sporadic precipitation years, and longer term persistence
requires either the necessary environmental conditions to maintain viable popula-
tions or high propagule pressure (Meyer et al. 2001 ). Resilience to disturbance is
generally low due to low productivity, infrequent plant establishment, and slow
recovery (Fig. 2.10b , and see Haubensak et al. 2009 ). Recent expansion of B. tecto-
rum and B. rubens into marginal salt desert environments has been attributed to
favorable years for establishment during El Niño–Southern Oscillation (ENSO)
events (Meyer et al. 2001 ; Salo 2005 ). Rising CO 2 levels (Smith et al. 1987 ) and
high genetic variability (Ramakrishnan et al. 2006 ; Haubensak et al. 2014 ) also may
help explain increased establishment and spread of B. tectorum and likely B. rubens.
Invasion of B. tectorum has increased fi ne fuels and fuel continuity in these types

High

Low

Resilience

High

Low

Resistance

a

b

B. rubens

B. tectorum

historical regime

altered regime

increased fire frequency

and/or overgrazing

Wyoming

sagebrush

Mountain

sagebrush

Mountain

brush

Salt

desert

Lower

Productivity

Hotter-Drier Colder-Wetter

Higher

Productivity

Fig. 2.10 Hypothetical ( a ) resistance to common Bromus species and ( b ) resilience to historical
and altered fi re regimes for the Cold Deserts ecoregion (primary altered regime characteristic
labeled in gray ). Figure modifi ed from Chambers et al. ( 2014a )

M.L. Brooks et al.