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where growing conditions are warmer and drier (Chambers et al. 2014a ). Despite
over 60 years of research, however, surprisingly little headway has been made in the
development of effective restoration tools and strategies for areas dominated by
B. tectorum (Mack 2011 ). Consequently, B. tectorum continues to expand its range
and gain site dominance by promoting recurrent wildfi res and altering soil moisture
and nutrient availability (Balch et al. 2013 ; Blank and Morgan 2013 ). Given its
propensity to invade under certain conditions (Chambers et al. 2007 ; Reisner et al.
2013 ), research has focused on reducing plant abundance (e.g., cover, density, and
biomass) and seed bank density in the short term to prevent the carryover of viable
caryopses (seed) from one year to the next (Young et al. 1969 ; Jones et al. 2015 ). In
contrast, relatively less research has emphasized how environmental conditions
(Young et al. 1969 ) and active control methods impact soil surface characteristics
(Young et al. 1972a , b , 1976 ).
12.2.6 Common Characteristics of Exotic Annual
Bromus Species
Based on our brief review, we suggest that population persistence from transient
seed banks, altered soil resources and litter production, displacement of native spe-
cies, and changed disturbance regimes are common characteristics associated with
ecosystems heavily invaded by Bromus. Due to the annual life form, persistence
within arid and semiarid ecosystems in the Western USA is primarily attributed to
suffi cient seed production and regeneration from seed banks to perpetuate popula-
tions. Interestingly, populations persist even when environmental conditions, patho-
gens, and active control treatments greatly reduce seed production and seedling
emergence (Smith et al. 2008 ; Baughman and Meyer 2013 ; Jurand et al. 2013 ).
Although this mechanism of persistence indicates that successful control strategies
will require multiple, consecutive-year efforts to diminish seed banks (Davies and
Johnson 2011 ), additional common characteristics should also be targeted. For
example, heavily invaded ecosystems may experience frequent disturbances that
increase resource availability and reduce the abundance of resident native species.
Consequently, annual grass invasion may leave legacies such as altered disturbance
regimes, soil microbial communities, and nutrient cycling (Belnap et al. 2005 ,
2015 ). When such legacies are present, simply removing Bromus may not result in
ecosystem recovery, especially for heavily invaded ecosystems where native species
have been displaced (Bauer 2012 ; Chambers et al. 2014a ). If positive feedbacks
among annual grass dominance, high litter production, and recurring wildfi res exist,
the frequency of disturbance must be lengthened in order for slower-growing native
perennial grass and shrub species to recover and establish from restoration seedings
(Knapp 1996 ; Gaertner et al. 2014 ). In addition, because increased litter abundance
generally favors germination and emergence of annual grasses, which is highly vari-
able—depending on site characteristics and interannual productivity and grazing
regime (Bansal et al. 2014 )—restoration efforts may need to address the roles of
T.A. Monaco et al.