171at least 18 species are widely introduced, 11 (or more) are crop weeds, 11 (or more)
are ruderal weeds, and at least eight are environmental weeds. Bromus hordeaceus
L. (soft brome) is one of the annual grasses that have replaced native bunchgrass
species in much of California (Howard 1998 ; Hobbs et al. 2007 ; Orrock and
Hoisington-López 2009 ). Bromus japonicus Houtt. (Japanese brome), a serious
crop weed in the North Central United States, is also invading natural environments
in northern mixed-prairie communities (Whisenant 1990 ; O’Connor et al. 1991 ;
Haferkamp et al. 1997 ). Some species in this section are associated with specifi c
crops. Bromus secalinus L. (rye brome) is a primarily a weed of cereal crops
(Tsvelev and Fedorov 1984 ; Cowbrough et al. 2007 ). Bromus interruptus (Druce)
grew with sainfoin, and Bromus grossus (Desf. ex DC.) was associated with spelt
wheat. Both are probably extinct in the wild (Ainouche and Bayer 1997 ; Rich and
Lockton 2002 ; Bilz 2011 ; Gigot 2011 ).
Genea and Bromus crop weeds reduce yields and increase weed control costs.
Many countries have restrictions limiting imports of commodities that contain their
seeds (Cowbrough et al. 2007 ; Walters 2011 ). Changes in agricultural methods may
be reducing populations of some species. B. secalinus is reported to be decreasing
in abundance in many areas, probably because of improved seed-cleaning tech-
niques and other control methods (Darbyshire 2003 ; Luneva 2003 –2009;
Cowbrough et al. 2007 ; Stace et al. 2005 ).
The perennial section Bromopsis contains far fewer weeds and invaders. Out of
89 species in the section Bromopsis , only B. inermis Leyss. (smooth brome) and
Bromus erectus Huds. (erect brome) are invasive. Taxonomists have suggested that
species in this section fall into two groups. The fi rst group is a mostly polyploid
Eurasian group with large anthers, small chromosomes, and a rhizomatous or
densely tufted growth pattern. Grasses in this group often cross-pollinate. This
group includes B. inermis , Bromus pumpellianus Scribn. (Pumpelly’s brome) ,
B. erectus , Bromus riparius Rehmann (meadow brome), and possibly South
American species Bromus auleticus Trin. ex Nees. The second group contains
mostly self- pollinating American species with small anthers and large chromo-
somes. These are less densely tufted and non-rhizomatous. Some Eurasian species
of Bromopsis , including Bromus ramosus Huds. (hairy brome) and Bromus benek-
enii (Lange) Trimen, are more similar to the North American species (Stebbins
1981 ; Armstrong 1983 ; Saarela 2001 ; Saarela et al. 2007 ; Sutkowska and Mitka
2008 ). The species in the Eurasian group have traits that suggest they may pose
more risk than the species in the American group. Rhizomes facilitate spread of
plants introduced into new environments, and rhizomatous species like B. inermis
may form monocultures (Otfi nowski et al. 2007 ; Otfi nowski and Kenkel 2008 ;
Sinkins and Otfi nowski 2012 ). The denser growth patterns of grasses in the Eurasian
group also make them useful for hay and for revegetation and soil stabilization.
The species in section Ceratachloa and Neobromus include annual, biennial, and
perennial species and are somewhat more likely to be widely introduced and weedy
than the species in section Bromopsis , but less so than the species in sections Bromus
and Genea. They may behave as short-lived crop weeds and ruderal weeds under
some circumstances. The Ceratochloa species of most concern, Bromus catharticus
6 Attributes That Confer Invasiveness and Impacts Across the Large Genus Bromus...