157Several studies have investigated invasiveness in annual Bromus grasses. Hulbert
( 1955 ) studied ten species that have been introduced into the Western United States
and suggested that winter hardiness, ability to germinate at lower temperatures,
rapid growth, spring maturation, and high seed output all help to make B. tectorum
a successful invader in semiarid areas. A more recent study of genetic and demo-
graphic characteristics of annual Bromus species (Roy 1990 ; Roy et al. 1991 ) found
a positive relationship between the number of climactic zones in the native range
and the number of regions with a Mediterranean climate occupied worldwide but
did not fi nd other clear differences between invasive and noninvasive species.
Databases for taxa that have high numbers of invasive species and that contain
information about characteristics of interest can be used in conjunction with predic-
tion and modeling tools to contribute to a better understanding of invasiveness and to
improve prevention and management of invasions. We developed and populated a
database of information about grass species in the genus Bromus to investigate the use
of databases for research into plant invasions and to develop an understanding of char-
acters associated with invasiveness in Bromus grasses. The database was developed in
conjunction with a Bromus Research, Education, and Extension (REEnet) project to
coordinate networking on the distribution and abundance, impacts, and management
of exotic and invasive Bromus species in the Western United States. It contains
information about range and distribution, habitat, plant traits, environmental interac-
tions, weediness and invasiveness, human uses, and management for over 150 species
( http://greatbasin.wr.usgs.gov/GBRMP/bromus/bromus.html ).
Grasses in the genus Bromus belong to the subfamily Pooideae and the tribe
Bromeae. There are two other genera in the Bromeae tribe, Boissiera (one species)
and Littledalea (four species) (Clayton and Renvoize 1986 ). The number of species
in genus Bromus is sometimes listed at over 400, but about 150 species are currently
considered to be valid (Soderstrom and Beaman 1968 cited by Saarela et al. 2007 ;
Clayton and Renvoize 1986 ). Bromus grasses use the C3 photosynthetic pathway ,
grow in temperate regions and at high elevations in the tropics, and are closely
related to important cereal grasses including wheat, rye, oats, and barley (Grass
Phylogeny Working Group 2000 , 2001 ). They differ from most other grasses in hav-
ing simple rounded starch grains rather than compound starch grains (Clayton and
Renvoize 1986 ).
Taxonomists have divided the grasses of the genus Bromus into sections (Smith
1970 ) or subgenera (Stebbins 1981 ). Tsvelev ( 1984 ) considered the differences
between Bromus subgroups to be so large that he separated the genus into separate
genera (Tsvelev and Fedorov 1984 ). All three classifi cations are used today, typi-
cally with Bromus species separated into six sections, subgenera, or genera (Table 6.1 ).
The section names will be used here. Grasses in three sections, Bromus (a section in
genus Bromus ), Genea , and Nevskiella , are annuals. Those in section Bromopsis are
perennials with one exception. The grasses in section Ceratochloa and Neobromus
tend to have somewhat intermediate life cycles and may be variously described as
annual, biennial, short-lived perennial, or perennial.
In the fall of 2012 and the spring of 2013, we used the database to investigate
patterns of introduction and weediness in order to develop and examine hypotheses
6 Attributes That Confer Invasiveness and Impacts Across the Large Genus Bromus...