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

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Vahl (rescue brome), a valuable forage species with many cultivars, has become an
invasive environmental weed in some areas where it has been introduced for human
use (Halvorson and Guertin 2003 ; Randall 2007 ; Wu et al. 2009 ).


6.3.3 Year (Hypothesis 3)

We included year of taxonomic recognition (naming) in analyses after noticing that a
number of invasive species had been named and described by Linnaeus. Hypothesis 3
(Table 6.2 ) says that species that were named earlier are more likely to be invasive
than species that were named later. Both the simple and multiple regressions showed
signifi cant correlations between early species names and both wide introduction and
weediness (Tables 6.3 and 6.4 ). We believe that species that were named early are
those that were common, distinctive, and accessible to botanists during the eighteenth
and nineteenth centuries. Many native European Bromus species that are invasive
were named before 1800 (Fig. 6.2 ). Most widely introduced or weedy non- European
species were named before 1850. One interesting exception is the annual European
weed Bromus lepidus Holmb. (slender soft brome). B. lepidus , which was described
in 1924, is interfertile with B. hordeaceus but has smaller seeds similar in size to those
of Fescue pratensis Huds. (meadow fescue). It is believed to have been introduced in
Britain as a contaminant of commercial lawn and turf seeds (Smith 1968 ). Recent
species that are spreading into new areas could be of concern , as they may have adap-
tations that help them to propagate and grow in human-dominated environments,


6.3.4 Seed Awns and Seed Mass (Hypotheses 4 and 5)

One feature of the Bromus grasses that stands out is the presence of short to long
subapical lemma awns. Hypothesis 4 states that species with long-awned seeds are
more likely to be invasive. Our analyses showed long awns to be signifi cantly
correlated with both wide introductions and weediness (Tables 6.3 and 6.4 ).
Genea species have medium to long awns, while those of species in other
sections are short to medium (Fig. 6.3 ). Long awns can aid in seed dissemination by
sticking in fur and wool and can discourage grazing. Some Genea have stiff awns
that can injure livestock and wildlife. Bromus tectorum is a useful forage grass early
in the season, but the dried awns can damage the mouths of grazing animals (Reid
et al. 2008 ; Germino et al. 2015 ). Bromus diandrus Roth (ripgut brome) has long
stiff awns, which can penetrate the skin, eyes, intestines, and feet of livestock
(Murrumbidgee Catchment Management Authority 2008 ). Awns may also help
orient seeds correctly during burial and promote germination in upper soil layers
(Peart 1984 ; Humphreys et al. 2011 ). However, awnless seeds may become buried
deeper in soil, remain dormant longer, and show better fi re survival (Peart 1984 ;
Humphreys et al. 2011 ).


S.Y. Atkinson and C.S. Brown
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