31the bunch grass Nassella pulchra (Hitchc.) Barkworth (purple needlegrass) while
reducing dominance of Bromus (Stahlheber and D’Antonio 2013 ). It has also been
suggested that spring fi res, which kill the exotic seed bank while still on the plant but
do not diminish the resprouting capacity of native perennial bunch grasses and forbs,
have potential (DiTomaso et al. 2006 ), although native grasses can still be highly
susceptible to m ortality from fi re (Marty et al. 2005 ). One experiment showed that
high-intensity fi res in mixed grasslands can improve the ratio of native versus exotic
( Bromus and Avena spp.) grasses for at least 2 years (Keeley and Brennan 2015 ).
However, a key to the success of this effort was that these native-dominated grass-
lands possessed substantial fuel loads because it had been many years since the last
fi re or other major form of disturbance. Large fuel loads contributed to extraordinary
fl ame lengths and presumably high soil temperatures that were suffi cient to kill the
exotic seed bank but allowed for the survival of native perennial grasses.
Studies from other vegetation types also illustrate the importance of the fuel load
in creating fi re behavior that can reduce Bromus dominance. For example, an inves-
tigation of spring burning on B. tectorum understory in a ponderosa pine forest
failed to displ ace this exotic invader (Keeley and McGinnis 2007 ). In that case the
low nutrient availability of the soils produced rather sparse grass populations and
fi eld measures of temperatures during the fi re revealed that they failed to reach high
enough levels to kill seed banks. The importance of high fuel loading to make this
approach effective has been noted in studies of other Bromus (Sweet et al. 2008 ).
Even when prescription burning to eliminate Bromus has been effective for the tar-
get species, they are replaced by other exotic annuals (Keeley et al. 2011 ). Other
studies have also shown that effects of a single fi re on annual grass r eduction can be
short-lived and repeated burning or burning followed by grazing may be necessary
for sustained reductions in annual grasses (Corbin et al. 2004 ).
2.4 Cold Deserts
Environmental conditions in the Cold Deserts are on average moderately warm and
dry in comparison to the range of soil temperature and moisture regimes across the
western United States (Fig. 2.3 ). However, the topographic diversity of this ecore-
gion results in high temperature and moisture variability associated with local ele-
vation gradients. The lower elevations dominated by salt desert and Wyoming big
sagebrush ecosystems are relatively hot and dry whereas the higher elevations typi-
fi ed by mountain sagebrush and mountain brush are cold and moist. The most wide-
spread invasive Bromus in the Cold Deserts ecoregion is B. tectorum which occurs
over a broad range of temperature and precipitation conditions (Fig. 2.2 ). Bromus
rubens also occurs in the Cold Deserts but appears less cold tolerant (Fig. 2.2a ) and
occurs at lower elevations and latitudes than B. tectorum. Colder winter conditions ,
in particular, severe and sudden freeze events, are lethal to B. rubens but are not
lethal to B. tectorum (Bykova and Sage 2012 ).
2 Exotic Annual Bromus Invasions: Comparisons Among Species and Ecoregions...