45and result in increases in B. tectorum growth and reproduction (Covington and
Sackett 1992 ; Monleon et al. 1997 ; Gundale et al. 2005 , 2008 ). Also, charcoal gen-
erated by fi re may adsorb phenolic compounds from pine litter inputs which can
otherwise impede understory plant growth (Gundale and DeLuca 2006 , 2007 ), and
increase nitrifi cation rates, further increasing the amount of plant available N in the
soil following fi re (DeLuca et al. 2006 ).
High severity human disturbances such a s clear-cut logging can similarly
decrease resistance of landscapes. Recruitment, survival, and reproduction of
B. tectorum can all be higher in clear-cuts across a range of western forest types
including those dominated by P. ponderosa , Pseudotsuga menziesii (Mirb.) Franco
(Douglas-fi r), Abies grandis (Douglas ex D. Don) Lindl. (grand fi r), and Thuja
plicata Donn ex D. Don (western redcedar) (Pierson and Mack 1990 ). Lower sever-
ity understory thinning can also decrease resistance to B. tectorum in these forest
types, but only where signifi cant understory cover of B. tectorum was present
before thinning. Even lower severity human disturbances such as cattle grazing
occurring at light intensity and short duration can lead to increased cover of
B. tectorum in ponderosa pine forests under drought conditions (Sorensen and
McGlone 2010 ), possibly due to competitive release of other vegetation such as
Elymus elymoides (Raf.) Swezey (bottlebrush squirreltail) and P. smithii caused by
preferential grazing by cattle.
Removal of litter and coarse woody debr is can increase exposure of mineral soil
and the probability that Bromus seeds can make contact with the soil and germinate
(Pierson and Mack 1990 ; Keeley and McGinnis 2007 ; Gundale et al. 2008 ). This is
partly due to the litter creating a physical barrier preventing seeds from coming into
contact with mineral soil, but also may be due to phenolytic compounds in new pine
litter that can inhibit germination.
Lower temperature limits may limit the upper elevation sites attainable by
Bromus species (Fig. 2.2a ), but climate change is increasingly recognized as a
mechanism by which high elevation habitats such as Western Forests may become
increasingly more invasible (Pauchard et al. 2009 ). There is evidence suggesting
that B. tectorum already exhibits the ability to adapt to higher elevation sites (Rice
and Mack 1991 ; Leger et al. 2009 ), and climatic warming may further reduce the
resistance of these habitats to Bromus invasion (Compagnoni and Adler 2014 ).
Rising CO 2 has also been shown to increase B. tectorum productivity through
increased water use effi ciency in Cold Deserts shrublands (Smith et al. 1987 ), and
the operative mechanism could further promote growth at higher elevations as they
warm.
2.6.2 Ecosystem Threats to Western Forests from Bromus
It is unclear how much of a threat Bromus pose to Western Forests compared to
other ecoregions of the western United States. No doubt Bromus can have some of
the same effects documented for other ecoregions, such as direct competition with
2 Exotic Annual Bromus Invasions: Comparisons Among Species and Ecoregions...