27Fire history plays a critical role in the prevalence of exotic grasses within shru-
bland ecosystems of the region. Short intervals between fi res can thin shrub density,
which can both increase grass abundance and reduce native shrub seed banks and
the vigor of potential resprouts (Zedler et al. 1983 ; Haidinger and Keeley 1993 ;
Talluto and Suding 2008 ; Keeley and Brennan 2012 ). These thinned stands lead to
reduced fi re intensity which may further favor survivorship of exotic propagules
(Keeley et al. 2008 ). A classic example of this dynamic is illustrated in Fig. 2.9
where a chaparral stand that burned three times over three decades was dominated
by B. madritensis compared to areas burned only once or twice. Howeve r, in a
region-wide remote sensing study only a few shrublands appeared to be undergoing
type conversion via fi res that occurred two times within 8 years (Meng et al. 2014 ),
so other factors may interact with fi re frequency to affect vegetation tra jectories.
Atmospheric pollution in the form of nitrogen deposition has been implicated in
the invasion of semiarid shrublands by annual grasses including Bromus (Westman
1979 ; Allen et al. 1998 ). In desert scrub, openings between widely spaced shrubs
are readily invaded by invasive grasses and this is enhanced by nitrogen pollution
(Brooks 2003 ). In contrast, within closed canopy shrublands there is no evidence
Fig. 2.9 Chaparral with three different fi re histories in southern California. The entire scene
burned in the 1970 Laguna Fire. The front and middle third burned in the 2001 Viejas Fire, and the
front third burned a third time in the 2003 Cedar Fire. The background illustrates mature chaparral
dominated by the native shrub Adenostoma fasciculatum , the middle third early seral chaparral
dominated by the native shrub Lotus scoparius , and the front portion a 1-year post-fi re scene domi-
nated by the nonnative annual grass B. madritensis (photo by Richard Halsey, Spring 2004)
2 Exotic Annual Bromus Invasions: Comparisons Among Species and Ecoregions...