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

26

perennial cover recovers more quickly. In a study of 250 chaparral sites (Keeley
et al. 2008 ), B. madritensis cover was negatively correlated with pre-fi re stand age
and previous fi re severity. In addition, 5 years post- fi re cover of exotic species, of
which Bromus was a major contributor, was best predicted by shrub cover and esti-
mated exotic seed bank at the time of fi re. In essence, the extent to which Bromus
and other exotics can invade disturbed Mediterranean shrublands is a race between
exotic propagules getting to the site (since most that were there before are killed by
high-intensity fi re) and shrub canopy reestablishment. Under historical disturbance
regimes, productivity and resilience covaried (Fig. 2.8b ). However, various anthro-
pogenic factors have altered disturbance regimes and reduced ecosystem resilience,
especially at the lowest and highest ends of the productivity gradient. Because chap-
arral tends to be more species rich with more post-fi re resprouting species compared
to sage scrub, it tends to be more resilient to increased fi re frequency (Fig. 2.8b ).
Exotic annual grasslands , most of which have a strong component of Bromus ,
owe their origins to either (1) disturbance that converts native woody vegetation or
mixed native perennial grass/shrub stands or (2) direct invasion of native grasslands
(Keeley 1990 ). Thes e exotic grasslands are markedly unlike native perennial grass-
lands in that the former are lower in functional diversity than the latter, being domi-
nated entirely by annual grasses and forbs with a different composition of native
forbs (Molinari and D’Antonio 2014 ). Where remnants of native grasslands persist
within larger exotic annual grasslands, Bromus are usually only co-dominants (e.g.,
Molinari and D’Antonio 2014 ).

High

Low

Resilience

High

Low

Resistance

a

b

Sage scrub Perennial Chaparral

grassland

Oak

woodland

Mixed

conifer

B. rubens

B. madtritensis

B. diandrus / B. hordeaceus

B. tectorum

historical regime

altered regime

overgrazing

increased

increased fire frequency

fire frequency increased

fire severity

Lower

Productivity

Hotter-Drier Colder-Wetter

Higher

Productivity

Fig. 2.8 Hypothetical ( a ) resistance to common Bromus and ( b ) resilience to historical and altered
fi re regimes for the Mediterranean California ecoregion (primary altered regime characteristic
labeled in gray )

M.L. Brooks et al.