Agriculture Reference
In-Depth Information
deep-rooted shrubs to shallow-rooted grasses, (4) increase in fine fuels and thus
lengthening the fire season, and (5) change in carbon storage capacity.
Although it appears that the majority of these grasslands arose from type
conversion of shrublands, on some sites they apparently displaced native perennial
grasslands (Hamilton
1997
) and on other sites annual forb-dominated
herblands
(Hoover
1936
). In all cases the origin of these alien annual grasslands was
dependent on increased frequency of fire and other disturbances such as intensive
animal browsing (Zedler
et al.
1983
; O'Leary & Westman
1988
; Haidinger &
Keeley
1993
; Cushman
et al.
2004
; Jacobsen
et al.
2004
). Air pollution has been
suggested as an alternative factor but there is no evidence that this alone is
sufficient; however, it is possible that type conversion has been enhanced by
ozone (Westman
1979
) and nitrogen pollution (Padgett & Allen
1999
; Allen
et al.
2000
). One study reporting for some sites a stronger relationship between
alien grasses and pollution than between alien grasses and fire (Talluto & Suding
2008
) requires further study as the researchers used a database of only larger fire
events; thus in the highly polluted portions of southern California they only
captured 5-10% of all fires.
Once established, annual grasslands can be very persistent and the return of
native shrublands is a rather slow process (Keeley
1990a
; Minnich & Dezzani
1998
; Stylinski & Allen
1999
; Rundel
2007
; Cox & Allen
2008
). On sites formerly
dominated by sage scrub, recolonization may occur within a decade or so if
disturbance is removed and seed sources are in close proximity (Freudenberger
et al.
1987
). This is promoted by the highly dispersed propagules in most sage
scrub species (Wells
1962
). When chaparral is type converted, natural restoration
is slow because shrubs with good dispersal establish poorly on open sites and those
capable of recruiting on open sites have poorly dispersed seeds (see
Chapter 5
).
The process where shrubs are observed to invade annual grasslands is often
referred to as
shrub encroachment
, but this is a misnomer since they are actually
recolonizing and restoring the natural communities (Keeley
2005
). Encroachment
is appropriate to describe the spread of alien shrubs into grasslands.
Some of the management practices that have long been used to reduce fire
hazard in California shrublands have now been shown to have a mixed benefit as
they also promote invasions of non-native grasses into native shrublands (Keeley
2006b
). This is particularly evident with programs to reduce hazardous fuels along
the urban-wildland interface. These programs reduce the amount of flammable
biomass in such areas and thus potential fire intensity but at the same time the
type conversion promotes the invasion of alien annual grasses and forbs into these
open habitats. Without careful management of these grassy areas, flammability is
locally enhanced because of the nature of these flashy fuels that ignite easily and
can carry fire through much of the year. These problems are also evident in other
fire-related management activities such as fuel breaks (Merriam
et al.
2006
; Potts &
Stephens
2009
). A related theme that has been studied relates to the management
implications of fuel breaks. Fuel breaks along ridgelines or across shrubland slopes
tend to promote alien plant invasion and play a role as a seed source for the
