Agriculture Reference
In-Depth Information
chaparral and sage scrub would likely have been in the range of every 30-100 yrs
or more (see
Chapter 5
). Today the region is dominated by an anthropogenic fire
regime and on most of the landscape fires are much more frequent.
Fires in undisturbed, relatively alien-free shrublands have limited alien presence
after fire due the lack of an alien seedbank. Even when aliens are present in gaps in
dense chaparral, fire intensities from these high fuel volume systems are capable of
killing alien seedbanks (Keeley
2001
). Those aliens that do colonize burned sites
are typically shaded out as the shrub canopy closes in the first decade or two after
fire. However, the native postfire annuals and other short-lived species in the early
seral communities form dense contiguous surface fuels when they die back during
the summer and autumn drought. With humans providing a ready source of
ignitions such communities are susceptible to reburning before the shrubs have
fully recovered and replenished their seedbanks. Given a source of alien grass and
forb propagules, these reburned sites are readily invaded, initiating an annual
grass fire cycle (
Box 12.1
) that can lead to degraded shrublands or complete type
conversion to grasslands (Zedler
et al.
1983
; Haidinger & Keeley
1993
; Jacobsen
et al.
2004
; Syphard
et al.
2006
; Cox & Allen
2008
; Fleming
et al.
2009
). Multivari-
ate modeling supports the hypothesis that the primary drivers of invasion are the
ready source of alien propagules at the time of fire and the speed of shrub canopy
closure (Keeley
et al.
2005d
).
Frequency of burning required to effect this type conversion varies with vegeta-
tion and site conditions. In California sage scrub, fires at less than 5-yr intervals
are often required for type conversion but longer-interval fires can also displace
sage scrub under more arid conditions (Keeley
et al.
2005d
). In chaparral, fire-
return intervals of 5-15 yrs often initiate invasion and set the community on a
trajectory toward type conversion. Over much of southern California there are
extensive areas of type conversion from sage scrub and chaparral to alien-
dominated annual grass and forb land. Sage scrub has suffered the greatest extent
of type conversion, primarily because it is distributed in the lower elevations and
subject to the greatest concentration of anthropogenic fires (Keeley
2006a
).
Jacobsen
et al.
(
2009
) suggest that water-use patterns of sage scrub species makes
this community more vulnerable to alien invasion.
Type conversion of chaparral communities to grasslands or herblands almost
certainly began in California with the activities of Native Americans (Keeley
2002b
). The vegetation cover of the Coast Ranges today comprises a mosaic of
chaparral, woodland and grassland. About a quarter of the landscape is covered
by annual grasslands that are largely dominated by alien grasses and forbs from
the Mediterranean Basin, which lack any obvious climatic or edaphic factors to
account for their distribution (Huenneke
1989
). Natural lightning ignitions in this
region would not have occurred frequently enough to allow the establishment and
maintenance of this open community, and thus they are undoubtedly an artifact of
an anthropogenic increase in fire frequency (Wells
1962
). We know that Native
Americans in California actively used fire as a management tool to open up
chaparral stands (Anderson
2006
), although prior to European introductions
