Agriculture Reference
In-Depth Information
Box 5.1 (
cont.
)
fires in those forests. However, the fire history data fail to support the idea that
fire suppression has excluded fires in shrubland crown fire ecosystems in
southern California (Keeley et al. 1999a ;Mensing et al. 1999 ;Keeley&Fother-
ingham 2001a , 2001b ). In fact rather than this landscape having massive stands
of anomalously old vegetation, quite the opposite is true (see Fig. 5.10 ).
Further evidence against the hypothesis that large fires are due to contempor-
ary fire management practices is the demonstration that large chaparral fires
were frequent prior to the modern fire suppression era (Keeley & Zedler 2009 ;
Lombardo et al. 2009 ).
Although there is clear evidence that fire suppression has not caused anomal-
ous changes in southern California fuels, Minnich ( 2001 ; Minnich & Chou 1997 )
has insisted that there is a marked change in the pattern of burning immediately
south of the border ( Fig. B5.1.1 ) and this must be viewed as support for their
model of how different fire management policies have changed fire behavior.
However, one of the alternative explanations for the differences in fire size north
and south of the USA/Mexican border identifies the very different demographic
patterns and rural land use (Keeley & Fotheringham 2001a , 2001b ). North of the
border most of the population is concentrated in coastal cities and much more of
the interior landscape is largely protected wildlands. However, south of the
border, population centers are distributed much further inland and there is a
striking change in social patterns immediately south of the border ( Fig. B5.1.2 ).
Intensive rural development has greatly fragmented fuels, with consequent
impacts on fire patterns. In addition, the warmer and drier conditions south of
the border, coupled with more intensive land use, contribute to more extensive
type conversion to flashy fuels of weedy alien annuals (see for example vegeta-
tion maps in Minnich & Franco-Vizcaino 1998 ), which, along with limited
attention to fire prevention, accounts for the much higher fire frequency in Baja
California.
In short, the model that burning patterns in Baja represent the natural pattern
in southern California is not supported by multiple lines of evidence:
(1) Large fires in southern California predate modern fire suppression.
(2) Fire suppression policy in this crown fire system has been unable to exclude
fire.
(3) Since the arrival of Euro-Americans, southern California landscapes have
burned more frequently than historically was the case; thus fuel loads are
lower than historically.
(4) Most large fires are driven by extreme Santa Ana winds (see Box 1.3 ) and
fuel age has limited control over fire spread.
(5) A mosaic of young age class chaparral fuels is incapable of stopping the
spread of many large fires.
Continued
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