Agriculture Reference
In-Depth Information
Fire Regime
The continuity of shrub canopies and lack of surface fuels ensures that these
shrublands burn in stand-replacing crown fires. Commonly crown fires will burn
large portions (i.e. 10
3
-10
4
ha) of the landscape, leaving few unburned patches.
Historically the fire cycle was rather long and likely limited more by ignitions
than by fuels (
Box 5.1
). In coastal mountain ranges this frequency was perhaps as
low as once or twice a century, but in interior mountain ranges the higher
lightning frequency would have promoted higher fire frequencies, perhaps several
fires a century (Keeley
2006a
). The peak season for fires was in the summer and
these often remained small and of low intensity due to the higher humidity and less
severe winds at this time of the year (Minnich
1987a
). Further, the peak for
lightning is in late summer and early autumn and it nearly overlaps with the
autumn Santa Ana wind season; thus it is inconceivable these landscapes would
have escaped large fire events (Keeley
2006a
; Keeley & Zedler
2009
). This model
only requires persistence of lightning-ignited fires for a few weeks to ensure being
caught by the annual gale-force Santa Ana winds. Thus, the historical fire regime
would have been a regime of summer lightning fires punctuated periodically by
huge Santa Ana wind-driven fires (see
Fig. B1.3.1
). In all likelihood the bulk of the
burning on these landscapes occurred under such conditions.
This historical fire regime is supported by the observation that summer fires
often continued for months and consumed no more than a few thousand hectares
(Minnich
1987a
) and at this rate it would have taken many centuries to burn this
landscape (Keeley & Zedler
2009
). However, Santa Ana wind-driven fires often
consume more than 10 000 ha in a single day (Keeley
et al
.
2004
,
2009b
) and thus
when an occasional lightning ignition carried over until the autumn the total area
burned would have increased by orders of magnitude. Fire scar dendrochronology
studies of one of our lowest elevation conifers,
Pseudotsuga macrocarpa
(see
Foothill coniferous trees section below), which is often juxtaposed with mid- to
high-elevation chaparral, reinforces this conclusion that large landscape fires were
historically commonplace on these landscapes (Lombardo
et al.
2009
). These
dendrochronology studies showed that since at least the seventeenth century, fires
covering 100 000 ha or more occurred roughly every 35-75 yrs on the Los Padres
National Forest in southern California.
In shrublands across the California MTC region there are large differences in
fire behavior related to climate, weather, topography and human demography.
These patterns translate into differences in fire hazard (Moritz
et al.
2004
). During
the latter half of the twentieth century most southern California counties have had
a fire rotation interval of 30-40 yrs, the central coastal region 40-80 yrs (Keeley
et al.
1999a
), the east bay area of San Francisco approximately 100 yrs (Keeley
2005
), and the foothill chaparral of the southern Sierra Nevada perhaps even
longer (Keeley
et al.
2005b
). The short fire rotation in southern California is due to
a longer period of annual drought, coupled with the annual autumn foehn
winds known locally as Santa Ana winds (see
Box 1.3
), plus the high population
