Agriculture Reference
In-Depth Information
40
Biomass
Fuel
30
20
10
0
0
30
10
20
Year
Fig. 5.5
Schematic model of temporal changes in biomass and fuels in chaparral. In the early
years the herbaceous ephemeral postfire flora dries completely during the summer drought and
contributes to substantial fuels during the early years, but as the shrub canopy closes in this
understory flora is shaded out by less flammable green shrub foliage
.
seedling recruitment (
Adenostoma
,
Arctostaphylos
and
Ceanothus
species) have a
marked tendency to retain dead branches in the canopy and this has important
effects on subsequent fire intensity (Schwilk
2003
). Other shrubs that recruit in the
absence of fire such as species of
Quercus
,
Prunus
,
Rhamnus and Rhu
s self-prune
dead branches, and are expected to have a higher live to dead ratio, although this
has not been well documented. The retention of dead branches is hypothesized to
be part of a character syndrome with evolutionary implications as discussed below.
Landscape patterns of fuels exhibit spatial variation as well. From south to
north, shrublands dominate a decreasing proportion of the landscape and thus
shrubland fires tend to be the largest in the southern half of the state. Even within
this region there are marked differences in fuel patterns that affect fire size
(Keeley & Zedler
2009
). The largest fires (
50 000 ha) have mostly occurred
either in San Diego County or further north in Santa Barbara/Ventura Counties
because the topography of both regions supports large contiguous east-west
swaths of shrubland fuels where both offshore and onshore wind flows can drive
fire over very long distances. An important exception is the 2009 Station Fire, the
largest fire in Los Angeles County. Unlike most large fires it was not driven by
Santa Ana winds (see
Box 1.3
) and this likely contributed to its extraordinary size.
Fires driven by these winds generally have a northeast to southwest trajectory and
in Los Angeles County this constrains ultimate fire size (Keeley & Zedler
2009
).
Due to the lack of Santa Ana winds, the Station Fire was a plume-driven fire (see
fronts. Although fuels were not outside the historical range of variability for this
type, about half of the landscape was older than typically the case in Los Angeles
County, and this, coupled with very rugged terrain and multiple years of intense
drought, contributed to this large fire event.
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