Fire in California - Fire in Mediterranean Ecosystems - page 115

Fig. 5.2 Chaparral and sage scrub mosaic in southern California. (Photo by Jon Keeley.)
1500 m. Although there are fine-grained differences in plant associations related to
slope aspects, fires generally burn in a coarse-grained manner (Keeley 2006a ).
Fuel Patterns
Mature shrublands typically range from 1-5 m in height and form a dense closed
canopy that excludes most herbaceous surface fuels. Dead surface fuels accumu-
late slowly because many species retain a substantial proportion of dead branches
in the canopy (Schwilk 2003 ). This fuel structure results in a fire regime dominated
by active crown fires with relatively little surface fire.
On most sites of moderate fertility a postfire ephemeral flora of annuals and
short-lived perennials germinate from seeds that survive fire and develop dense
stands during the early seral stage of postfire recovery. This ephemeral flora
produces a significant load of fine fuels that die back each summer ( Fig. 5.3 )
and, coupled with dead skeletons from the last fire, comprise fuel loads often in
excess of 10 Mg ha 1 (see Table 2.1 ). These fine fuels are easily ignited and are
sufficient to carry fire ( Fig. 5.4 ), but when they occur within the first 10 yrs they
are generally detrimental to the recovery of the shrub dominants.
Typically within the first decade after fire the shrub canopy closes and the
ephemeral flora remains primarily as dormant seedbanks. Over the subsequent
decade, as shrub canopies expand, the ratio of live to dead fuel may be too high to
carry fire except under severe weather conditions. Fuel loads tend to increase with
stand age although there is substantial species-specific variation in the relationship
between stand age and live/dead ratio that may be an important determinant of
flammability under all but the most extreme conditions.