Fire in California - Fire in Mediterranean Ecosystems - page 131

higher incidence of lightning-ignited fires than the lower chaparral-dominated
foothills (Show & Kotok 1923a ; Court 1960 ; Keeley 1982 ). Today these fires still
represent the bulk of fire starts in some regions, and a significant source of
burning throughout most forested regions ( Table 5.1 ) . Lightning-ignited fires
are concentrated in summer months of June, July and August and arise from
monsoonal storms that are concentrated in the southwestern USA, but often
affect interior mountain ranges in the interior of California.
Lightning ignitions in California are spatially quite variable. Throughout the
region lightning strikes increase with elevation but exhibit little preference for
slope aspect. Prior to human occupation fires in the lower foothills may have been
ignition limited, whereas in the higher mountains forests were saturated with
natural lightning ignitions (van Wagtendonk & Cayan 2008 ) and under conditions
of saturating ignitions, fuels play a larger role in controlling fire frequency. In the
higher mountain ranges lightning ignitions typically peak at mid-elevations
(Vankat 1985 ).
Fire Regime
Fires in these forests typically spread by low-intensity surface burning but may
include patches of high-intensity passive crown fire. This mixed surface and crown
fire regime varies in the proportion of these two modes of burning both spatially
and temporally. Factors that affect the proportion of surface to crown burning
include climate, weather, topography and past management practices. In forests
with herbaceous fuels, crowning is often dependent on ladder fuels, whereas in
other forests localized accumulations of dead branches and leaves may be suffi-
cient to cause fire behavior to switch from surface to crown fire.
Cross sections of trees with fire-scarred bases (see Fig. 4.8a ) typically reveal
numerous embedded fire scars that can be accurately dated by association with
annual growth rings (see Figure 4.8b ) . These studies show that historically these
forests were subjected to frequent fires, and since the trees survived (and have no
resprouting capacity), these were obviously low-intensity surface fires (Kilgore &
Taylor 1979 ). Fire-scar dendrochronology studies have been done in forests through-
out California and the western half of the USA and reveal a common pattern of
frequent fires prior to the twentieth century, but nearly total fire exclusion over the
past century (Skinner &Chang 1996 ). As a consequence, this subsectionwill focus on
the historical fire regimes, and in a later subsection we will discuss how these patterns
have been perturbed by land management practices of the twentieth century.
The historical fire frequency varied spatially and temporally at different scales.
In the Sierra Nevada Range the lower-elevation Pinus ponderosa forests had a
higher fire frequency than the upper-elevation Abies concolor dominated mixed
conifer forests (Swetnam & Baisan 2003 ). Also, within the mixed conifer forest at
the same elevation, equator-facing slopes had a higher fire frequency than more
mesic pole-facing slopes (Caprio 2004 ). It is unlikely that ignitions were a factor in
these patterns due to the characteristics of lightning distribution noted above. Fuel