Fire in California - Fire in Mediterranean Ecosystems - page 118

Table 5.1 Lightning- and human-ignited fires in selected sites in California

Total fires (#/million ha/yr) Percentage due to lightning

Coastal southern California 4290 < 1

Coastal central California 117 17

Coastal northern California 507 3

Interior southern California 2803 6

Interior central California 117 17

Interior northern California 456 55

Source: Based on data from Keeley ( 1982 ).

Ignition Patterns

In California humans have been a source of ignitions for only slightly more than
10 000 yrs, but the early Holocene populations may not have been large enough to
have had a very widespread influence on fire regimes. However, very little is
known about these early stages of New World colonization. By the mid Holocene
the expanded utilization of seeds, in particular acorns from oak trees, was coupled
with increased populations. For the past 5000 yrs Native Americans potentially

affected significant portions of the California landscape (Erlandson & Glassow
1997 ), and on many shrubland landscapes at low elevation they undoubtedly
increased fire frequency over that due to lightning alone (Timbrook et al . 1982 ;
Keeley 2002b ).
Prior to the colonization by Europeans and Americans in the nineteenth cen-
tury, lightning would have been the primary source of ignitions over vast stretches
of rugged and uninhabited parts of the chaparral region. Even today lightning
contributes to some significant fires in remote areas. Lightning-ignited fires vary
spatially because summer thunderstorms are rare near the coast and most frequent
at higher elevations in the interior (Keeley 1982 , 2006a ; Greenlee & Langenheim
1990 ; van Wagtendonk 1993 ). In coastal southern California lightning-ignited
fires are uncommon ( Table 5.1 ) and tend to be concentrated in the late summer
just prior to the Santa Ana wind season (see Box 1.3 ). In the northern California
San Francisco Bay area, lightning fires are quite rare (four lightning fires per
decade per 100 000 ha, Keeley 2005 ).
Today most ignitions in coastal California are started by people and com-
monly near the wildland-urban interface. These ignitions increased in frequency
during the twentieth century, concomitant with population growth (Keeley &
Fotheringham 2003a ). However, toward the latter part of the century ignitions
in many parts of the state apparently have reached a threshold and leveled off or
even declined. This change has been ascribed to patterns of housing development
in the wildland-urban interface, particularly as the early development stages that
form intermixes with wildland areas transformed to more classical interface
zones (Syphard et al. 2007 ).