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50
40
30
Interior ranges
20
10
C oastal ranges
0
0
800
1600
2400
3200
Elevation (m)
Fig. 2.6 Elevational distribution of lightning strike density for the interior and coastal ranges
of southern California. (From Keeley 2006a .) Similar elevational patterns of lightning-ignited
fires are reported for other MTC regions (e.g. Horne 1981 ;V ´zquez & Moreno 1998 ;
Mazarakis et al. 2008 ).
(Ne'eman et al. 2004 ), whereas that impact has been about one tenth as long in
California. Prior to human occupation in California, ignitions in coastal regions
were potentially less frequent and fires were dependent on foehn or other offshore
winds (see Box 1.3 ) spreading lightning-ignited fires from the interior to the coast.
On many landscapes humans have altered fuel structure in ways that also might
impact lightning-ignited fires, contributing to new patterns of distribution (e.g.
Arienti et al. 2009 ). It has been suggested that the local probability of ignition by
lightning is reflected in the evolutionary development of vegetation responses
across broad landscapes (Manry & Knight 1986 ).
Estimating past frequency on a landscape is an important means of assessing
ecosystem tolerances to fire return intervals. Techniques for measuring fire
frequency vary in their strengths and weaknesses. The method applicable to
most landscapes is estimating fire frequency from observational records of past
fire events. Although such records may extend back to the nineteenth century or
slightly earlier (Keeley & Zedler 2009 ), systematic records of comparative value
generally began sometime in the twentieth century, with timing varying in
different MTC regions. In California the U.S. Forest Service (USFS) has one
of the longest periods of documented records, with most forests established
sometime between 1906 and 1911 (Cermak 2005 ). Although it is a widely held
view by many reputable scientists that the record keeping on these forests in the
early years was unreliable, there is no documented evidence for this opinion.
Early twentieth-century records likely lacked a full accounting of wilderness
fires, most of which were lightning-ignited fires, because these events typically
occurred in isolated areas that were not readily accessed and often burned out
before detection. However, a study of early USFS records suggests there was a
very careful reporting of fires on those landscapes where rangers had access
by foot or horseback.
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