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Historical patterns of wildfire ignition sources in California
ecosystems
Jon E. Keeley
A
,
B
and Alexandra D. Syphard
C
A
US Geological Survey, Western Ecological Research Center, Sequoia–Kings Canyon Field
Station, 47050 Generals Highway, Three Rivers, CA 93271, USA.
B
Department of Ecology and Evolutionary Biology, University of California, 612 Charles E. Young
Drive East, Los Angeles, CA 90095, USA.
C
Conservation Biology Institute, 10423 Sierra Vista Avenue, La Mesa, CA 91941, USA.
D
Corresponding author. Email: jon_keeley@usgs.gov
Abstract. State and federal agencies have reported fire causes since the early 1900s, explicitly for the purpose of helping
land managers design fire-prevention programs. We document fire-ignition patterns in five homogenous climate divisions
in California over the past 98 years on state Cal Fire protected lands and 107 years on federal United States Forest Service
lands. Throughout the state, fire frequency increased steadily until a peak c. 1980, followed by a marked drop to 2016.
There was not a tight link between frequency of ignition sources and area burned by those sources and the relationships
have changed over time. Natural lightning-ignited fires were consistently fewer from north to south and from high to low
elevation. Throughout most of the state, human-caused fires dominated the record and were positively correlated with
population density for the first two-thirds of the record, but this relationship reversed in recent decades. We propose a
mechanistic multi-variate model of factors driving fire frequency, where the importance of different factors has changed
over time. Although ignition sources have declined markedly in recent decades, one notable exception is powerline
ignitions. One important avenue for future fire-hazard reduction will be consideration of solutions to reduce this source of
dangerous fires.
Additional keywords: arson, debris burning, equipment, lightning, powerlines, smoking.
Received 20 February 2018, accepted 25 September 2018, published online 7 November 2018
Introduction
Increasing concern over wildfires has prompted a re-emphasis
by the federal government to stop this trend (https://www.doi.
gov/pressreleases/secretary-zinke-directs-interior-bureaus-take-
aggressive-action-prevent-wildfires, accessed 1 April 2018;
Bedard 2017). For many decades, the focus of fire management
has been on fuel modification with success on certain landscapes
(Kalies and Kent 2016) but limited improvement on others
(Keeley and Safford 2016). Because humans are a dominant
ignition source over the majority of North America (Balch et al.
2017;Syphard et al. 2017) there is reason to believe improve-
ments in fire prevention may be a key to reducing fire impacts.
Indeed, the United States Forest Service (USFS) has been
reporting fire causes since it began collecting systematic data on
fires in 1905, with the explicit purpose of helping land managers
design fire prevention programs (Donoghue 1982a).
Effective fire-prevention requires a sound understanding of
the patterns and causes of fire ignitions, which are closely
aligned with both human and biophysical-landscape character-
istics (Syphard et al. 2008). Prestemon et al. (2013) suggested a
conceptual model that linked ignitions to changes in biophysi-
cal, societal, prevention and management variations that
illustrates the complexities of ascertaining relationships
between different sources and how they change over time.
An important characterisation of anthropogenic ignitions is
that the most abundant ignition sources are not always associ-
ated with the greatest area burned (Syphard and Keeley 2016).
Thus, a topic in need of further study is how to sort out those
ignition sources that are most damaging, how those have
changed over time, and in light of future needs, how climate
change is likely to affect different ignition sources and losses.
For example, it has been demonstrated for the state of Victoria,
Australia, that some ignition sources, such as electrical distribu-
tion lines, may be limited in number but result in much more
severe fire consequences (Miller et al. 2017). In addition, these
fires are more likely during periods of elevated fire danger. If
some ignition sources play a larger role in area burned, these
might be targets for closer scrutiny and fire-management plan-
ning. This potential has been demonstrated for parts of southern
California over recent decades, where powerlines have been
shown to cause a substantial amount of area burned in both
subregions in southern California (Syphard and Keeley 2015).
Other important factors were arson in one subregion and
equipment in another.
CSIRO PUBLISHING
International Journal of Wildland Fire 2018,27, 781–799
https://doi.org/10.1071/WF18026
Journal Compilation ÓIAWF 2018 Open Access CC BY-NC-ND www.publish.csiro.au/journals/ijwf
The goal of the present research is to expand that approach to
include the entire written history of fires in the state. Our focus
was on the spatial and temporal patterns of different ignition
sources and the relationship between type of ignition and area
burned. We took a long-term historical approach utilising data
from 1910 to 2016 for USFS lands and from 1919 to 2016 for
state protected Cal Fire lands. First, we examine the spatial and
temporal patterns of natural lightning-ignited fires v. human
fires in the state, and their contribution to area burned. Next, we
investigated which anthropogenic causes are most frequent,
their distribution within the state, their change over time and
their contribution to area burned. Based on a study of state-
protected lands in California, Syphard et al. (2007) found that
fires increased from 1931 to the 1980s, but then decreased over
the subsequent decades. A similar pattern for the whole state was
also reported by Keeley and Syphard (2017) on both Cal Fire and
USFS lands. Thus, the present study contrasts fire-ignition
patterns within climatically homogenous sub-regions for the
period before 1980 and for the period 1980–2016.We also
investigated the extent to which seasonal climate parameters
could explain patterns of ignitions and area burned for each type
of ignition source.
Methods
Fire-history data for numbers of fires and area burned, by cause,
were analysed separately for state-protected Cal Fire and federal
USFS lands. Data for counties, forests and climate divisions
were all normalised by the area protected each year and within
each unit and expressed as number of fires, or hectares burned,
per million hectares.
Cal Fire data included 51 of the state’s 58 counties (see Fig
S1, available as Supplementary material to this paper) as 7
counties had limited fire activity or records. Fire statistics were
from direct protection areas (DPA), which are mostly state-
responsibility lands with smaller amounts of federal lands, and
included the years 1919–2016, summarised by county. The term
DPA was first used in 1986 and the area included was equivalent
to what was called State Zone (1972–1985), Zones I and II
(1945–1971) and Zones 1, 2, 3 (1919–1944). Cal Fire data from
1919 to 1930 are unpublished and were only available as typed
reports at the California State Archives in Sacramento. Data
from 1931 to 2016 were available in annual reports variously
named, Forest Fire Summary, Fire Statistics, Fire Activity
Statistics, and Wildfire Activity Statistics, often referred to as
the Redbook series, available from research libraries or directly
from the agency. Only 30 counties had complete data (excluding
1927 for nearly all counties and a few additional years in other
counties) beginning in 1919, and an additional 21 counties had
continuous data beginning in 1945 (or slightly later in a few
cases) (see Table S1 for years of records for each county). Area
protected has changed through this period of record and thus
data were normalised to the hectares protected for that year
presented with the annual reports. There was a period from 1941
to 1952 area where protected-area data were not included in
annual reports and, as best we can determine, those data are no
longer available, so we used the areas protected in 1940. In all
cases, the changes between 1940 and 1953, when such data were
again available, were minor.
USFS fire data covered 17 national forests (Fig S2) and
included the years 1910–2016 (see Table S2, two forests were
created after 1910 carved out of area from adjoining forests).
Area protected has changed through this period of record and
thus data were normalised to the hectares protected for that year.
Data for USFS lands through the 1980s were from annual fire
statistics reports for Region 5, available in the Forestry Library
(most of which was transferred to the Bioscience Library) at
the University of California, Berkeley. More-recent data, 1970
to 2016, were from the National Wildfire Coordinating Group
(see http://fam.nwcg.gov/fam-web/weatherfirecd/state_data.
htm, accessed 1 June 2017).
Most investigators are unfamiliar with these historical fire
records and are sometimes sceptical of their accuracy. For
example, Stephens (2005) contended that USFS data before
1940 were inaccurate, but cited a source (Mitchell 1947) that
provided no evidence of this. Likely, the idea comes from
Donoghue’s (1982b)comment ‘1940 marked the modern era
of fire reporting’. However, that comment was in reference to
the fact that ‘the report issued at this time was the first designed
for automated data processing and easy readability’ and was not
in reference to reliability.
Historians have generally been confidant in these early
California fire records (Brown 1945;Show 1945;Clar 1969;
Cermak 2005). The first author, J. E. Keeley, examined all of the
California fire-related materials stored at the state and federal
archives and believes collectively they show managers have
always been conscientious about reporting accuracy and
completeness. For example, beginning in 1905, USFS record-
keeping required 15 items of information on the fire reporting
Form 944, including the specific cause (Donoghue 1982b).
On state-protected lands there was an incentive in that the
1911 Federal Weeks Law provided fiscal aid to states based
on statistics of fire protection (see http://www.calfire.ca.gov/
about/about_calfire_history2, accessed 23 May 2018). In 1919,
the California state legislature appropriated money for fire
prevention and suppression, and records in the state archive
show that, by 1920, there were more than 400 fire wardens
distributed throughout the state who were charged with fire-
fighting and fire reporting. In 1920, there were 800 flights of the
Army’s 9th Aero Squadron fire patrol that covered 426 500 km
during the 5-month California fire season (Cermak 1991).
One complication in studying ignition sources is that
reported categories have changed over time. Certain causes
have persisted over the entire period of record, including
lightning, smoking and camping, but other categories have
changed their names. For example, arson fires are a relatively
new category as intentionally set fires have, in the past, been
labelled as ‘incendiary’, and seem to have been more rural than
contemporary urban arson fires (Kuhlken 1999) and in this paper
they are all recorded as arson fires. Other changes include the
term ‘brush burning’ being changed to ‘debris burning’, and the
categorisation of brush burning has been folded into debris
burning. Causes that were unknown or represented minor
categories have been included as miscellaneous fires (Donoghue
1982a), but are not addressed here.
Cal Fire data were spatially explicit at the level of the county
and USFS data at the level of the individual forest. However, for
analysis, these were grouped into climatically homogenous
782 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
areas as defined by the National Oceanic and Atmospheric
Administration’s (NOAA) National Climatic Data Center
(NCDC) California Climate Divisions (Fig. 1), comprising the
main fire-prone landscapes in the state (see http://www.ncdc.
noaa.gov/temp-and-precip/time-series/index.php?parameter=pd-
si&month=1&year=2008&filter=p12&state=4&div=5, accessed
15 June 2016). These include, from north to south, Division 1
(North Coast), 2 (North Interior), 5 (Sierra Nevada), 4 (Central
Coast), and 6 (South Coast). Where boundaries did not match
precisely, counties or forests were placed in the climate division
comprising the majority of land area in that unit.
In 1919, Cal Fire-protected lands were 11.7 10
6
ha and
increased to 12.5 10
6
ha in 2016. USFS lands comprised
9.8 10
6
ha in 1919 and decreased to 9.5 10
6
ha in 2016.
Vegetation on state lands was dominated by grasslands and
shrublands in the south and with significant woodlands and
coniferous forests farther north (see Keeley and Syphard 2017
for more detailed vegetation data). USFS lands were dominated
by coniferous forests, except in the southern part of the state
where they were dominated by shrublands.
To evaluate climate impact on fire activity, we utilised
PRISM climate for each county on Cal Fire-protected lands
and each forest on USFS lands (Fig. 1). For every year in the
analysis, we extracted 2.5 arc-minute PRISM data (PRISM
Climate Group, Oregon State University, see http://prism.
oregonstate.edu, accessed 15 February 2017) for areas within
the boundaries of the Cal Fire and USFS lands. For each county
and forest, we computed area-weighted averages of monthly
mean precipitation and temperature, summarised by season –
winter being December (prior year), January and February,
spring being March, April and May, summer being June, July
and August, and autumn being September, October and
November.
Analysis was conducted with Systat software (ver. 11.0,
Systat Software, Inc., San Jose, CA, http://www.systat.com/).
For the climate analysis, we developed multiple regression
models explaining area burned for USFS and Cal Fire based
on seasonal temperature, precipitation and prior-season precipi-
tation variables. To ensure multicollinearity would not be an
issue, we calculated correlation coefficients among all potential
explanatory variables and eliminated those that were strongly
correlated (P,0.05) with other variables in the model.
Results
Long-term averages show that on a per-unit-area basis fires were
approximately twice as frequent or more on Cal Fire lands as on
USFS lands in all five NOAA climate divisions (Table 1).
However, the relationship between ignitions and area burned
varied markedly between Cal Fire and USFS lands and between
divisions. In the North Coast division, Cal Fire dealt with twice
as many fires as the USFS but the average area burned was very
similar. In contrast, in the interior from the Sierra Nevada
northward, Cal Fire experienced approximately double the
number of fires and nearly double the area burned. In the coastal
N
0 45 90 180
Kilometers 270 360
Fig. 1. NOAA climate divisions and Cal Fire protected and USFS
protected lands in California for the five climate divisions with long-term
fire history.
Table 1. Fire frequency and area burned on state and federal lands in California
Cal Fire (1919–2016) USFS (1910–2016)
NOAA division Fire frequency Area burned Fire frequency Area burned
(n/year/10
6
ha) (ha/year/10
6
ha) (n/year/10
6
ha) (ha/year/10
6
ha)
North Coast 317 7780 150 7559
North Interior 421 9642 207 5914
Sierra Nevada 356 8436 169 4709
Central Coast 277 5496 66 18860
South Coast 656 15278 369 24442
Fire ignition sources Int. J. Wildland Fire 783
areas from San Francisco to San Diego, there were substantially
more fires on Cal Fire-protected lands but the area burned was
substantially greater on USFS lands.
For Cal Fire-protected landscapes, the area-based average
number of fires per year (1919 to 2016) varied from 1645 in
Placer County to 87 in Glenn County (Table 2). Humans were
Table 2. Cal Fire counties total fires, percentage due to human ignitions and regression coefficients for population density v. number of fires
(per year per million ha) for years 1919–2016
Division County Total Percentage human ,1980 $1980
rPrP
North Coast Del Norte 330 97 0.39 0.007 0.07 0.667
Humboldt 219 93 0.31 0.018 0.62 0.000
Lake 446 98 0.72 0.000 0.66 0.000
Marin 1155 100 0.10 0.526 0.07 0.699
Mendocino 255 94 0.46 0.000 0.46 0.004
Napa 493 99 0.79 0.000 0.83 0.000
Siskiyou 256 63 0.44 0.000 0.07 0.681
Sonoma 488 99 0.90 0.000 0.71 0.000
Trinity 229 75 0.05 0.709 0.27 0.102
North Interior Butte 929 95 0.89 0.000 0.51 0.001
Colusa 115 95 0.44 0.003 0.60 0.000
Glenn 87 93 0.28 0.069 0.64 0.000
Lassen 178 50 0.55 0.000 0.39 0.017
Modoc 113 51 0.42 0.020 0.04 0.811
Nevada 936 97 0.72 0.000 0.80 0.000
Placer 1645 98 0.88 0.000 0.89 0.000
Plumas 489 74 0.67 0.000 0.08 0.654
Shasta 469 90 0.77 0.000 0.38 0.020
Solano 406 99 0.55 0.000 0.54 0.000
Tehama 196 93 0.90 0.000 0.48 0.000
Yolo 226 97 0.61 0.000 0.32 0.051
Yuba 723 97 0.49 0.000 0.26 0.118
Sierra Nevada Amador 507 99 0.60 0.000 0.47 0.003
Calaveras 337 99 0.59 0.000 0.57 0.000
El Dorado 213 98 0.77 0.000 0.26 0.123
Fresno 100 96 0.84 0.000 0.69 0.000
Inyo-Mono 255 98 0.67 0.002 0.63 0.000
Kern 804 99 0.84 0.000 0.51 0.002
Kings 321 99 0.29 0.146 0.32 0.107
Madera 823 99 0.65 0.000 0.48 0.003
Mariposa 617 97 0.70 0.000 0.45 0.005
Merced 602 95 0.21 0.180 0.42 0.010
San Joaquin 850 97 0.05 0.789 0.46 0.005
Stanislaus 237 95 0.35 0.024 0.20 0.244
Tulare 180 93 0.77 0.000 0.33 0.048
Tuolumne 280 93 0.89 0.000 0.23 0.174
Central Coast Alameda 117 98 0.65 0.000 0.68 0.000
Contra Costa 447 93 0.59 0.000 0.21 0.213
Monterey 306 93 0.84 0.000 0.83 0.000
San Benito 151 93 0.62 0.000 0.81 0.000
San Luis Obi 322 99 0.77 0.000 0.64 0.000
San Mateo 158 97 0.74 0.000 0.08 0.055
Santa Clara 242 93 0.38 0.002 0.63 0.000
Santa Cruz 814 97 0.80 0.000 0.57 0.000
South Coast Los Angeles 778 98 0.41 0.007 0.53 0.001
Orange 1198 100 0.85 0.000 0.62 0.000
Riverside 790 98 0.82 0.000 0.82 0.000
San Bernardi 791 96 0.73 0.000 0.81 0.000
San Diego 576 97 0.63 0.000 0.60 0.000
Santa Barbar 347 99 0.70 0.000 0.64 0.000
Ventura 713 99 0.84 0.000 0.42 0.010
784 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
responsible for most of fires, accounting for 95% or more of the
ignitions in two-thirds of the counties. However, certain north-
ern California counties stood out as notable exceptions, e.g. in
Siskiyou, Trinity, Lassen, Modoc and Plumas counties lightning
accounted for one-quarter to more than half of all ignitions,
patterns illustrated in Fig. 2a. Regions with the lowest lightning-
ignited fires extended through the coastal ranges from north of
San Francisco to Santa Barbara. Area burned by lightning-
ignited fires generally followed a similar pattern, although it
was the source for significant burning in the San Bernardino
County of southern California (Fig. 3a).
For USFS lands, the area-based average number of fires per
year (1910 to 2016) varied from 478 in San Bernardino to 67 in
Eldorado National Forest (Table 3). Humans accounted for far
fewer fires than on Cal Fire lands. In the South Coast division,
humans were responsible for 74–88%; however, in half of the
other forests, humans accounted for less than 50% of the fires.
As with Cal Fire landscapes, USFS lightning-ignited fires were
most common in the north-east part of the state and declined
markedly in coastal central and southern California (Fig. 4a).
Area burned by lightning-ignited fires generally followed a
similar pattern with the exception that parts of the North Coast
and Central Coast, despite having few such ignitions, had
substantial area burned by this source (Fig. 5a).
On both Cal Fire- and USFS-protected lands, humans played
a substantial role in fire ignitions. During the first two-thirds of
the 20th century there was a very strong positive relationship
between population density and fire frequency in nearly 90% of
the counties (Table 2) and more than 75% of the forests
(Table 3). However, from 1980 to 2016, although population
growth continued throughout the state, in most counties and
forests, population density exhibited a highly negative relation-
ship with fire frequency (Tables 2,3).
On both Cal Fire- and USFS-protected lands, human-ignited
fires derived from both intentional and accidental causes. The
highest number of fires was from equipment, arson, debris
Lightning
(a)
2.7–3.4 2.4–5.8 2.7–8.0 3.3–9.6
9.7–15.9
16.0–23.7
23.8–32.2
32.3–41.0
41.1–58.0
58.1–74.1
74.2–103.0
103.1–137.0
137.1–215.8
0.4–2.0
2.1–3.2
3.3–4.6
4.7–5.8
5.9–7.8
7.9–9.6
9.7–13.8
13.9–18.2
18.3–22.9
23.0–35.2
120.7–201.3
86.0–120.6
64.8–85.9
47.8–64.7
40.9–47.7
23.6–40.8
20.9–23.5
14.9–20.8
7.4–14.8
2.7–7.3
14.9–27.2
0.9–3.0
3.1–8.5
8.6–12.6
12.7–19.5
19.6–24.8
24.9–31.2
31.3–54.6
54.7–79.6
79.7–136.8
136.9–270.2
27.3–43.2
43.3–57.2
57.3–81.5
81.6–97.5
97.6–118.9
119.0–162.2
162.3–205.8
205.9–259.4
259.5–528.9
8.1–15.5
15.6–26.3
26.4–32.1
32.2–44.2
44.3–55.9
56.0–73.8
73.9–125.9
126.0–189.2
189.3–250.2
5.9–11.2
11.3–23.0
23.1–39.0
39.1–47.0
47.1–62.1
62.2–80.1
80.2–97.9
98.0–162.2
162.3–261.6
3.5–4.2
4.3–4.7
4.8–7.2
7.3–10.6
10.7–15.2
15.3–22.0
22.1–32.9
33.0–56.5
56.6–126.9
(e)(f)(g)(h)
(b)(c)(d)
Arson Debris Smoking
Playing Equipment Vehicle Powerline
Fig. 2. Fire frequency for different ignition sources on Cal Fire protected lands in California for the years 1919–2016 (n/year/10
6
ha); note change in scales
for each source.
Fire ignition sources Int. J. Wildland Fire 785
burning, children playing with fire, smoking, vehicles and
powerlines (Tables S3–S6). Sources, such as railroads and
lumber practices, did cause many fires in the early part of the
record, but are of minor significance today (this change not
shown).
These ignition sources exhibited marked geographical varia-
tion in their importance (Fig. 2–5). On lower elevation Cal Fire
landscapes, arson was responsible for much of the area burned in
the northern Sierra Nevada Mountains. (Fig. 3b), whereas debris
burning was responsible for much of the area burned north and
south of San Francisco (Fig. 3c), vehicles were the cause of
much of the burning in the central coastal ranges (Fig. 3g) and
equipment fires in southern California (Fig. 3f). Powerlines
were responsible for significant number of fires in the north
bay area of San Francisco and coastal communities from Santa
Barbara south to the border (Fig. 3h).
In USFS forests, area burned in the South Coast was most
heavily affected by arson and powerlines (Fig. 5b,h), but
equipment and debris burning dominated in the Central Coast
(Fig. 5c,f). Forests adjacent to high density metropolitan areas in
Los Angeles and western San Bernardino counties had substan-
tial burning due to smoking, children playing with fire, and
powerlines (Fig. 5d,e,h).
Changing ignition patterns over time
The historical pattern of fire frequency on lower elevation Cal
Fire-protected lands for 97 years and USFS lands for 107 years is
illustrated in Fig. 6 for the five climate divisions. There was a
common pattern across both Cal Fire and USFS lands and
consistent within each of the five climate divisions – a highly
significant increase in fire frequency from the beginning of
records to 1979, and a switch to a highly significant decline in
fires from 1980 to 2016 (Fig. 6), the single exception being the
South Coast USFS lands (Fig. 6r). Despite a significant fit of
these data to the linear regression models, there were some
marked departures on Cal Fire lands during the early record.
Plotting of linear regression residuals from 1919 to 1979 shows
Lightning
0.8–14.1 0.6–206.0 5.7–69.0 5.7–171.6
171.7–386.4
386.5–613.1
613.2–1049.0
1049.1–1190.7
1190.8–1394.0
1394.1–2074.5
2074.6–2762.0
2762.1–3463.1
3463.2–6316.6
0.0–8.1
8.2–21.5
21.6–38.7
38.8–95.5
95.6–174.8
174.9–263.8
263.9–658.2
658.3–2060.9
2061.0–3619.7
3619.8–5551.4
3066.0–4585.1
1625.0–3065.9
516.9–1624.9
304.0–516.8
156.6–303.9
90.7–156.5
36.9–90.6
15.6–36.8
3.2–15.5
0.6–3.1
0.7–117.2
117.3–286.7
286.8–412.7
412.8–523.9
524.0–649.4
649.5–877.7
877.8–1049.0
1049.1–1353.1
1353.2–3013.5
3013.6–4671.9
339.5–953.1
191.5–339.4
110.2–191.4
79.4–110.1
46.3–79.3
24.0–46.2
11.7–23.9
7.7–11.6
3.2–7.6
0.3–3.1
69.1–198.9
199.0–393.2
393.3–505.8
505.9–621.9
622.0–723.9
724.0–889.2
889.3–1206.0
1206.1–1788.5
1788.6–2436.8
206.1–521.6
521.7–769.1
769.2–1402.3
1402.4–1929.2
1929.3–3040.5
3040.6–3871.0
3871.1–4830.5
4830.6–6519.3
6519.4–12775.1
14.2–38.0
38.1–85.2
85.3–177.7
177.8–282.5
282.6–486.3
486.4–693.5
693.6–915.8
915.9–1448.1
1448.2–2059.2
(a)(b)(c)(d)
(h)
(g)
(f)
(e)
Arson Debris Smoking
Playing Equipment Vehicle Powerline
Fig. 3. Area burned by different ignition sources on Cal Fire protected lands in California for the years 1919–2016 (ha burned/year/10
6
ha); note change in
scales for each source.
786 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
Table 3. USFS forests total number of fires, percentage ignited by humans and regression coefficients for population density v. number of fires (per
year per million ha) for years 1910–2016
Division Forest Total Percentage human ,1980 $1980
rPrP
North Coast Klamath 192 28 0.23 0.057 0.33 0.045
Mendocino 97 51 0.26 0.032 0.36 0.030
Six Rivers 137 65 0.46 0.007 0.11 0.528
North Interior Lassen 271 38 0.66 0.000 0.43 0.008
Modoc 118 21 0.42 0.000 0.06 0.730
Plumas 280 46 0.36 0.002 0.37 0.024
Shast-Trinity 188 50 0.15 0.142 0.41 0.012
Tahoe 257 54 0.22 0.067 0.44 0.007
Sierra Nevada Eldorado 67 58 0.43 0.000 0.25 0.129
Inyo-Mono 237 40 0.85 0.000 0.67 0.000
Sequoia 79 39 0.75 0.000 0.76 0.000
Sierra 213 48 0.63 0.000 0.68 0.000
Stanislaus 201 49 0.50 0.000 0.55 0.000
Central Coast Los Padres 203 82 0.51 0.000 0.50 0.002
South Coast Angeles 367 87 0.58 0.000 0.49 0.002
Cleveland 292 88 0.71 0.000 0.05 0.773
Sbernardino 478 74 0.89 0.000 0.50 0.002
Lightning
(a)(b)(c)(d)
(h)
(g)
(f)
(e)
Arson Debris Smoking
Playing Equipment Vehicle Powerline
11.7 0.9 1.6
1.7–3.3
3.4–4.7
4.8–6.2
6.3–7.7
7.8–8.0
8.1–11.9
12.0–12.6
12.7–13.7
13.8–17.0
7.3–8.3
8.4–11.5
11.6–13.1
13.2–17.9
18.0–20.2
20.3–24.7
24.8–29.7
29.8–31.3
31.4–33.7
33.8–50.5
0.3–0.4
0.5–0.9
1.0
1.1–1.3
1.4–1.7
1.8–1.9
2.0–2.2
2.3–2.5
2.6–6.6
6.7–10.4
55.0–95.4
10.3–54.9
8.3–10.2
7.0–8.2
6.1–6.9
5.3–6.0
5.2
3.4–5.1
1.7–3.3
1.6
2.5–3.1
3.2–5.6
5.7–5.9
6.0
6.1–7.1
7.2–8.7
8.8–11.8
11.9–12.9
13.0–25.1
25.2–53.6
40.8–59.0
9.8–40.7
8.1–9.7
5.9–8.0
5.2–5.8
4.3–5.1
3.7–4.2
2.2–3.6
0.5–2.1
0.4
1.0–8.5
8.6–8.9
9.0–9.9
10.0–10.7
10.8–12.0
12.1–13.1
13.2–18.8
18.9–45.1
45.2–51.9
11.8–36.7
36.8–48.2
48.3–93.2
93.3–93.6
93.7–99.8
99.9–103.2
103.3–127.6
127.7–149.5
149.6–165.1
Fig. 4. Fire frequency for different ignition sources on USFS protected lands in California for the years 1910–2016 (n/year/10
6
ha); note change in scales for
each source.
Fire ignition sources Int. J. Wildland Fire 787
marked and consistent diversions in most divisions (Fig. 7).
Although the residuals early in the record are closely aligned
with the regression line, during the 1920s and 1930s from the
Sierra Nevada north there was a marked increase in ignitions.
This pattern was less obvious in coastal central and southern
California. In the 1950s and 1960s, there was a marked
depression in ignitions in all climate divisions. It is worth noting
that in the former period it was drier than the long-term average
and in the latter period wetter (Fig. S3).
Changes in area burned did not closely follow changes in fire
frequency (Fig. 6) – while fire frequency increased in the first
three-quarters of the 20th century, area burned declined or
stayed more or less constant. USFS forests in the northern part
of the state showed a tendency for increased area burned in the
last 4 decades (Fig. 6d,l) but in general there were no strong
trends in area burned after 1980.
Of particular interest is how specific ignition sources
have changed and, in order to simplify this presentation, we
have consolidated climate divisions in the north (North Coast,
North Interior and Sierra Nevada) and in the south (Central
Coast and South Coast), which is justified by the marked
similarities in ignition patterns within these two regions
(see Fig. 2–5).
On Cal Fire-protected lands, it is noteworthy that changes in
number of ignitions for lightning-ignited fires matched that of
many human ignition sources, specifically increased ignitions
during the first part of the record and decreased ignitions in
recent decades (Fig. 8a,e). Numbers of lightning ignitions were
more than double in the north than in the south, and substantially
fewer than the leading anthropogenic causes. On USFS forests,
lighting fire frequency (Fig. 9a,e) followed a temporal pattern
similar to Cal Fire lands but were ,3 times more abundant than
on Cal Fire landscapes and were one of the dominant ignition
sources in forests. Despite changes in number of lightning-
ignited fires, the area burned by this source did not exhibit
consistent trends, although, in northern California forests, area
burned by lightning-ignited fires has increased since 1980
(Fig. 8i,m,9i,m).
Lightning
114.5
114.6–726.5
726.6–1378.6
1378.7–1634.9
1635.0–1738.0
1738.1–1920.2
1920.3–2355.4
2355.5–3103.0
3103.1–3816.3
3816.4–5525.1 5990.2–6830.1
3609.7–5990.1
1205.1–3609.6
1147.9–1205.0
1041.1–1147.8
773.3–1041.0
558.1–773.2
364.6–558.0
98.9–364.5
98.8 37.5–55.2 34.2–84.7
1.7
13.5–40.0
44.1
0.4–0.6
0.7–1.6
1.7–2.2
2.3–4.0
4.1–6.2
6.3–10.1
10.2–32.7
32.8–254.1
254.2–324.4
324.5–2539.6
44.2–50.6
50.7–98.4
98.5–149.2
149.3–209.5
209.6–287.7
287.8–498.4
498.5–952.3
952.4–1353.7
1353.8–5006.0
40.1–88.2
88.3–114.5
114.6–142.2
142.3–384.1
384.2–485.0
485.1–548.8
548.9–633.4
633.5–808.1
808.2–1484.6
1.8–2.3
2.4–5.7
5.8–27.5
27.6–40.0
40.1–49.6
49.7–135.6
135.7–353.4
353.5–1157.6
1157.7–15377.3
84.8–143.4
143.5–373.9
374.0–473.6
473.7–529.2
529.3–573.6
573.7–865.6
865.7–1245.5
1245.6–1465.6
1465.7–2828.6
55.3–96.5
96.6–104.2
104.3–115.8
115.9–191.1
191.2–258.8
258.9–330.7
330.8–469.8
469.9–859.7
859.8–1651.2
(a)(b)(c)(d)
(h)
(g)
(f)(e)
Arson Debris Smoking
Playing Equipment Vehicle Powerline
Fig. 5. Area burned by different ignition sources on USFS protected lands in California for the years 1910–2016 (ha burned/year/10
6
ha); note change in
scales for each source.
788 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
Cal Fire USFS Cal Fire USFS
r
2
0.65
r
2
0.54
P 0.001
P 0.001
Fire frequency (#/yr/million ha)
(q)(r)(s)(t)
South Coast
(m)(n)(o)(p)
Central Coast
(i)(j)(k)(l)
Sierra Nevada
(e)(f)(g)(h)
North Interior
(a)(b)(c)(d)
North Coast North Coast
South Coast
Central Coast Central Coast Central Coast
Sierra Nevada Sierra Nevada Sierra Nevada
North Interior
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
Area burned (log ha/yr/million ha)
South Coast South Coast
North Interior North Interior
North Coast
(p)
North Coast
1000
800
600
400
400
500
200
200
300
100
00
400
500
200
300
100
0
400
500
200
300
100
0
400
500
200
300
100
0
1000
800
600
400
200
0
1000
800
600
400
200
0
1000
800
600
400
200
0
1200
1600
2000
800
400
0
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
1000
800
600
400
200
0
r
2
0.69
P 0.001
r
2
0.60
P 0.001
r
2
0.64
P 0.001
r
2
0.62
P 0.001
r
2
0.68
P 0.001
r
2
0.56
P 0.001
r
2
0.49
P 0.001
r
2
0.80
P 0.001
r
2
0.
10
P 0.
050
r
2
0.
07
P 0.
023
r
2
0.
25
P 0.
002
r
2
0.
21
P 0.
001
r
2
0.
50
P 0.
001
r
2
0.
63
P 0.
001
r
2
0.
10
P 0.
009
r
2
0.
29
P 0.
001
r
2
0.
04
P 0.
254
r
2
0.
58
P 0.
001
r
2
0
P 0.715
r
2
0.31
P 0.001
r
2
0.28
P 0.001
r
2
0
P 0.800
r
2
0.19
P 0.001
r
2
0
P 0.924
r
2
0.34
P 0.001
r
2
0.39
P 0.001
r
2
0.03
P 0.351
r
2
0.03
P 0.130 r
2
0.03
P 0.299
r
2
0
P 0.973
r
2
0
P 0.985
r
2
0
P 0.878
r
2
0.05
P 0.058
r
2
0.29
P 0.001
r
2
0.29
P 0.001
r
2
0.09
P 0.070
r
2
0.14
P 0.021
r
2
0.16
P 0.001
Year
Fig. 6. Fire frequency for Cal Fire protected lands (a,c,i,m,q) and USFS lands (b,f,j,n,r) and area burned on Cal Fire (c,g,k,o,s) and USFS (d,h,l,p,t)
lands. Note for frequency the change in scale between the South Coast and other divisions and between Cal Fire and USFS lands.
North Coast North Interior Sierra Nevada Central Coast South Coast
1910
1920
1930
1940
1950
1960
1970
1980
2
1
0
1
2
3
Standardized residual
1910
1920
1930
1940
1950
1960
1970
1980
2
1
0
1
2
3
1910
1920
1930
1940
1950
1960
1970
1980
Year
3
2
1
0
1
2
3
1910
1920
1930
1940
1950
1960
1970
1980
2
1
0
1
2
3
1910
1920
1930
1940
1950
1960
1970
1980
2
1
0
1
2
3
Fig. 7. Ignition sources recorded throughout the period 1919–2016 on Cal Fire protected lands by frequency (a–h) and area burned (i–p) in the north
(climate divisions North Coast, North Interior, and Sierra Nevada) and the south (Central Coast and South Coast) with lines for significant regressions from
1919 to 1979 and from 1980 to 2016; note the change in scale between lightning and anthropogenic sources for fire frequency.
Fire ignition sources Int. J. Wildland Fire 789
The main anthropogenic ignition sources on Cal Fire lands
were arson, debris burning and smoking, and all showed
a significant decrease in recent decades (Fig. 8b–d,f–h). Also,
area burned by these ignition sources mostly showed a marked
decrease in recent decades (Fig. 8j–l,n–p).
On USFS lands, arson and smoking were very important but
camping was also a significant cause (Fig. 9). Arson fires
exhibited remarkable similarity in the south of both jurisdictions
with a marked decline in frequency and area burned since 1980
(Fig. 9f,n).
On both Cal Fire and USFS lands, some ignition sources,
such as children playing with fire, equipment, vehicles and
powerlines, were not specifically recorded during the early
years (Fig. 10,11). Children playing with fire declined
significantly in both jurisdictions in the north and south
(Fig. 10a,b,11b,f) as did area burned by this source
(Fig.10i,m,11j,n). Equipment-ignited fires increased
markedly between 1960 and 1979 on Cal Fire lands
(Fig. 10b,f) but, during the same period, declined on USFS
lands (Fig. 11c,g). Since 1980, this source of ignitions has
declined sharply on Cal Fire lands in the north and south
(Fig. 10b,f) but increased on USFS lands in the south
(Fig. 11g). In contrast to all other ignition sources, powerline
fires on Cal Fire and USFS lands in both the north and south
have not declined in the last 4 decades (Fig. 10d,h,11d,h)nor
has area burned by this ignition source (Fig. 10p,11p).
0
20
40
60
80
100
0
20
40
60
80
100
r2 0.54
r2 0.38
r2 0.06
r2 0.09 r2 0.33
r2 0.32 r2 0.61
r2 0.34
r2 0.14
r2 0.01
r2 0.29
r2 0.48 r2 0.29 r2 0.09 r2 0.71
r2 0.17 r2 0.29 r2 0.74
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001 P 0.001
P 0.001
P 0.001 P 0.001
P 0.011
P 0.001
P 0.001
P 0.579
0
40
80
120
160
200
0
40
80
120
160
200
0
40
80
120
160
200
0
40
80
120
160
200
0
40
80
120
160
200
0
40
80
120
160
200
(a)
(e)
(i)
(m)(n)(o)(p)
(j)(k)(l)
(f)(g)(h)
(b)(c)(d)
North - Lightning North - Arson North - Debris North - Smoking
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Lightning South - Arson South - Debris South - Smoking
Fire frequency (#/yr/million ha)
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
North - Lightning North - Arson North - Debris North - Smoking
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Lightning South - Arson South - Debris South - Smoking
Area burned (log ha/yr/million ha)
Year
Year
P 0.001 P 0.001 P 0.001
P 0.001
P 0.001
r2 0.01 r2 0.01
r2 0.72
r2 0.72
P 0.491
P 0.020 P 0.348
r2 0.02 r2 0.38
P 0.001
P 0.066
P 0.010 P 0.152
P 0.147
P 0.001
P 0.030
P 0.075
P 0.021
P 0.590
r2 0.12 r2 0.06
r2 0.04 r2 0.57 r2 0.22
r2 0.27
r2 0.09
r2 0.44
P 0.001
Fig. 8. Ignition sources not reported separately before 1960 on Cal Fire protected lands by frequency (a–h) and area burned (i–p) in the north (climate
divisions North Coast, North Interior, and Sierra Nevada) and the south (Central Coast and South Coast) with lines for significant regressions before 1980
and 1980–2016; note the change in scale for fire frequency.
790 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
Climate relationships to ignitions
Based on the sharp change in ignition patterns through the
period of record, it is critical to understand to what extent
climate variation may have played a role. Considering the
marked changes in climate over the period of this study
(illustrated as decadal anomalies in seasonal temperature and
precipitation in Fig S3), it is reasonable to expect climate
variation has some explanatory value in understanding changes
in ignition sources.
Multi-variate models used mean temperature and total pre-
cipitation for winter, spring, summer and autumn plus the prior-
year winter–spring precipitation. Presented in Table 4 are those
ignition sources with a significant P,0.05 model. Not all
ignition sources exhibited a significant climate mode and the
models determining fire frequency were not the same as those
for area burned. We note that, before 1980, the biggest driver of
debris and railroad fires was prior-year precipitation. Since
1980, there was a negative relationship with summer tempera-
ture for debris burning, playing with fire, smoking and railroad
fires. In the south, where lightning-ignited fires were uncommon
(Fig. 2a), before 1980, they were strongly associated with high
summer temperatures and autumn precipitation.
Total area burned on Cal Fire lands in the north before 1980
was significantly tied to low winter precipitation and high spring
temperatures. In the south, area burned by both arson and
powerlines was significantly tied to climate variation.
0
60
120
180
240
300
0
60
120
180
240
300
r2 0.40
r2 0.37
r2 0.04
r2 0.01
r2 0.06
r2 0.45
r2 0.22 r2 0.18 r2 0.04 r2 0.07 r2 0.86
r2 0.65
P 0.001
P 0.001
P 0.001 P 0.001
P 0.001 P 0.001 P 0.001
P 0.004
P = 0.050
P 0.152
P 0.124 P 0.110
0
30
60
90
120
150
0
30
60
90
120
150
0
15
30
45
60
75
0
15
30
45
60
75
0
15
30
45
60
75
0
15
30
45
60
75
(a)
(e)
(i)
(m)(n)(o)(p)
(j)(k)(l)
(f)(g)(h)
(b)(c)(d)
North - Lightning North - Arson North - Camping North-Smoking
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Lightning South - Arson South - Camping South - Smoking
Fire frequency (#/yr/million ha)
Year
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
North - Lightning North - Arson North - Camping North - Smoking
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Lightning South - Arson South - Camping South - Smoking
Area burned (log ha/yr/million ha)
Year
r2 0.08
P 0.019
r2 0.64 r2 0.03 r2 0.07
P 0.124 P < 0.001 P 0.001
r2 0.32 r2 0.76
P 0.090
P 0.494 P 0.778
P 0.656 P 0.925 P 0.016
P 0.921
P 0.002
P 0.005 P 0.100 P 0.436
r2 0.21 r2 0.08
r2 0.23
r2 0.02
r2 0.24
r2 0.52
r2 0.47
P 0.001 P 0.001
P 0.001
P 0.001
P 0.001
r2 0.00
r2 0.00
r2 0.24
r2 0.00
r2 0.00
r2 0.32
r2 0.08
Fig. 9. Ignition sources recorded throughout the period 1910–2016 on USFS protected lands by frequency (a–h) and area burned (i–p) in the north (climate
divisions North Coast, North Interior, and Sierra Nevada) and the south (Central Coast and South Coast) with lines for significant regressions from 1910 to
1979 and from 1980 to 2016; note the change in scale between lightning and anthropogenic sources for fire frequency.
Fire ignition sources Int. J. Wildland Fire 791
On USFS lands, total number of fires and area burned in both
the north and south exhibited many significant climate models
(Table 5). However, the patterns are complicated and not easily
summarised as the specific climate models varied both spatially
and temporally, as well as being different for different ignition
sources.
For example, lightning fire frequency and area burned in
both the north and south and before and after 1980 were
significantly associated with climate variation, but, before
1980 in the north, the frequency of lightning fires was posi-
tively associated with summer precipitation, but the area
burned was negatively associated with summer precipitation.
After 1980 in the north, the model switched and there was
a very strong effect of prior-year precipitation and summer
temperature.
Since 1980, one of the strongest climate variables affecting
both frequency and area burned was a positive relationship with
prior-year precipitation. Although higher summer temperatures
were associated with increased frequency of arson fires, it was
noteworthy that lower summer temperatures were associated
with an increased incidence of smoking, camping and children
playing with fire in the north. Frequency of powerline fires were
associated with elevated autumn temperatures and higher prior-
year precipitation.
Cal Fire area-burned data were also presented by vegetation
type and showed that in the north, forest and shrubland area
0
40
80
1
20
1
60
2
00
0
40
80
1
20
1
60
2
00
0
40
80
1
20
1
60
2
00
0
40
80
1
20
1
60
2
00
r2 0.88
r2 0.76
r2 0.43 r2 0.92
r2 0.22
r2 0.81
r2 0.44
r2 0.91
r2 0.64 r2 0.75
r2 0.57
r2 0.88
r2 0.77
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001 P < 0.001 P 0.001
P 0.001
P 0.148
P 0.001
P 0.001 P 0.001
P 0.001
0
20
40
60
80
100
0
20
40
60
80
100
0
10
20
30
40
50
0
10
20
30
40
50
(a)
(e)
(i)
(m)(n)(o)(p)
(j)(k)(l)
(f)(g)(h)
(b)(c)(d)
North - Playing North - Equipment North - Vehicles North - Powerlines
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Playing South - Equipment South - Vehicles South - Powerlines
Fire frequency (#/yr/million ha)
Year
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
North - Playing North - Equipment North - Vehicles North - Powerlines
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Playing South - Equipment South - Vehicles South - Powerlines
Area burned (log ha/yr/million ha)
Year
r2 0.72
P < 0.001
P 0.001 P 0.002
P 0.682
r2 0.68
r2 0.35 r2 0.23
r2 0.67 r2 0.36 r2 0.01
P 0.181 P 0.034
P 0.193 P 0.052
r2 0.05 r2 0.17
r2 0.18 r2 0.10
P 0.001
P 0.019 P 0.314 P 0.354 P 0.665
r2 0.15 r2 0.04 r2 0.10 r2 0.01
Fig. 10. Ignition sources variously recorded throughout the period 1910–2016 on USFS lands by frequency (a–h) and area burned (i–p) in the north (climate
divisions North Coast, North Interior and Sierra Nevada) and the south (Central Coast and South Coast) with lines for significant regressions before 1980 and
1980–2016; note the change in scale for fire frequency.
792 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
burned had significant relationships with climate variation
before 1980 but not afterwards (Table 6). In the south, grass-
lands had a significant climate model after 1980.
Discussion
Particularly striking about California ignitions is the steady
increase in number of fires since the early 1900s until a peak c.
1980, followed by a marked drop in fire frequency up to 2016.
This happened on both lower-elevation Cal Fire-protected
lands and higher-elevation USFS lands, and in most climate
divisions (Fig. 6). Despite a significant increase in fires during
the first three-quarters of the 20th century, there were marked
departures from this linear model, with accelerated ignitions
during the 1920s and 1930s and a marked drop in the 1950s
and 1960s (Fig. 7). Climate may have had some role in
these changes since the former decade was drier and the latter
was wetter (Fig S3) and during this period total fires on USFS
lands did have a significant climate model largely driven by
high summer temperatures and low summer precipitation
(Table 4). What is particularly striking is the disconnect
between number of ignitions and area burned; during the first
three-quarters of the 20th century, although ignitions were
increasing, area burned was steadily decreasing through much
of the state.
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
0
10
20
30
40
50
r
2
0.35
r
2
0.02
r
2
0.10
r
2
0.17
r
2
0.21 r
2
0.54
r
2
0.00 r
2
0.01
r
2
0.01
r
2
0.38
r
2
0.03
r
2
0.13 r
2
0.59
r
2
0.36 r
2
0.45 r
2
0.16
r
2
0.03
r
2
0.02
r
2
0.00
r
2
0.03
r
2
0.76
r
2
0.17
r
2
0.07
r
2
0.78
P 0.001
P - 0.718
P - 0.303
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001
P 0.001
P
0.011
P
0.120
P
0.011
P
0.360
P
0.246 P
0.097 P
0.733 P
0.303
P
0.216
P - 0.185
P
0.305 P
0.615
P
0.938 P
0.491
P
0.067 P
0.001
P
0.650
P
0.729
P
0.735
P
0.014
0
5
10
15
20
25
0
5
10
15
20
25
(a)
(e)
(i)
(m)(n)(o)(p)
(j)(k)(l)
(f)(g)(h)
(b)(c)(d)
North - Debris North - Playing North - Equipment North - Powerlines
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
1900 1930 1960 1990 2020
South - Debris South - Playing South - Equipment
South - Powerlines
Fire frequency (#/yr/million ha)
Year
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
1.0
1.8
2.6
3.4
4.2
5.0
North - Debris North - Playing North - Equipment North - Powerlines
1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020 1900 1930 1960 1990 2020
South - Debris South - Playing South - Equipment South - Powerlines
Area burned (log ha/yr/million ha)
Year
r
2
0.14
P - 0.022 P
0.337
r
2
0.02
r
2
0.16 r
2
0.15 r
2
0.02 r
2
0.03
r
2
0.04
r
2
0.28
P
0.001
Fig. 11. Ignition sources variously recorded throughout the period 1910–2016 on USFS lands by frequency (a–h) and area burned (i–p) in the north
(climate divisions North Coast, North Interior, and Sierra Nevada) and the south (Central Coast and South Coast) with lines for significant regressions prior
to 1980 and 1980-2016; note the change in scale for fire frequency.
Fire ignition sources Int. J. Wildland Fire 793
In contrast, since 1980, ignitions have steadily declined, yet
area burned has either not changed or, in some northern parts of
the state, has increased. In short, the number of ignitions does
not directly explain area burned. However, as discussed below,
this conclusion does not apply to individual ignition sources,
and, in this respect, there may be particular sources worth
targeting for fire management purposes.
Factors that may have played a role in these historical
patterns of ignitions and area burned are changes in: population
density, infrastructure development, fire-prevention success,
fire-suppression effectiveness, vegetation-management prac-
tices, climate, and possibly record-keeping accuracy. The dri-
vers behind changes in ignition patterns are quite possibly
different for different sources, different parts of the state and
at different times. First, we consider the patterns for natural
lightning-ignited v. human-caused wildfires.
Lightning-ignited fires
In California, natural lightning-ignited fires decreased from
north to south and from high (USFS) to low (Cal Fire) elevation
(Fig. 2,4). On USFS lands, Lassen and Plumas forests in the
north-east averaged over 150 lightning-ignited fires per year per
million hectares, whereas the coastal Los Padres Forest aver-
aged one-tenth as many (Table S5). In northern California for-
ests, such as the Klamath, Lassen, Modoc, Inyo-Mono and
Sequoia, lightning accounted for the majority of fires, and on
many others it is about equally important as human-ignited fires
(Table 3). Notable exceptions are the coastal Los Padres and
southern California Angeles, Cleveland and San Bernardino
forests, where lightning accounted for less than one-quarter of
all fires. In contrast, on lower-elevation Cal Fire-protected
lands, lightning accounted for less than 10% of all fires in most
counties, and in coastal areas from Sonoma County south, typ-
ically ,1% of all ignitions (Table 2). These patterns closely
follow the distribution of lightning strikes in the state (van
Wagtendonk and Cayan 2008). In general, lightning-ignited
fires in coastal California were substantially less than that
observed over much of the USA (Prestemon et al. 2013). Thus,
the report that extreme fire events driven by high winds are
commonly due to human ignitions and not lightning (Abatzo-
glou et al. 2018) should not be too surprising in California
because these extreme winds are largely restricted to coastal
areas in southern California and the San Francisco Bay Area.
Area burned by lightning-ignited fires approximately paral-
leled these geographical patterns with a couple noteworthy
exceptions. In the northern and central part of the state, more
coastal USFS forests had low lightning-ignited fire frequency
but these accounted for a substantial amount of area burned,
although this was less evident on lower-elevation Cal Fire lands
(Fig. 2–5). For interior forests where lightning is the dominant
ignition source, fires have proven to be reasonably easy to
extinguish, in large part because they typically occur in forests
with a low-intensity surface-fire regime, and during lightning-
storm weather conditions (van Wagtendonk and Cayan 2008),
are conducive to rapid fire control. As a consequence, less than
1% of these forest lands burn each year and these landscapes
have a fire-rotation interval of 100–200 years (Table 1), very
different with what is believed to be the natural fire interval
(Stephens 2005;Van de Waterand Safford 2011;Saffordand Van
de Water 2014). In coastal central and southern California,
lightning accounts for verylittle area burned, in large part because
lightning strikes are very low, but also because human-ignited
fires often occur under weather conditions more conducive to fire
spread, contributing to a shorter fire-rotation interval, e.g. 40–50
years on southern California forest lands (Table 1).
Lightning fires have increased markedly over most of the
20th century on both Cal Fire and USFS lands, in the north and
south (Fig. 8,9). A possible explanation for this pattern is
improvement in detection, as lightning-ignited fires often occur
in remote areas and detection may have been less effective in the
early part of the 20th century and improved in the latter part of
the 20th century. However, there is there is reason to retain some
level of scepticism that this pattern is an artefact of reporting
(see the ‘Methods’ section), primarily because state and federal
agencies have put in extraordinary effort at fire detection since
the early 1900s (Clar 1969;Cermak 2005), including hundreds
of thousands of kilometres of wilderness aircraft fire patrols
beginning in 1919 (Cermak 1991).
Another reason for not simply dismissing historical patterns
as an artefact of reporting is that there are physical factors that
could account for such changes. For example, one potential
factor for a 20th-century rise in lightning fires could be changes
in forest fuel structure, which has been shown to affect light-
ning-ignited fire frequency on other landscapes (Krawchuk et al.
2006). In California, this would be expected based on the
marked drop in area burned following the burning peak in the
1920s – for both Cal Fire and USFS lands, three times more area
burned in that decade relative to the decadal average burned
from 1950 to 1980 (Keeley and Syphard 2017). Thus, during the
mid-20th century there was potentially an increase in fuels that
Table 4. Significant climate models (P,0.05) explaining frequency of
ignitions and area burned for the period ,1980 and $1980 for Cal Fire
protected lands
Models tested mean temperature and total precipitation in winter, spring,
summer and autumn, and prior year winter þspring precipitation
(,1PptWinSpr)
Variable Era Adjusted R
2
PModel
Frequency in North
Debris ,1980 0.18 0.021 ,1PptWinSpr
Debris $1980 0.24 0.050 - TempSum
Playing $1980 0.27 0.035 - TempSum þTempWin
Smoking $1980 0.32 0.016 - TempSum
Railroad ,1980 0.18 0.020 ,1PptWinSpr þPptSpr
Railroad $1980 0.25 0.045 - TempSum þTempWin
Frequency in South
Lightning ,1980 0.40 0.001 TempSum þPptAut
Debris ,1980 0.18 0.021 ,1PptWinSpr – PptAut
Area in North:
Total ,1980 0.23 0.007 - PptWin þTempSpr
Area in South:
Arson $1980 0.25 0.045 - TempWin
Powerlines ,1980 0.99 0.046 - TempSum – PptSpr –
PptAut
Powerlines $1980 0.24 0.050 - TempAut
794 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
may have contributed to a greater chance of lightning strikes
igniting fires.
Changes in reporting standards is also not likely to explain
the pattern of decreased lightning-ignited fires from 1980 to
2016 (Fig. 8,9). On Cal Fire and southern USFS lands, this did
not produce any significant trend in area burned by this fire
source, although northern California (including the Sierra
Nevada) USFS lands showed an increase in area burned by
this source, a pattern also seen in the northern Rocky Moun-
tains (Stephens 2005). Climate is strongly implicated in this
change (Table 5) as two-thirds of the annual variation in area
burned by lightning-ignited fires is explained by a combination
of prior-year precipitation and current-year summer tempera-
ture. Although the latter variable most likely affects fuel
moisture at the time of fire, the former is thought to increase
fires through its effect on herbaceous fuels in the following
year (Littell et al. 2009; Crimmin and Comrie 2011; Keeley
and Syphard 2016). A similar conclusion was drawn by Knapp
(1995) for the climatic control of lightning-ignited fires in the
Intermountain West.
The future projections are that lightning strikes will
increase 50% over this century (Romps et al. 2014), but this
is not easily translated into future lightning fire risks in
California. Some landscapes, such as forests in the north-
eastern part of the state, may already be saturated with
lightning ignitions and coastal landscapes have very few
strikes and thus a 50% increase may not significantly change
lightning-ignited fire risk. In addition, changes in lightning-
strike frequency will have very different impacts dependent on
which season those changes occur in as well the state of future
fuel conditions.
Human-ignition sources
The fact that in all climate divisions, the number of ignitions is
not a monotonic function of time over the past 100 years sug-
gests a complex model of how ignition sources affect burning
activity. Prestemon et al. (2013) presented a conceptual model
of biophysical, social, prevention and management drivers in
controlling human ignition sources. These factors are not static,
Table 5. Significant climate models (P,0.05) explaining frequency of ignitions and area burned for the period ,1980 and .1980 for USFS
protected lands
Models tested mean temperature and total precipitation in winter, spring, summer and autumn, and prior year winter þspring precipitation (,1PptWinSpr)
Variable Era Adjusted R
2
PModel
Frequency in North
Total ,1980 0.27 0.001 TempSum – PptSum þPptAut
Lightning ,1980 0.21 0.005 TempSum þPptSum þPptAut
Arson ,1980 0.13 0.040 –PptAut – PptSpr
Arson $1980 0.28 0.030 TempSum
Smoking $1980 0.46 0.001 –TempSum – PptSum
Debris $1980 0.43 0.002 – PptWin – TempAut – PptSum
Camping $1980 0.26 0.039 –TempSum
Playing $1980 0.47 0.001 –TempSum – PptSum
Railroad ,1980 0.28 0.007 PptAut
Railroad $1980 0.32 0.017 –PptWin þPptSpr
Equipment $1980 0.32 0.016 –TempSum – PptWin – PptSum
Powerlines $1980 0.35 0.010 ,1PptWinSpr
Frequency in South
Total $1980 0.29 0.026 Ppt–1WinSpr
Lightning ,1980 0.18 0.012 TempSum þPptAut
Powerlines $1980 0.28 0.029 TempAut þPptWin
Area in North:
Total ,1980 0.35 ,0.001 –PptWinSpr – PptSum
Total $1980 0.63 ,0.001 ,1PptWinSpr þTempSum – PptAut
Lighting ,1980 0.17 0.018 –PptSum
Lighting $1980 0.64 ,0.001 ,1PptWinSpr þTempSum
Arson ,1980 0.38 ,0.001 –PptWin – PptAut – TempAut – TempWin
Debris ,1980 0.15 0.031 – PptWin –PptAut
Smoking ,1980 0.16 0.022 TempAut þTempSpr
Playing $1980 0.34 0.010 –TempSum –TempAut
Equipment $1980 0.28 0.030 ,1PptWinSpr
Vehicles $1980 0.37 0.007 ,1PptWinSpr
Powerlines $1980 0.47 0.001 –TempWin þ,1PptWinSpr
Area in South
Total ,1980 0.20 0.006 – PptAut – PptWin
Total $1980 0.24 0.048 TempSum – TempWin
Lighting $1980 0.25 0.043 –TempSpr – PptSpr
Fire ignition sources Int. J. Wildland Fire 795
as illustrated by Guyette et al.’s (2002) dynamic anthropogenic
fire regime model for the Ozark Mountains in Missouri. In their
model, they found that the landscape changed over time from
being ignition limited to fuel limited followed by stages
dependent on fuel fragmentation and ultimately a culture-
dependent stage. These temporal changes in drivers could
explain a lot about the temporal changes observed in California
ignitions.
It may be that the marked rise in ignitions during the first
three-quarters of the 20th century in California is the result of
increasing effectiveness of reporting, but this seems unlikely
because the steepest rise in ignitions was in the latter part of the
20th century, i.e. 1960–1980 (Fig. 6). The 20th-century increase
in ignitions was very strongly correlated with population growth
(Tables 2,3), but we believe that more is involved than just
increasing population growth translates into more fires. This
early–mid-20th-century growth spurt was correlated with road
expansion throughout the state, which was bringing more people
in contact with highly flammable fuels (Show 1945;Lockmann
1981;Keeley and Fotheringham 2003). In addition, because of
migration patterns, growth included populations from less fire-
prone parts of the US, and thus a population relatively naı¨ve
about the dangers of fire use in wildland areas (e.g. Zahn 1944;
Show 1945). In addition, fire-prevention education was in its
infancy and the population was slow to recognise their role in the
fire problem. Included too is the widespread use of outdoor
equipment that contributed to the sharpest rise in fires on Cal
Fire landscapes between 1960 and 1980 (Fig. 10). On top of that,
development of fire-response actions were far from perfect (Clar
1969). Also, during the period from 1940 to 1970 the State
Resources Agency was actively involved in promoting burning
of chaparral shrublands for the express purpose of type-
converting native shrublands to exotic grasslands of greater
economic value as rangelands (unpublished records in the State
Archives). Indeed, the state was funding type conversion of
private lands as this was perceived as a fire hazard reduction
strategy, with an economic incentive of increasing rangeland.
Not directly related to changing demography is the signifi-
cant decline in fires in the last several decades – while popula-
tions continued to grow after 1980, fire frequency was
negatively related to population density (Tables 2,3). This is
consistent with the pattern of fire activity peaking under inter-
mediate population density (Syphard et al. 2009). That is, the
relationship between population density and ignition frequency
is likely a function of finer-scale spatial processes regulating the
degree of interspersion between development patterns and
wildland vegetation. In other words, as both population and
development expand into wildland areas, ignitions increase up
to a point at which the area of development, or, impervious
surface, far exceeds the area of wildland, and at that point, the
relationship becomes negative. However, the timing of this
switch varies with regions, e.g. south-east Australia continues
to see a positive relationship in between population density and
fire frequency (Collins et al. 2015).
Thus, these broad-scale patterns observed across the state may
be reflecting macro-scale urbanisation trends over time. Massive
areas of wildland vegetation have been developed and fragmen-
ted in California over the course of the 20th century (Hammer
et al. 2007;Syphard et al. 2017), and the resulting extent and
fragmentation of fuel surely has affected ignition trends and area
burned. It may therefore be important to monitor areas that are
becoming newly developed, as these may be the most fire-prone
areas on the landscape, with sufficient people to start fires and
wildland vegetation to carry fires (Radeloff et al. 2018).
Patterns such as these have been interpreted as indicating
fires are not limited by human ignitions (Knorr et al. 2013;
Moritz and Knowles 2016). This has prompted some to conclude
that fire activity during the last several decades has been driven
largely by climate change (Westerling et al. 2011). It is apparent
that in mid–high-elevation forests in California seasonal climate
variation has been an important factor in determining annual
area burned (Table 5) and that global warming may exacerbate
the fire situation on those landscapes (Keeley and Syphard
2016). However, in coastal California, climates are capable of
generating large fire events most years (Keeley and Syphard
2017), with one exception being years with anomalous late
spring rains (e.g. Dennison et al. 2008). In these coastal loca-
tions, big fire events occur during extreme wind events, how-
ever, these Santa Ana, Diablo or North Wind events occur
predictably every year and yet big fires occur at unpredictable
intervals, being determined by the coincidence of a human
ignition with a wind event (Keeley and Zedler 2009).
During the first two-thirds of the 20th century more people
translated into more fires, and greater fire activity. However, in
recent decades the relationship between human population
growth and fire activity has become more complex, nicely
captured in Prestemon et al.’s (2013) model. In California in
recent decades, increasing population density has increased the
probability of ignitions under the worst weather conditions, either
intentionally by arson for example or accidentally by powerline
failures. This appears to be a widely seen situation throughout the
USA where human-related ignitions are associated with condi-
tions resulting in large wildfires (Nagy et al. 2018).
Decreasing ignitions over the last 4 decades is potentially
reflective of increasing efficiency of fire prevention. However,
it also likely reflects changes in human infrastructure; new roads
in this era were tied to development projects that required
demonstration of adequate fire response capabilities. In addi-
tion, an important factor behind declining ignitions is quite
possibly the emergence of the California Fire Safe Council in
the early 1990s (http://www.cafiresafecouncil.org/about-us/,
accessed 11 August 2016), which made significant contributions
to fire-safety education.
Arson has long been a major source of intentional human
ignitions on both Cal Fire (Fig. 8) and USFS (Fig. 9) lands and on
both jurisdictions arson ignitions increased during the first part
of the 20th century and then dropped markedly in recent
decades. Arson fires have always been one of the largest sources
of area burned, although it was much higher in the early 20th
century than in recent decades. This category comprises igni-
tions motivated for diverse reasons. Early in the 20th century,
these were termed incendiary fires and were often motivated by
goals of maintaining traditional burning practices (Coughlan
2016). As such practices became less socially (and legally)
acceptable, the category was labelled arson fires. Arson fires
exhibit interesting distribution patterns. On low-elevation Cal
Fire lands they are a major ignition source in the northern Sierra
Nevada (Fig. 2,3) but on USFS lands they dominate in the
796 Int. J. Wildland Fire J. E. Keeley and A. D. Syphard
southern part of the state (Fig. 4,5), suggesting a need for more
concentrated anti-arson prevention measures in those regions.
This clustering of arson fires has been observed in parts of the
Mediterranean basin and has prompted an early alert system
(Gonzalez-Olabarria et al. 2012).
One of the real success stories illustrated by these data is the
marked decline since 1980 in frequency and area burned by
arson fires on both Cal Fire and USFS lands (Fig. 8,9). This
reduction in arson fires is a pattern observed for other parts of the
country (Prestemon et al. 2013). In California, this may be
attributed to better neighbourhood-watch programs, which
include patrols during red-flag warnings, but broadcasted fire
prevention messaging may also be a factor. Another factor may
be increased penalties for arson; e.g. the person found guilty of
starting the 2003 Old Fire in southern California was sentenced
to death, as was the arson convicted of the 2006 Esperanza Fire
(Gabbert 2012).
Another source of burning on both Cal Fire and USFS lands
has been smoking. This was a significant cause in the earliest
records, recording even ignitions from cigarettes thrown from
open cockpit planes. Throughout the first half of the 20th
century, smoking was a major cause of wildfires and was the
focus of one of the earliest fire prevention campaigns. In 1942,
over 100 000 ‘fag bags’ were distributed to persons entering the
Angeles National Forest, bright red bags designed to carry
smoking materials and with a prominent fire-safety reminder
stamped on them (Show 1945). The late 20th-century decline in
smoking caused such fires to decline at a much faster rate (Fig. 8,
9) than due to simple reduction in smoking (Prestemon et al.
2013). Reductions in smoking-caused fires are due to a combi-
nation of less smoking, more fire-resistant cigarettes, and
improved fire prevention (Butry et al. 2014).
Children playing with fire has been an important ignition
source and it has exhibited a marked decline in frequency in
recent decades on both Cal Fire (Fig. 10) and USFS (Fig. 11)
lands. Increased fire-prevention effectiveness through better
messaging and development of childproof lighters are potential
factors. Perhaps stricter ordinances in power-tool usage in
wildlands under red-flag warnings may be a factor as well as
requirements for more effective spark arrestors.
Vehicles present another accidental fire source that has
declined sharply on Cal Fire protected lands. Catalytic conver-
ters, which were first required in 1975, are thought to have been
a significant ignition factor (Bertagna 1999; http://www.cbs8.
com/story/35871110/how-a-cars-catalytic-converter-can-spark-
a-massive-fire, accessed 1 June 2017) when they overheated,
igniting roadside vegetation. However, modern vehicles have
warning lights when they overheat, which has the potential
for reducing vehicle fires and could be a factor in the decline
of such fires. Another factor potentially reducing vehicle fires is
improved vegetation treatment along roadside verges.
Electrical powerlines have been reported ignition sources
since 1905 (Show 1945). In the present study, this source of
ignition stands out in that, unlike many other human ignition
sources, powerline fires and area burned by this ignition source
have not declined in recent decades (Fig. 10,11). Although
powerlines do not account for many fires, they often account
for substantial area burned, and some of substantial size
(Keeley et al. 2009;Syphard and Keeley 2015). One reason
that powerline fires are so dangerous is that they commonly
occur during high winds and there are three effects of these
winds: tree contact, line arcing, and metal fatigue resulting in
lines down (Mitchell 2009). These winds create extremely
dangerous fires capable of rapid spread over long distances.
This is a serious problem in other regions such as southern
Australia where it was found that electricity-caused wildfires
are over-represented when fire danger is high (Miller et al.
2017) and similar conclusions were drawn by Ganteaume
and Guerra (2018). Powerline distribution tends to follow
roads and this may be part of the reason burning patterns
are closely correlated with road distribution in southern
California (Faivre et al. 2014). Also, they burn larger areas
than fires ignited by most other causes and are associated with
more significant impacts on lives and property (Collins et al.
2016).
Because these powerline failures typically occur in known
extreme-wind corridors, it has been proposed that wiring these
corridors with underground power could minimise the problem
(Keeley et al. 2009). However, utility companies have shown a
reluctance to accept this solution. One company in southern
California, San Diego Gas & Electric, has opted for an alterna-
tive plan whereby they monitor weather throughout the county
and use these data to shut down portions of the power grid when
that area experiences high winds (https://www.cnbc.com/2017/
12/13/southern-california-utilities-shut-off-power-to-prevent-
wildfires.html, accessed 1 June 2017). In initial attemptsto deal
with fire hazards there have been significant complaints about
the process of shutting down the power grid as it creates many
unanticipated problems (https://www.nbcsandiego.com/news/
local/Supervisor-Demands-State-Investigation-of-Power-Shut-
offs-During-Lilac-Fire-467782743.html, accessed 1 June 2017).
Other approaches have been to replace wooden poles with metal
poles, however, this seems to be a distraction since wooden poles
have not been blamed for starting fires (https://www.voiceof-
sandiego.org/topics/science-environment/sdge-environmentalists-
are-at-opposite-poles-on-one-fire-prevention-method/, accessed 1
June 2017).
Climate change impacts on anthropogenic ignitions is rather
difficult to parse out because climate affects both fire behaviour
and human behaviour. For example, in forests, fire activity is
enhanced by higher spring and summer temperatures through
effects on fuel moisture (Westerling et al. 2006;Littell et al.
2009;Keeley and Syphard 2017). However, in the present study,
fires started by camping, children playing with fire, and smoking
were negatively correlated with summer temperatures, suggest-
ing the possibility that cooler temperatures may have encour-
aged greater outdoor activity.
In general, climate variation exhibited a closer relationship
with fire activity in the higher-elevation USFS lands in the
northern part of the state, consistent with the flammability limited
fire regimes in these regions (Keeley and Syphard 2017). Of
particular significance is the importance of prior-year rainfall
as this is well known to be due to increased fuel production
in grass dominated ecosystems (Crimmins and Comrie 2004;
Keeley and Syphard 2016). We found that this climate variable
was strongly tied to powerline fires, suggesting perhaps that
fine flashy fuels may be a marked hazard in association with
powerlines and may be an additional management target.
Fire ignition sources Int. J. Wildland Fire 797
Conclusions
Throughout California, fire frequency has increased steadily
until a peak c. 1980, followed by a marked drop to the present.
There was not a tight link between frequency of ignition sources
and area burned by those sources and the relationships changed
over time. Natural lightning-ignited fires decreased from north
to south and from high to low elevation. Throughout most of the
state human-caused fires dominated the record and were posi-
tively correlated with population density for the first two-thirds
of the record, but this relationship reversed in recent decades.
Most ignition sources have declined markedly in recent decades
with one notable exception, powerline ignitions. One important
avenue for future fire hazard reduction will be consideration of
solutions to reduce this source of dangerous fires.
Conflicts of interest
The authors declare that they have no conflicts of interest.
Acknowledgements
We thank colleagues for helpful feedback; Hugh Safford (USFS), Mike
Rohde (Orange Co Fire Authority, retired), Dave Passovoy, Dave Sapsis and
Tadashi Moody (Cal Fire), Gary Gilbert (Cal Fire, retired) and Stephen Pyne
(Arizona State University). Thanks also go to Anne Pfaff, Kate Dobrinsky
and Ken Ferschweiler for assistance with data gathering.
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