CSIRO PUBLISHING
International Journal of Wildland Fire 2008, 17, 72–83 www.publish.csiro.au/journals/ijwf
Long-term relations among fire, fuel, and climate in the
north-western US based on lake-sediment studies
Cathy WhitlockA,E, Jennifer MarlonB, Christy BrilesB, Andrea BrunelleC,
Colin LongD and Patrick BartleinB
ADepartment of Earth Sciences, Montana State University, Bozeman MT 59717, USA.
BDepartment of Geography, University of Oregon, Eugene, OR 97403, USA.
CDepartment of Geography, University of Utah, Salt Lake City, UT 84112, USA.
DDepartment of Geography and Urban Planning, University of Wisconsin Oshkosh,
Oshkosh, WI 54901, USA.
ECorresponding author. Email: whitlock@montana.edu
Abstract. Pollen and high-resolution charcoal records from the north-western USA provide an opportunity to examine
the linkages among fire, climate, and fuels on multiple temporal and spatial scales. The data suggest that general charcoal
levels were low in the late-glacial period and increased steadily through the last 11 000 years with increasing fuel biomass.
At local scales, fire occurrence is governed by the interaction of site controls, including vegetation, local climate and fire
weather, and topography. At subregional scales, patterns in the long term fire-episode frequency data are apparent: The
Coast Range had relatively few fires in the Holocene, whereas the Klamath–Siskiyou region experienced frequent fire
episodes. Fire regimes in the northern Rocky Mountains have been strongly governed by millennial- and centennial-scale
climate variability and regional differences in summer moisture. At regional scales, sites in present-day summer-dry areas
show a period of protracted high fire activity within the early Holocene that is attributed to intensified summer drought in
the summer-dry region. Sites in summer-wet areas show the opposite pattern, that fire was lower in frequency than present
in the early Holocene as result of strengthened monsoonal circulation then. Higher fire-episode frequency at many sites
in the last 2000 years is attributed to greater drought during the Medieval Climate Anomaly and possibly anthropogenic
burning. The association between drought, increased fire occurrence, and available fuels evident on several time scales
suggests that long-term fire history patterns should be considered in current assessments of historical fire regimes and
fuel conditions.
Additional keywords: charcoal data, fire history, Holocene, pollen data, western US.
Introduction
The western USA is currently experiencing dramatic environ-
mental changes in the form of recent drought, rapid glacial
recession, and severe wildfires (Gedalof et al. 2005; Westerling
et al. 2006; Pederson et al. 2007). Recent attention on wild-
fires began in 1988 with the large area burned in Yellowstone
National Park (600 000 ha), and this event has been followed by
several years of record-setting fires in the western US and Canada
(National Interagency Fire Center data, http://www.nifc.gov,
accessed 4 January 2008). With each conflagration, the issue of
historical precedence is raised: have fires of this scale occurred
before, what are the prospects for ecological recovery, and are
these events a predictable consequence of climate changes or
fire-related management practices? To address these questions
requires information on fire history and a clear understanding of
how humans and climate have interacted to shape present forest
ecosystems through the disturbance of fire.
Fire-history information comes from two primary sources.
Dendrochronological records, including fire-scars on tree-rings
and forest stand-age structure data, comprise one source and
are available for most western forests (see the National Oceanic
and Atmospheric Administration National Climatic Data Center
(NOAA NCDC) International Multiproxy Paleofire Database for
available data: http://www.ncdc.noaa.gov/paleo/impd/paleofire.
html, accessed 4 January 2008). Tree-ring records are spatially
specific and also temporally precise, and it is possible to iden-
tify individual years and even seasons of burning. Their primary
limitation is that the oldest trees are relatively young, generally
<500 years, which does not provide a good analogue for pro-
jected changes in climate as a result of increased greenhouse
gases (IPCC 2007).
The second source of fire-history information comes from
stratigraphic records of charcoal preserved in the sediments of
lakes and wetlands and other geologic deposits. This charcoal
is produced during a forest fire, carried aloft, deposited on the
lake surface, and eventually buried in the sediments. Informa-
tion on modern fires as well as theoretical and modelling studies
provide the basis for interpreting charcoal data in terms of past
fire (e.g. Whitlock and Millspaugh 1996; Peters and Higuera
2007). The geographic coverage of charcoal records in western
USA is less extensive than that of tree-ring data, and they lack
the spatial resolution or temporal precision. They do extend fire
© IAWF 2008 10.1071/WF07025 1049-8001/08/010072

Fire, fuel, and climate in the north-western US Int. J. Wildland Fire 73
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Fig. 1. Location of study sites in the north-western USA. Site information is available in Table 1.
reconstructions back thousands of years (i.e. the age of many
western lakes). In addition, it is possible to compare charcoal and
pollen records from the same cores to examine the interactions
between fire and vegetation during periods of major environ-
mental reorganisation. Thus, the insights gained from these long
records have direct bearing on our understanding of current fire
regimes in the face of changing climate.
The objective of the present paper is to compare the fire and
vegetation history of the north-western USA based on char-
coal and pollen records from 15 lake-sediment records. Other
high-resolution fire records are available from the region and
used for comparison (e.g. Hallett et al. 2003; Daniels et al.
2005; Gavin et al. 2006; Minckley et al. 2007), but our focus
here is on the 15 sites as representative examples. We describe
(1) the methods used for high-resolution fire-history studies;
(2) the fire history of specific subregions of the north-western
USA; and (3) a regional comparison of the temporal trends
in charcoal data. We then discuss the implications of these
long fire records for understanding present-day fire regimes and
consider the importance of this information in current forest
management.
The 15 sites are located in the Oregon Coast Range (OCR), the
Klamath–Siskiyou Mountains (K-S), and the Northern Rocky
Mountains (NRM) (Fig. 1; Table 1) and span an area of varied
topography, geology, and present-day vegetation, climate, and
fire regimes. Present-day vegetation ranges from temperate rain-
forest in the OCR to diverse conifer forests in the K-S, and lower
steppe parkland communities to subalpine parkland in the NRM.
The vegetation history includes periods of tundra, parkland, for-
est, and dry grassland over the last 17 000 years. Present fire
regimes vary from infrequent high-severity (crown) fire in the
OCR to mixed-severity fires in the K-S and NMR, and frequent
low-severity (surface) fires at lower treeline in the NRM.
Seasonality of precipitation is an important site characteris-
tic related to long-term changes in fire activity and fire-episode
frequency in particular (Whitlock and Bartlein 2004; Brunelle
et al. 2005). The sites can be broadly grouped into two cate-
gories according to the ratio of summer to annual precipitation.
‘Summer-dry’ sites lie in areas where summer precipitation
is currently suppressed by the north-eastern Pacific subtrop-
ical high-pressure system. This high-pressure system causes
widespread subsidence over the north-western USA, leading
to especially dry summer conditions in the OCR, K-S and
parts of the NRM. ‘Summer-wet’ sites exist east of the Con-
tinental Divide where recirculated monsoonal moisture is pre-
cipitated during mid–late summer convectional thunderstorms.
In summer-wet areas, moisture is drawn northward from the
Gulf of Mexico and eastern subtropical North Pacific into the
Southwest and farther north into the NRM during the summer.
Twelve of the 15 sites exhibit summer-dry patterns, whereas
three are summer-wet (Baker, Pintlar and Slough Creek Lakes),
and the geographic location of these precipitation regimes does
not seem to shift substantially during the last 11 000 years (the
Holocene) (Whitlock and Bartlein 1993).