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Fire-induced erosion and millennial-
scaleclimatechangeinnorthern
ponderosa pine forests
Q1
Jennifer L. Pierce
1
, Grant A. Meyer
1
& A. J. Timothy Jull
2
1
Department of Earth and Planetary Science, University of New Mexico,
Albuquerque, New Mexico 87131, USA
2
NSF-Arizona AMS Facility, The University of Arizona, Tucson, Arizona 85721,
USA
.............................................................................................................................................................................
Western US ponderosa pine forests have recently suffered exten-
sive stand-replacing fires followed by hillslope erosion and
sedimentation
1–4
. These fires are usually attributed to increased
stand density as a result of fire suppression, grazing and other
land use, and are often considered uncharacteristic or unprece-
dented
1–3
. Tree-ring records from the past 500 years indicate that
before Euro-American settlement, frequent, low-severity fires
maintained open stands
1–3
. However, the pre-settlement period
between about AD 1500 and AD 1900 was also generally colder
than present
5–10
, raising the possibility that rapid twentieth-
century warming promoted recent catastrophic fires. Here we
date fire-related sediment deposits in alluvial fans in central
Idaho to reconstruct Holocene fire history in xeric ponderosa
pine forests and examine links to climate. We find that colder
periods experienced frequent low-severity fires, probably fuelled
by increased understory growth. Warmer periods experienced
severe droughts, stand-replacing fires and large debris-flow
events that comprise a large component of long-term erosion
11
and coincide with similar events in sub-alpine forests of Yellow-
stone National Park
12
. Our results suggest that given the powerful
influence of climate, restoration of processes typical of pre-
settlement times may be difficult in a warmer future that
promotes severe fires.
Fire regimes in western US conifer forests vary spatially with
local climate
2,13,14
. Cool, moist, high-elevation spruce-fir forests
experience severe, infrequent stand-replacing crown fires, whereas
frequent light surface fires are considered typical of warm, xeric
ponderosa pine (Pinus ponderosa) forests
1–3,13
. Over the twentieth
century, fire size and severity have increased in most ponderosa pine
forests, including those in the northern Rocky Mountains. From
1908 to 2000, canopy fires burned 50% of the Boise National Forest
in central Idaho, mostly during severe droughts in 1926–35 and
1986–2000. Recent stand-replacing fires in this area have led to
numerous large debris flows and sediment-charged floods in steep
mountain drainages
4
. Sediment yields from single large sedimen-
tation events following severe 1989 and 1994 fires
4
are three orders
of magnitude greater than annual sediment yields measured with-
out large events
11
.
To assess the role of longer-term climatic variations on fire and
erosion in northern ponderosa pine forests, we date fire-related
deposits in tributary alluvial fans of the South Fork Payette River
area, central Idaho (SFP; Fig. 1). These fans receive sediment from
small (0.01–6 km
2
), steep basins in weathered Idaho batholith
granitic rocks, where post-fire erosion is likely. Annual precipitation
is ,600–1,000 mm, with a marked summer drought conducive to
fires. Ponderosa pines are common at lower elevations (,850–
1,300 m) and on dry south-facing slopes. Mixed Douglas-fir
(Pseudotsuga menziesii) and ponderosa pine stands dominate
mid-elevation (,1,500–1,800 m) and north-facing slopes, and
mixed conifers dominated by Douglas-fir and Engelmann spruce
(Picea engelmannii) characterize high elevations around 2,000 m.
Post-fire debris-flow events and sediment-charged floods are
produced via two primary mechanisms in the SFP area
4
. (1) After
severe burns, reduced infiltration and smooth soil surfaces greatly
increase surface runoff, typically during moderate- to high-intensity
convective storms
15
. Extreme discharges entrain sediment through
slope wash, rilling and gullying
4,12
. In lower-severity burns, discon-
tinuous runoff generation limits sediment yields from post-fire
storm events
16
. (2) Loss of root strength a few years after tree
mortality promotes shallow landslides
17
, usually during major
winter storms. Storms that strike severe burns often produce
thick, bouldery, charcoal-rich debris-flow deposits, but large debris
flows may exist as only thin marginal facies at some alluvial fan
sites
4,12
. Discrete layers of charred forest litter provide another
stratigraphic indicator of fire. These well-preserved burned soil
surfaces imply rapid burial by post-fire sediments before bioturba-
tion. We define large fire-related events (‘large events’) conserva-
tively as debris-flow units with abundant coarse angular charcoal
(Supplementary Fig. 1) that are coarser grained than other units in a
stratigraphic section, and comprise at least 20% of its thickness;
most overlie burned soil surfaces. Deposits that do not meet these
criteria but that contain abundant charcoal and (or) overlie burned
surfaces are considered small fire-related events (‘small events’).
Multiple events that occurred hours to several years after a single fire
may appear as a single thick deposit that would be recognized as a
‘large event’, and still represent a major geomorphic response to a
severe fire.
Individual charcoal fragments were AMS
14
C-dated, using seeds,
needles and twigs when available to reduce the probability of dating
a sample much older than the fire in which it burned (yielding a
large ‘inbuilt age’). We obtained 133 dates from 33 stratigraphic
Figure 1 The SFP and northern Yellowstone National Park (YNP) field areas. Right, grey
shading shows distribution of Pinus ponderosa (ponderosa pine) in the western USA and
Canada (ponderosa pine distribution available at http://climchange.cr.usgs.gov/data/
atlas). Left, red dots on shaded relief map of the SFP study area show locations of alluvial
fan stratigraphic sites. Elevations range from ,850 m in Garden Valley to .2,000 m on
the eastern border of the map.
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sections(sites)in32differentfans, with 1–15 dates per site
(Supplementary Table 1). Calibrated probability distributions
18
for 91 fire-related sedimentation events were summed to produce
a probability spectrum (Fig. 2); multiple dates for the same fire-
related event and stratigraphically inverted ages (Supplementary
Table 2) were excluded. Small events dominate the overall record
and are primarily responsible for major peaks except at ,750–1,000
and 1,700 calibrated calendar years before present (cal. yr BP). At-a-
site frequency values were calculated to represent the frequency of
fire-related events in a single fan’s basin. Deposits of single events
usually cover only part of a fan, and low- to moderate-severity fires
often result in little or no deposits, thus at-a-site frequency of fire-
related sedimentation is much lower than fire frequency in the
associated basin. Nonetheless, at-a-site recurrence intervals for
small sedimentation events are as low as 33–80 yr. Maxima in
small events (Fig. 2a) at ,350, 1,200–1,500, 2,800–3,000, ,5,000
and 6,800–7,400 cal. yr BP correspond with peak at-a-site frequency
over the study area, indicating periods of frequent low- to mixed-
severity fires that produce limited sedimentation.
In contrast, about 24–27% of the total dated fan thickness was
emplaced by only 9 large fire-induced debris-flow events ,1,000–
800 cal. yr BP (Fig. 3). The importance of infrequent large events is
supported by Holocene-average sediment flux estimates for Idaho
batholith basins an order of magnitude greater than modern rates
measured without major debris-flow events
11
. Most fans in the
lower-elevation ponderosa pine environment contain debris-flow
units interbedded with multiple thin sheetflood deposits. Thick
(0.5–2 m) fire-related debris-flow deposits dominate higher-
elevation fans, consistent with a greater importance of severe
stand-replacing fires in mixed-conifer forests.
We compare SFP fire-related sedimentation with a similar record
from high-elevation forests in northern Yellowstone National Park
(Fig. 1), a cooler and wetter environment than the SFP area, where
dense lodgepole-pine-dominated forests burn primarily in large,
severe fires with recurrence intervals of 150–400 yr (refs 12, 14, 19).
Notably, maxima in small-event probability (Fig. 2a) in the SFP area
generally correspond with prominent minima in fire-related sedi-
mentation in Yellowstone (,350–500, 1,200–1,400 and 2,800–
3,000 cal. yr BP), suggesting a regional millennial-scale climatic
control. In Yellowstone, these minima in fire-related sedimentation
are associated with colder, effectively wetter, climates and increased
runoff in streams, including during the Little Ice Age (LIA)
12
. LIA
climate was not uniformly cold in either time or space, but glacial
advances occurred at ,650–100 cal. yr BP in the northern Rocky
Mountains
6–8
, with colder than present conditions in many
Northern Hemisphere localities
8–10
.
We infer that over much of the LIA, colder conditions maintained
high canopy moisture, inhibiting stand-replacing fires in both
Yellowstone lodgepole pine forests and SFP ponderosa pine
forests
12–14
. Concurrently, effectively wetter conditions increased
understory grass growth and provided fuel for low-severity
burns
13
in the SFP, where summers are warmer and drier than in
Yellowstone. In ponderosa pine forests, non-lethal surface fires
typically occur when several cool, moist years produce abundant
fine fuels, followed shortly by a mild to moderate drought
year
13,20–22
. Without prolonged severe droughts, wet–dry cycles on
annual to multiyear timescales promote frequent fires in the SFP,
and an increase in storm frequency during wetter intervals would
also augment the number of small fire-related sedimentation events
recorded in alluvial fans.
In contrast, intervals of stand-replacing fires and large debris-
flow events are largely coincident in SFP ponderosa pine forests and
Yellowstone, most notably during the ‘Medieval Climatic Anomaly’
(MCA), ,1,050–650 cal. yr BP (ref. 23; Fig. 2b). Although often
Figure 2 Summed calibrated probability distributions for radiocarbon ages on fire-related
sedimentation events in the SFP Idaho area (this study) and in Yellowstone National Park
12
(YNP, grey-filled curves). Probability distributions were smoothed using a 100-yr running
mean to reduce the influence of short-period variations in atmospheric
14
C, but retain
major probability peaks representing the most probable age ranges. Calibrated years
before present (cal. yr BP) are equal to the number of calendar years before AD 1950. The
general trend of decreasing probability over time, and overall lower probability values
before ,4,000 cal. yr BP reflect lesser exposure and preservation of older deposits.
a, SFP ‘small events’ (blue line) in Idaho are thin deposits probably related to low- or
moderate-severity burns; note correspondence of peaks with minima in YNP fire-related
sedimentation, and major peak during the ‘Little Ice Age’ (LIA), ,650–50 cal. yr BP.
(Fewer near-surface deposits in the 400 cal. yr BP–present range were selected for dating
because of bioturbation and large uncertainties in
14
C calibration). b, SFP ‘large events’
are major debris-flow deposits probably related to severe fires; note correlation with the
YNP record, and prominent peak in large-event probability during the ‘Medieval Climatic
Anomaly’ (MCA), ,1,050–650 cal. yr BP.
Figure 3 Variations in the thickness of fire-related deposits over time. Relative
stratigraphic thickness of dated fire-related fan deposits in the SFP are expressed as a
percentage of the total thickness for all dated deposits and plotted by calibrated age
interval. Individual deposit thicknesses are shown by stacked bar segments and are
calculated as the average of maximum and minimum possible thickness. This range in
thickness results from measured variations in deposit thickness and from uncertainty in
defining deposit boundaries. Average range of uncertainty in deposit thickness is ^10%.
Red bar segments represent ‘large events’ based on criteria described in the text.
Because of calibration problems for very young radiocarbon samples, deposits
,100 cal. yr BP are not shown, and data older than 4,000 cal. yr BP are too sparse to be
informative. Across the study area, ,50% of the total measured thickness of fan
sediments deposited over the past 2,000 yr is probably or possibly related to fire.
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called the ‘Medieval Warm Period’, times of anomalous Northern
Hemisphere warmth are confined to shorter intervals within this
period
10,23–25
. In the western USA, the MCA included widespread,
severe multidecadal droughts
23,26
, with increased fire activity
across diverse northwestern conifer forests
12,13
. Some unusually
wet decades also occurred, at least locally
27
. Despite high fire
frequency in the LIA, large SFP debris-flow events during the
MCA account for most of the dated fire-related sediment produced
over the last millennium (Fig. 3).
Although climate-induced changes in forest composition could
drive changes in fire regimes, identification of carbonized macro-
fossils in fan deposits indicates that at least after ,4,500 cal. yr BP,
the same conifer species comprising present forests existed in
tributary basins (Supplementary Table 1; Supplementary Fig. 2).
The inferred changes in fire frequency and severity may have
stemmed in part from changes in relative species abundance
—
for
example, infilling of open ponderosa pine stands by Douglas-fir as
in recent decades in central Idaho
3
—
but we cannot resolve such
spatial changes in forest composition. At site LO1, now character-
ized by ponderosa pine and minor Douglas-fir, only Douglas-fir was
identified at 7,400–6,800 cal. yr BP. Nonetheless, frequent small
sedimentation events during this interval suggest a low- to mixed-
severity fire regime, similar to ponderosa-dominated stands in the
1600s–1800 s (ref. 3).
Along with previous studies covering shorter timescales, our
results illustrate how superposed centennial- to millennial-scale
climate variations influence fuel conditions, wildfire, and fire-
related erosional events. Within multi-centennial cool-moist
periods such as the LIA, moderate annual to multi-annual droughts
produced frequent fires in xeric SFP ponderosa pine forests, con-
trolled primarily by growth and desiccation of grass and other
fine surface fuels. Concurrently, a cooler and effectively moister
regional climate inhibited most fires in the high-elevation forests of
Yellowstone.
In contrast, extreme droughts of up to multi-decadal length were
associated with severe stand-replacing fires and large fire-related
erosional events in both the SFP and Yellowstone during the ,400-
yr MCA. Denser stands possibly aided these severe fires; perhaps
prolonged droughts limited grass growth and surface fires, allowing
survival of understory trees as ladder fuels
1–3,13
. Alternatively,
decades of unusually wet summers allowed densities to increase.
In any case, extreme drought promotes critically low canopy
moisture
13
and is associated with MCA stand-replacing fires, despite
the absence of fire suppression or land-use effects. Relatively severe
droughts within the LIA may have resulted in some stand-replacing
fires, as suggested by tree-ring studies in other ponderosa pine
forests
22,28,29
. Peak SFP fire-related sedimentation at ,350 cal. yr BP
(,AD 1600) may partly reflect a prolonged drought in the late AD
1500s
26
. Nonetheless, limited geomorphic response indicates that
fires in the LIA were overall less severe than in either the MCA or
recent decades.
Instrumental and proxy records reveal a rapid rise in Northern
Hemisphere temperatures over the last century
5,8,9
that is probably
in part anthropogenic
24
, and current warmth is probably greater
than in the MCA
25
. Warming of 0.6–2 8C has affected much of the
western USA during the last century, and has been accompanied by
a 5–20% decrease in precipitation
30
in northern ponderosa pine
environments. Fire-season Palmer drought severity index correlates
strongly with burn area in both central Idaho and Yellowstone
12,20
,
and shows a significant (P¼0.01) increase in drought severity
between 1895 and 2002. In addition, twentieth-century fire sup-
pression and to a lesser extent grazing have limited surface fires,
allowing seedling survival and increased densities in SFP ponderosa
pine stands
3
. Alteration of fuel conditions combined with increased
incidence of extreme drought have probably produced the recent
spate of extensive severe fires in the SFP and other northern
ponderosa pine forests.
Fire management and ecological restoration strategies in ponder-
osa pine forests typically aim to prevent large stand-replacing fires
by reproducing pre-settlement conditions with low tree densities
1,3
.
Climate exerts a powerful control on fire regimes, however, and the
rapidity and magnitude of twentieth-century global climate change
is probably greater than has occurred for millennia
24,25
. Efforts to
return to fire regimes typical of a generally colder pre-settlement era
will need to adapt to changing vegetation and fire activity in a
warmer and drought-prone future. A
Received 17 February; accepted 15 September 2004; doi:10.1038/nature03058.
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Supplementary Information accompanies the paper on www.nature.com/nature.
Acknowledgements Research funding was provided by the NSF, the University of New Mexico
(UNM) and the UNM Department of Earth and Planetary Sciences. We thank L. Huckell for
assistance with macrofossil identification; K. Pierce, B. Huckell and L. McFadden for comments
on the manuscript; and S. Wood, T. Lite, L. Rockwell, S. Caldwell, C. North, K. Grover-Wier,
K. Pierce and W. Andersen for discussions and field assistance.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to J.P. (jpierce@unm.edu).
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