Int. J . Wildland Fire 2(3): 139-144,1992
O IAWF. Printed in U.S.A.
...
When It's Hot, It's Hot or Maybe It's Not!
(Surface Flaming May Not Portend Extensive Soil Heating)
Roberta A. Hartford and William H. Frandsen
U S . Department of Agriculture, Forest Service, Intermountain Research Station
Intermountain Fire Sciences Laboratory P.O.Box 8089, Missoula, MT 59807
Tel. 406 329 4820; Fax 406 329 4863
Abstract. Fire effects on aplant community, soil, and air
are not apparent when judged only by surface fire intensity. The fire severity or fire impact can be described by
the temperatures reached within the forest floor and the
duration of heating experienced in the vegetation, forest
floor, and underlying mineral soil. Temporal distributions of temperatures illustrate heat flow in duff and
mineral soil in three instrumented plots: two with slash
fuel over moist duff and one with litter fuel over dry duff.
Fires in the two slash fuel plots produced substantial
flame lengths but minimal heating in the underlying
mineral soil. In contrast, smoldering combustion in the
dry duff plot produced long duration heating with nearly
complete duff consumption and lethal temperaturesat the
mineral soil surface. Moisture content of duff and soil
were key variables for determining f i e impact on the
forest floor.
Keywords: Temperatures; Duff; Smoldering; Northern Rocky
Mountains; Larix occide~alis;
Abies lasiocarpa.
Introduction
Flaming slash gives the visual impression of intense
heating while smoke from smoldering duff may be the
only clue to its combustion, giving the impression of an
innocuous fire. Temperatures measured in the forest
floor over time (temperature histories) provide a direct
means or observing the downward heat pulse. This paper
illustrates temperature histories in forest floor duff and
mineral soil underlying burning slash and ground fuel,
and also shows the contribution of moisture in limiting
fire impact. We describe temperature data from three
plots in areas with similar duff depth and surface fuel
moisture contents but differing surface fuel load and duff
moisture content.
Common descriptions of fire behavior may not provide adequate information for fire effects on vegetation
and soil. For example, fire intensity, the rate of heat
released by a fire per unit length of fire front, can be
estimated from flame length (Byram 1959), the most
readily observed fire behavior characteristic. While it is
a good indicator of aboveground fire effects (Brown
1984), it does not relate to the heat released by fuels that
continue to bum after the flame front passes (Alexander
1982). The effect of fire on the forest floor is indicated by
duff and large woody fuel consumption and the physical
condition of the mineral soil.
Ryan and Noste (1985) describe fire severity as an
expression "of the effect of the fire on the ecosystem."
They characterized fire severity by classifying the combined effects of fire intensity and the depth of ground char
(depth of duff consumed or depth of duff andlor soil
altered by fire). The equivalent Canadian forest fire
management term, fire impact (Merrill and Alexander
1987), is similarly defined as "the immediately evident
effect of fire on the ecosystem in terms of biophysical
alterations" including both effects on crown and ground
fuel.
A brief review of fire terminology and descriptions of
some fire processes will help prevent confusion. The
forest floor includes the surface litter and duff (fermentation and humus layers) that overlay mineral soil and often
contain decomposing large woody material. Three types
of forest fires--crown, surface, and ground fire (Davis
1959)--can involve the forest floor to varying extents.
Crown fires bum in forest crowns independently or are
supported by surface fire that bums in fuels such as litter,
slash, shrubs, or small trees. Both crown and surface fires
bum by flaming combustion as pyrolyzing fuel releases
volatile gases that mix with air and are heated to ignition
(Countryman 1976). These fires consume duff as heat
from burning fuels causes subsurface or ground fuels
(duff and rotted wood) to dry, then ignite. Ground fire
characteristicallyburns without flame in subsurface fuels
by smoldering combustion as duff is slowly pyrolyzed to
char. The highly reactive char combines with oxygen at
temperatures below those of flaming gases, evolving CO
140
Hartford, R.A. and Frandsen, W.H.
and CO, and producing heat. This pyrolyzes adjacent
organic material evolving incompletely oxidized products and generating more reactive char (Shafizadeh
1984). Volatile gases, tar droplets, and soot particles are
released as smoke at relatively low temperatures and do
not ignite. The tars and other unburned organics can
condense on cooler surfaces such as litter above the duff.
Large temperature gradients occumng in the mineral
soil underlying the combustion zone can cause downward movement and condensation of these hydrophobic
substances on soil particles (DeBano et al. 1977).
Many authors have related duff consumption to actual moisture contents (or indexes that reflect moisture
content) of slow drying fuel. Van Wagner (1972) related
duff consumption in eastern pine stands to duff moisture
content as estimated by the Duff Moisture Code, a
component of the Canadian Fire Weather Index System.
Sandberg (1980) correlated duff consumption in western
Washington and Oregon with duff moisture content,
components of the National Fire Danger Rating System,
and fuel load. Sandberg (1980) stated that duff burned
independently of surface fuel combustion if the duff
moisture content was below 30%. FurtRermore, no duff
consumption occurred, regardless of the amount of surface fuels, if the duff moisture content was over 120%.
Brown et al. (1985) used data from several studies in
Northern Rocky Mountain forests to develop relationships between duff depth reduction or minerd soil exposure and duff moisture contents, National Fire Danger
Rating System's 1000-hrmoisture index, Canadian Duff
Moisture Codes, and surface fuel loads.
Less work has been reported relating fire behavior to
subsurface temperatures. Mineral soil heating due to
burning slash fuel is influenced by duff depth and duff
and soil moisture content (Frandsen and Ryan 1986).
The presence of inorganics as well as moisture can stop
smoldering combustion in duff (Frandsen 1987). Once
the combustion limit is reached, the unburned duff layer
acts as a banier to heat flow from the burning material
above to the underlying mineral soil.
moisture content, loading of various size classes, duff
depth, etc. preclude extrapolation from one or two plots
to a burn unit. Three of the 43 plots had similar forest
floor characteristics but were dramatically different in
fire behavior and resulting soil heat treatment. These
three plots illuminate the amount and duration of heating
resulting from duff combustion.
Two plots were established within prescribed burr:
units, MooseRidge in northern Idaho and Uhler Creek in
western Montana. Criteria for plot location included
undisturbed duff, slash fuel loading representative of the
unit, and fuel continuity sufficient to assure spread of the
surface fire over the plot. Areas with fuels greatcr than
7.6 cm in diameter were avoided. Both units originally
had a mixed short needle conifer ovcrstory. Larix
occidentalis Nutt. (western larch) needles, twigs, bark
flakes, etc. were a major component of the litter and duff
at Moose Ridge while Abies lasiocarpa (Hook) Nutt.
(subalpine fir) was the dominant component at Uhler
Creek. The plots at these two sites are referred to as the
slash fire plots with site names used when necessary for
differentiation.
By comparison, the third plot location, West Side Bypass in western Montana, was selected for lack of reccnt
disturbance and lack of downed woody surface fuels.
Litter was the only surface fuel, and the duff was dry
enough that an independent ground fire was anticipated.
The overstory was mixed Larix occidentalis and Pinus
contorta v. latifolia Engelm. (lodgepole pine), but Larix
occidentalis dominated the litter and duff. We referrcd
to this plot as the ground fire plot.
Preburn data included fuel moisture, soil moisture,
duff depth, and downed woody load over each plat
Table 1. Prebumconditions at onenorthernIdaho plot (Moose
Ridge) and two western Montana plots (Uhler Creek and West
Side By-pass).
Slash fire plots
Moose Ridge
Uhler Creek
ABLANEFE
PSMED'ACA
Nine
Six
12.8 kg/m2
0.0 kg/m2
5%
60%
10%
Duff depth
6 cm
6 cm
6.5 cm
Moisture content
Slash fines
Litter
Fermentation
Humus
Soil
(dty weight basis)
8%
9%
11%
12%
3 1%
16%
64%
39%
Habitat type' ABLAICLUN
Methods
Observed fire behavior wascompared to forest floor
heating on 43 plots with undisturbed duff layers and
diverse loads of surface fuel (dataon file at Intermountain
Fire Sciences Laboratory). These data were collected
opportunisticallyin western Montanaand northern Idaho
by instrumenting selected sites within prescribed bum
units or by burning smaller plots when weather permitted. The variety of weather and fuel conditions encountered makes replication of conditions unlikely. Natural
variability and natural fuel bed characteristics such as
Ground fire plot
West
Side
Ry-Pass
-
Fire groupZ
Nine
Approx. slash
10.3 kg/m2
load on plot
Slope at plot
NA
11%
12%
'Habitat types (Cooper et al. 1987) and (Pfister et al. 1977).
Fire groups (Fischer and Bradley 1987).
14 1
- When It's Hot, It's Hot (Table 1). Litter and duff depths are an average of three
samples taken within 0.5 m of the instrumented location.
On all three plots the litter was about 1 cm in depth, duff
depth around 6 cm, and rotted wood was common in the
humus layer. Slash loads, estimated by line intercept
over the plots (Brown 1974),were 10and 13kg/m20nthe
plots at Moose Ridge and Uhler Creek. There was no
surface woody fuel on the ground fire plot. Moisture
contents were measured by ovendrying paired samples
collected adjacent to the instrumented plots, and we
reported these as a percentage of the dry weight. Fine
woody fuels were 8% and 9% moisture content at Moose
Ridge andUhler Creek; litter moisture content was 11%
to 12% on all three plots. We observed differences in
moisture content between plots of the duff and mineral
soil. Duff at the ground fire plot was considerably drier
than at theother two plots. The percentage slope at Uhler
creek is much greater than the other two sites.
All bums were conducted in August. Ignition began
at dusk on the prescribed bum units containing the slash
fire plots. They were ignited in cross-slope strips from
the top of the units. One strip ignited fuel down slope of
the instrumented plots, allowing the fire to burn upward
through the plots. The ground fire plot was ignited by
litter surface fire. Weather was similar for all burns.
Afternoon temperatures were near 30°C, dropping to
near 4°C at night. The minimum relative humidity was
near 22% and the maximum near97%. Winds werecalm
throughout the burns and skies were clear.
Temperature histories of the forest floor from the
litter surface, through the duff, and into the mineral soil
were recorded with a data logger at 1-minute intervals
with chromel-alumel thermocouple probes. The probes
were positioned parallel to the litter surface at increasing
depths to form a vertical comb-like array (Fig. 1, insets).
The uppermost thermocouple was placed at the upper
surface of the litter. Probes were more widely spaced in
the slash fire plots than in the ground fire plot in anticipation of deep heating. We measured the exact placement following burning because the probes were sometimes deflected during insertion.
All plots were visible throughout the burns. We
estimated flame lengths by comparing them to objects of
known height near the plot, and we used photographs to
verify field estimations. We also noted the duration of
flaming in surface fuels at the plot location.
Postfire data collechon included measurement of
duff reduction at the thermocouple m a y position, measurement of exact thermocouple position, and description of the condition of the remaining duff and woody
matter at the plot location.
Table 2. Post-bum analysis at one northern Idaho and two
western Montana plots.
Ground fire plot
Slash fire plots
Moose Ridge Uhler Creek West Side By-Pass
Duff reduction
5 cm
5 cm
6.5 cm
Max temperatures
Litter surface
Duff
Soil surface
690°C
625°C
40°C
460°C
400°C
80°C
300°C
1 hr
2.5 hrs
1 6 hrs
Duration
Temp> 100°C in duff
515°C
4Oo0C
Results and Discussion
Maximum flame lengths on the slash f r e plots reached
3 m and flaming lasted up to 30 minutes at the location
of the instrumented plot. Flames on the ground fire plot
were only a few centimeters in length and had a duration
of less than 3 minutes. Table 2 summarizes duff reduction, duration of elevated temperatures, and peak temperatures.
On all three plots, all fermentation and humus layers
with moisture contents around 30% or less were consumed. Litter, surface woody fuel, and most of the duff
were totally consumed on the slash fire plots. About 1cm
of unburned duff, charred only on the surface, remained
on the mineral soil surface. All duff was reduced to white
ash on the ground fire plot. A layer of charred litter with
a resinous coating of condensed organics remained on
the surface of the plot and was commonly observed on
other ground fire plots.
Figure 1 shows temperature histories of the three
plots. The most noteable feature is the difference in the
duration of heating to temperatures above 100°C in the
duff. At the slash fire plots at Moose Ridge and Uhler
Creek, temperaturesin excess of 100°C lasted 1hour and
2.5 hours. In contrast, much of the lower duff was still
smoldering at the ground fire plot 16 hours after the
upper portion of the duff first reached 10O0C,
The peak temperature observed at the surface of the
litter was affected by the presence of slash. Temperatures reached 460°C to 690°C on the slash fire plotsinfluenced by surface fire-while the maximum litter
surface temperaturebarely attained 300°C on the ground
fire plot. Peak temperatures at the litter surface were
realized while the slash was flaming and then burning out
following passage of the flame front. The litter surface
fire on the ground fire plot influenced the litter temperature sensor only slightly, reaching less than 100°C.
These differences coarsely correspond to the extreme
differences in surface fire intensity, exhibited by the
flame lengths in the slash fire plots compared to the
ground fire plot. The maximum surface temperature
Hartford, R.A. and Frandsen, W.H.
r la
'0°
1
MOOSE RIDGE
'0°
r
UHLER CREEK
WEST SIDE BY PASS
ELAPSED TIME, h
Figure 1. Temperature histories in the litter (L), fermentation (F), humus (H), and mineral soil. Insets show vertical placement of
temperature sensors relative to the forest floor profile.
occumd on the ground fire plot 6 hours after the surface
fire passed when the fermentation layer was smoldering
under the charred litter.
The maximum temperature reached at the mineral
soil surface appeared to be influenced by the moisture
content of the duff and soil. In the slash sites, where the
humus and mineral soil were moist, the mineral soil
surface remained less than 100°C. While the peak
temperature at the mineral soil surface was sufficient to
cause damage to some organisms,it was too low to cause
physical changes in the mineral soil or to consume or
pyrolyze the organic material in the mineral soil. About
4 to 7 cm below the daminera1 soil interface, the
mineral soil was barely warmed. By contrast,theground
fire plot duff was dry (520% moisture content) and
burned independently of the surface fire, raising the
mineral soil surface temperature above 300°C. Temperatures above 100°C were maintained for more than 10
hours. Though no thermocouples were positioned below
the mineral soil surface, heating beneath this surface is
inferred. Temperature histories reported by Frandsen
and Ryan (1986) show a decreasein peak temperature of
about 200°C between thermocouples positioned at the
dry mineral soil surface and those positioned 2 cm below
the mineral soil surface. Though there was some moisture (9%) in the mineral soil of the ground fire plot, it
would be reasonable to assume an approximate peak
temperature of 200°C at a depth of 2 cm in the mineral
-When It's Hot, It's Hot soil and a duration of several hours given the duration of
heating.
A temperature plateau that occurred while moisture
was present in duff or mineral soil is delineated by the
"'knee" in temperature plots (Fig. 1, slash fire (Uhler
Creek) trace "b"at 1 to 2 hours and ground fire plot traces
"c", "d","em,and "f' at 2 to 7 hours). This characteristic
observation is supported in work by Frandsen and Ryan
(1986) and Chinanzvavanaetal. (1986). A large amount
of heat must be absorbedby water to cause it to evaporate
(latent heat of vaporization). As long as moisture is
present, the temperature of the duff and soil will remain
below 100°C. On thermocoupletrace "b"from the slash
fire plot at Uhler Creek and throughout the duff at the
ground fire plot temperatures rose to around 84OC and
held at that temperature until moisture in the layer
evaporated before continuing to rise to ignition. Where
duff moisture was sufficient to prevent ignition (Moose
Ridge "c" and Uhler Creek "c") temperatures remained
below 100°C.
Temperature-inducedfire effects have been reported
within the range of temperatures observed here. Boyer
and Dell (1980) and Wright and Bailey (1982) have
summarized literature on temperature effects on plants
and soils. A few benchmark figures are given for
comparison.Plant tissue death generally occurs between
50°C and 60°C though factors such as moisture content,
sugar content, or bark thickness may prolong the plant's
ability to withstand heating. Most microorganismscannot survive temperatures greater than 100°C. Grass
seeds can survive temperatures up to 150°C for 5 minutes, and some shrub seeds with hard or thick seed coats
are able to survive temperatures up to 150°C. Seeds in
cones can resist even higher temperatures but for a
duration of only a few seconds. Organisms could have
survived in the lower portion of the humus in both of the
slash fire plots. However, they would have been killed
and consumed throughout the duff of h e ground fire plot
and may not have survived in the upper few centimeters
of the mineral soil. As temperatures rise above 150°C,
chemical and physical changes occur in organic matter
and soil nutrients. Rapid pyrolysis occurs between
300°C and 390°C (Shafizadeh, 1985). At these temperatures soil pH increases, and up to 75% of soil nitrogen
may be lost. Long duration heating at 400°C to 500°C
will cause ashing of organics as was seen on the ground
fire plot. At still higher temperatures, structural changes
of the soil can occur.
Summary
Fires of sufficient intensity to consume slash fuels
and produce flamelengths adequate to scorch treecrowns
143
and consume shrubs appeared "hot" but produced little
heating in the mineral soil when the duff layer was not
completely consumed. Combustion in duff occurred as
a smolderingprocess at temperatures lower than flaming
woody materials. Unburned duff provided good heat
insulation to the underlying soil. In contrast, a thick duff
layer, adequately dry to bum, provided long-term lethal
heating near the mineral soil surface with temperatures
capable of pyrolyzing organic matter in the soil and
causing physical and chemical changes in the mineral
soil. A very low intensity surface fire spread over the
forest floor and ignited the duff, which slowly burned as
a ground fire, appearing "cool", but actually producing
considerable heating of the soil.
In this illustration moist duff (60% to 80% humus
moisture content) under surface fire in small diameter
slash fuel did not totally bum while fermentation and
humus layers with moisture contents ranging up to about
30% were consumed even when not aided by surface fire
in slash fuels. Moist duff gave considerable protection
from mineral soil heating.
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