When It's Hot, It's Hot... or Maybe It's Not! (Surface Flaming May Not Portend Extensive Soil Heating)

Transcription

1 Int. J. Wildland Fire 2(3): ,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 US. Department of Agriculture, Forest Service, Intermountain Research Station Intermountain Fire Sciences Laboratory P.O. Box 8089, Missoula, MT Tel ; Fax 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 temperatures at the mineral soil surface. Moisture content of duff and soil were key variables for determining fie 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 characteristically burns 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

2 140 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. 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 opportunistically in 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 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 Ground fire plot Moose Ridge Uhler Creek West -- Side Ry-Pass Habitat type' ABLAICLUN ABLANEFE PSMED'ACA Fire groupz Nine Nine Six Approx. slash load on plot 10.3 kg/m kg/m2 0.0 kg/m2 Slope at plot 5% 60% 10% Duff depth 6 cm 6 cm 6.5 cm Moisture content (dty weight basis) Slash fines 8% 9% NA Litter 11% 12% 11% Fermentation 31 % 16% 12% Humus 64% Soil 39% 'Habitat types (Cooper et al. 1987) and (Pfister et al. 1977). Fire groups (Fischer and Bradley 1987).

3 - 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 10 and 13 kg/m20n the 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 may 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. Slash fire plots Ground fire plot Moose Ridge Uhler Creek West Side By-Pass Duff reduction 5 cm 5 cm 6.5 cm Max temperatures Litter surface 690 C 460 C 300 C Duff 625 C 400 C 515 C Soil surface 40 C 80 C 4Oo0C Duration Temp> 100 C in duff 1 hr 2.5 hrs 16 hrs Results and Discussion Maximum flame lengths on the slash fre 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 1 cm 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, temperatures in excess of 100 C lasted 1 hour 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 temperature barely 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

4 Hartford, R.A. and Frandsen, W.H. r la MOOSE RIDGE '0 r UHLER CREEK '0 1 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 decrease in 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

5 -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 Chinanzvavana etal. (1986). A large amount of heat must be absorbed by 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 thermocouple trace "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-induced fire 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 microorganisms cannot 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 he 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 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 smoldering process 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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