Heat-Transfer in the Soil During Very Low-Intensity Experimental Fires - the Role of Duff and Soil-Moisture Content

The aim of this study was to analyse the effects of duff thickness and moisture content, and of soil moisture content on the transfer of heat in the soil. The experimental design used intact soil blocks with their duff layer, subjected to controlled fires of variable very low intensities of up to 100 kW m-1. The fuel on the surface was composed of needles and twigs of Pinus pinaster. The maximum temperatures measured within the fuel were of the order of 650 degrees C and were independent of the fireline intensities. For fires with fireline intensity of the order of 30 kW m-1, the presence of the duff layer reduced from 330 degrees C the temperature rise at the soil surface. Duff thickness played only a secondary role, but increasing moisture content reinforced its insulating effect, so that the temperature rise was 2.5 times less at 1 cm depth in the duff when the moisture content exceeded 70% dry weight, than when the moisture content was less than 30%. For more intense fires (> 50 kW m-1) that produced longer-lasting surface heating, duff thickness and moisture content played an important role in significantly reducing the temperature rise at the soil surface (range 140 degrees C to 28 degrees C). Because of low soil thermal conductivity, temperature attenuation with increasing depth was noticed. In the case of low intensity fires ( 60 degrees C) were measured in the upper few centimetres of the duff layer in very low-intensity fires, and in the upper few centimetres of the soil (where nutrients are most concentrated and biological activity most intense) in the slightly more intense fires. The fire intensities were always very moderate, and of the order of magnitude df those encountered in the prescribed burns conducted on fuel-breaks of the french Mediterranean area. Their impact on the surface of the forest soil, in terms of lethal temperatures transmitted to the horizon rich in organic matter, are not negligible. In contrast, below 3 to 5 cm depth, prescribed burns, conducted under the conditions of the experiments, would not lead to significant change to nutrients or microfaunal or microfloral activity; in particular, root tips would not be subjected to heat stress sufficient to kill them.

[1]  Ross A. Bradstock,et al.  Soil temperatures during bushfires in semi‐arid, mallee shrublands , 1992 .

[2]  B. Blackwell,et al.  Conversion of dense lodgepole pine stands in west-central British Columbia into young lodgepole pine plantations using prescribed fire. 1. Biomass consumption during burning treatments , 1992 .

[3]  William C. Fischer,et al.  Predicting Duff and Woody Fuel Consumption in Northern Idaho Prescribed Fires , 1991, Forest Science.

[4]  Henry A. Froehlich,et al.  Infiltration, water repellency, and soil moisture content after broadcast burning a forest site in southwest Oregon , 1989 .

[5]  R. Hobbs,et al.  Spatial variability of experimental fires in south‐west Western Australia , 1988 .

[6]  William H. Frandsen,et al.  The influence of moisture and mineral soil on the combustion limits of smoldering forest duff , 1987 .

[7]  K. Ryan,et al.  Soil moisture reduces belowground heat flux and soil temperatures under a burning fuel pile , 1986 .

[8]  R. J. Raison,et al.  Soil temperatures during and following low-intensity prescribed burning in a Eucalyptus pauciflora forest , 1986 .

[9]  Park S. Nobel,et al.  Physiological Plant Ecology I: Responses to the Physical Environment , 1981 .

[10]  P. Rundel.  Fire as an Ecological Factor , 1981 .

[11]  Range Experiment Station,et al.  Effects of burning moist fuels on seedbed preparation in cutover western larch forests , 1978 .

[12]  D. Hamilton,et al.  The Transfer of Heat and Hydrophobic Substances During Burning1 , 1976 .

[13]  A. Gill,et al.  Coupled soil moisture, heat and water vapour transfers under simulated fire conditions , 1976 .

[14]  D. Scholl.  Soil Wettability and Fire in Arizona Chaparral 1 , 1975 .

[15]  S. Savage.  Mechanism of Fire‐Induced Water Repellency in Soil , 1974 .

[16]  R. M. Rice,et al.  WATER-REPELLENT SOILS: THEIR IMPLICATIONS IN FORESTRY , 1973 .

[17]  C. E. Van Wagner,et al.  Duff Consumption by Fire in Eastern Pine Stands , 1972 .

[18]  D. Packham.  Heat transfer above a small ground fire. , 1970 .

[19]  D. A. Hamilton,et al.  Translocation of Hydrophobic Substances into Soil by Burning Organic Litter , 1970 .

[20]  C. E. V. Wagner.  Temperature gradients in duff and soil during prescribed fires. , 1970 .

[21]  James R. Sweeney,et al.  Quantitative Studies of the Removal of Litter and Duff by Fire under Controlled Conditions , 1961 .

[22]  N. Beadle.  Soil temperatures during forest fires and their effect on the survival of vegetation. , 1940 .