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 hea t in the soil. The experimental design used intact soil blocks with th eir duff layer, subjected to controlled fires of variable very low int ensities of up to 100 kW m(-1). The fuel on the surface was composed o f needles and twigs of Pinus pinaster. The maximum temperatures measur ed within the fuel were of the order of 650 degrees C and were indepen dent 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 thicknes s played only a secondary role, but increasing moisture content reinfo rced 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 mor e intense fires (> 50 kW m(-1)) that produced longer-lasting surface h eating, duff thickness and moisture content played an important role i n significantly reducing the temperature rise at the soil surface (ran ge 140 degrees C to 28 degrees C). Because of low soil thermal conduct ivity, temperature attenuation with increasing depth was noticed. In t he case of low intensity fires (< 30 kW m(-1)) in the absence of a duf f layer, the maximum temperatures were reduced from 350 degrees C at t he surface to 7 degrees C at 3.5 cm. The temperature rise in the soil decreased with depth according to a negative exponential relation. The rate constant of this relation was greater when the initial surface t emperature and the soil moisture content were higher. For the soil stu died, and under the moisture conditions encountered (between 7 and 19% of dry weight), the rate constant could be predicted with acceptable precision (r(2) = 0.67), if the surface soil temperature rise and the soil moisture content were known. In these experimental fires, which w ere carried out when the air temperature did not exceed 20 degrees C, lethal temperatures (> 60 degrees C) were measured in the upper few ce ntimetres of the duff layer in very low-intensity fires, and in the up per few centimetres of the soil (where nutrients are most concentrated and biological activity most intense) in the slightly more intense fi res. 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 sur face of the forest soil, in terms of lethal temperatures transmitted t o the horizon rich in organic matter, are not negligible. In contrast, below 3 to 5 cm depth, prescribed burns, conducted under the conditio ns of the experiments, would not lead to significant change to nutrien ts or microfaunal or microfloral activity; in particular, root tips wo uld not be subjected to heat stress sufficient to kill them.