Soil Physical Properties Regulate Lethal Heating during Burning of Woody Residues

Temperatures well in excess of the lethal threshold for roots (60°C) have been measured in forest soils when woody fuels are burned. Whether this heat pulse is strongly moderated by soil moisture or soil texture is not fully understood, however. We measured soil heat profiles during 60 experimental burns, identifying changes in maximum soil temperature and heat duration above 60°C as a function of soil moisture and soil texture. Experimental treatments included a factorial combination of soil moisture (∼5, 15, 30, and 45% v/v) and soil textural (sandy loam, sandy loam–pumice, loam, clay loam) gradients, with a surface fuel load comprised of a dense layer of masticated wood. Soil moisture had a strong influence on heat transfer. A volumetric moisture content of 20% or greater quenched the heat pulse in all soils at depths of 2.5 cm and lower. In comparison, soil temperatures in dry soil far exceeded the lethal threshold to a depth of 10 cm. Differences in heating characteristics among the four soil types were minor despite their dissimilarities in texture, porosity, bulk density, and presumed thermal conductivity. It was also shown that intact soil cores were required to produce accurate heat profiles during burning, as maximum soil temperatures in the surface 5 cm were overestimated by 40 to 100°C using disturbed soil (sieved and packed). The empirical results along with a simple predictive model of soil heating show that burning of woody fuels when underlying soils have 20% volumetric moisture or greater is an effective means for limiting lethal heating in a variety of soils.