Effects of controlled burns on the bulk density and thermal conductivity of soils at a southern Colorado site

Fire can play an important role in managing and maintaining ecosystems in many areas of the world. However, if the fire is sufficiently intense, soil can be irreversibly altered. The present study outlines an experiment to determine the effect that controlled burns can have on the bulk density and thermal conductivity of the soils at the Manitou Experimental Forest in the Rocky Mountains of southern Colorado. Previous studies indicate that soil bulk density nearly always increases as a result of fire, but virtually no studies exist on the impact of fire on soil thermal conductivity. Vertical profiles of soil bulk density (ρ_b), thermal conductivity (λ), and volumetric soil moisture (Θ_v) were obtained at two burn locations. Because soil moisture is the primary determinant of thermal conductivity and bulk density the second, both are necessary to study thermal conductivity. Soil samples, centered at 5, 10, 15 and 20 cm depth, were obtained and analyzed in the lab for bulk density and soil moisture. Thermal conductivity was measured in situ at the same depths with a heated needle probe. Measurements were made before and after the burns within the burn areas, as well as outside the burn areas (controls). The first slash pile burn was performed in an area that had previously been used as an access road, whereas, the second burn site had not. Results indicate:

(1) No detectable change between post-fire bulk densities within the burn area and the controls at the first burn site (the old road bed). However, this site did show significantly higher bulk densities than the second burn site (approximately 1.5 gm/cm³ vs. 1.3 gm/cm³) and comparison to the controls at the first burn site indicated that only the upper 2 - 5 cm of the old road were compacted relative to the surrounding (less disturbed) areas. Both burn sites showed bulk densities that increased with depth. Samples obtained at pre- and post-fire within the burned area at the second burn site indicated that bulk density increased from about 1.1 gm/cm³ to about 1.3 gm/cm³; however, comparisons between the controls and burned area did not show any statistically significant difference. Therefore, results at the second burn site are suggestive, but not conclusive, of an increase in bulk density as a result of the fire.

(2) Thermal conductivity changed significantly, but in an unexpected way, at both burn sites as a result of the fires. Regressing the measured thermal conductivity against the corresponding bulk densities and soil moistures (i.e., λ = Aρ_b+BΘ_v) for the controls and burn areas yielded: λ_control = 0.123ρ_b+8.12Θ_v and λ_burn = 0.486ρ_b+2.70Θ_v. Although no unambiguous change in bulk density was detectable at either burn site, both sites showed that thermal conductivity to be about 4 times more sensitive to bulk density after the burn than before. On the other hand, thermal conductivity appears about a third less sensitive to soil moisture after the fire than before. These results would seem to indicate some change in soil structure as a result of the burns. The nature of these changes is not clear from this particular data set. However, in a semi-arid region, such as Manitou Experimental Forest, where Θ_v < 0.1 much of the time, present results suggest that the daily and annual soil heating within the burned areas will exceed the unburned areas in both amplitude and depth of penetration.