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

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 f i e impact on the forest floor.

How wildfires affect soil properties. A brief review

“…In general, fire severity is an indirect measure of the magnitude of changes in the soil or the ecosystem as a result of a fire. Evaluating fire severity should not take into account only the effect on soils, since the intensity of the disturbance in the ecosystem may be very high despite a low impact on soil (Vasander and Lindholm, 1985;Frandsen and Ryan, 1986;Hartford and Frandsen, 1992;Ryan, 2002). Most systems for classifying fire severity are arbitrary, but are selected from previous experience, and implicitly recognize that even in the case of high severity fires, large spatial variability exists due to the irregularity of the medium or influencing factors (fuel, weather variables or spot morphology).…”

Mitigating old tree mortality in long-unburned, fire-dependent forests: a synthesis

“…Crown and surface fires consume surface and canopy fuels by active flaming. These two types can cause crown injury through convective and radiant heating, but typically cause little to no soil heating (Byram 1959;Hartford and Frandsen 1992). Ground fires burn through duff and organic soils by smoldering combustion.…”

“…Long-term smoldering can cause extended high soil heating, frequently above 140 °F (60 °C), which is the temperature required to kill living tissue. Hartford and Frandsen (1992) reported soil temperatures under smoldering duff mounds of 750 °F (400 °C), with duff temperatures above 212 °F (100 °C) for over 16 hours, compared to soil temperatures of less than 176 °F (80 °C) and duff temperatures above 212 °F (100 °C) for 1 hour under burning slash. Temperatures in smoldering duff mounds exceeded 572 °F (300 °C) for 2 to 4 hours during a prescribed burn in Glacier National Park, resulting in 45 percent mortality of the cambium sampled (Ryan and Frandsen 1991).…”

Evaluating the effectiveness of postfire rehabilitation treatments

“…Burn severity (fire severity) is a qualitative measure of the effects of fire onsite resources (Hartford andFrandsen 1992, Ryan andNoste 1983). As a physicalchemical process, fire produces a spectrum of effects that depend on interactions of energy release (intensity), duration, fuel loading and combustion, vegetation type, climate, topography, soil, and area burned.…”