Depth of Burn

Depth of burn is the amount the forest floor (organic layers above mineral soil) is reduced during a fire. This description of fire severity can be expressed as the actual thickness of the layer removed (vertical depth), a percentage of the total layer, or the percentage of mineral soil exposed. When expressed as an actual thickness, depth of burn is often measured by inserting metal pins level to the surface of the forest floor before burns and measuring how much of the pins are exposed after burns.

Correctly interpreting depth of burn generally requires knowledge of forest floor conditions before burns, particularly when a burn results in complete consumption. For example, in fuel types where only a thin forest floor accumulates or where this layer has been recently removed (by fire, for example), even very low intensity fires will completely consume this fuel layer (Wade 1986).

Depth of burn is influenced by the moisture and porosity (compactness) of the forest floor (see also Ground fuels). Moisture in the forest floor has to be evaporated before the layers can be heated to ignition temperature. Therefore, the moister the forest floor is, the poorer it will burn. Similarly, the more compressed the litter and humus, the poorer the forest floor will burn (Wade 1986). Since depth of burn depends largely on the moisture and compactness of the forest floor, it is more-or-less independent of rate of spread and fireline intensity (Alexander 1982). If the forest floor is the primary fuel consumed, depth of burn is a good analog of the total heat energy released by the fire (Wade 1986).

While depth of burn is more useful than fireline intensity for estimating damage to belowground plant tissues and soil (Wade 1986), relating depth of burn to downward heat fluxes should be done cautiously. The lower layer of humus can be an effective barrier to downward heat fluxes, plus moisture in these layers can delay or prevent ignition. When gaseous water vapor from these layers moves downward, it further complicates the prediction of lethal time-temperature patterns (Wade 1986).