Fire Severity

Fire severity is a qualitative indicator of the effects of fire on an ecosystem, whether it affects the forest floor, canopy, or some other part of the system. Like fire intensity, fire severity reflects the amount of heat released by a fire, and therefore it is also dependent on fuels and fire behavior. But fire severity also integrates fuel and soil conditions before a fire, energy released during and after flaming combustion, and visible effects after a fire. Assessing and mapping burn severity is important for monitoring fire effects.

Prescribed fires are usually low severity fires by design, because they are conducted when fuel loads, fuel moisture, and weather conditions are favorable for a low intensity fire (DeBano 1998). Wildfires, on the other hand, are usually high severity fires because they usually occur when temperature, wind speed, and fuel loading are high, and humidity and fuel moisture are low (DeBano 1998). Due to these burning conditions, wildfires often have major effects on ecosystems.

Fire severity is traditionally expressed by classifying the magnitude of a fires impact on a particular resource into broadly defined classes ranging from low to medium to high fire severities. Crown scorch, bark char height, residence time, and depth of burn are often used as indicators of fire severity. Fuel moisture has also been used to predict fire severity before a burn or help to define it after a burn (Robichaud and Waldrop 1994).

Very often, fire severity is used to characterize fires effects on soil. Since moisture can buffer the effects of fire, pre-burn soil and/or duff moisture can also help to predict fire severity before a burn or help to define it after a burn. Hungerford (1996) provides an example of a fire severity classification system based on the postfire appearance of litter and soil (Table:Fire Severity Classification System).

Classifications systems that define fire severity at relatively small scales (plots, for example) can be scaled-up to classify entire burns by determining the percentage of the total area that is burned at low, moderate, and high fire severities (see Table:Burn Severity Classification; Wells at al. 1979).

Usually, fire-severity classification systems like Hungerfords are resource-specific and site-specific, so they should be modified to fit local conditions. A recently developed fire severity rating system called the Composite Burn Index (CBI) can be used across broad geographic regions because it is not site-specific (Key and Benson 1999). The Composite Burn Index (CBI) uses 7 possible burn severity classes to rate 23 attributes. Unlike most fire severity classification systems that have 3 or 4 possible classes (low, medium, and high severity) the CBI computes an index that can range from 1 to 69. This index is used in FIREMON- the fire effects monitoring system widely used by federal and state agencies to monitor fire effects.