Summary Descriptions of General Burn Severity Categories

To achieve a 4-level classification of severity based on rating factors from the CBI:

Summary definitions are based on conditions averaged over the area of a 30-meter diameter plot, assuming the entire area was potentially burnable. In each case, significantly different effects may produce intermediate overall ratings. For example, some low indicators and some high indicators may combine to yield a site-wide moderate level of severity. Commonly, various levels of effect occur along with unburned patches to create mottled burn patterns, and low or moderate severity when considering the whole plot. The resultant classification of a plot depends on the area occupied by the various fire effects. For the observations below, the benchmark timeframe typically is during the first post-fire growth period when vegetation survivorship and latent mortality can be estimated. The categories are intended as generalized descriptors, recognizing there is overlap between them. The specifics vary between ecosystems, and many times classes can be more precisely defined and recognizable when considering only one system.

Increased greenness or enhanced productivity – Areas that burned but display more vegetation cover, density, and/or productivity (vigor) usually within one growing season after fire than before fire. This is a fire-caused effect from release of nutrients into soil, and/or reduced competition for nutrients, light and water (much like a thinning or pruning effect). Such conditions may persist only 1-3 years after fire. The affected areas are usually herbaceous or low shrub communities that undergo little change in species composition after fire.

Unburned to Very Low Severity – Either unburned, or when visible fire effects occupy less than 5% of the site. If more than 5% of the site is burned, then effects are very slight and limited to very few biophysical components. For example, sites burned 20% in area that show only modest litter consumption, and essentially no impacts to duff, woody fuels, or pre-fire vegetation. The class may also include areas that recover very quickly after fire, such as grassland, or very light surface burns under dense, non-impacted canopies.

Low Severity – Usually more than 15% of the site burned. Collectively, understory and overstory conditions are slightly altered from the pre-fire state, but some individual components may show pronounced burn effects. For substrates, only litter normally exhibits fairly high consumption (up to 100%). Duff, woody fuels and newly exposed mineral soil typically exhibit some change, but combined, the effects are less than about 30% change from pre-fire amounts. Low vegetation (<1 meter) and taller shrubs or small trees (1-5 meters) may show significant aboveground scorch, char or consumption, and vegetation density or cover may be greatly altered temporarily. But plants present before fire generally are still viable, and recover quickly within a year or two with little change in species composition. Western conifers are exceptions, where sapling-sized trees may exhibit 50% or more mortality. Depending on the site, these effects may actually lead to increased density or cover when compared to pre-fire vegetation within one or two growing seasons; if for example, more nutrients become available or there is reduction in competition from overstory trees. Both intermediate-sized and big overstory trees may exhibit up to 25% mortality evidenced by up to 35% crown char or scorch, with 65-100% of the pre-fire crown volume remaining green and essentially unchanged. Where charring does not kill tree crowns, as is common in the southeast, higher percentages of black char may occur, with extensive resprouting evident from burned branches. Char height from ground flames is typically less than 3 meters.

Moderate Severity – This class is difficult to describe in brief, because of the variety of effects and associations that can lead to so called moderate severity. Indicators may be fairly consistent and intermediate between the low and high severity across all biophysical components. On the other hand, numerous potential combinations of distinctly low and high effects may occur to yield a moderate classification overall for the site. New seed-germinated plants may readily colonize at this level, since some degree of soil exposure and reduced competition is likely. The amount of new colonization may depend on distance to available seed sources. Moderate fire intensity may not sterilize soil or remove nearby seed sources to as large an extent as when high intensity burning occurs. Because of the array of possibilities, no further description will be attempted, except that conditions are transitional in magnitude and/or uniformity between the low and high characteristics described.

High Severity – This class is characterized by fairly consistent traits across the CBI rating factors (severity indicators). Overall biomass consumption and changes in carbon balance are significant. For substrates, litter is at least near-totally consumed; duff is typically over 70% consumed; medium and heavy woody fuels are 35-65% consumed and at least deeply charred. Mostly ash and carbon remain from pre-fire organic substrate material. Sites normally exhibit over 50% cover of newly exposed mineral soil or rock fragments. Herbaceous plants and shrubs are over 90% charred or consumed above ground, often with notable branch loss on taller shrubs, which may be reduced to small stubs and difficult to locate. Resprouting from perennial plants, except grasses, is strongly reduced, with less than 35% viability and at least 80% reduction in cover. Grass viability and cover may be somewhat higher. Of plants <5 meters tall, species composition is often markedly changed, while the occurrence of seed-propagated colonizers is variable. The latter can be fairly high, depending on the proximity to seed sources, size of high-severity patches, and the degree of soil sterilization controlled by original fire intensity. Such effects are generally evident for several years after fire, with herbaceous plants and shrubs taking a few growing seasons to recover, depending on climate, soil conditions, and other post-fire factors. Overstory trees are typically strongly impacted, with over 75% mortality resulting from over 80% crown char or scorch. Crown Char is frequently 100% from torching fire, and significant branch loss is evident at highest levels. Where crown torching does not occur, char height from ground flames often exceeds 4 meters. At the lower end of high severity, big trees tend to show slightly lower impacts than the intermediate-sized trees. Sites occupied by some tree species, like spruce and hemlock that are very sensitive to fire, may show high mortality without the associated indicators of high percent char, scorch or organic carbon consumption. Overstory tree effects are generally long lasting. New tree establishment may occur within 1-3 years post-fire, but forest development usually takes many decades.

C. Key and N. Benson 2008.

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