Soil burn severity
n. The fire-induced changes in physical, chemical, and biological soil properties that impact hydrological and biological soil functions; the classification of post-fire soil based on fire-induced changes in properties.
Soil burn severity is an important category of fire effects included in the broader definition of fire severity or burn severity. In the post-fire community, particularly among the Burned Area Emergency Response (BAER) teams, there has been an intentional effort to use the term “soil burn severity” to differentiate post-fire soil properties from fire characteristics (such as flame length), fire effects on vegetation (such as tree mortality), and/or general fire effects on long-term ecosystem health.
Some fire effects resulting from the consumption of organic material near and on the soil surface lead to changes in soil properties that reduce soil infiltration, such as: 1) the formation or strengthening of soil water repellency; 2) change in soil structure (for example, consumption of fine roots increases micro and macro pores); and 3) change in bulk density (for example, collapse of aggregates and clogging of voids by ash). Other effects include: 1) changed soil color (darkens due to charring and reddens due to formation of iron oxides); 2) decreased quantity of organic matter; 3) temporarily increased pH and nutrient availability; 4) decreased cation exchange capacity; and 5) decreased microbial and invertebrate biomass (Certini 2005)
The degree of soil burn severity is dependent on the peak temperatures and duration of those temperatures within the soil. Observable post-fire ground parameters are often used to classify soil burn severity (see table below).
BAER teams, when evaluating the need for post-fire stabilization treatments, are particularly interested in the post-fire soil properties that impact soil hydrological functions as these changes are associated with increased potential for flooding and erosion. Satellite imagery is often used to produce the first post-fire burn severity maps, and, as a result, the maps generally reflect the post-fire condition of the canopy rather than the soil. Since the need for erosion mitigation treatments is related to soil burn severity, post-fire assessment teams often need to verify that soil burn severity is correctly mapped. Field verification of soil burn severity may include a general inspection of ground parameters (see table 1), testing for soil water repellency, and examination of changes in fine root characteristics.
Table 1. Hungerford (1996) and DeBano (1998) used post-fire ground characteristics (for burned conifer forests) to classify soil burn severity into three classes: low, moderate, and high (after Robichaud and others, 2000).