SIMPLE QUESTION; DIFFICULT ANSWER: HOW MUCH FUEL IS ACCEPTABLE?

Fire Management Today, Fall 2007 by Alexander, M E

Fire managers commonly want to know what quantity of wildland fuel is acceptable (Noble 1979). But this question - simple as it may seem - is difficult to answer. A host of factors are involved.

Fire behavior depends not only on fire potential at one location, but on a range of associated factors that include the distribution and characteristics of the individual and collective elements comprising the fuel complex (table 1) and fire behavior potential across surrounding areas that could encompass one or more drainages.

Acceptable fuel loads depend on resource values, management objectives for the land, pattern of land ownership, and suppression capability (fig. 1). In some stands, acceptable fuel load might depend on the resistance of trees to crown scorch and cambium kill (Outcalt and Wade 2004; Weatherspoon and Skinner 1995). Sound professional judgment (Haas 2003) is certainly needed to determine what can be considered acceptable fuel loads.

Decision Steps

To decide how much fuel is acceptable requires the integration of many factors (fig. 1). This can be done systematically in a three-step process (Brown and others 1977):

Step 1: Consider management objectives and values-at-risk. For the latter, both resource values and risk of a fire during periods of critical fire weather and fire danger causing damage are jointly considered.

Step 2: Appraise fuels by (a) describing fuels from inventory (Brown and others 1982), prediction (Brown and others 1977), or ocular estimation using a photo series (Wendell and others 1962; Fischer 1979) and (b) interpreting fire behavior and fire impact potential such as rate-of-spreading, intensity, flame length, crown scorch height, and degree of flame defoliation.

Step 3: Consider other fire-related factors, such as fuel and fire behavior potential on adjoining lands, suppression capability, frequency and severity of historical fires, and fire's ecological role.

Acceptable fuel loads can depend to a high degree on the factors considered in Step 3. For example, a very high fuel load would be acceptable on a unit surrounded by sparse fuels with little chance of ignition than on a unit surrounded by very heavy fuel loads with a more certain probability of ignition. Once management objectives have been specified, Orni (1996) has suggested a conceptual framework that can be used for assessing the viability of landscape-scale fuel treatments (fig. 2).

Fuel Appraisal

Appraising the potential fire behavior of fuels is often termed "fuel appraisal" (Brown 1972, 1978) and is the process of: (1) describing fuel characteristics, such as quantity and size (table 1) and (2) interpreting the fuel in terms of fire behavior, such as rate-of-spread, fire intensity, and flame length (fig. 3).

Thus, the appraisal process attempts to answer the question: Given steepness of slope and weather conditions, what is the expected fire behavior for different fuels?

This question is difficult to answer, partly because the answer is made up of different elements of fire behavior (Anderson 1974) - including rate-of-spread, intensity, flame dimensions, torching potential, crowning potential, spotting potential, blowup potential, and duration of heat (flame front residence time and burn-out time, or smoldering potential).

One or more of these elements may have to be appraised when a specific fuel management situation is being evaluated. Furthermore, the overall assessment of fire behavior potential must ultimately be interpreted in terms of the implications for fire suppression (Murray 1983).

Potential fire behavior of litter, downed woody debris, and understory vegetation can be appraised by (fig. 3): (1) mathematical modeling, (2) experienced judgment, and (3) comparison (such as case studies). Mathematical modeling of fire behavior characteristics such as rate-of-spread, fire intensity, flame length, crowning potential, and fire size (Alexander 2006; Anderson 1974; Brown 1974; Brown and Johnson 1987; Hirsch and others 1979; Lavoie 2004; Radloff and others 1982; Salazar and Bevins 1984) offers the most objective means of appraising potential fire behavior.

Systems have been developed specifically for this purpose (Hirsch and others 1981; Puckett and others 1979; Radloff and others 1982; Roussopoulos and Johnson 1975).

Such modeling, however, does have its limitations (Albini 1976; Alexander 2004a). Furthermore, too much reliance can be placed on models (Alexander 2004b). After all, predicting fire behavior is a science and an art (Alexander and Thomas 2004).

Experienced Judgment

Experienced judgment is an important means of appraising fuels. An experienced person can integrate many factors that elude quantification. Even when more sophisticated methods are available, judgment is still important. However, even experienced judgment has its limitations (Gisborne 1948).

One way of using experienced judgment is to establish a reference fuel load that can be used to compare against other fuel loads. The reference fuel load could represent fuels for which a consensus of fire control-experienced land managers could agree on a rating. This might be in terms of resistance to fireguard construction or resistance to control (Murphy and Quintilio 1978; Ponto 1990).