Methods for Estimating Fuel Loading

Along with fuel moisture, fuel loading is among the most important variables affecting fire behavior and consumption (Hough 1968). Fuel loading is a critical input in all fuel models, with bearing on flaming and smoldering fire combustion and fire severity. Estimating fuel loads is therefore a critical step in planning prescribed fires or assessing risk of fire danger. Fuel loading is a function of site productivity, decomposition rate, and time since last disturbance (e.g., fire, harvesting, past land-uses). Along with the rate of fuel accumulation, the standing crop of fuel (fuel loading) varies over the southeastern landscape.

Fuel loading is measured or estimated in several ways: field sampling, use of photo series, tabular or statistical correlations, and remote sensing (Pyne et al. 1996). Fuel loads measured by any of these techniques are generally expressed as mass per unit area (kg/ha, lb/ft², lb/acre, or tons/acre) of live and dead fuel components.

Field sampling methods

Field sampling can be performed by several methods. The most commonly accepted methods are line transect sampling and quadrat methods.

• The Downed Woody Material Inventory System, also called Browns method is the most commonly used line transect method for sampling fuels. In this method, several plots are installed at a site. At each plot, three 11 m (35 ft. or 1/2 chain) long, 1.8 m (6 ft) tall planar intercept transects are installed at three azimuths. Along each transect, all 10-hour timelag fuels are counted within 3 m (10 ft) of the endpoint, and 100-hour timelag fuels are counted along the entire 10 m transect. The diameter at the point of intersection, species, and decay status of 1000-hour timelag fuels are measured along the entire 11 m transect as well (Brown 1974). Additional information on this technique is available at the FIREMON website.

• The WWU tree-based transect method (Taylor and Fonda 1990) is a similar method that is applicable to areas with fuels clustered around trees, as in long fire return interval ecosystems (e.g., sand pine scrub) or long-unburned fire-prone forests (fire excluded longleaf pine – hardwood ecosystems).

• Living and dead understory, shrub, and tree fuels can be measured along line transects (as above) or by quadrat methods. In quadrat sampling, herbaceous and forest floor material are harvested from known areas (usually 1 m² for vegetation, smaller for forest floor material), ovendried, and weighed. Shrub and canopy fuels are estimated with using line transects, allometric equations, or using crown densiometers to estimate foliage fuel biomass.

Photo series

A sequence of photos called a "photo series" can provide a quicker and easier means of quantifying fuel loads than field sampling methods, particularly when exact fuel amounts are not required (Reeves 1988, Ottmar and Vihnanek 2000). Photo series consist of a site’s photograph and the fuel loading data associated with the conditions in that specific photograph. Fire managers can then utilize the photographs (often interpolating between more than one) to visually estimate their site’s fuel loading and fire behavior.

Photo series are available for the following southeastern ecosystems:

• Longleaf pine sandhills (Ottmar and Vihnanek 2000, Ottmar et al. 2003)
• loblolly pine, eastern white pine, pitch pine, and Virginia pine (Lynch and Horton 1983)
• poscosins  (Ottmar and Vihnanek 2000)
• freshwater marsh ecosystems (Ottmar and Vihnanek 2000)
• southern pine: post-hurricane (Wade et al. 1993)
• hardwoods with white pine (Ottmar et al. 2003)
• Southern pine and hardwoods: pre and post fire (Scholl and Waldrop 1999)
• pine-hardwood logging slash (Sanders and Van Lear 1987, 1988)

More information on photo series can also be found at the USDA Forest Service Fire and Enviromental Research Applications team website.

Tabular methods

The following tables can also be used to estimate fuel loadings (tons/acre) on an area.

Litter weights:

• (Total litter weight under shortleaf pine stands as affected by stand basal area and age of rough)

• (Total litter weight under slash and longleaf pine stands as affected by stand basal area and age of rough)

• (Total litter weight under loblolly pine stands as affected by stand basal area and age of rough)

• (Litter and duff weight accumulations in developing undisturbed loblolly pine plantations from 6 to 23 years)

• (Litter buildup in upland hardwood stands )

• (Estimated available litter fuel weights for different total litter weights and moisture contents)

Vegetation:

• (Understory vegetative dry weight in the palmetto-gallberry type as related to age of rough and understory height)

• (Total available grass accumulations under pine overstories)

• (Estimated total available fuel (litter + vegetation) as a function of total litter layer moisture content, total litter weight, and understory vegetative dry weight)

• (Dry weight of foliage and branches to 1-1/2 inches in diameter for individual, shortleaf pine trees )

Slash:

• (Available fuel 1 inch in diameter and less in the unpiled pine logging debris type)

Remote sensing

Recently, remotely sensing has been used for fuel characterization and monitoring. This technique is based on the fact that a high correlation exists between spectral variation in remote sensing imagery and fuel variation. This technique involves a significant amount of data exploration to establish relationships between the imagery and fuel features on the ground. Computer algorithms are then used consistently classify the imagery based on the identified relationships.