Physical Fuel Properties

The primary physical fuel properties influencing combustion and fire behavior are size, shape, loading, and arrangement. Here, we define these four properties and discuss the physical properties of typical fuels found in the southeastern US.

Size

Particle size is one of the most important fuel characteristics affecting combustion and fire behavior (Byram 1959). Large particles have high heat capacities, requiring more heat to ignite and combust the particle. Smaller particles have low heat capacities, so they require smaller amounts of heat energy for ignition and combustion. For dead fuels, particle size is also related to the rate at which fuel moisture content changes, and therefore size classes of fuels are also referred to as timelag classes. Different time-lag classes burn differently: 1-hour fuels (needle litter, hardwood leaves) ignite quickly and combust at rapid rates. Progressively larger particles (10-, 100-, 1000-hour and larger fuels) require more heat for ignition and combustion. Fires usually start and spread in dead fines fuels (< ¼ in. diameter), which ignite increasingly larger size classes of fuels. If fine fuels are reduced or missing, a fire may not ignite or spread.

Shape

Fuel shape (surface area-to-volume ratio) is related to particle size: the more finely divided the material, the higher the ratio. Fuel surface area is measured in cm²/ m³ (or ft²/ ft³). Fuels with high surface area-to-volume ratios (pine needle litter, most foliage fuels) have lower heat capacity and require less pre-heating for ignition (Byram 1959). The increased surface area of these fuels provides more surface area for heat oxidation and combustion. Further, these fine fuels dry out and ignite more rapidly than coarser fuels. Small surface area-to-volume ratio fuels (downed logs and other 1,000-hour fuels) resist ignition and combust slowly. There are many examples of fuels from southeastern ecosystems that have high surface area-to-volume ratios.

Loading

Fuel loading is the amount of live and dead fuel, expressed in weight per unit area (kg/m² or tons/acre). Total fuel is all fuel, both living and dead, present on a site. Available fuel is the amount of fuel that will burn under a specific set of fire conditions (Pyne et al. 1996). Fuel loadings are usually grouped by particle size class (or timelag classes).

Fuel loading is an important characteristic of southeastern fuel complexes. Fuel loads vary considerably depending on site productivity, recent disturbance history, and fire regime. While the generally warm and humid Southeastern climate provides optimal growing conditions, some systems are more productive than others. Fuel production varies from low in the xeric sand pine scrubs, Appalachian ridgeline ecosystems, and Piedmont granite outcrops, to high in mesic pine forests and most wetland communities. Recent disturbances may increase or decrease existing fuel loads, by either removing fuels (in the case of fire) or adding new fuels in the form of coarse woody debris (in the case of hurricanes). For this reason, long-unburned stands typically have higher fuel loads than stands managed with frequent prescribed fire. In these long-unburned stands, midstory and overstory fuels generally increase at the expense of fine fuels in the understory.

See Methods for Measuring Fuel Loads.

Arrangement

Fuel arrangement is another important physical property of fuels. Both the packing ratio and fuel placement describe different aspects of fuel arrangement:

Packing ratio is a measure of the compactness of the fuel bed. It is expressed as a percentage of the fuel bed composed of fuel, with the remainder being air space. Densely packed fuels prevent moisture evaporation and oxygen diffusion into the fuelbed, thereby suppressing ignition and flaming combustion. Conversely, loosely packed fuels allow rapid evaporation and oxygen diffusion, and hence rapid ignition and flaming combustion. However, fuels that are very open can burn slowly because little heat is transferred among widely spaced particles. For every size of fuel particle, there is an optimum packing ratio at which heat transfer and oxygen produce the most efficient combustion (Burgan and Rothermel 1984). Draped pine needles and upper layer forest floor litter are examples of fuels with low packing ratios. Live branchwood is a classic example of densely packed fuel. Fuel bulk density is a related measure of the compactness of a fuel or fuel bed. It is calculated by dividing the weight per unit area by the fuel bed depth, and is expressed as g/cm³ or lb/ft³. In general, the higher the bulk density of fuel is, the higher the spread rate (Miller 1994).

Fuel placement and fuel continuity describe the horizontal and vertical distribution of fuels (Pyne et al. 1996). Fuels placed within the flaming zone are available for combustion, whereas fuels out of the combustion zone are not. Fuels with horizontal and/or vertical continuity pre-heat adjacent fuels. Conversely, fuels lacking continuity do not transmit heat to adjacent fuels. Horizontal continuity is a critical factor for surface fires; bare patches and patches of sparse vegetation act as fuelbreaks. In crown fires, vertically continuous fuels facilitate crown ignition and crown-to-crown horizontal continuity sustains crown fire (Pyne et al. 1996).