Fire Triangle

Fire behavior is controlled by three interacting components: fuels, weather, and topography. Fuels provide the energy source for fire. Fuel availability, which depends on both fuel arrangement and fuel moisture, determines if fires will burn as ground, surface, or crown fires. Weather elements, such as temperature, relative humidity, wind, precipitation, and atmospheric stability, also combine to influence fire behavior by regulating fuel moisture and rate of spread. Topography can influence fire indirectly, by mediating wind patterns, or directly- fires burning upslope spread faster than fire burning on flat land.

The variety of fuel, weather, and topographical conditions that exist in the South create fires that vary in the amount of fuels that burn, the rate at which these fuels burn, the depth of burns, and whether living plants become fuel. This variation in fire behavior, in turn, influences the effects of fire on natural communities and people. This section of the encyclopedia provides a background on how fuels, weather, and topography influence fire behavior.

• Fire, Weather, and Fire Behavior- Overview

• Fuels: This section defines fuel categories (ground, surface, ladder, and canopy fuels); explains how physical fuel properties, such as size, shape, loading, and arrangement, combine to influence combustion and fire behavior; explains how chemical fuel properties affect heat content and flammability; describes the importance of fuel moisture and other factors that control fuel availability.

• Fire Weather: This section defines important components of fire weather and explains how each of these components influence fire behavior and risk of wildfire.

• Fire Behavior: This section explains how fuels, weather, and topography combine to influence fire behavior. It includes a discussion of combustion and heat transfer, fire intensity, fire growth and spread, extreme fire behavior, and fire behavior prediction systems.

It is important to understand what controls fire behavior and how to predict it. This knowledge will help predict fire effects, conduct prescribed burns, predict wildfire risk, and control wildfires. Fire behavior is controlled by three interacting components: fuels, weather, and topography. Fuels provide the energy source for fire. Fuel availability which depends on both fuel arrangement and fuel moisture determines if fires will burn as ground, surface, or crown fires. Weather elements such as temperature, relative humidity, wind, precipitation, and atmospheric stability combine to influence fire behavior by regulating fuel moisture and rate of spread. Topography can influence fire indirectly, by mediating wind patterns, or directly- fires burning upslope spread faster than fire burning on flat land.

It is important to understand the physical-chemical process of fire to understand how heat is generated by fire. Fire releases heat through combustion. Oxygen, heat, and fuel- often called the fire triangle- must be present in the proper ratio for a fire to ignite and sustain combustion. Once a fire has ignited, the heat must be transferred to surrounding fuel in order for the fire to grow and spread. This occurs through one of several heat transfer processes, usually convection, radiation, and/or conduction, although vaporization and mass transport may also play roles.

Once a fire has ignited, its shape and rate of spread will continually change. Rarely, a fire can continue to increase its rate-of-spread and intensity, resulting in extreme fire behavior- a level of fire behavior that goes beyond human methods of fire control and prediction.

Characterizing flame attributes such as flame height, length, depth, angle, and char height can help predict fire effects and make comparisons among different fires possible. Fire intensity, which describes the rate of heat release, and rate of spread play significant roles in characterizing fire behavior.

Computer models can be used to predict fire behavior based on differences in fuels, weather, and topography. These fire behavior prediction systems are used to support fire management decisions, as a training tool to improve fire management skills, and can help display and explain fire behavior and fire management strategies to the general public. Fire danger rating systems produce qualitative and/or numeric indices of fire potential based on fuels, topography and weather. These rating systems allow fire managers to estimate present and future fire danger for a given area. Both fire behavior prediction systems and fire danger rating systems require mathematical descriptions of fuel models and their respective fuel properties as input.

Fuel is all living and dead plant material that can be ignited by a fire. Fuel characteristics strongly influence fire behavior and the resulting fire effects on ecosystems. Fires vary widely in the kind of fuels that burn (e.g., live vs. dead fuels, surface vs. ground fuels), the total amount of fuels that burn, and the rate or intensity at which these fuels burn. These characteristics of fuel consumption, in turn, determine peak temperatures reached, the duration of heat, and the stratification of heat above and below the soil surface (Miller 1994). The following sections discuss concepts that will help users understand how fuels affect fire behavior.

• Fuel Categories defines ground, surface, ladder, and canopy fuels and explains factors controlling the availability of each fuel type.
• Physical Fuel Properties explains how fuel size, shape, loading, and arrangement combine to influence combustion and fire behavior. Several techniques for measuring fuel loads are also described.
• Chemical Fuel Properties describes how the chemical composition of fuels affects their heat content and flammability.
• Fuel Moistureexplains how fuel moisture determines fuel availability, what controls fuel moisture, defines time-lag classes, and describes several techniques for measuring fuel moisture.
• Fuel Availability and Consumption explains what factors control the availability of both live and dead fuels.
• Plant Flammability discusses species-specific factors that control the flammability of live plants and presents flammability indices for common southern plant species.

Fuels can be classified into four broad categories based on their vertical distribution:

• Ground fuels: organic soils, forest floor duff, stumps, dead roots, and buried fuels
• Surface fuels: litter layer, downed woody materials, and dead and live plants to 2 m (6 ft.) height
• Ladder fuels: vine or liana fuels and draped foliage fuels
• Canopy fuels: tree crowns

These fuel categories are not to be confused with the fuel types used in fuel models (such as grasses, brush, timber litter, and logging slash). Fuel models are more specific classes of fuels used in fire behavior modeling.

Ground fuels are those forest fuels that lie below the litter layer or within the soil, including organic soils, forest floor duff, stumps and dead roots, and buried fuels. Ground fuels can ignite and smolder for days to months following flaming front passage. Ground fires produce persistent and harmful smoke and can re-ignite surface fuels making them a bane for fire managers.

The forest floor is the layer of organic matter overlying the mineral soil and has both surface and ground fuel components. The forest floor fuel complex contains distinct horizons, each with different moisture relationships, particle sizes, chemical composition, densities, and depths. The surface fuel component of the forest floor is the litter (Oi) horizon. The ground fuel component, duff, is beneath the litter horizon. It is comprised of the fermentation (Oe) and humic (Oa) horizons. In long-fire interval ecosystems the duff layer can become well-developed, however in frequently burned systems it may be intermittent or nonexistent. Duff is created by litter decomposition, so many volatile compounds are lost, particle sizes are reduced, and it is shaded by the overlying litter horizon. Similar to 1,000-hour timelag fuels, duff is slow to absorb moisture. Therefore, when duff moisture is low, smoldering phase combustion often consumes this horizon, resulting in high fire severity and copious amounts of smoke.

Organic soils are important forest fuels in several southeastern ecosystems. Organic soils contain the duff layer overlying a variety of soils (see earlier discussion) and true histosol organic soils. Histosols are dark-colored soils consisting of large amounts of organic peat and muck, underlying poorly-drained forested and nonforested wetlands (e.g., cypress domes, pitcher plant bogs, and bay swamps). Available fuel in organic soils is defined by three factors: moisture, packing, and mineral soil content (Frandsen 1987). Increases in any of these factors decreases flammability and retards combustion. However, following extended droughts, organic soils can ignite and burn for days to months, often smoldering beneath the surface (so called “muck fires”). Organic soil fires are serious concerns in many southeastern wetland communities; they are difficult to control, and have serious ecosystem effects (see: Prescribed Burning in Organic Soils).

See also: Moisture Content of Ground Fuels.

Surface fuels are the primary fuel of interest for fire behavior in most southeastern ecosystems (Wade et al. 2000). Surface fuels include understory plants < 2 m (6 ft.) tall (dead and alive), the litter layer, downed woody materials, and often midstory tree and shrub fuels. Surface fuel availability for consumption is determined by moisture content, particle size, horizontal continuity, compactness, and fuel type (particularly fuels with high volatile compounds). Under most burning conditions in most southeastern ecosystems these fuels carry surface fires.

The understory is the layer of living and dead vegetation from the soil surface to 2 m (6 ft.) tall. Many southeastern ecosystems (e.g., open pine savannas and forests, freshwater marshes, pitcher plant bogs, prairies) contain a dominant understory with abundant grass, forb, small woody shrub and litter fuels. Both grasses and their allies (sedges and rushes) and forbs have high surface area-to-volume ratio, low fuel moisture, are within the flaming zone of most surface fires, and retain abundant dead leaves making them ignite and combust rapidly (exceptions to this are succulents and large-leaved species). Understory fuel availability in southeastern ecosystems is controlled by fuel moisture, horizontal fuel continuity, and fuel loading.

Small woody shrubs can be important understory surface fuels (Blackmarr and Flanner 1975, Hough and Albini 1978). Pocosins, flatwoods, sand pine scrubs, and bogs contain large loadings of shrubby fuels. Many southeastern shrubs have high surface area-to-volume (e.g., saw-palmetto, Serenoa repens), high volatile contents (e.g., gallberry, Ilex glabra), grow within the flaming zone of surface fires, and are highly flammable. In some ecosystems, shrubs and small trees grow into the midstory (between 2 and 5 m; 6 and 16 ft.) and carry surface fires into lower canopy fuels. Midstory fuel availability is regulated by vertical fuel continuity, fuel moisture, and fire behavior. Low-intensity fires with low flame lengths often don’t ignite midstory shrub fuels.

The forest floor is the layer of organic matter overlying the mineral soil and has both surface and ground fuel components. The surface fuel component of the forest floor is the litter (Oi) horizon. The ground fuel component of the forest floor is the duff layer. Litter horizons are fuels in almost all forested southeastern ecosystems, and are therefore somewhat diverse in their composition and structure. Most litter horizons contain recently deposited litter, small woody fuels (10-, 100-, and few 1,000 hour timelag fuels), cones, and other dead plant parts. Litter fuels have reduced volatile content, low fuel moisture content (often 5 to 15%), and are usually loosely packed. Surface fires can be carried solely by litter fuels. Litter fuels may also ignite live understory fuels, pre-heat larger woody fuels, and initiate smoldering of underlying ground fuels, if present. Forest floor fuel availability is determined primarily by fuel moisture content and fuelbed bulk density. Separation of available and unavailable fuel is made on depth to moisture, with all dry fuel included as surface fuel and the remaining wet included as ground fuel.

Understory and shrub fuels are measured using quadrat, point-quarter center, and line transect sampling methods (see Measuring Fuel Loads). Loads (measured in dry kg/m² or lb/acre) are calculated and extrapolated to larger areas or can be input into fire behavior models (e.g., BEHAVE). Forest floor surface fuels are measured by harvesting small quadrats (in kg/m² or lb/acre, and drying for moisture content) and by determining fuelbed bulk density (in kg/cm² or lb/ ft³).

Ladder fuels are those that provide vertical continuity between understory or midstory surface fuels and canopy fuels. Ladder fuels consist of vine or liana fuels, draped foliage fuels, and hanging broken branches. Most surface fires in southeastern ecosystems involve isolated ladder fuels, though in particular circumstances ladder fuels can accumulate and lead to high severity fires.

Vine fuels include several southeastern species that are important in surface fires, such as yellow jessamine (Gelsemium sempervirens), greenbriers (Smilax spp.) and wild grape (Vitus spp.). Vines ascend trees and shrubs creating vertically continuous fuels. Dead and live foliage, stems, and flower structures have low fuel moisture, are bathed in convective heat, and contain volatile compoundscrown fires. Vine fuel availability is governed by fuel moisture, flame height, windspeed, and the live-to-dead ratio. Vine fuels are usually measured as presence/absence, height in canopy, vine loads, and live to dead ratios.

Draped dead foliage (especially pine needle litter) on vines and living or dead shrubs is another important ladder fuel type. Draped fuels have very low fuel moisture (wind, sun, and humidity effects are increased) and are highly flammable. Draped fuels increase the height of the combustion zone, linking understory and midstory fuels to canopy fuels. Southeastern pine plantations and long-unburned forested ecosystems with well-developed vine and/or shrub layers are especially prone to draping.

Hanging broken branches become important ladder fuels in forests following hurricanes, tornadoes, ice storms, and other disturbances.

Canopy fuels are the crowns of trees that form the overstory. The receptivity of the canopy fuels to crown fire is based primarily on three factors: canopy base height, canopy bulk density, and, to a lesser degree, foliar moisture content (Fieldhouse and Dickinson 2003). Canopy base height relates the bottom of the overstory tree crowns to the top of the understory fuel bed and ladder fuels. Canopy bulk density is a measure of the amount of fuel contained in a unit volume of the canopy. High bulk densities present large fuel loads for a fire.

Canopy or crown fuels are typically not consumed during fires in the southeastern US except in isolated cases of "torching" which affect individual trees. Crown foliage is commonly scorched, but rarely is it consumed (i.e., combusted) in crown fires. Particular exceptions are the stand-replacing fires common in sand pine scrub forests in central Florida, in stand-replacing fires in non-indigenous melaleuca forests in south Florida, and limitedly in Table mountain pine forests in the southern Appalachians. General exceptions to this statement occur in fires, either prescribed or wildfire, with extreme fire behavior (caused by low moisture levels, erratic winds, or high fuel loadings).

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).

Many fuels typical of southeastern ecosystems are highly flammable due to their high surface area-to-volume ratios (Brown 1970). In particular, long-needle pine litter and grasses have very high surface areas and are responsible for carrying surface fires in many southern ecosystems. This page provides examples of plant species with high surface area-to-volume ratios.

Grasses, Sedges, Rushes

Grasses and their grass-like allies (sedges and rushes) have extremely high surface area-to-volumes and are highly flammable fuels that carry surface fires in many southeastern ecosystems. Notable examples of these flammable fuels are:

• wiregrasses (Aristida stricta and others),

• bluestems and broom sedges (Andropogon and Schizachyrium spp.),

• giant cane (Arundinaria gigantea),

• panic grasses (Panicum and Dichanthelium spp.),

• muhly grasses (Muhlenbergia spp.),

• the invasive cogon grass (Imperata cylindrica), and

• sedges (particularly sawgrass, Cladium jamaicense) and rushes.

Conifers

Conifers in the southeastern US contain a large group of species with highly flammable litter (Fonda 2001) due to the high surface area-to-volume of conifer needles.

• Long-needle pines have the highest surface area-to-volume ratios of conifers. Examples are: longleaf pine (P. palustris), south Florida slash pine (P. elliottii var densa), slash pine (P. elliottii var elliottii), loblolly pine (P. taeda), pond pine (P. serotina).

• Short-needle pines have lower surface area-to-volume ratios but are still flammable. Examples include: shortleaf pine (P. echinata), Ocala sand pine (P. clausa var. clausa), Choctawhatchee sand pine (P. clausa var. immuginata), spruce pine (P. glabra), Virginia pine (P. virginiana), Table Mountain pine (P. pungens), eastern white pine (P. strobus), and pitch pine (P. rigida).

• Other southeastern conifers have reduced surface area-to-volume, but still much higher than small woody fuels (10- and 100-hour timelag fuels) and many hardwood foliage fuels. Examples include: eastern redcedar (Juniperus virginiana), southern redcedar (J. silicicola), Atlantic white-cedar (Chamaecyparis thyiodes), bald cypress (Taxodium distichum), pond cypress (T. ascendens), eastern hemlock (Tsuga canadensis), Florida torreya (Torreya taxifolia), Fraser fir (Abies fraseri), and red spruce (Picea rubens).

Forbs, shrubs, palms, vines, ferns, lichen, and bromeliads

Other southeastern fuels that are notable for their surface area-to-volume and resulting flammability are forbs, palms, lichens, ferns, vines and bromeliads.

• There is a great diversity of forbs and shrubs with high surface area-to-volume ratios and make up a large portion of the available fuels in several southeastern ecosystems.

• Palm leaves have high surface area-to-volume and are highly flammable; saw-palmetto (Serenoa repens), cabbage palm (Sabal palmetto), and scrub palm (Sabal etonia) are notable examples.

• Lichen fuels (foliose lichens; the reindeer “mosses” Cladonia and Cladina spp. and draped Usnea spp.) have extremely high surface area-to-volume, and combust readily where they dominate patches in pine sandhill and sand pine scrub ecosystems (Brown 1970).

• Understory ferns (particularly bracken fern, Pteridium aquilinum) also have high surface area: volume and combust readily in southeastern surface fires.

• Several draped vines (many Smilax spp. and yellow jessamine, Gelsemium sempervirens) have high surface area-to-volume and contribute to surface fires and localized crown torching.

• Spanish moss (Tillandsia usneoides) and Tillandsia recurvata can be important firebrand fuels, contributing to long-distance spotting (Clements 1976) and spill-over.

Hardwoods

Hardwood foliage has a great diversity of surface area-to-volume, so flammability varies drastically in these fuels. Generally speaking, hardwood litter can burn under dry conditions, but is much less flammable than needle litter and standing dead herbaceous fuel.

• Among hardwoods, most oaks [notably turkey oak (Quercus laevis), post oak (Q. stellata and Q. margaretta), and white oak (Quercus alba)], many other deciduous tree and shrub species [black cherry (Prunus serotina), blackgum (Nyssa sylvatica), hawthorns (Crataegus spp.), and heaths) have high surface area-to-volume and are the most flammable.

• Other deciduous hardwoods and most evergreen hardwood species [e.g., live oaks (Quercus virginiana and Q. geminata) and southern magnolia (Magnolia grandiflora)] have lower surface area-to-volume and require more pre-heating for ignition and combustion.

There are other species-specific characteristics that determine flammability in addition to surface area-to-volume ratios, such as the concentration of organic volatiles. For information on the flammability of individual plant species that take into consideration all of these characteristics, see Plant Flammability.

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.

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.

Chemical properties of fuels affect their heat content and the types of emissions released during a fire (Hough 1969, Shafizadeh et al. 1977, Rundel 1981, DeBano et al. 1998). Forest fuels are composed of living plants and dead plant parts, both of which are constructed of:

• cellulose (from 35 to 55%): an important source of combustible volatiles and is made up of polymers of hexose sugar D-glucose in a linear structure. Cellulose pyrolizes rapidly, producing much of the gaseous fuel in flaming combustion.

• hemicellulose (15 to 25%): a carbohydrate polysaccharide that is similar to cellulose and is found in the cell walls of many plants.

• phenolics such as lignin and tannins (15 to 30%): lignin is an aromatic polymer that does not degrade as easily as cellulose and hemicellulose. This material gives wood its hardness and structural integrity. If heated to temperatures above 400-450° C, 50% of lignin will volatize while the rest becomes char residue or pyrolizes slowly during smoldering combustion.

• extractives (from 2 to >45% in foliage fuels): extractives consist of organic compounds such as sugars, resins, alcohols, hydrocarbons, and fatty acids. Extractives can be highly flammable and are often the first hydrocarbons to be volatilized (during pre-ignition) and consumed in combustion. Extractives that are volatile compounds, such as oils and resins can increase heat release because of their high energy content.

• mineral ash: minerals (Si, Ca, Mg, K, etc) are found in low concentrations and are not combustible.

The relative proportions of these compounds can affect a fuel’s ignition, combustion, and extinction characteristics through particle-level flammability. Woody fuels are high in cellulose, lignin, and hemicellulose, while foliage contains high amounts of extractives.

Heat content is an important aspect of fuel chemistry influencing fire behavior (Miller 1994). A fuel’s heat content (also termed heat yield, energy content, caloric value, or heat of combustion) is the potential heat energy of the particle or the reaction heat resulting from complete combustion, measured in kilojoules per gram (or KJ/g or Btu/lb). A forest fuel with a higher heat content will burn at a higher temperature and more rapidly than a low heat content fuel.

While standard values of heat contents are often used (18,620 KJ/kg), forest fuels vary in their heat contents (see Table: Plant flammability). The presence of volatile compounds in some live fuels increases heat content, and thus flammability (Miller 1994). For example, resinous pine heartwood has almost twice the heat content of oak wood. Foliage heat content is strongly affected by extractive content, so can vary by species and by season. Heat contents change with age, with some species increasing contents with aging and others decreasing with time (Burgan and Susott 1991, Hough 1969). Heat contents are important inputs into Rothermel’s fire spread equations and several fire models (e.g., BEHAVE).

Adjustments and reductions for heat of combustion

Total, gross, or high heats of combustion describe the caloric content of a fuel as measured by an oxygen bomb calorimeter and expressed as calories per gram of dry fuel weight. The high heat of combustion sets a theoretical maximum on the amount of potential energy available for combustion. The average value for wildand fuels is 4500 cal/g (Pyne 1984). Since ideal burning conditions are seldom approached in the field, the high heat of combustion is usually adjusted or reduced to account for fuel moisture, radiation, and incomplete combustion (Alexander 1982). The reduced value is usually called heat yield. Although heat of combustion is often used interchangeably with heat yield, the heat yield for a particular fuel will vary with the heat of combustion (Pyne 1996).

The first reduction of high heat of combustion is for the latent heat expended in evaporating adsorbed water (Byram 1959). Since this latent heat cannot be spent in pyrolysis and combustion, it reduces the amount of energy returned for the heat invested (Pyne 1984). The high heat of combustion reduced by this standard amount, 1263 kJ/kg, then becomes the low heat of combustion (Alexander 1982). In practice, low heat of combustion varies so little from fuel to fuel (roughly 10%) that a basic value of 18,620 kJ/kg has been used as a constant (Van Wagner 1973, Albini 1976). A second reduction, for fuel moisture content, is 24 kJ/kg per moisture content percentage point (Van Wagner 1972b).

Although Byram (1959) also adjusted heat of combustion for radiation losses in his equation for fireline intensity, there are two arguments against this reduction: 1) there is no sound basis available for estimating radiation heat as a proportion of the total energy output of individual fires of different intensities and, 2) radiation is not really a loss, but contributes greatly to fire behavior (Van Wagner 1973). This reduction is suggested if some special purpose requires an estimate of only convective heat output (Van Wagner 1972b).

Another possible reduction is for incomplete combustion or char formation. Heats of combustion that have been adjusted to account for these heat losses are also called effective heat yields. Effective heat yields can range from 34-78% of high heat yields (Pyne 1984). Since incomplete combustion is so variable and difficult to measure, use of effective heat yields remain a matter of subjective judgement (Alexander 1982).

Fuel moisture content is among the most important fuel characteristics affecting fire behavior (Byram 1959, Pyne et al. 1996). It determines how much fuel is available for burning, and ultimately, how much is consumed (Miller 1994). Moisture absorbs heat released during combustion, making less heat available to preheat fuel particles to ignition (Burgan and Rothermel 1984). By raising a fuel’s heat capacity, fuel moisture content influences ignition. At high moisture contents, the heat required to evaporate moisture in fuels is more than the amount of heat available in the firebrand (Simard 1968 in Miller 1994), and combustion can be stopped. This point is termed the moisture of extinction (also called extinction moisture content). Moisture of extinction is a function of the fuel type. For most dead fuels (forest floor duff is an exception), the moisture of extinction is between 12 and 40 percent. For live fuels the moisture of extinction generally exceeds 120 percent (fuel moisture is expressed per unit of dry fuel weight, making moisture contents >100 percent possible).

Both live and dead fuels can slow, stop, or contribute to fire spread, depending on their moisture content. The factors that regulate fuel moisture differ among live and dead fuels. The primary determinants of live fuel moisture content are: internal factors that regulate diurnal and seasonal changes, climate, site factors that affect the fuel environment, phylogenetic differences among species groups (evergreen vs. deciduous), and differences among plant tissues (leaves vs. stems). Fuel moisture in dead fuels ranges widely based on particle size, short and long-term weather changes, topography, decay class, and fuel composition (Byram 1959). How these factors regulate live fuel moisture is briefly explained in the following sections:

• Factors Regulating Moisture Content of Live Fuels

• Factors Regulating Moisture Content of Dead Fuels

Fuel moisture content is a primary variable in all fire behavior prediction models, affecting ignition, combustion, amount of available fuel, fire severity, and smoke generation. Estimating fuel moisture is therefore a critical step in planning a prescribed fire or assessing risk of fire danger. There are several ways to measure fuel moisture, including volumetric analysis, gravimetry, moisture indicator sticks, formulas and graphs, and ocular methods, explained briefly in the following section:

• Determining Fuel Moisture

Land managers can also obtain quick estimates of live and dead fuel moisture from the Wildland Fire Assessment System which produces daily maps of both live fuel moisture and dead fuel moisture across the U.S.:

• Map of estimated live fuel moisture

• Map of estimated 10- hour fuels

• Map of estimated 100-hour fuels

• Map of estimated 1000-hour fuels

Living plants and dead fuels respond quite differently to weather changes. The moisture content of a living plant is closely related to its physiology. The major variations in moisture are seasonal in nature, although shorter term variations are also brought about by extreme heat and drought. The Wildland Fire Assessment System produces daily maps of live fuel moisture across the U.S.

Internal factors

Fuel moisture fluctuates diurnally, with fluctuations greatest in fine living fuels and least in large living tissues. Fuel moisture also fluctuates seasonally, with young foliage having water contents two to three times the content in senescent foliage.

Climatic variation

Climate not only influences the length of the growing season, but can also cause periods of cold-induced dormancy or heat or drought induced quiescence. Long-term climate patterns (particularly extended droughts) can override weather events, creating a lag in changes in living plants’ fuel moisture contents.

Site factors and fuel environment

Site conditions such as soil and canopy cover can cause differences in moisture content within the same species. Elevation and aspect affect local microclimate and produce local differences in seasonal development of many plant species (Schroeder and Buck 1970). In mountain topography, for example, lower elevations and southern exposures favor the earliest start of the growing season.

Species groups

Seasonal moisture patterns vary among deciduous and evergreen species and woody and herbaceous species. Deciduous species generally have higher moisture content than evergreen species. Moisture content in deciduous species also varies more throughout the season, reaching a peak soon after bud break and decreasing after seasonal growth has finished. Because they retain old leaves for several years, evergreen shrubs have a more complex pattern of seasonal moisture content. As with evergreen shrubs, conifers have complex annual patterns of moisture content because they retain needles for several years. In general, old needles reach their lowest moisture content when new needles are being formed. In the southeastern U.S., conifers may flush more than once during the growing season. Moisture content for herbaceous species may be more variable throughout the year than any other species group. With annual species, all of the plant is new tissue at the beginning of the growing season, and all of the plant can become cured at the end. Once cured, herbaceous species respond to atmospheric conditions as a dead fuel. This is particularly true for grasses in areas with hot, dry summers (Byram 1959, Blackmarr and Flanner 1975).

Plant part

The major live fuel categories are foliage, twigs, branches, stems, cones and fruits, and roots. Foliage and other non-woody fuel moisture content are generally very high (>100 – 300 percent), varying among and within species. Foliage moisture is greatest in new foliage, decreasing as the foliage ages. Southeastern pocosin shrub species foliage emerges with 200 to 300% moisture contents and declines rapidly to 100-150% at the end of the growing season. Retained foliage on evergreen shrubs has 100% moisture content, varying little during the year (Blackmarr and Flanner 1975). Cones and other fruiting structure fuels have moisture contents that vary tremendously by season and developmental stage. Small woody fuels (twigs and branches) have lower fuel moisture contents, but these values are considered stable. Stems of pocosin shrub species maintain moisture contents at 100% throughout the year (Blackmarr and Flanner 1975). Still larger living woody fuels (small stems and boles) have lower, albeit more stable moisture contents.

Evergreens growing in climates having marked seasonal changes generally have seasonal growth cycles. Leaves that have lived through a dormant period increase in moisture content at the beginning of the new season from a minimum of perhaps 80-100 percent to a maximum of perhaps 120 percent within a few weeks. These values are typical, but do not necessarily apply to all species and regions. Moisture decreases slowly after this modest increase until the minimum is again reached at the onset of dormancy.Within a few days of the initial increase in moisture in old leaves, twig and leaf buds open and a new crop of leaves begins to emerge. Their initial moisture may exceed 250 percent. Leaves may emerge quickly, or over an extended period, depending on species and the character of the weather-related growing season. The average moisture content of the new growth drops rapidly to perhaps 150 percent, as the new leaves grow in size until about midsummer, and then more slowly, matching the moisture content of the older foliage near the end of the growing season.

Different species of evergreen trees and shrubs characteristically retain a season's crop of foliage for different periods of years. This may vary among species from one season to five or more. There are also differences within species, due partly to age, health, and stand density, but mostly to the weather-dictated character of the growing season. Thus, in years of poor growth there is normally little leaf fall, and in years of lush growth the fall is heavy. As crown canopies become closed, leaf fall tends to approximate foliage production. The oldest foliage, that closest to the ground, is the first to fall, and, in time, the lower twigs and branches that supported it must also succumb and add to the dead fuel supply.

There are exceptions, of course, to the normal, seasonal growth and leaf-moisture cycle, and to the annual replenishment of foliage. Particularly striking are the variations found in the drought-resistant brush and chaparral species in the semiarid West. It is not uncommon for midseason soil-moisture deficiency to cause cessation of growth in these species, with foliage moisture lowering to between 40 and 50 percent. Usually, these plants retain the ability to recover after the next rain. Prolonged severe drought, however, can prove fatal to major branches or even to whole shrubs. Conflagration potential is then at its peak.

The live foliage of evergreens as a class is usually more combustible than that of deciduous species. There are several reasons, but differences in their moisture regimes are most important. All deciduous foliage is the current year's growth, and it maintains relatively high moisture content during most of the growing season. Evergreens, on the other hand, and particularly those that retain their foliage for a number of years, have much lower average foliage moisture during the growing season. Old-growth foliage with its lower moisture may constitute 80 percent or more of the total evergreen foliage volume. Among the evergreens, too, there is greater tendency toward a mixture with dead foliage, branches, and twigs.

Annual range grasses are much more sensitive to seasonal and short-term weather variations than are most other fuels. These grasses are shallow-rooted and thus depend primarily on adequate surface soil moisture for full top development. At best, annuals have a limited growth season. They mature, produce seed, and begin to cure or dry. But deficient surface moisture at the beginning of the season, or its depletion by hot, dry weather may shorten the growth period. Similarly, because of the weather, the curing time may vary from 3 weeks to 2 months after noticeable yellowing.

Perennial grasses have deeper, stronger root systems than annuals and are somewhat less sensitive to short-term surface soil moisture and temperature changes. In regions that have marked growing seasons limited by hot, dry seasons or cold winters, the perennial grasses have, however, a growth and curing cycle similar to annuals, but dieback affects only leaves and stems down to the root crowns. The principal differences in moisture content result from a later maturing date and a slower rate and longer period of curing. In warm, humid areas, some stems and blades cure and die while others may remain alive, although more or less dormant. Often, such mixtures will burn in dry weather.

Dead fuels absorb moisture through physical contact with water (such as rain and dew) and adsorb water vapor from the atmosphere. The drying of dead fuels is accomplished by evaporation. These drying and wetting processes of dead fuels are such that the moisture content of these fuels is strongly affected by fuel sizes, weather, topography, decay classes, fuel composition, surface coatings, fuel compactness and arrangement:

Particle size

Small fuels can gain and lose moisture faster than large fuels. Based on this principle, dead woody fuels are divided into 4 diameter classes, also called timelag classes: < 1/4, 1/4-1, 1–3, and > 3 in. See Timelag classes.

The Wildland Fire Assessment System produces daily maps of dead fuel moisture across the U.S. based on time-lag classes: Map of estimated 10- hour fuels, Map of estimated 100-hour fuels, and Map of estimated 1000-hour fuels.

Weather

Short-term and seasonal/annual patterns in weather determine fuel moisture changes, and thus fuel consumption. Primary weather factors that determine fuel moisture changes are sun, wind, precipitation, relative humidity, and air temperature (Schroeder and Buck 1970). Sunny skies, elevated wind speeds, lack of precipitation, low humidity, and warm temperatures all act to dry fuels. Cloudy or hazy skies, still winds, precipitation, elevated humidity, and cool temperatures either act to increase fuel moistures or prolong their present moisture state. Long-term events like droughts affect 100- and 1000-hour fuels significantly. For more information see: Effects of weather and topography on fuel moisture.

Topography

Topography affects fuel moisture indirectly by influencing microclimate. Fuel moisture content is generally higher on north facing slopes than south slopes because there is less direct exposure to sun. Fuels are also moister at high elevations due to lower temperatures and higher relative humidity (Pyne et al. 1996). For more information see: Effects of weather and topography on fuel moisture.

Decay class

Moisture can be gained and lost more rapidly in decayed wood because particle size is reduced, surface area-to-volume ratio is increased, and moisture-holding capacity is lost. Dry decayed wood combusts and is consumed rapidly. Decay may also remove flammable volatile compounds from dead fuels.

Fuel composition

The major dead fuel categories are dead standing herbaceous material, leaf litter, cones, fallen twigs and branches, fallen logs, and standing snags. Fuel moisture response varies among dead woody fuels, deciduous leaf litter, grass litter, and coniferous needle litter (see Timelag classes). The moisture content of the forest floor complex also has unique characteristics based on its thickness, composition, arrangement, and compactness. Slash (logging debris) presents a special fuel complex.

Surface covering

A covering of organic material, such as bark, waxes, or cutin, can slow the movement of moisture in or out of dead woody fuels. For example, dead woody fuel with bark can gain and lose moisture at 2/3 the rate of fuels without bark (Simard et al. 1984).

Dead vegetation retains its original structure of cells, intercellular spaces, and capillaries. It can soak up liquid water like a blotter, only more slowly, until all these spaces are filled. Dead vegetation may hold two or more times its own dry weight in water. Fine materials may absorb that much in a matter of minutes, while large logs may require a season or more of heavy precipitation. In some climatic regimes, the centers of large materials may never become completely saturated. One reason is that the rate of penetration slows down with increasing distance from the surface.

A second and equally important consideration in understanding fuel-wetting processes is that the materials making up the dead cell walls are hygroscopic. Hygroscopic materials have an affinity for moisture that makes it possible for them to absorb water vapor from the air. This process is one of chemical bonding.

Molecules of water are attracted to, penetrate, and are held to the cell, fiber, or walls by the hygroscopic character of the cell material. The water molecules that penetrate and the few molecular layers that adhere to the cell walls are called bound water. The hygroscopic bond between the cell walls and the water molecules is strong enough to effectively reduce the vapor pressure of the bound water. The layer of water molecules immediately in contact with a cell wall has the strongest hygroscopic bond and lowest vapor pressure. Successive molecular layers have progressively weaker bonds until the cell walls become saturated. At that point, the vapor pressure in the outer layer of water on the cell wall is equal to that of free water, or saturation pressure. The amount of bound water at the fiber-saturation point varies with different materials. For most plant fuel it is in the range of 30 to 35 percent of the fuel dry weight.

The result of the bonding phenomenon is that free water cannot persist in a cell until the cell walls become saturated. Then free water can pass through the cell walls by osmosis. Below the saturation level, moisture is evaporated from cell walls of higher moisture content and taken up by cell walls of lower moisture content until the moisture in each cell attains the same vapor pressure. In this manner, much of the moisture transfer within fuels is in the vapor phase and always in the direction of equalizing the moisture throughout a particular piece of fuel.

Dead fuels will extract water vapor from the atmosphere whenever the vapor pressure of the outer surface of the bound water is lower than the surrounding vapor pressure. In a saturated atmosphere, this may continue up to the fiber-saturation point. Full fiber saturation rarely persists long enough in the absence of liquid water to permit the necessary internal vapor transfer.

See also: Fuel-drying Processes.

Fuel drying is accomplished only by evaporation to the atmosphere. The moisture content of dead fuels thoroughly wetted with free water within and on the surface decreases in three steps in a drying atmosphere, with different drying mechanisms dominant in each period:

1. Constant-rate period: The rate here is independent of both the actual moisture content and the hygroscopic nature of the fuel. It ends at the critical moisture content, the condition in which the total fuel surface is no longer at or above fiber saturation.

2. Decreasing-rate period: During this period, there is a decreasing saturated fuel surface area and an increasing proportion of moisture loss through the slower removal of bound water. The period ends when all the fuel surface reaches the fiber-saturation level.

3. Falling-rate period: During this period, the hygroscopic nature of dead fuel becomes dominant in the drying process.

The process of moisture loss in the constant-rate period is somewhat simpler than those of the succeeding steps. Drying takes place by evaporation exactly as from any freewater surface. It will proceed whenever the surrounding vapor pressure is less than saturation pressure, and at a rate generally proportional to the outward vapor-pressure gradient. Wind speed during this period does not affect ultimate attainment of the critical moisture content level. But it does affect the time required to reach that point. When there is evaporation from a water surface in calm air, a thin layer next to the interface between the free water and air tends to become saturated with water vapor. This saturation near the water surface decreases the evaporation rate and dissipates only by relatively slow molecular diffusion in the air. Wind breaks up this thin layer and blows it away, thereby speeding up the evaporation process.

The intermediate decreasing-rate period may best be described as a transition step in which there is a variable change in moisture loss rate. This rate begins changing slowly within the defined limits from the linear rate of the constant-rate period to the orderly decreasing rate characteristic of the falling-rate period. Variations in the rate of drying during the decreasing-rate period are caused by fuel and environmental factors that are difficult to evaluate and for which no general rules are available. This period is often considered as part of what we have called the falling-rate period when the error involved in calculations is considered tolerable. It is separated for our purposes because it applies only to drying and is not reversible in the sense of vapor exchange between fuel and air as is the case in the falling-rate period. Wind speed still plays a significant role in the drying process during this period.

The falling-rate period of drying depends upon an outward gradient between the bound-water vapor pressure and the ambient vapor pressure in the atmosphere. As moisture removal progresses below the fiber-saturation point, the bound-water vapor pressure gradually declines, and the vapor-pressure gradient is gradually reduced. Either of two conditions must prevail to assure continued significant drying: One is to maintain a surrounding vapor pressure appreciably below the declining bound-water vapor pressure; the other is addition of heat to the fuel at a rate that will increase its temperature and correspondingly its bound-water vapor pressure. Both processes operate in nature, sometimes augmenting and sometimes opposing each other (see Moisture Equilibrium).

As drying progresses toward lower moisture-content values, a vapor pressure gradient is established within the fuel. The external vapor pressure needed to maintain this gradient must therefore be quite low. Under these conditions, molecular diffusion into the atmosphere is more rapid than that within the fuel. This results in a lesser and lesser tendency for thin layers of higher vapor pressure to form at the fuel surface. For this reason, the effect of wind speed on drying gradually decreases at moisture levels progressively below fiber saturation. The effect may never be eliminated, but at low moisture levels it has little practical significance.

See also: Fuel-wetting Processes.

Dead fuels are categorized into fuel diameter classes named according to the timelag principle (Pyne et al. 1996). This principal is based on the fact that the proportion of a fuel particle exposed to weather is related to its size. Small diameter fuels can change rapidly in response to weather changes, while larger diameter fuels are slower to respond. A timelag is the time required for a fuel particle to reach 63% of the difference between the initial moisture content and the equilibrium moisture content (or equilibrium with changed atmospheric conditions). The categories are named for the “midpoint” of the response time of each fuel category: 1-hour fuels respond in less than 2 hours, 10-hour fuels respond in 2 to 20 hours, 100-hour fuels respond in 20 to 200 hours, and 1,000 hour fuels respond in greater than 200 hours. Below are typical fuels and fire behavior for each of these 4 time lag classes.

The Wildland Fire Assessment System produces daily maps of dead fuel moisture across the U.S. based on time-lag classes: Map of estimated 10- hour fuels, Map of estimated 100-hour fuels, and Map of estimated 1000-hour fuels.

1-hour time lag fuels (< 0.625 cm (0.25 in.) diameter)

1-hour time lag fuels are the most important for carrying surface fires and their moisture content governs fire behavior. One-hour fuels include fallen needle and leaf litter, grassy fuels, lichens, and small twigs. Within this category, response times vary by fuel type. Lichen, grass, and well-cured needles respond to changes faster than freshly fallen needles and hardwood leaves. Due to their high surface area to volume, low moisture content, and location in the combustion zone, they produce little smoke and have low flame residence time. One-hour fuels are consumed by both flaming and smoldering combustion, regularly undergoing complete consumption in most surface fires.

10-hour timelag fuels (0.625 - 2.5 cm (0.25 to 1 in.) diameter)

Common 10-hour fuels include small branches and woody stems. Due to their resistance to drying and greater heat capacity, 10-hour fuels often do not combust in low-intensity surface fires. When moisture is low, however, 10-hour fuels can carry hot fires and help ignite larger (100- and 1000-hour) fuels. Ten-hour fuels are readily consumed when fuel moistures are low.

100-hour timelag fuels (2.5 cm - 7.6 cm (1 - 3 in.) diameter)

Larger downed woody debris is common 100-hour forest fuels. These fuels take longer to dry, deterring their consumption under most conditions. Likewise, 100-hour fuels are slow to gain moisture, so they can combust after prolonged drought, even with recent precipitation. When 100-hour fuels ignite they can burn for hours, in mixtures of flaming and smoldering combustion. Decay of 100-hour fuels can alter their response and makes them combust more readily than intact fuels.

1000-hour timelag fuels (> 7.6 cm (3 in.) diameter)

These fuels, which include large downed branches, logs, and tree stumps, burn only under prolonged dry conditions, or when sufficiently pre-heated by adjacent fuels. Since they do not commonly burn, 1000-hour fuels can act as firebreaks and cause fire shadows. When they do burn, 1000-hour fuels are common smoldering fuels and can burn for days after ignition, creating air quality and re-burn hazards.

One method of expressing absorption and drying rates based on both equilibrium moisture content and fuel characteristics makes use of the timelag principle. According to this principle, the approach to equilibrium values from moisture contents either above or below equilibrium follows a logarithmic rather than a straight-line path as long as liquid water is not present on the surface of the fuels.

If a fuel is exposed in an atmosphere of constant temperature and humidity, the time required for it to reach equilibrium may be divided into periods in which the moisture change will be the fraction (1-1/e) ≈ 0.63 of the departure from equilibrium. The symbol, e, is the base of natural logarithms, 2.7183. Under standard conditions, defined as constant 80° F temperature and 20 percent relative humidity, the duration of these time periods is a property of the fuel and is referred to as the timelag period. Although the successive timelag periods for a particular fuel are not exactly equal, the timelag principle is a useful method of expressing fuel-moisture responses if average timelag periods are used.

To illustrate the moisture response, let us assume that a fuel with a moisture content of 28 percent is exposed in an environment in which the equilibrium moisture content is 5.5 percent. The difference is 22.5 percent. At the end of the first timelag period, this difference would be reduced 0.63 x 22.5, or about 14.2 percent. The moisture content of this fuel would then be 28 -14.2, or 13.8 percent. Similarly, at the end of the second timelag period the moisture content would be reduced to about 8.6 percent, and so on. The moisture content at the end of five or six timelag periods very closely approximates the equilibrium moisture content.

The average timelag period varies with the size and other factors of fuels. For extremely fine fuels the average period may be a matter of minutes, while for logs it ranges upward to many days. Using the timelag principle, we can describe various fuels--irrespective of type, weight, size, shape, compactness, or other physical feature--as having an average timelag period of 1 hour, 2 days, 30 days, and so on. Dead branchwood 2 inches in diameter, for example, has an average timelag period of about 4 days. Logs 6 inches in diameter have an average timelag period of about 36 days. A 2-inch litter bed with an average timelag period of 2 days can be considered the equivalent, in moisture response characteristics, of dead branchwood (about 1.4 inches in diameter) having a similar timelag period if there is no significant moisture exchange between the litter and the soil (see Timelag classes).

Moisture equilibrium has meaningful application to forest-fuel moisture only in the range of moisture-content values between about 2 percent and fiber saturation. This is the range covered by the falling-rate period of drying. Fuel will either gain or lose moisture within this range according to the relative states of the fuel and its environment. The amount, rate, and direction of moisture exchange depend on the gradient between the vapor pressure of the bound water and the vapor pressure in the surrounding air. If there is no gradient, there is no net exchange, and a state of equilibrium exists.

The equilibrium moisture content may be defined as the value that the actual moisture content approaches if the fuel is exposed to constant atmospheric conditions of temperature and humidity for an infinite length of time. The atmospheric vapor pressure is dependent upon the temperature and moisture content of the air. The vapor pressure of the bound water in fuel depends upon the fuel temperature and moisture content.

Assuming that the fuel and the atmosphere are at the same temperature, then for any combination of temperature and humidity there is an equilibrium fuel-moisture content. At this value, the atmospheric vapor pressure and the vapor pressure of the bound water are in equilibrium. This point almost, but not quite, exists in nature. Small vapor-pressure differences can and do exist without further moisture exchange. This is demonstrated by the fact that a dry fuel in a more moist environment reaches equilibrium at a lower value than a moist fuel approaching the same equilibrium point from above. For this reason also, reduction of humidity to zero does not reduce fuel moisture to that value. Vapor exchange involving bound water is not as readily attained as is free water and atmospheric vapor exchange. At low vapor-pressure gradients involving bound water, there is not sufficient energy at normal temperatures and pressures to eliminate these small gradients.

Equilibrium moisture content has been determined in the laboratory for numerous hygroscopic materials, including a variety of forest fuels. The usual procedure is to place the material in an environment of constant temperature and humidity, leaving it there until the moisture content approaches a constant value. The process is then repeated over the common ranges of humidity and temperature encountered in nature. Continuous or periodic weighing shows the changing rates at which equilibrium is approached from both directions. Different fuel types usually have different equilibrium moisture contents, but for most fire-weather purposes it is satisfactory to use the average determined for a number of fuels.

The rates at which moisture content approaches the equilibrium value vary not only with the kind of fuel material, but with other characteristics such as fuel size and shape, and the compactness or degree of aeration of a mass of fuel particles. For any one fuel particle with a moisture content below fiber saturation, the rate of wetting or drying by vapor exchange is theoretically proportional to the difference between the actual moisture content and the equilibrium moisture content for the current environmental conditions.

This means, for example, that when actual fuel moisture is 10 percent from its equilibrium value, the rate of increase or decrease is 10 times as rapid as if the moisture were within 1 percent equilibrium. This relationship indicates that moisture content approaching equilibrium follows an inverse logarithmic path.

Use of the equilibrium moisture-content concept makes it possible to estimate whether fuel moisture is increasing or decreasing under a particular environmental situation, and the relative moisture stress in the direction of equilibrium. This by itself, however, is a poor indicator of the quantitative rate of moisture-content change. To it, we must also add the effect of size or thickness of the fuel in question. Applying the time-lag principle allows us to divide fuels into time-lag classes based on their size and thickness.

The moisture content of live and dead vegetation is a product of the cumulative effects of past and present weather events. Fuel moisture changes as weather conditions change, both seasonally and during shorter time periods. This fact, coupled with known attributes of different fuels, provides a useful basis for estimating fire potential in any forest or range area. Fuel moisture content limits fire propagation. When moisture content is high, fires are difficult to ignite, and burn poorly if at all. With little moisture in the fuel, fires start easily, and wind and other driving forces may cause rapid and intense fire spread. Successful fire-control operations depend upon accurate information on current fuel moisture and reliable prediction of its changes.

Living and dead fuels have different water-retention mechanisms and different responses to weather. Live fuel moisture is closely related to its physiology. The major variations in moisture are seasonalheat and drought.

Dead fuels absorb moisture through physical contact with liquid water such as rain and dew and adsorb water vapor from the atmosphere. The drying of dead fuels is accomplished by evaporation. The nature of the drying and wetting processes of dead fuels is such that dead fuel moisture is strongly affected by weather elements such as precipitation, air moisture, air and surface temperatures, wind, and cloudiness. Dead fuel moisture contents are also influenced by fuel factors such as surface-to-volume ratio, compactness, and arrangement.

During clear weather, fuel-bed surfaces exposed to full midday sun may reach temperatures as high as 160° F. or more. Not only does this greatly increase the bound-water vapor pressure, but it also warms the air near the surface and reduces relative humidity. The combination often results in surface fuel moistures 4 to 8 percent below those in adjacent shaded areas. Similarly at night, cooling of these exposed fuel surfaces may cause dew to form on them, while it does not form under the tree canopy. Surface fuel moistures and accompanying changes in moisture gradients are thus commonly much greater, and at the same time much more spotty, in open forest stands than under forests having closed-crown canopies. Clouds also tend to reduce the diurnal extremes in fuel moisture.

Wind can increase drying processes by moving moist air away from fuel surfaces. But wind can also have the opposite effect. Moderate or strong winds may affect surface temperatures of fuels in the open and thereby influence surface fuel moisture. During daytime heating, wind may replace the warm air layers immediately adjacent to fuel surfaces with cooler air. This in turn raises the relative humidity in that area and lowers the fuel-surface temperature. Fuel drying is thereby reduced. At night, turbulent mixing may prevent surface air temperatures from reaching the dew point, thus restricting the increase of surface fuel moisture.

North-facing slopes do not receive as intense surface heating as level ground and south exposures, so they do not reach the same minimum daytime moistures. The highest temperatures and lowest fuel moistures are usually found on southwest slopes in the afternoon. In mountain topography, night temperatures above the nighttime inversion level ordinarily do not cool to the dew point; therefore, surface fuel moistures do not become as high as those at lower elevations.

The moisture content of ground fuels is influenced by three different moisture gradients, or layers of differing water potential: one between the fuel and the air, another between the fuel and the soil, and still another between the top and bottom of the fuel bed itself. Only the upper surface is exposed to the free air while the lower surface is in contact with the soil. In deep and compact fuel beds, air circulation in the lower layers may be nearly nonexistent.

Precipitation soaking down through the fuel into the soil may produce relative humidities near 100 percent at the lower levels, and this can persist for appreciable times. Subsequent drying starts at the top and works downward. In deep fuels, it is not uncommon for the surface layer to become quite flammable while lower layers are still soaking wet.

Reverse gradients also occur after prolonged drying, resulting in the topsoil and lower duff becoming powder dry. Then morning dew on the surface, high relative humidity, or a light shower may cause a downward moisture gradient.

These changes in upward and downward moisture gradients are common in most compacted fuel beds. In some situations, they may even be part of the diurnal cycle of moisture change in response to diurnal changes in temperature and relative humidity. This is particularly true in open forest stands where much of the surface litter is exposed to direct solar heating during midday and to direct radiant cooling to the sky at night.

The amount of fuel available for combustion is often determined by these interior moisture gradients. In some cases, for example, fire may only skim lightly over the surface; in others, the entire dead-fuel volume may contribute to the total heat output of the fire.

Slash (trunks, branches, and tree tops) from thinning or harvest cutting of coniferous forests is a special and often particularly hazardous kind of dead fuel. Often, it is flammable from the time it is cut, but it is particularly hazardous if added to significant quantities of flammable dead fuels already on the ground. As the slash dries, it becomes more and more flammable. The slash of different species dries at different rates, and within species the drying rates depend on degree of shading, season of cutting, weather, and size of material. Needles and twigs dry faster on lopped than on unlopped slash. Within a matter of weeks, however, it is not necessary to consider slash needle and twig moisture different from that of older dead fuels. Stems, of course, require longer periods of seasoning to approach the fuel moisture of their older counterparts.

Fuel moisture content can be determined in several ways, from cheap and approximate to expensive and precise. Methods discussed here by no means include all field or laboratory methods, but do include: titrimetry (volumetric analysis), gravimetry, moisture indicator sticks, solvent distillation, statistical correlations, formulas and graphs, ocular methods, and remote sensing.

Titrimetry

Titrimetry or volumetric water content measures percent water content of a known volume of fuel. This analysis is performed by:

• Measuring the dimensions of the fuel volume (the length, width, and height of a fuel volume, i.e., 1 m³) of interest. This known volume is collected and weighed as a “wet” weight.

• The wet sample is then sealed and transported to a drying oven and dried (usually between 70 and 100^oC) to a constant weight.

• The “dry” sample is weighed and subtracted from the “wet” field weight.

• This difference is divided by the original wet weight and the resulting value is the volumetric water content (%).

The advantages of this method are its ability to integrate different fuels for a better estimate of a stand or site fuel moisture and its low cost (Pyne et al. 1996).

Gravimetry

Gravimetric fuel moisture content measures the percent water content of a known weight of fuel. Sampling is very similar to that used for volumetric water content, except that the original volume of fuel does not need to be measured:

• Collecting field fuels and weighing their “wet” weight.

• The wet sample is then sealed and transported to a drying oven and dried (usually between 70 and 100^oC) to a constant weight.

• The “dry” sample is weighed and subtracted from the “wet” field weight.

• This difference is divided by the original wet weight and the resulting value is gravimetric fuel moisture content (%).

The advantages of gravimetric analysis are its ease of sampling (1-, 10-, 100-, and 100-hour fuels can be “grabbed” and weighed without measuring a fuel’s dimensions), low cost, and subjectivity of fuels sampled (Pyne et al. 1996). It is often used to measure moisture content for specific classes of fuels: grass, forbs, woody debris, etc...

Moisture indicator sticks

Moisture indicator sticks are another method of determining fuel moisture. The sticks are 0.625 cm (¼”) diameter x 50 cm (20 in.) long wooden (pine sapwood) dowels that mimic water absorption of 10-hour timelag fuels. Moisture sticks are placed on support brackets 25 cm (10 in.) above a fuelbed and allowed to equilibrate for some time prior to observation. Measurements of moisture are made on a standard scale and applied as a site average. Moisture sticks have the advantage of being site specific and quickly measured.

Tabular, Statistical correlations, and graphs

Tabular or statistical correlation and graphical methods are a popular method of fuel moisture determination. Many tables have been generated that integrate relative humidity, air temperature, and wind speed to yield fine fuel (1-hour timelag fuel) moisture content. These measures are easily acquired (through field, belt weather kit, or weather station measurements) and tables are quick. This method has utility for fine fuels and for occasions when quick determination of fuel moisture is critical. For example, see the following tables to predict litter moisture.

• (Fine fuel moisture content of dead grass (1-hour timelag))

• (Moisture content of pine needle litter)

• (Average moisture of hardwood litter under an upland oak stand in Eastern Tennessee)

• (Total litter moisture content from duration of (last) rain and days since (last) rain data)

Ocular methods

Ocular methods of determining fuel moisture are another method of fuel moisture determination. Ocular methods include observation of vegetation color, either directly or by using remote sensing imagery (Pyne et al. 1996). Experienced burners in the Southeast can manually bend, twist, and break litter fuels (1-hour timelags) to estimate relative fuel moisture contents. These methods can be inexpensive (even analysis of remote sensing data is inexpensive over large landscapes), but require extensive experience and can lack utility in fire management situations.

Solvent distillation

Solvent distillation is another method of determining fuel moisture content. Distillation consists of heating or boiling a fuel sample of known weight or volume. The volatilized gases and vapor are captured, cooled, and collected. The weight or volume of the collected liquid is subtracted from the original weight or volume, and then divided by the original weight or volume. The resulting value is the distillation fuel water content. This method is precise, but is labor and equipment intensive.

Instrumental techniques

Several fuel moisture determining instruments