Encyclopedia of Wildfires and Wildland-Urban Interface (WUI) Fires

Living Edition
| Editors: Samuel L. Manzello

Activity Fuels

  • Roger D. OttmarEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-51727-8_232-1



An activity fuel is defined as live or dead vegetative matter that has accumulated or has been altered through active vegetation management such as timber harvest, thinning, pruning, mastication, chipping, or a combination of approaches.


Wildland fuels are defined as the live and dead vegetative matter that contributes to wildland fire (Davis 1959). The fuels include the boles and canopies of trees, shrub stems and leaves, grass and herbaceous vegetation, sound and rotten woody material, needle and leaf litter, moss, lichen, and decayed organic material often referred to as duff. The vegetative biomass is often arranged into distinct categories including trees, shrubs, grasses, wood, litter, and duff (Fig. 1, Ottmar et al. 2007). The array of these categories is defined as a fuel bed and has unique characteristics of loading, depth, height, and bulk density that, along with moisture content, wind, and topography, determine how fire will behave if these fuels combust.
Fig. 1

Horizontal quantification of fuel beds by stratum and categories with the Fuel Characteristic Classification System

Fuels and the characteristics associated with each fuel bed are constantly changing in space and time as natural ecological processes continuously interact with the live and dead vegetation (Keane 2015). For example, as a tree grows, the stem and canopy generally will become larger, adding bole, bark, limb, needle, and leaf biomass. This biomass can fall to the ground as the tree ages or falls over if there is a wind or ice event or when the season changes, adding to the woody and litter layers. The woody and litter layers in turn begin to decay and will often initiate a denser, organic layer called duff. All of this biomass can become available to consume and contribute to the fire behavior under favorable weather and fuel moisture conditions. If altering the fuel bed complex is a result of natural, ecological processes, it is referred to as a natural fuel.

If the fuel bed results from or is altered through a vegetation management activity such as timber harvest, thinning, pruning, mastication, chipping, or a combination of approaches, then it is referred to as an activity fuel (Fig. 2). The debris left after these activities adds to the fuel bed biomass including logs, branches, needles and leaves, chips, bark, small trees, and shrub pieces and is often referred to as activity fuel or slash. Depending on the type and extent of activity, the additional or manipulated biomass can lead to an increase or decrease in fire behavior as compared to accumulation of vegetative matter from natural processes.
Fig. 2

Land management activities resulting in activity fuels including (a) timber harvest, (b) thinning, (c) piling, and (d) mastication

Types of Activity Fuels

Activity fuels are generated from a variety of hand and mechanical manipulations of live and dead vegetation. The types of manipulations include timber harvest (logging), tree thinning, land clearing, fire hazard reduction, wildlife habit improvement, or initiating an ecological shift such as maintaining or eliminating a specific vegetation type. Often these activities will target one or several fuel bed components depending on the management objective. For example, thinning of a forested stand for ecological benefit, reduction in crown fire potential, or improving wildland habitat can create large amounts of surplus logging debris. This debris includes unmerchantable material such as broken logs, tree limbs, and twigs with attached needles, rotten logs, stumps, and fractured shrub debris. Although the trees have been partially removed and thus reducing the tree crown density, the material remaining on the ground is generally small twigs and needles with a high surface-to-volume ratio (see “Surface Area to Volume Ratio” chapter) and higher fuel bed depth, resulting in an increase in surface fire behavior potential. In order to mitigate the surface fire hazard, this material may be prescribed burned, piled and burned, chipped, chopped, or collected for bioenergy generation to reduce available fuel biomass. Land management activities such as chaining or felling less desirable species such as the encroachment of western juniper (Juniperus occidentalis) in a sage grouse basin sagebrush (Artemisia tridentata) habitat can lead to additional surface woody fuels if the trees are felled and left on site. Using prescribed fire or piling the material and burning will reduce the fire potential.

Activity Fuel Longevity

If activity fuels have increased fire hazard, they can remain hazardous for many years depending on the total mass of material and rate of decay. For example, timber harvest in a mature Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata) stand located on the western slopes of the Cascade Mountains can produce hundreds of tons per hectare of large and small unmerchantable woody debris during the logging operation. As these activity fuels cure and dry, the fire behavior potential increases resulting in a hazardous fuel bed across the timber harvest area. However, as the needles fall off the twigs, the woody debris begins to decay and compact, new live vegetation is established, and the fuel bed becomes less hazardous. It generally takes approximately 10–15 years for the activity fuels to be less fire prone and be classified as natural fuels. In the southeastern United States, the climate is hotter and more humid, activity fuels will compact and decay much more rapidly, and the fire hazard will be reduced from these fuel components after 4–5 years.

Estimating Fuel Loading and Fire Behavior Potential of Activity Fuels

Mass, volume, and depth are critical characteristics needed to assess fire potential of activity fuels. Inventory methods for measuring these critical characteristics are outlined in the Monitoring Inventory System (FIREMON) sampling protocols (Lutes et al. 2006) and the Fuel Characteristic Classification System Field Guide (Prichard et al. 2019), which were developed for use in fire monitoring applications with an emphasis on repeating plot measurements. The photo series is a land management tool that can also be used to assess activity fuels through visually appraising living and dead woody material and vegetation and stand characteristics with the use of photos and accompanying field collected data (Ottmar and Hardy 1989). The Fuel Characteristic Classification System (FCCS; Prichard et al. 2013) can be used to build an activity fuel bed based on default or collected field data. By adding environmental variables such as fuel moisture, wind speed, and temperatures, the FCCS will estimate fire behavior, fuel consumption, energy release, and smoke production so that the fire potential of the fuel bed can be assessed.

As new fire behavior and fire effect models come to the forefront, new and improved methods are being developed to improve the measurement of activity fuels. The use of terrestrial and aerial Light Detection and Ranging (LiDAR), Unmanned Aerial System (UAS) photogrammetry, and three-dimensional fuel modeling is fast becoming a reality to improve the measurement of activity fuels fuel bed depth, loading, and arrangement which are critical for the next generation of fire behavior modeling.

Activity Fuel and the Wildland Urban Interface (WUI)

As previously mentioned, activity fuels result from human activity that alters the fuel bed. These activities may increase or lessen the fire behavior potential. These management activities are often used to reduce the fire threat near homes and urban areas. For example, if a dense stand of trees located near homes has substantial ladder fuel to carry the fire into the canopy, crown fire potential will be high, and the homes will be at risk. To reduce crown fire potential, the forest is often thinned to reduce the basal area, increase the canopy base height, and reduce ladder fuels, resulting in a reduction of crown fire potential. However, if the tree harvest or thinning results in an accumulation of surface fuel biomass of stems with needles, logs, and litter, the surface fire behavior may actually increase. When assessing potential fire hazard reduction activities, it is essential that all existing and altered fuel bed components and how they interact are considered when estimating the final fire behavior reduction potential.


When fuels are altered through vegetation management such as timber harvest, thinning, pruning, mastication, chipping, or a combination of approaches, it is often referred to an activity fuel. The additional or altered debris remaining after these activities including logs, branches, needles and leaves, chips, bark, small trees, and shrub pieces can lead to an increase or decrease in fire behavior. Prescribed fire, pile and burn, chipping, and mastication are several of the land management treatment methods used to reduce the fire hazard potential that may result from these activity fuels. If land management activities are prescribed for areas near WUI to reduce overall fire potential, it is imperative that all existing and altered fuel bed categories be considered during the planning process.



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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.US Forest Service, Pacific Northwest Research Station, Pacific Wildland Fire Science LaboratorySeattleUSA

Section editors and affiliations

  • Sara McAllister
    • 1
  1. 1.USDA Forest ServiceRMRS Missoula Fire Sciences LaboratoryMissoulaUSA