Wildfire ecology of Washington’s east-side Cascade Mountain low-elevation forests

Wildfire ecology of Washington’s east-side Cascade Mountain low-elevation forests

 SourceThe Wildfire Box curriculum created by Susan Ballinger and funded by the U.S.D.A. Forest Service, Okanogan & Wenatchee National Forests Conservation Education Program

The Fire Triangle and Wildland Fire Behavior

Fire describes the very rapid release of energy stored in fuel.

 Fire is a rapid chemical reaction that combines fuel and oxygen to produce heat and light:


       Fuel + heat + oxygen        –>         carbon dioxide + water vapor + heat + light                    

An external source of heat is usually required to start the reaction.  Once the fire has started, it produces the heat needed to continue burning.

The three parts needed for a fire (Fuel + heat + oxygen) can be represented by a fire triangle.  To stop a fire, the triangle must be broken; if any one of the three components is missing, a fire cannot occur.

Heat is provided in nature in the form of lightning or volcanoes.  Matches & untended cigarettes, campfires, and outdoor debris burning are the leading sources of heat in human-caused wildfires in Washington.  Heat sources used to start intentional fires include spark plugs in cars, pilot lights in natural gas-burning appliances, and a lit match.

 Oxygen is found in the air.  The amount of oxygen available to fire is often influenced by wind.  Wind also helps the fire by blowing the heat toward more fuel, carrying sparks to unburned areas, and drying out the fuel through evaporation.  Blowing on a campfire provides extra oxygen and blows the heat toward unburned fuels.  If you blow too hard, you scatter the heat so much that the fire goes out.

 Fuels are anything that will burn in a fire.  In a car, gasoline is the fuel used by an internal combustion engine. Candles are fueled by melted wax, campfires are fueled by wood.

To slow down or stop a fire, the triangle must be broken by reducing or removing one of the three legs:

  • Ways to remove heat: throw water or retardant on the fire (both cut off oxygen and absorb heat). When you “Blow” out a candle, you are removing so much heat away from the wick, that it goes out.
  • Ways to remove oxygen: You can stop the flow of oxygen by throwing dirt on a fire, using a fire extinguisher, or by dropping fire retardant from planes.  “Stop, drop, and roll” is a way to reduce oxygen available to burning clothes.
  • Ways to remove fuels: The fuel supply can be removed by clearing a fire line around a fire (remove all burnable plant parts down to mineral soil).

Types of Fuels

In wildlands, nature’s fire fuel is plant materials.


  • fire_ecologyCanopy fuels are found high above the   ground in the tree crowns. Includes tree branches, leaves, and needles, dead standing trees, hanging beard lichens, dwarf mistletoe, and tall brush.
  • Surface fuels lie on or right above the ground.  Include needles, leaves, grass, dead wood, partially decomposed plants, downed logs, stumps, low brush.
  •  Ground fuels, called duff, make up the top active layer of soil.  Duff consists of decaying leaves and wood, and roots.  It is sometimes mixed with fine rock particles which won’t burn.


Three Types of Fires

  1.  Surface:      Burns fuels on the ground, as well as shrubs and trees.  Fuels like branches, bark, dead shrubs, downed trees, and grass burn quickly.  A surface fire moves over an area rapidly, not damaging soils and trees.  Surface fires can help keep fuels from building up and will stimulate grass, wildflower, and shrub re-growth.
  2. Ground:  Creeps slowly through the duff, burning the roots of living trees and plants.
  3. Crown (Canopy) Fire:  Fire usually begins as a surface fire, spreading upward into dry tree branches and up into the tree tops, often pushed by strong winds.  Fuels like dry branches and dwarf mistletoe allow the fire to climb up this “fire ladder.”  Crown fires create their own winds as they use up large amounts of oxygen and as heat from the fire rise.

Types of Fire Intensity

Fire intensity is a term used to describe the amount of heat a fire produces, making it a cool (low-intensity), moderate, or hot (high-intensity) fire.  Several factors determine fire intensity:

  • Amount of sun.  Slopes that face south, southwest, or west tend to be warmer and drier because they get more sun.  Fires on these slopes burn more readily that fires on north-facing slopes.
  • Slope of the land.  Fires will burn up a steep slope more rapidly than on level ground because the fire and heat rise quickly and dry out the vegetation.
  • Size of fuels: Fuels that are small and very dry (grass, twigs) produce cool fast fires.  Larger woody fuels burn much hotter and produce higher intensity fires.
  • Weather: The more air moisture (humidity), the cooler the fire will be.  The higher the air temperature, the drier the fuels will be.  Fires that happen in spring burn less intensely that fires during a hot dry summer due to increased moisture in the air, fuels, and soil.  Rain can cause a fire to cool down and lessen the intensity.  Wind can fan a fire, increasing its intensity.

 Outcomes of Low-intensity fires

  • Roots:  not killed.  Shrubs/grasses/wildflowers: re-sprout quickly.
  • Tree crowns & branches:  not damaged by heat.  Plant nutrients in the form of ash are put back into the soil. Soils not damaged by heat.  Prevent the build-up of fuels and invasion of plant species not adapted to fire and dry conditions.

Outcomes of high-intensity fire

  • All plants and roots killed.  New growth depends on unburned seeds buried in soil or seeds brought in by wind or animals.  Nutrients in soil lost due to high heat of fire.  Land subject to erosion.  Potential invasion of invasive weeds.

East-side Forests:  Pre-1900 vs. today

 Over 100 years of effective suppression of wildfire in our east-side Cascade forests has resulted in dramatic changes in the tree density and tree species composition.  Forests of today are very over-crowded with fire-prone species and carry great loads of fuel, ready to transform surface fires into high intensity canopy firestorms.

Pre-1900 forests experienced frequent, low-to moderate-intensity wildfires (varied greatly, often every 6-20 years).  The main forest trees, shrubs, and wildflowers adapted by being able to either endure or resist wildfire.  Endurers like grasses, wildflowers, and many shrubs quickly re-sprout new stems from below-ground buds and roots.  The dominant trees were the long-lived resisters like ponderosa pine, western larch, and (to a lesser extent) Douglas-fir, with thick bark, the absence of low limbs, and a well-spaced growth pattern.  Their seeds sprout in full sun.  With short intervals between fires, few fuels (downed limbs, logs, shrubbery) accumulated on the forest floor, so fires quickly moved on.  Generally, an area needed to be fire-free 6-12 years to allow young trees enough time to grow large enough to be able to resist fire.  A pattern of well-spaced groves of similar-aged trees resulted, creating an open, park-like forest.

In the late 1800s, settlers arrived to build homes and towns in these fire-adapted forests.  Logging selectively removed the biggest trees, often the ponderosa pines, leaving behind a higher proportion of less fire-adapted species like Douglas-fir and grand fir.  Several decades of intense sheep and cattle grazing reduced the ground cover in forests, which slowed down the natural spread of wildfire.  Several early-1900 catastrophic wildfires in the Western U.S. gave political and social momentum for governments to institute fire suppression policies.  We, as people, became extremely effective at spotting and extinguishing wildfires before they could spread.  We stopped the natural pattern of frequent low-intensity fires typically triggered by summer and early fall lightning storms.

The fire-resistant forest of the 20th Century has been transformed into the fire-prone forest of the 21st Century.  Thick groves of grand firs and Douglas-fir create a shady forest floor where sun-loving ponderosa pine seeds can’t sprout.  Grand fir’s low thick branches now provide a ladder for surface fires to climb into the tree tops.  Because fires have become so infrequent, there is a great buildup of downed fuels on the forest floor that support higher-intensity burns.  The greater density of trees like grand fir allow insects and pathogens, that weaken or kill living trees, to spread.  The climate pattern of hot, dry summers and frequent lightning has not changed.  Fire suppression has reduced the frequency of fires, but increased the opportunity for large, high-intensity fires.  Even plants that have adapted to resist fire cannot tolerate the high temperatures created by these canopy burns.

Another type of Western U.S. forest is dominated by lodgepole pine. These landscapes have always experienced larger-scale, higher-intensity wildfires.  These thin-barked trees burn easily and completely.  Lodgepole seeds require full sun.  Young trees grow very quickly and can begin making seeds when only 5-10 years old, and often die at age 100.  They produce two kinds of cones.  Some cones open when ripe and the wind disperses their seeds.   Others, called serotinous cones, keep their seeds inside, sealed by resin, often for decades.  When the heat of wildfire comes and kills the parent tree, it melts the cones’ resin and millions of seeds are released to start a new forest.  Since 1900, more and more people are living within lodgepole pine forests.  The term, urban interface, is used to describe this phenomenon.  It is the people, not the forest, than cannot tolerate infrequent, high-intensity crown fires.

Determining Fire History using Tree Rings

 Tree rings and scars on trees tell the history of a forest.  Fire has been a part of the history of most forests in North America for thousands of years. Dendrochronology is the science of learning about trees and climates from tree growth rings. At first, it may seem an easy task to count tree rings, but in actuality it requires special training and magnification.  A team of scientists at the U.S.F.S. Research Station in Wenatchee have provided the dated & polished tree cross-section (“cookie”) in the Wildfire Box.  The digitally produced poster of “cookies” is also provided by this lab.  Most tree cookies are obtained from dead standing trees and from stumps, but a partial cookie can be cut from a living tree without causing the tree’s death.

A fire scar is formed when a part of the tree’s cambium (the ring of living cells under the bark) is killed by heat from a fire.  If the cambium is damaged only part-way around the tree, the tree often survives.  Fire scars are made by surface fires that are not severe enough to kill the tree.

In years after the fire, new wood forms at the edge of the damaged area.  Year after year, new rings of cambium are formed that gradually curl over the edges of the damaged area and begin to cover it.

From the outside, the fire scar looks like a triangular wound coming up from the ground and it is sometimes called a cat’s face.  Typically, wildfire burns hotter and longer on the uphill side of a ponderosa pine.  Thus, a fire scar “cat’s face” forms in the same place, repeatedly, with each successive wildfire.  If a tree lives a long time without more fire damage, new wood from both sides of the scar may eventually cover the scar completely.  We call these hidden scars:  they are not visible from the outside of a tree, but are seen on a cross-section sample of the tree.

Historians recognize two important sources of wildfire prior to European Settlement of North America:  lightning and Native American burning.  It is hard for us to know the relative frequency of these two sources of ignition.

By examining this tree cookie, we know that the last crown fire in its forest happened before this tree began to grow.  By counting the years between fire scars, we calculate the interval of years between surface fires for this tree.  The center of the cross-section is the pith and is the tree’s first year of growth.  The last year of growth is the outer ring adjacent to the bark.

Scientists have studied the fire intervals in hundreds of cookies from many forests in the Western U.S.  In past centuries (prior to 1900), surface burns happened often and were very seldom were there crown fires in ponderosa pine/Douglas-fir forests.  Very little ground fuels built up on the forest floor and trees were well spaced.  Especially since 1900, people in the West have done a good job of putting out most wildfires, so people have changed the fire interval pattern.  The ponderosa pine/Douglas-fir forests have become denser (more trees packed together) and more ground fuels have built up.  After 100 years, these forests are susceptible to hot crown fires that kill all trees due to the increased fuels.

Fire as a Management Tool

 The intentional, planned use of fire is known as prescribed burning. Prescribed burning is not a new practice.  Native Americans used prescribed burning for many centuries all across North America.  Fire itself was used for cooking, heating, and light.  Forests were burned to enhance agriculture or to create habitat desirable for game animals or plant harvest.  Fire was used to make it easier to travel through forests and even as a war weapon.

In the early 1900s, U.S. forest land managers implemented a policy to suppress all fires.  Fire was viewed as an enemy.  Effective Smokey Bear messages urged people to prevent forest fires.   Preventing and suppressing natural fires has contributed to changes in forest ecosystems and has contributed to the severity of recent forest fires.

Fire is now recognized as a natural agent, neither good nor bad, that has created and maintained much of our landscapes.  In the right place at the right time, fire can be an asset and a tool. Forest managers today are now looking at ways to use fire as a management tool in fire-dependent ecosystems to help protect the ecosystems diversity, productivity, and stability.  One technique is the use of prescribed burning.  It can be used to duplicate the historic cycle of natural fire (the fire return interval).  Thinning of small trees and limbing of low fire-ladder branches often precedes a prescribed burn as a way to reduce the fuel load.

By analyzing weather conditions, fuel types, and the topography of an area, a professional fire manager can begin to predict how fast the fire will spread, how high the flames will go, and how intensely the fire will burn the area.  Prior to setting a prescribed burn, managers complete a burn plan.  These plans consider such things as the purpose of the burn, fuel load of an area, public notification plans, ignition source and patterns, pre-fire surveys, and people and equipment needed.  Sometimes, prescribed burns “escape” and cause damage.  This is usually due to an unexpected change in the weather.

Production of air-polluting smoke is a problematic result of both natural and prescribed fires.  Forest managers strive to minimize smoke production from a prescribed burn, but natural burns often result in severely poor air quality.

Urban Interface:  Issues and Actions

The term urban interface describes places where people built homes in close proximity to flammable fuels found naturally in wildlands.  These wildlands can be forests, hillsides, and valleys.  Across most of the western U.S., we see increasing numbers of vacation and permanent homes being built in urban interface areas.

Most forest and urban areas in Washington are served by two types of fire protection agencies.  These differ in their response time, equipment, and fire-fighting strategies:

  1. City or rural fire departments funded by taxes paid by property owners to protect structures.
  2. Wildland government agencies with fire-fighters trained to control wildland fires, with a lower priority to protect structures.

As more homes are built in the urban interface, increasing demands are placed on both types of fire protection agencies.

People living in an urban interface region must understand three essential facts:

  1. The presence of their home is a threat to wildlands.  People can cause fire.
  2. Even one home that is improperly protected from fire can put an entire community or wildland at risk.
  3. Living near wildlands involves fire risks not found in urban areas.

People living in urban interface areas have a responsibility to their neighbors and to the environment to protect their home.    Home-owners must choose a fire-safe location, design and build fire-safe structures, and protect their surrounding property by practicing fire-safe landscaping and home maintenance.  The term defensible space is used to describe the property surrounding a home that has been made fire-safe.

Learn more about homes and fire safety at the Internet site:  www.firewise.org.


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