©2013 Delmar Cengage Learning. All Rights Reserved. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in

©2013 Delmar Cengage Learning. All Rights Reserved. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Chapter 7Chapter 7Fire and the ForestFire and the Forest

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Former 10


Chapter Highlights

Ways that fire is both beneficial and destructive

Three elements required for fire Surface, ground, and crown fires and fuels Major causes of destructive forest fires Effects of wildfires


Chapter Highlights (Continued)

Use of prescribed burns Methods of fire suppression Government policy regarding forest fires


Fire as Friend and Foe

Benefits of fire Cleans debris (plant material) from forest floor Controls brush and thins stand

Destructive fires result from buildup of fuel on forest floor May burn with such heat as to ignite foliage and

cause trunks to explode Second only to insects as cause of timber loss


Three Elements Essential to Fire Interaction of these elements

controls the rate and intensity of a fire: Availability and concentration

of flammable fuels Heat energy sufficient to raise

fuel to combustion temperature Adequate supply of oxygen

Change in any ingredient alters nature of fire


Types of Fuel That Cause Fires

Surface fuels: Undecayed dry plant materials and live plants on

soil surface Ground fuels:

Duff or litter composed of decaying plant material just below surface

Peat from bogs where decay is limited


Types of Fuel That Cause Fires(Continued)

Aerial fuels: More than 6 ft. above ground, upper foliage or

canopy This type of fire is called a crown fire


Fires Resulting from Different Fuels

Surface fire: moves quickly across surface, seldom intense Reduces risk of more serious fires

Ground fire: can smolder for many weeks, limited by amount of oxygen available

Crown fire: ignited by intense surface fire


Major Causes of Destructive Fires

Lightning: especially in drought conditions, aggravated by wind

Volcanic activity: rare in North America Human activities: campfires, internal

combustion engines, etc. Loss of control of planned (prescribed) burns Incendiaries: willfully set; more than one-

fourth of all U.S. fires each year


Effects of Wildfires on Forests

Massive damage due to size and unpredictability Most serious in dry regions in periods of drought

that follow periods of precipitation Fire intense enough to reach crown kills trees

Some wildlife may benefit as forage plants fill in areas of former tree coverage Endangered species at greater risk


Effects of Wildfires on Forests (Continued)

Serious damage to soils as organic matter is destroyed, increasing possible erosion

Water quality damaged by resulting ash and silt


Prescribed Burns Improve Forest Health

Best approach to dealing with wildfires is prevention Goal of prescribed burns is to eliminate fire

hazards Reduces surface litter, which can fuel more

destructive fires as it builds up


Prescribed Burns Improve Forest Health (Continued)

Prescribed burns can be used to cleanse forest Controls losses due to insects and disease However, it may weaken trees and increase

susceptibility to insects and disease


Policy of Short-Duration Fire Cycle

Forest Service policy in recent years: broader scale use of prescribed burns

Will take many years to return entire forest to a short-duration cycle because of excessive fuel buildup on many forest floors Safer to wait until after next harvest cycle before

applying this practice


Fire Suppression Policy of U.S. Forest Service

Congress first funded fire control in 1911, 1924

1935: intense fire suppression policy Impossible to implement for lack of equipment

and personnel Mid-1900s: fire priority approach

Suppression in high-value timber areas


Fire Suppression Policy of U.S. Forest Service (Continued)

Today: fire management Surface fires allowed to burn if consistent with

long-term objectives Cost of suppressing fires versus cost of letting

them burn


Effects of Wind on Fire

Wind provides oxygen to fire As wind direction changes, turbulence often

evident Can cause surface fire to become crown fire Increased intensity can create strong winds

Fire creates its own draft (supply of fresh air) Resulting firestorm may jump natural barriers and fire

lines (known as spotting)


Effects of Wind on Rate or Spread

Wind speed of 10 mph increases to 20 mph. How much does the increase in wind speed affect the rate at which the fire can be expected to spread? Calculate R for a wind speed of 10 mph:

1. R = 1022. R = 100


Effects of Wind on Rate or Spread(Continued)

The rate of spread at 10 mph has an index of 100 Calculate R for a wind speed of 20 mph:

1. R = 2022. R = 400

The rate of spread at 20 mph has an index of 400 or four times the rate of spread when compared to a wind speed of 10 mph. The rate of spread quadrupled when the wind speed doubled


Indirect Attack Method of Fire Suppression

Indirect attack: isolates fire from fuel supply Create firebreak or fire line in pathway of fire

Removal of fuel from a strip of terrain Set backfire to gain control of crown fire

A fire set along inside edge of firebreak to burn fuel back toward fire

Establishes a wide barrier May force fire to change direction


Indirect Attack Method of Fire Suppression (Continued)

Backfires remove fire fuel from an area, so when an advancing fire meets the burned out area, it will have nothing to feed on


Direct Fire Attack

Water and fire retardant chemicals are applied to fires from aircraft, trucks, and backpacks. It is a common practice to make airdrops of fire retardant chemicals


Direct Attack Method of Fire Suppression

Direct attack: employs water or fire retardant chemicals to suppress fire Lowers temperature of fire below kindling point Interrupts oxygen supply to fire

Applied from aircraft, trucks, backpacks


Direct Attack


Government and Industry Efforts to Prevent Forest Fires

Educating public about forest fire prevention with a themed campaign Keep America Green Smokey Bear

Providing educational materials to schools Signs, brochures, other media promotions Elimination of fire hazards in forests

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©2013 Delmar Cengage Learning. All Rights Reserved. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in