Introduction to Fire Ecology

Introduction to Fire Ecology

• Consider the statement: “Fire is bad”–What do you think? Why?– Can you think of examples of when fire

is good and bad? What are they?

• Fire is a natural component to many ecosystems– Both plants and animals – Interactions among organisms

What is Fire Ecology?• A branch of ecology that studies the

origins of wildland fire and its relationship to the biotic and abiotic environment.

• Fire is studied as a natural process operating as a component of an ecosystem.

• Fire, similar to floods, earthquakes, and storms, is just one way of promoting change in an ecosystem.

• Fire ecologists study: – the origins of fire– what influences the spread and

intensity– fire’s relationship with ecosystems– how controlled fires can be used to

maintain ecosystem’s health

Physical and Chemical Properties of Fire

• What does it take to ignite a fire?– Oxygen at ignition point or kindling

temperature• Woods has to reach 800◦F to burst into

flames– As wood is heated, it is dried of water while

oil and resins are boiled away– Chemical structure of wood is broken down

and flammable gases are produced • Flaming combustion

Physical and Chemical Properties of Fire

• Flaming combustion causes the surface of the wood to become charcoal

• At cooler temps, glowing combustion consumes 3 molecules– Charcoal, water, carbon dioxide

• What influences the probability of ignition and combustion?– Fuel–Weather– Topography– Fire history

Forest Fuel Layers

Fire Behavior• Mostly described by its intensity and

the area it spreads to

Factors influencing intensity• Available fuel– Quantified by size and arrangement–More fuel, more intense

Factors influencing intensity• Moisture and temperature– Cool, moist fuels combust more slowly

than hot, dry fuels

Factors influencing intensity• Fuel composition– Can make fire more or less intense– Oils and resins increase heat yield of

reaction and burn more intensely– High concentrations of minerals can

reduce flammability

Factors influencing intensity• Wind– Increases oxygen supply– Can produce “spot” fires

Factors influencing intensity• Topography– If fire at top of slope, fire will burn slowly

downhill– If fire at bottom of slope, fire will burn

quickly uphill

• Factors influencing spread– Fires in dry, windy conditions with ample

fuel spread rapidly– Fuel continuity– Topography• Rivers and lakes can act as fire breaks

– Composition of plant community• Some plants are more flammable than

others

Effects of Fire on Ecosystems

1. Mosaic Patterns– Burn Mosaic• Wildfires create areas of total burn,

partially burned, and unburned sections– Result from factors like wind shifts,

chemical composition, temperature changes

– Varied regrowth rates of vegetation

2. Soil Conditions– Fires can be both good and bad for soil– Nutrient levels can increase due to:• Mineral content of ash and charcoal• Warm, moist conditions which increase

microbial activity– Intense heat can cause soil to become

water repellant• Causes run off and erosion

3. Animal Populations– Some will perish during fire especially smaller,

older, weaker animals and insects– Larger animals can leave area– Populations tend to suffer more after fires due to

loss of food source• Most fires tend to be before winter

– Predators have advantage since groundcover is gone

– Scavengers find increase in food– Some animals, like deer, will eat nutrient rich

charcoal– Birds benefit

• Increase in seed availability• Nesting sites increase due to snags (dead standing trees)

4. Plant Populations– Vegetation composition determines how fire

behaves• Fire behavior in turn determines how plant

population is affected– Fire decreases competition to survivors– Increases nutrient, water, and light availability– May rid plants of their parasites (e.g. mistletoe)– Plant adaptations allow to survive and

reproduce• Sequoia has thick bark to protect• Chaparral shrub needs fire to open seeds

Thick bark of Sequoia tree

Chaparral seeds need fire to be released

Fire Regimes• Regimes are patterns in wildland fires

that include frequency, extent, intensity, type, and season

• Varies among ecosystems due to their composition and structure– Influenced by climate, vegetation types,

and ignition source

• Ecologists will study evidence to determine historical fire records or natural fire regimes– Sample fire scars on trees

• Sequence of fires on growth rings– Sample sediments from lakes and

reservoirs– Current weather–Written and oral histories– Fuel build up– Lightning fires

• Based of those factors and the ecosystem, ecologists will use the most appropriate management regime.

• Various management techniques will be used. One technique does not fit all habitat types.

Human influence on wildland fires

• Humans have one of the greatest influences on ecosystems

• Humans have altered natural fire regimes– Native Americans and early settlers

used fires in their management practices

– Today will use fire to clear lands for housing, agriculture, and roads

Prescribed Burning • Fire Suppression– One of the fires management practices– 1930’s researchers began to challenge

policy• Possible benefits to ecosystems

– 1950s and 1960s the national forests began to experiment with prescribed burning• Prescribed burning = starting fire on

purpose

Prescribed Burning (continued)

• Prescribed burning– Restore natural fire regime in area

where suppression had been practiced–When reintroduced to ecosystem, takes

time for ecosystem to adjust– Use low intensity fires– In areas where there is potential for high

intensity fires (fuel and wind), this practice is not has useful

What effects does Fire have on an ecosystem?

• Depends on its intensity• Hotter, longer lasting fires have more

extensive effects than cooler, short-lived fires

ProsReturn nutrient to the soilEncourage plant diversity

Prevent insect attacksBurned areas provides snags for

wildlife

ConsSoil erosion

Food for wildlife, humans destroyedKills trees, vegetation, wildlife

Very hot fires remove all roots, seeds, and soil organisms for tree growth