Lecture 9: Wildfires
• Wildfire dates to the time when trees first evolved 350 million years ago.
• Many fires start naturally as a result of lightning or volcanic eruptions
• After a fire, vegetation completes a cycle from early colonizing plants to mature ecosystem.
• The ecosystem that evolves adapts to the climate at that particular location and particular time
Adaptation to Wildfires
• Many species have evolved to withstand fire or promote the life of the species after a fire event.
o oak and redwood trees have bark that resists fire damage
o some pine trees have seeds that only open after a fire
Wildfires Through History
• The geologic record shows an increase in the amount of charcoal in sediment beginning
approximately 10,000 years ago.
• This suggests high amounts of wildfire activity at the time. Why might there be more fire activity?
A warmer and or/ drier climate
increased use of fire by humans for clearing land and for heat, cooking, etc.
Elements of Wildfires
• Wildfire requires three elements: fuel, oxygen, and heat. If any of these are lost, the fire will dissipate
• Plants accumulate carbon dioxide and store carbon in their tissues.
• During a wildfire, this carbon dioxide is released back into the atmosphere
• There are 3 phases to a wildfire: preignition, combustion, and extinction.
o During this phase, vegetation reaches a temperature at which it can ignite.
o As vegetation is heated, it often loses water
o This is a chemical process describing the degradation of large hydrocarbon molecules into
o The process occurs in the presence of heat (i.e. from heat radiating off of nearby flames)
• The two processes of preheating and pyrolysis produce the first fuel gases that ignite.
• The combustion phase begins with ignition
• Ignition is not a single process, but occurs repeatedly as the fire moves.
• Not all ignitions will result in a wildfire (the vegetation must be dry)
Types of Combustion
• Flaming combustion is the rapid, high temperature conversion of fuel into
• It is characterized by flames and large amounts of unburned material.
• Smouldering combustion occurs in areas with burned material and ash
that covers new fuel
Transfers of Heat
• As a wildfire moves across the land, • three processes control the transfer of heat:
• Conduction: transfer of heat by molecule to molecule
• Radiation: transfer of heat in the form of invisible waves
• Convection: transfer of heat by mvmt of a liquid or a gas
Transfers of Heat by Wildfires
• In wildfires, heat transfer is mainly by radiation and
• Heat from radiation increases the surface temperature of the
• As air is heated, it becomes less dense and rises
• The rising air removes heat from the zone of flaming and is replaced by fresh air.
• The fresh air (oxygen) sustains the combustion
• This is the point at which combustion ceases.
• There is no longer sufficient heat or fuel to sustain a fire.
• Types of fuel include leaves, woody debris, decaying organic material, grasses, shrubs, etc.
• If diseases or storms down large number of trees, the decaying material dries and burns easily.
• The density of the forest plays a role:
• In western North America, dense boreal forests contain abundant fuel supply
• The fuel content can vary by slope orientation.
• In the Northern Hemisphere, southfacing slopes are relatively warm and dry.
• Slopes exposed to prevailing winds are often drier
• Wildfires burning on steep slopes preheat fuel upslope from the flames.
• Large wildfires are common following droughts.
• ‘Dry thunderstorms’ with lightning can produce wildfires but the rain evaporates before reaching the
• Wind can help preheat unburned materials
• Wind carries embers that can ignite spot fires ahead of the front
Types of Fires
• Wildfires are classified according to the layer of fuel that is allowing the fire to spread: surface or
• Surface fires travel close to the ground and burn shrubs, leaves, twigs, grass
• They vary in intensity but most move relatively slowly.
• Crown fires move rapidly through the forest canopy by flaming combustion.
• They can be fed by surface fires that move up limbs on tree trunks, or they may spread independently
of surface fires
• They are driven by strong winds and are common in boreal forests. Crown Fires
• Intermittent crown fires consume the tops of some trees in an area.
• Continuous crown fires consume the tops of all trees
Regions at Risk
• In Canada, the hazard is greatest in British Columbia and in the boreal forests of the Canadian Shield