Geography 2152F/G Lecture 11: Geo 2152 Week 11 Lecture-Wildfires
14 Feb 2019
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Geo 2152 – Week 12 Lecture
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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 time.
Adaptation to Wildfires
• Many species have evolved to withstand fire or promote the life of the species after a fire event.
• Examples:
• Oak and redwood trees have bark that resists fire damage
• 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 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:
o pre-ignition, combustion, and extinction.
Pre-Ignition Phase
• Pre-heating
o During this phase, vegetation reaches a temperature at which it can ignite.
o Vegetation is heated, it often loses water
o The presence of heat radiating from flames of a wildfire can pre-heat nearby vegetation
• Pyrolysis
o This is a chemical process describing the degradation of large hydrocarbon molecules into smaller ones.
o The process occurs in the presence of heat (i.e. from heat radiating off of nearby flames.)
Combustion Phase
• The two processes of pre-heating and pyrolysis result in fuel that is prone to ignite.
• The combustion phase begins with ignition
• Ignition is not a single process; it can occur 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 heat.
• It is characterized by flames and large amounts of unburned material.
• Smoldering combustion occurs in areas with burned material and ash covers new fuel
o There is no more new fuel to burn
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 contact (solid object to solid object)
• Radiation: Transfer of heat in the form of invisible waves
Geo 2152 – Week 12 Lecture
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• Convection: Transfer of heat by movement of liquid or gas
Transfers of Heat by Wildfires
• In wildfires, heat transfer is mainly by radiation and convection.
• Heat from radiation increases the surface temperature of the fuel.
• 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.
• This flush air (Oxygen) is sustained by combustion
Extinction Phase
• This is the time when which combustion has ceased.
• There is no longer sufficient heat or fuel to sustain a fire.
Fuel
• 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 NA, dense boreal forests contain abundant fuel supplies
Topography
• The amount of fuel can vary by slope orientation.
• In the North Hemisphere, south facing slopes are relatively warm and dry (more risk – sun, dry) (faces afternoon
sun)
• Slopes exposed to prevailing winds are often drier (winds coming for the west – slopes facing west = stronger
winds)
• Wildfires burning on steep slopes preheat fuel upslope from the flames.
• This results in rapid spreading of fire upslope
• Southwest is overall the worst
Weather
• Large wildfires are common following droughts.
• ‘Dry thunderstorms’ with lightning can produce wildfires but the rain evaporates before reaching the ground.
• Wind can help preheat unburned materials.
• Wind carries embers that can ignite spot fires ahead of the fire front.
Regions at Risk
• In Canada, the hazard is greatest in British Columbia and in the boreal forests of the Canadian Shield region.
• The geographic region most at risk changes annually with the weather and corresponds to areas experiencing
drought.
Yellowstone National Park Wildfire
• A series of lightning strikes caused 50 fires in the park in 1988.
• Park officials have a policy that allows naturally caused fires to burn
• This became controversial as hot, dry weather that summer allowed the fires to spread and merge.
• Officials responded to political pressure and eventually called in nearly 10,000 firefighters
• The fires were beyond the control of the crews and burned for several months.
• It is believed that the fires became uncontrollable because many years of fire-suppression policies in the past
had allowed fuel amounts in the park to reach dangerous levels.
• The fires of 1988 revitalized ecosystems in the park.
• The officials remain committed to natural burn policy today; this is a common policy in national parks.
Document Summary
Some pine trees have seeds that only open after a fire. Adaptation to wildfires: many species have evolved to withstand fire or promote the life of the species after a fire event, oak and redwood trees have bark that resists fire damage. Wildfires through history: a warmer and/or drier climate. Elements of wildfires: wildfire requires three elements: fuel, oxygen and heat. If any of these are lost, the fire will dissipate: during a wildfire, this carbon dioxide is released back into the atmosphere. Increased use of fire by humans for clearing land and for heat, cooking, etc. Plants accumulate carbon dioxide and store carbon in their tissues. The geologic record shows an increase in the amount of charcoal in sediment approximately 10,000 years ago. This suggests high amounts of wildfire activity at the time. There are 3 phases to a wildfire: pre-ignition, combustion, and extinction.
