Lecture 10 - Wildfires Wildfire dates to the time when...

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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.
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Wildfires 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.
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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
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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.
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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: pre-ignition, combustion, and extinction.
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Pre-Ignition Phase Pre-heating During this phase, vegetation reaches a temperature at which it can ignite. As vegetation is heated, it often loses water. Pyrolysis This is a chemical process describing the degradation of large hydrocarbon molecules into smaller ones. The process occurs in the presence of heat ie: from heat radiating off of nearby flames
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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).
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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 that covers new fuel.
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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 Radiation Transfer of heat in the form of invisible waves Convection Transfer of heat by movement of a liquid or gas
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