Post on 20-Jul-2020
transcript
Fire Intensity & Scale
Carl N. SkinnerProject Leader
USDA Forest Service
Pacific Southwest Research Station
Redding, CA
PSW
Intensity vs. Severity
• Intensity = A measure of the rate of heat (energy) released by a fire.
Fire intensity is directly proportional to the fuel condition, amount and rate of fuel consumed. Thus, fuels, weather, and topography are very important in determining the rate of heat released by a fire.
• Severity = Degree to which a site has been altered or changed by fire.
Loosely, a product of fire intensity, vegetation susceptibility, and residence time.
Young Trees Under Old
Only Young Trees
Fire with <4 ft flames in both stands.
Fire severity class = Low
Fire severity class = High
Young trees killed by scorch in both stands.
Severity vs. Intensity
Fire Severity Distribution
Sources: Weatherspoon & Skinner 1995, Taylor & Skinner 1998, USDA Forest Service 2003
Low
High
Mod
Fire
Severity
Patterns
~Hancock
Fire
2006
Fire
Severity
Patterns
~Biscuit Fire
2002
Fire & Climate Change:
Where are we headed?
• Fire season getting longer.
• Fuels keep growing.
• Greater probability of intense fires.
Fuel
Fire
Triangle
Mediterranean Climate
• Cool/wet Wintersvegetation/fuel
• Warm/dry Summerannual fire season
• Lightning
Fire Climate - Precipitation PatternsFrom: Schroeder & Buck 1970, p. 203.
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F
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A
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Inches
Prescott, AZ - Avg. 20.7"
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F
M
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A
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Inches
Toronto, Canada - Avg. 32.1
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M
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M
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A
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Inches
North Platte, NB - Avg. 18.2"
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Inches
Sonora, CA - Avg. 32.2"
20th Century Fire Trends
Sierra Nevada
Area Burned
0
100
200
300
400
500
600
700
800
1910-51 1952-93
Area
1000s of Acres
Weatherspoon & Skinner 1996
20th Century Fire Trends
Sierra Nevada
Area Burned
0
100
200
300
400
500
600
700
800
1910-51 1952-93
Lightning
Human
1000s of Acres
Weatherspoon & Skinner 1996
20th Century Fire Trends
Sierra Nevada
Max Fire Size
0
10
20
30
40
50
60
1910-51 1952-93
Area
1000s of Acres
Weatherspoon & Skinner 1996
20th Century Fire Trends
Sierra Nevada
Max Fire Size
0
10
20
30
40
50
60
70
80
1910-51 1952-93
Lightning
Human
1000s of Acres
Weatherspoon & Skinner 1996
Red dots =
Lightning Caused Fires
Blue dots =
Human Caused Fires
From: Jones et al. 2001 Science 292: 662-667
Much colder than 20th or 21st Century.
Tree-ring Based Fire Histories
Mostly cover the period of
~1600 to 1900
Climate Change & Fire
Regimes?
• TemperatureFire Frequency
• MoistureFire Extent
Climate Variation?
Swetnam 1993
Chang 1999
Miller 2003
Climate Change & Fire?
Hasn’t Climate Warmed Before?
Never
when accompanied by
50-100 Yrs of
fire suppression!
Restoration???
Resilience
“…the capacity of a system to absorb
disturbance and reorganize while
undergoing change so as to retain
essentially the same function,
structure, identity, and feedbacks…”
Folke, C. and others 2004. Regime shifts, resilience, and
biodiversity in ecosystem management. Annual Review of
Ecology, Evolution, and Systematics 35: 557-581.
Resilience
Forested systems characterized by fire
regimes of frequent, mostly low-
moderate intensity fires, dominated
by large, long-lived trees, are
considered resilient if the forested
landscape exhibits a generally
forested condition, including larger
trees, shortly following a disturbing
event such as fire.
Hancock Fire 2006
Biscuit Fire 2002
Scale of Effects
Fire:
A Catalyst for
Change
Severity Patterns – Alternative Pathways
Stanislaus NF - 1929
“The virgin forest is uneven-aged, or at best even-aged by small groups,
and is patchy and broken; hence it is fairly immune from extensive
devastating crown fires.”
“fire creates a patchy scattered distribution of reproduction”
(Show and Kotok 1924)
Climate Change…
Where are we headed?
• Fire season getting longer.
• Fuels keep growing.
• Greater probability of intense fires.
Fuel
Fire
Triangle
Thank You!