Fire regimes and the World’s
biomes
23 September 2010
What is a fire regime?
• Characterization of the features of the
historic (“natural”) fires for a particular
ecosystem or ecoregion.
Fire Regime Components:
Examples…?
•
•
•
•
•
•
Extent (size)
Frequency
Seasonality
Duration
Intensity
Severity
Magnitude
Extent
• Range of burned area
• Can influence seed dispersal from
neighboring systems.
• Barriers to spread?
Fire Frequency – Data sources
• Dendrochronology (tree scars from non-lethal fires,
cross-dating)
• Paleoecology (charcoal in lake & soil sediments; wounds
on tree boles)
• Data bases: reports, aerial photos, maps, atlases,
satellite images, remote sensing
Fire frequency:
Fire return interval
• The number of fire events at a point or
within a specific area.
• Short fire return intervals
– < 25 years
• Intermediate fire return intervals
– 25-100 years
• Long fire return intervals
– >100 years
Seasonality
• How likely fire might occur during
each time of year
• Affects plant survival and flowering
Duration
• The length of time a fire burns within a
particular area
• For example…
– Fast-moving prairie fires
– Slow moving ground fires
• Important effect on intensity and severity…
Intensity
Direction of
spread
Burned
Area
One
Foot
Flame
depth
Severity
• Effect of fire on the ecosystem
• Some measures of severity…
–
–
–
–
–
–
% of organic biomass consumed by fire
% soil organic matter consumed
Mortality of plants and animals
Depth of heat penetration into the soil
Change in color of ash and soil
Description of fire behavior (surface, ground,
crown)
• Most common measure of severity
– Mortality in overstory vegetation
Fire classification:
severity and frequency (return interval)
• Short fire return intervals (<20 years)
– Low-severity surface fires are common .
– Fire tolerant herbaceous species or shrubs dominate.
– Species composition often similar.
• Intermediate fire return intervals (20-75 years)
– Fuel buildup and continuous in distribution
– Moderate-severity fires (patchy crown fires) or some high-severity fires
– Greater changes in plant composition
• Long fire return intervals (>100 years)
– Very high fuel loads possible
– High-severity fire commonly occur (stand-replacing crown fires)
– Postfire & prefire vegetation can be very different
• Mixed-severity fires
– Combination of frequent low-severity & infrequent high severity fires
How do historical (pre-settlement) and modern
(current) fire regimes differ – and why?
• In ecosystems with high frequency, low intensity
fire regimes (e.g., dry forests, grasslands,
woodlands, savannas), significant changes due to:
– Land use change (agriculture, urban)
– Fire suppression and fuel accumulation
– Change in vegetation type and structure
• Forests with low frequency, high severity standreplacing fire regimes:
– Much less change from historical fire regimes – Why?
Fire Regime Exercise
• Brief description of several major
ecosystems in the world
• TTYP: describe the fire regime for each
ecosystem
Northern boreal forests
• Boreal forests and
tundra
• Long winters, short
summers
• Lots of biomass
Mediterranean shrublands
• Generally
shrublands, also
trees and grass
• Wet, mild winters /
hot, dry summers
• Fire adapted biota
Tropical rainforests
• High biomass
• Ever-wet with
occasional droughts
Tropical savannas (Africa)
• Cyclic wet/ dry
periods
• Grazing x fire
interactions
• Shifting grasslands
– savannas –
forests
Brazilian cerrado
• Fire-adapted grasses & shrubs
• Dry winters
• Summer rains
Ponderosa pine – Douglas fir
• Mountains in the West/Southwest
• Dry summers, but also cooler and moister at high
elevations
• Without fire, D-fir establishes in understory of
Ponderosa pine