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

Burned Area

Direction of spread

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 stand-replacing 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