Fire Regimes

1. Evidence of past fires

Evidence of historic forest fires may be obtained by examination of:

• fossil charcoal
• charcoal in aquatic sediments
• charcoal layers in soil horizons
• forest stand structure
• historical records
• burned stumps
• photographs

2. Determination of patterns in time and space of past fire occurrence

Definitive historical fire frequencies can be determined by:

• analyzing annually laminated lake and bog sediment
• analyzing stand age class distributions over the landscape
• examining written records
• the age structure of fire-initiated forests
• dendiochronological analysis of fire-scarred living trees or dead snags

3. Fire Regimes

Fire types in forests can be divided into three categories (Brown and Davis 1973):

• ground fire: fires which consume the organic matter beneath the surface litter of the forest floor, spreading within rather than on top of the organic mantle
• surface fire: fires which burn the surficial litter and debris of the forest floor as well as low vegetation; fire behaviour is variable depending on conditions, may sometimes reach into the tree crowns
• crown fire: fires which travel through the top layers of trees or shrubs, more or less independent of surface fires.

Classification of fire regimes

These three basic types of fire can occur singly or in combination, both in space and time. Consideration of the:

• type and intensity of fire
• fire size
• and the fire frequency or return interval

has lead to the definition of seven distinct fire regimes (Heinselman 1981):

0 - No natural fire (or very little)

1 - Infrequent light surface fires (more than 25 year return intervals)

2 - Frequent light surface fires (1 to 25 year return intervals)

3 - Infrequent, severe surface fires (more than 25 year return intervals)

4 - Short return interval crown fires and severe surface fires in combination (25 to 100 year return intervals)

5 - Long return interval crown fires and surface fires in combination (100 to 300 year return intervals)

6 - Very long return interval crown fires and severe surface fires in combination (over 300 year return intervals)

Ecosystems can be placed into four general categories which relate to the presence or absence of fire and its influence:

Fire independent ecosystems

These are environments which are virtually free from fire. The species possess no adaptations to fire and when fire occurs the effects are dramatic and long lasting and recovery is slow. These ecosystems include tropical and sub-tropical forests; temperate zone marshes and swamps; some floodplain forests; coastal dunes; open desert; alpine fellfields; and subarctic bogs, barrens, and heaths.

Fire dependent ecosystems

Fire is common in these systems and fuel conditions are conducive to fire spread. The plant species are adopted to fire and require it for their survival and continuance. Included here are tropical and subtropical grasslands, savannas, and dry forests; temperate grasslands, savannas, chapparal, and shrublands; some deciduous forests (aspens, oaks) and coniferous forests (some pines). Post-fire recovery is immediate and fire exclusion is unnatural.

Fire-initiated ecosystems

Fire is infrequent and catastrophic - it simultaneously terminates and initiates long-lived species. These types are common in temperate and boreal regions and include some pioneer species which are shade intolerant, require mineral soil for germination, and minimum competition for survival. These die out to be replaced by other species if the fire-free interval is too long (typically more than several hundreds of years). Examples are western white pine, western hemlock, western redcedar, and coastal Douglas-fir. Initial revegetation is rapid but the post-fire recovery period is lengthy -up to hundreds of years.

Fire maintained ecosystems

Fire is frequent (annually to decadal), usually as surface fires, intensity is light, and crown fires are uncommon. Fire serves to decrease fuel buildups and control plant succession, often this means keeping out invading species. Fire favours the faster growing trees with higher branches and thick bark while the fire intolerant species are selected out. These ecosystems include deciduous and dry evergreen monsoon forests; other tropical, subtropical, and temperate savannas. Examples from North America are redwood, sequoia, ponderosa pine, longleaf, and semiarid oak savannas in the southeast. The exclusion of fire from these types leads to fuel buildups and vegetative change - fire intolerant species become more abundant and woody species invade grasslands.

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