II.H._FIRE
(Transition from Tree Autecology to Forest Synecology)
Outline:
Causes of Fire
Types of Fire and Nomenclature
Ecological Effects of Fire
Effects on Soil
Effects on Plants
Fire Adaptations
I. Causes of fire
A. Natural – volcanism, spontaneous combustion, and lightning (e.g. 10,000
lightning-caused fires in the U.S. per year)
Key questions:
1. How do numbers of lightning-ignitions and area burned vary from year to year
in relation to broad-scale climate mechanisms? At annual, decadal and centennial
scales?
2. Is the fire regime lightning-saturated? Are there typically sufficient lightning
ignitions each year to burn all the area that can burn given fuel types and weather
conditions (i.e. desiccation of the fuel)? Or, is the lack of lightning a limiting factor?
B. Anthropogenic -- (importance recognized by C.O. Sauer and Omer Stewart)
Aboriginal use of fire for:
1.
2.
3.
4.
5.
Warfare
Religion
Hunting tool
Vegetation management (e.g. forage improvement)
Accidental
Key questions about anthropogenic ignitions:
1. How does the frequency of anthropogenic ignitions vary among biomes and
locally with variables such as elevation, precipitation, vegetation type,
distance from habitation or trails?
2. How is the total area burned per year altered by anthropogenic ignitions in
comparison to natural ignitions?
II.
Fire regime and types of fire
Fire regime includes the type (kind) of fire, frequency and timing (season) of fires,
and intensity (heat generated).
Fire severity is a fire effect used as substitute for intensity.
Three basic fire types:
A. Surface fires
B. Ground fires
C. Crown fires (a fire that burns in the canopy of live and dead foliage and
branches). Three types:
Active Crown Fire is a crown fire in which the entire fuel complex – all fuel strata
– become involved, but the crowning phase remains dependent on heat released
from the surface fuel strata for continued spread (also, a running crown fire or
continuous crown fire).
Passive Crown Fire is a crown fire in which individual or small groups of trees
torch out (candle), but solid flaming in the canopy fuels cannot be maintained
except for short periods.
Independent Crown Fire is a crown fire that spreads through the aerial fuel strata
without the aid of a supporting surface fire. This crown fire type rarely occurs and is often
mistaken for an active or passive crown fire with a surface fire component that is lagging
behind the crown fire component. The most common occurrences of independent crown
fire are early spring burns where active crowning occurs in dry aerial fuels over snow in
the understory.
D. Classification based on severity in woodland and forest environments:
1. Low-severity-- non-lethal to mature trees.
2. High-severity—lethal to mature trees (also called stand-replacing; mostly
crown fires, but could be severe surface or ground fires).
3. Variable severity—includes an ecologically significant component of both lowseverity and high-severity fires (also called mixed severity); variable severity
may refer to a single fire event or to different events over time.
III. Fire effects on the environment depend on intensity and spread
A. Fire intensity depends on:
1. Amount of fuel (grams per square meter); a function of productivity and
decomposition.
2. Heat yield (kcal per gram of fuel) as determined by fuel quality (chemical and physical
properties).
B. Potential for fire spread depends on fuel characteristics (horizontal
distribution, vertical distribution [ladders], density of fuels in canopy,
live vs. dead fine fuels, etc….forest type and structure.
C. Rate of spread (m/sec) is especially determined by weather and topography.
IV. Fire effects on soil
A. Effects on soil organic matter
combustion of organic matter (e.g. 85% lost at 250C)
decline in CEC
loss of organic matter reduces porosity, increases bulk density, etc.
B. Physical effects on soil:
1. decline in water holding capacity
2. possible changes in soil "wettability" (can translocate water repellant
chemicals towards greater depth)
C. Effects on soil pH and on soil microorganisms
Short-term increase in pH.
Generally fungal pathogens decline (due to lack of litter) and bacteria increase
(due to less acid conditions).
ADAPTATIONS TO FIRE
Two alternatives:
a. Survival of fire (e.g resistance)
b. Population recovery following fire
1. Post-fire germination (scarification of seed, ability to germinate and survive in open post-fire
sites)
2. Fire-resistant foliage (e.g. low resin content)
3. Thick fire-resistant bark
4. Evanescent lower branches
5. Sprouting behavior
latent buds on tree bole (epicormics)
root collar or stump sprouting
new shoots from lateral roots (suckering)
new shoots from lignotubers (partially buried swollen stem)
6. Cone serotiny
7. "Grass stage" of longleaf pine (Pinus palustris)