Agriculture Reference
In-Depth Information
with deep peat layers such fires may smolder for months until extinguished by
rainfall. In forests, such fires play a potential role in collecting ignitions at one
point in time, and later irrupting into surface or crown fires when the weather
changes.
Some ecosystems are characterized by either surface fires or crown fires, but in
many systems mixtures of both fire type occur. The proportion of landscape
burning in one or the other fire type is a function of the time since last burning,
rate of fuel accumulation, antecedent drought and severity of fire weather. Some-
times a single fire will comprise a mixture of surface and crown fires and has been
termed
stand-thinning
fires (Keeley & Zedler
1998
). Some ecosystems experience a
temporal mix of surface fires alternating over time with high-intensity crown fires
(Zimmerman & Omi
1998
). These mixed fire regimes may perform important
ecosystem functions by creating landscape mosaics critical for plant regeneration
and animal habitats as discussed in
Chapter 3
.
Fire Intensity and Severity
Fire intensity describes the physical combustion process of energy release from
organic matter. In physical terms it is the energy per unit volume multiplied by the
velocity at which the energy is moving, measured as watt m
2
. This represents the
reaction intensity in Rothermel's (
1972
) fire spread models, which forms the basis
for most fire behavior models. However, in fire science the term fire intensity often
takes on other meanings.
One example is Byram's (
1959
) fireline intensity, which is the rate of heat
transfer per unit length of the fireline:
I
ΒΌ
HWR
;
where
H
equals heat of combustion (kJ kg
1
of fuel),
W
is consumed fuel (kg m
2
),
and
R
is the rate of fire spread (m s
1
), giving a fireline intensity (
I
)inkWm
1
.
This is the radiant energy release in the flaming front and is a good measure of fire
propagation, which is critical for fire suppression activities and has been incorpor-
ated into fire danger rating calculations (Hirsch & Martell
1996
; Weber
2001
).
It can vary from 1000 kW m
1
in forests with understory burning of surface fuels
to 20 000 kW m
1
in shrublands with active crown fires (Agee
1993
; van Wilgen
et al.
1985
).
Direct measurement of fireline intensity requires that one distinguish fuels
consumed by the flaming front from the total fuel consumption. However, prac-
tical measures of fuel consumption are based on the difference between prefire and
postfire fuel inventories, and this inflates estimates of fireline intensity (Alexander
1982
; Scott & Reinhardt
2001
). As a result flame length is used as a surrogate for
fireline intensity since there is a significant relationship between these parameters
in forest and shrubland ecosystems (van Wilgen
1986
; Burrows
1995
; Fernandes
et al
.
2000
). However, in vegetation with a mixture of fine fuels and woody fuels or
