Agriculture Reference
In-Depth Information
density affects airflow and heat propagation in ways that can have diverse effects
on fire behavior. Alternatively, widely spaced canopy branches may inhibit fire
spread relative to more densely packed canopies; for example, the dense canopies
of many
Pinus
make them more susceptible to high-intensity burning compared
with the open spreading canopies of many
Eucalyptus
species.
One of the key factors affecting both ignition and combustion is canopy
porosity, measured as the canopy volume/leaf-and-stem volume (Rundel
et al.
1980
). High canopy porosity allows fine fuels to react more rapidly to changes in
relative humidity and also increases oxygen flow around the fuel. Although high
canopy porosity increases flammability, it leads to lower bulk density (mass/
volume) and fuel loading (mass/area), reducing the total energy available for
combustion. For example, the high canopy porosity of the needle-leaved chaparral
shrub
Adenostoma fasciculatum
increases flammability under a wide range of
conditions, whereas the lower canopy porosity, but higher bulk density, of the
associated shrub
Quercus berberidifolia
limits the range of conditions suitable for
burning, although under the severest conditions
Quercus
fuels should be expected
to generate higher fire intensities. Chemical composition of fuels may likewise
affect flammability by volatilizing at lower temperatures, and combustion of these
gases contributes to heating of structural fuels (Pyne
et al.
1996
). However, the
chemical make-up of foliage, and the potential heat energy content, exhibits far less
variation between species than structural characteristics (e.g. Dimitrakopoulos &
Panov
2001
), and the latter may contribute more to different patterns of fire
behavior (Dimitrakopoulos
2001
).
As elaborated in
Chapter 9
, some species possess structural characteristics that
make them more flammable than others and this may have adaptive significance
(Bond & Midgely
1995
). For example, retention of dead branches contributes to
spreading fire into canopies of some trees and shrubs. When this trait is coupled
with fire-dependent regeneration it may be viewed as an adaptive trait, as in
serotinous pines (Keeley & Zedler
1998
), and when supported by phylogenetic
analysis it may be interpreted as an adaptation selected for by fire (Schwilk &
Ackerly
2001
); see
Chapter 9
.
Antecedent Climate
Climate in the months or even years prior to a fire affects fire behavior by
(1) altering fuel moisture and (2) altering fuel loads. Fuel moisture is particularly
critical because it has a major effect on combustion and is often a major determin-
ant of fire spread and fuel consumption (e.g. Bilgili & Saglam
2003
; Anderson &
Anderson
2010
). Factors affecting fuel moisture vary between dead and live fuels.
Dead fuel moisture is largely affected by weather and fuel thickness, with smaller
fuels requiring less time under low humidity and high temperatures to dry suffi-
ciently for combustion. Live fuel moisture is affected not just by climate but by
access to soil moisture, which is determined by rooting depth and water use
