Agriculture Reference
In-Depth Information
characteristics. Deeply rooted plants are likely to have greater access to soil mois-
ture far longer into the dry season than shallow-rooted plants and this can affect
flammability (Green
1981
). Fuel moisture can also vary with patterns of stomatal
control and water use (e.g. Jacobsen
et al
.
2008
). Even on the same sites there may be
substantial variation in fuel moisture of particular species (Weise
et al.
1998
). Since
live stems can withstand lower moisture levels than leaves, overall plant fuel
moisture will be affected by structural characteristics such as the leaf to stem ratio.
In MTC regions such as the eastern Mediterranean Basin or California where the
summer droughts are particularly severe (see
Box 1.1
), the live fuel moisture of many
shrubs in most years is at the lowest level of tolerance (
60%) during late summer
and autumn. However, in certain years late spring rains can reduce the length of the
fire season through effects on live fuel moisture in summer and autumn (Dennison
et al.
2008
). In the MTC Cape region of South Africa and in southwestern Australia,
the summer drought on average is not as dry as in California and thus live fuel
moisture levels are generally higher (van Wilgen
et al.
1990b
).
In addition to affecting fuel moisture, antecedent climate plays a key role in
producing short-term changes in fuel loads. For example, in open forests with
surface fire regimes driven by herbaceous understory, high-rainfall years will
increase these fuels and lead to increased burning in subsequent years (Westerling
et al.
2002
; Pausas
2004
). The potential for short-term changes in fuels is largely a
function of fuel structure. For example, in California conifer forests the lower-
elevation ponderosa pine community typically has fires driven by herbaceous fuels
whereas the higher-elevation and more closed-canopy mixed conifer forest has
surface fires that feed on dead leaves and branches and other forest floor litter. In
the ponderosa forest historical fire scar records show significantly elevated fire
activity 1 to 2 yrs following high-precipitation years (
Fig. 2.8b
) (Swetnam &
Betancourt
1998
). However, the mixed conifer forests lack herbaceous surface
fuels and do not exhibit such a lag effect (
Fig. 2.8a
). Similar patterns have been
observed in Australian semi-arid landscapes (Bradstock & Cohn
2002a
).
In shrubland crown fire regimes, drought can alter dead fuel loads and this
plays a key role because the dead fuels dry rapidly and combust readily, whereas
live fuels absorb heat and tend to suppress fire. Thus, fire behavior is markedly
affected by the ratio of dead/live fuel. Although it is widely accepted that this ratio
increases with stand age (Baeza
et al.
2002
; Regelbrugge & Conard
2002
), less well
appreciated is the extent of short-term changes in dead fuels due to drought. In
assessing the contribution of dead fuels on fire behavior this is generally considered
to be captured by stand age, which is how it is commonly factored into fire danger
indices. However, this has limitations in shrublands because the rate of dead fuel
accumulation is often not a linear response to age (Keeley & Fotheringham
2003a
;
Baeza
et al
. 2011) and can experience short-term changes due to drought-caused
dieback (Keeley & Zedler
2009
). Altering fire danger indices by including fuel
inventories (Woodall
et al.
2005
) are an improvement, but to truly capture periodic
drought-induced changes in dead fuels we ultimately will need to develop remote
imagery that can capture the dead to live signal
in shrublands. There are
