Agriculture Reference
In-Depth Information
highest density of fires occur in June whereas the greatest area burned usually
occurs in October or November (Keeley & Fotheringham
2003a
).
Seasonality in some fire-prone landscapes is not annual but rather arises from
decadal-scale droughts that create high fire hazard in some years but not in others.
These aseasonal climates have on average significant rainfall each season, but in
anomalous years there may be an extended drought conducive to widespread
fires (Vines
1974
). Although lacking a predictable annual drought as is present
in MTC regions, these aseasonal climates have a predictable drought associated
with El Nin˜ o/Southern Oscillation (ENSO) events that occur once or twice a
decade and create a major fire risk (Verdon
et al.
2004
).
One of the potentially important impacts of humans is to provide ignition
sources during seasons when lightning ignitions are unlikely. The impact of out-
of-season fires is only beginning to be studied but is potentially very important. In
western coniferous forests it affects the types of fuels consumed (Knapp
et al.
2005
). Out-of-season winter burning has potential safety advantages for control-
ling fires in these highly hazardous fuels (Keeley
2002a
). However, there are
noteworthy examples of unexpectedly poor regeneration following such out-of-
season burns (Keeley
2006b
). Seasonality effects on postfire recovery are evident
in most MTC ecosystems (Papanastasis
1980
; Bond
1984
; Hobbs & Atkins
1990
).
In MTC regions one impact of out-of-season burning is that it cuts short the
postfire growing season; for example, natural summer or autumn fires are
followed by an approximately six month growing season for resprout growth
and seedling recruitment, whereas winter burns may cut this growing season in
half. Out-of-season spring fires also may have negative consequences on the fauna,
due to disruptions of nesting season, an impact not experienced during normal
summer and autumn fires.
Emergent Properties of Fire Regimes
Fire regimes are not just the sum total of the five factors described above, but
an emergent property of these and other biotic and abiotic ecosystem attributes
(Gill & Bradstock
2003
). As a result, each site has its own unique fire regime;
however, broadly speaking there are three categories worth recognizing for the
purposes of contrasting ecosystems:
surface
fire,
crown
fire and
mixed
(surface and
crown) fire regimes. Although there are multiple parameters that define a fire
regime, two factors that play key roles are site productivity and disturbance
frequency (Keeley & Zedler
1998
; Pausas & Bradstock
2007
). These two factors
play key roles in circumscribing fire regimes (
Fig. 2.7
) and lead to distinct patterns
of fuel consumption and fire spread. For example surface fire regimes are usually
constrained to generating low-intensity fires whereas crown fire regimes, due to
the higher fuel loads being consumed, generate much higher intensity fires. These
in turn both affect and are affected by fire frequency, which in turn may be
constrained by seasonality. Generally surface fires are smaller and patchier than
