Agriculture Reference
In-Depth Information
However, some chaparral wilderness areas have been cut off from the heavy load
of anthropogenic ignitions and there may be some resource benefit to adding fire.
One way this could be accomplished is illustrated by the 2007 Zaca Fire, which
burned over 94 000 ha (Keeley
et al.
2009b
), with about 38 000 ha due to a single
backing fire ignited by fire crews (Vives & Boxall
2009
). Although there are
perhaps some resource benefits to this action, this amount of prescription burning
represents a significant resource challenge. Fires in this vegetation are largely
ignition limited and unless there is a substantial reduction in human ignitions,
there is a high probability that a significant portion of this landscape will reburn
within the next two decades, thus posing a major immaturity risk for some taxa.
Other regions have not embraced the notion of restoring historical fire
regimes but rather designed management strategies using direct evidence of fire
regimes required to support target species. One is to determine prescription burn
intervals based on time to flowering maturity (Burrows & Friend
1998
) or time to
produce a viable seedbank (Keith
1996
; Menges
2007
). Another approach may
focus on maximizing the density of target populations or on minimizing the risk of
extirpation of species in the system (Bradstock
et al.
1995
,
1998a, 1998c
; Gill &
Bradstock
1995
; Keith
1996
). Still another is to use prescription burn intervals that
are compatible with the widest range of flora and fauna thresholds of tolerance,
with the expectation that this will lead to maximum biodiversity (Gill &McCarthy
1998
). Sometimes this will require including a range of fire regime parameters
beyond just fire frequency (van Wilgen
et al.
1994
). Critical to these approaches is
the recognition of a need for regional approaches since interactions between fire
and environmental heterogeneity mean that prescriptions for some sites may not be
suitable for others (Williams
et al.
1994
; Bradstock & Kenny
2003
; Burrows
2008
).
There is some evidence that regional and spatial variability in application of
different prescription regimes will maximize regional diversity.
Biodiversity is just one of a number of resource costs that can be associated with
fuel treatments such as prescription burning. For example, in kwongan heathlands
of southwestern Australia, the conflict between reducing fire hazard and main-
taining sustainable resources has a somewhat different character. Here protection
of agricultural lands from fire spread from wildlands is thought to be enhanced by
frequent burning of surrounding heathlands. However, these wildlands are a rich
nectar source for the apicultural industry and it is believed that frequent burning is
detrimental to optimum honey production (Bell
et al.
1984
).
Other ecosystem processes that may be affected by frequent fires include soil
nutrient depletion. Fire increases losses both through volatilization as well as
greater postfire soil leaching due to reduced plant cover (DeBano & Rice
1971
).
Such treatments may diminish ecosystem nutrients and alter the competitive
balance in ways that affect community assembly. Frequent burning may also
affect forest structure. For example, in jarrah forests of Western Australia 25 years
of frequent prescription burning reduced soil nutrients, but the thinning of
understory trees apparently reduced competition for water and increased tree
diameter growth (Burrows
et al.
2010
).
