Agriculture Reference
In-Depth Information
spot fires ignite fine surface fuels, but in shrublands with dense canopies of fine
dead fuels it is possible to ignite spot fires in the canopy.
Another prefire treatment designed to reduce fuels is fuel breaks, which are
typically elongated areas of 50-1000 m in width in which woody fuels are cleared
or at least greatly reduced by prescription burning, mechanical treatment or
herbicides (see Fig. 12.3). Unlike prescription burns, which sometimes utilize fire
return intervals compatible with sustaining native flora and fauna, fuel breaks
oftentimes are designed to permanently alter fuel structure, and in shrublands they
usually result in permanent loss of native flora and fauna (Keeley
2002a
). In
forests with surface fire regimes, fuel breaks do not require complete removal of
overstory trees; rather forest thinning and removal of understory fuels produce
what is referred to as a
shaded fuel break
(Agee
et al.
2000
).
A potential cost to fuel breaks is that they are readily invaded by non-native
species and due to their typical configuration of following ridgelines are capable of
moving alien species deep into wildland areas (Merriam
et al.
2006
). On moder-
ately fertile soils grass and forb invasion results in flashy fuels that are more easily
ignited than woody fuels. In forests, however, shaded fuel breaks typically leave
substantial overstory tree cover and this may inhibit alien plant invasion (Agee
et al.
2000
).
Benefits of fuel breaks depend a lot on severity of fire weather and location.
Sometimes under moderate weather conditions these fuel breaks may constrain
fire spread by causing fires to die out due to lack of sufficient fuel to carry fire
through the fuel break. However, under most weather conditions sufficient to
create destructive wildfires, such breaks are highly ineffective as barriers (Cheney
1996
). Under these conditions their primary value is as access areas of defensible
space that can be used for backing fires or related fire-fighting activities. Studies of
fuel break effectiveness in southern California have found that fuel breaks are
relatively ineffective as barriers to fire spread and primarily of value for providing
access to fire-fighting activities (
Fig. 13.3
).
Understanding costs and benefits of fuel treatments may be improved by
economic analysis. A simple model of cost-benefit analysis (
Fig. 13.4
) would show
that as the amount of management effort increases, implementation costs likewise
rise but the damage from fires decreases. Rideout
et al.
(
1999
) predict that the total
cost, that is, the sum of prefire implementation plus postfire damage, would not be
a monotonic function of management effort but rather there will be some opti-
mum point where the total costs are minimized. This is a welcome approach
although several considerations are in order. One is that total management effort
per se
may not be the important metric but rather the strategic pattern of effort;
that is, not
total area treated
but where they are located may be more critical.
Ultimately there needs to be a clear articulation of resource costs associated with
fires as well as with fire management practices.
On many landscapes fuel treatments have the greatest impact on fire outcomes
when they provide conditions that reduce fire threats around values at risk,
particularly at the wildland-urban interface (WUI) or areas compatible with
