Agriculture Reference
In-Depth Information
No predictable
fire
Unpredictable
stand-replacing
fire
10 0 0
Predictable
crown
fire
10 0
10
Predictable
mixed
fire
Predictable
surface fire
1
1
10
10 0
Relative site productivity
Fig. 2.7
Fire regimes generated by patterns of productivity and disturbance frequency. (From
Keeley & Zedler
1998
.)
crown fires, in part because fire spread in crown fires is often facilitated by, and
sometimes dependent upon, high winds capable of overcoming potential barriers
to fire spread such as patches of reduced fuel loads, or physical barriers such as
streams, rock outcrops and fuel breaks.
Fire regimes may be altered over time as climates change; however, human
impacts are presently one of the major drivers of fire regime change. MTC regions
in particular may be characterized as having experienced substantial shifts in fire
regime due to human occupation (Keeley & Zedler
2009
; Pausas & Ferna´ ndez-
Mun˜ oz
2011
). Anthropogenic fire regimes are often thought of as disruptions in
natural fire regimes and exhibit two extremes. On the one hand humans are
responsible for greatly increasing fire frequency on landscapes where lightning-
ignited fires are infrequent, and on the other hand humans suppress fires and
greatly diminish the historical or
natural
fire frequency (Keeley
et al.
2009a
). The
relative importance of these two impacts varies markedly with the fire regime. For
example, in conifer forests with low-intensity surface fires, fire suppression has
been highly effective and consequently has accomplished near total fire exclusion
for a century or more. However, in crown fire shrublands, high-intensity fires
driven by high winds have proven very difficult to suppress and thus fire exclusion
has happened only on a very limited scale. Indeed, on these landscapes, since they
are often juxtaposed with urban environments, they have experienced an increase,
not decrease, in fire frequency (see
Fig. 5.10
).
Physical scientists interested in finding common underlying patterns in complex
systems have turned their attention to biological systems including ecosystem fire
activity. Across a wide range of fire regimes in North America and Australia,
wildfires exhibit a power law distribution of frequency vs. area (Malamud
et al.
1998
). Such patterns suggest to some that there is a common underlying mechan-
ism that explains fire activity. Malamud
et al.
(
1998
; Malamud & Turcotte
1999
)
