Agriculture Reference
In-Depth Information
structure and fuel moisture are the primary drivers behind these fire regime
differences and these can often vary over very short distances (e.g. Stephens
2001
).
There are many other sources of variation in historical fire frequency. At a local
scale fire frequency is often highest on ridgetops, followed by mid-slope forests and
then lower slope forests (Skinner
et al.
2006
). There is also substantial regional
variation. Drier southern California mixed conifer forests have substantially longer
fire intervals (Everett
2003
) than Sierra Nevada forests, due to lower primary
productivity and slower fuel accumulation. More mesic forests in the northern part
of the state also have longer intervals than Sierra Nevada forests, due to a combin-
ation of a shorter growing season for fuel production and a shorter fire season when
fuels are available to burn (Agee
1991
; Taylor & Skinner
1998
).
In addition to spatial variation there is a clear record of temporal variation in
response to climate. Not surprisingly, drier years are associated with more wide-
spread burning (Taylor & Beatty
2005
). Utilizing the 2000-yr record in
Sequoia-
dendron giganteum
, Swetnam (
1993
) showed that fire frequency increased during
warmer periods and was reduced during cooler periods. These patterns also
affected fire intensity since the longer fire-free intervals resulted in greater fuel
volumes that contributed to higher fire intensity.
On the basis of these fire-scar records, the frequency of low-intensity surface fires is
the best-understood fire regime parameter in these forests. Patches of high-intensity
crown fire that kill even the largest trees and create gaps in the forest canopy have
occurred in the past, but we know relatively little about the size distribution of such
gaps and how they have varied relative to historical climate changes (Stephenson
et al.
1991
). The size of these gaps may vary from an area the size of a single tree to
hundreds or thousands of hectares (Show & Kotok
1923b
). There is some evidence
that different forest types are prone to different sized gaps (Collins & Stephens
2010
).
Because all trees in gaps are killed in high-intensity crown fires, there is little record of
the past distribution of gap sizes. Past gaps can be inferred from localized patches of
similar aged trees but extending such an analysis over broad areas has not been done.
This is an important area of research for understanding historical fire regimes and
because gap size plays a critical role in the recruitment of many tree species.
Another fire regime parameter relatively well documented by fire scar records is
seasonality. Studies show that most fires occurred late in the growing season,
perhaps August-September (Swetnam & Baisan
2003
). This interval is offset from
the peak lightning-ignited fire season so it seems likely that this seasonal distribu-
tion was controlled by the state of the surface fuels, in particular fuel moisture.
Postfire Community Response
Tree mortality from surface fires or mixed surface and crown fires varies largely by
tree size and fuel accumulation and less by species. Seedlings and saplings are
often killed by low-intensity surface fires but high-intensity crown fires may kill
even mature individuals. Where high surface fuel loads and or ladder fuels have
accumulated, surface fire will spread to passive crown fires. These fires result in
