Fire History in Southern California’s Shrublands and Forests
Janet Franklin,
Professor, Department of Biology,
Adjunct Professor,
Department of Geography, San Diego State University
For a very
interesting perspective on California fire ecology and management I recommend
Dr. Jon Keeley’s publications
http://www.werc.usgs.gov/seki/keeley.asp as well as the fire-related briefs
issued by the USGS Western Ecological Research Center
http://www.werc.usgs.gov/fire/firerelated.html
(Except from manuscript in
preparation, Oct 30, 2003)
An ecosystem’s
disturbance regime is defined by parameters describing the recurrence,
location, and severity of natural disturbance such as fire, e.g., size, return
interval, intensity (Pickett and White 1985, Turner et al. 1989, Johnson 1992).
When disturbance is infrequent, average parameters can be hard to define as the
frequency of the disturbance is of the same order of magnitude as changes in the
causal factors such as decadal fluctuations and longer-term trends in climate,
and human impacts (Zedler 1995, Keeley and Fotheringham 2003).
In southern
California, the plant communities comprise species that tolerate fire by
various methods of resistance or resilience, or in some cases require
fire for regeneration (Keeley and Zedler 1978, Keeley 1986, Keeley, Zedler
1995). The shrublands that cover the greatest area in the foothills and
remaining undeveloped coastal plain (Stephenson and Calcarone 1999) burn in
stand-replacing fires (Keeley and Fotheringham 2001). Therefore, it is
difficult to reconstruct fire histories and vegetation response using
dendroecological methods (fire scars) that, when combined with palynological
data, have proved so useful in forests (e.g., McBride and Laven 1976, Agee et
al. 1978, Anderson and Carpenter 1991).
Charcoal in
sediment cores, historical, and ethnographic data (Lewis 1973, Mensing et al.
1999, Keeley 2002) provide insights into long-term trends in the shrubland fire
regime in southern California, but lack spatial detail. Detailed fire mapping
has only been carried out for the 20th century, primarily during the
modern period of fire suppression, while people have affected the fire regime in
this region in various ways for 400 years during the historic period, and
prehistorically throughout the Holocene (reviewed by Keeley and Fotheringham
2003).
The available
data suggest that the in the second half of the 20th century the
frequency of small fires increased in southern, coastal California, while their
average size decreased (Keeley et al. 1999, Moritz 2003). In San Diego County,
this has resulted in an increased rate of burning (area burned per decade) in
coastal shrublands (especially the coastal sage scrub formation), although no
trend could be detected for the chaparral formation of the foothills and
mountains, more distant from urban development (Wells et al. submitted). This
pattern is consistent with a causal explanation of increased human-caused
ignitions along the lower-elevation wildlands-urban interface (with suppression
efforts keeping many of those fires small), contrasting with effective fire
suppression in the upper-elevation montane forests (Keeley and Fotheringham
2003, Wells et al. submitted).
It has also been
suggested that 20th century fire suppression has resulted in fewer,
larger, high intensity fires in chaparral owing to fuel accumulation (Minnich
and Dezzani 1991, Chou et al. 1993, Minnich 1995, Minnich 2001) but the evidence
for this has been refuted (Keeley and Fotheringham 2001, Moritz 2003). For
example, based on charcoal deposited in marine sediments in the Santa Barbara
Channel, it appears that for at least 560 years (predating the Spanish period)
large fire events in the region have been associated with extreme fire
weather, and most of the land area burned is consumed in those large fires
(Moritz 1997, Mensing et al. 1999). This suggests that modern fire suppression
efforts have not increased the risk of large fires by allowing fuel to build up.
For example, data
for over 600 fires in the San Diego Foothills and Mountains (Figure 1) from
1910-1999 show that the area burned per decade during the historical period of
fire suppression does not show any increasing trend (Figure 2). Nor does fire
size (Figure 3) – at least there does not appear to be an increase in the number
of very large fires (>10,000 ha). More carefully analysis is required to
determine if the trend towards increasing numbers of small fires found in other
parts of southern California holds here. It may be because this study area
excludes the low elevation, coastal areas and therefore much of the urban-wildland
interface, that we do not see such a trend in these data. When fire history
data from all of San Diego County is considered, a trend towards
increasing area of low elevation coastal shrublands burned per decade is seen
(Wells et al. in press). Data are from the California Department of Forestry
Fire and Resource Assessment Program (http://frap.cdf.ca.gov/projects/fire_data/fire_perimeters/index.asp).
Figure 2.The San Diego
County portion of the Foothills and Mountains ecoregion (Stephenson and
Calcarone 1999) showing the area burned by decade, 1910-1999.
Figure 2. Area burned
in the San Diego Foothills (Low Elev ~1000-4500 ft) and
Mountains (High Elev <4500 ft) per decade (1=1910s… 9=1990s).
Figure 2. Fire size
(ha, Log scale) in the San Diego Foothills 1910-1999 (618 recorded fires).
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