Densification, stand-replacement wildfire, and extirpation of mixed conifer forest in Cuyamaca Rancho State Park, southern California
Introduction
After a century of fire suppression management, dense stands of mixed conifer forest (MCF) have fueled high-severity wildfires over extensive areas of the southwestern United States (e.g., Graham, 2003). Wildfires were far less destructive prior to fire suppression management initiated ca. 1900. In California, open park-like stands were maintained by pre-suppression surface fires that cleared the understory of sapling and pole-sized trees (reviewed in Van Wagtendonk and Fites-Kaufman, 2006, Minnich, 2007). In October 2003, massive wildfires burned ∼300 000 ha across southern California, including approximately 10 000 ha of MCF. A crown fire stand-replaced nearly the entire 4000 ha “sky island” of MCF on Cuyamaca Mountain in the Peninsular Range. Dense forest stands were completely charred, and patches of severely burned soil, conspicuously reddened with thermally produced iron oxides, covered as much as 15% of the land surface (Fig. 1; Goforth et al., 2005).
The fire affected Cuyamaca Rancho State Park (CRSP) represents a microcosm of the MCF covering approximately 5.5 × 106 ha (14%) of California (Barbour and Major, 1988). Previous studies have assessed how fire suppression altered the structure of MCF in California, by relying upon indirect evidence of change, interpreted from late nineteenth century written accounts, photographs, and survey records (e.g., Minnich, 1988, McKelvey and Johnston, 1992, Stephens and Elliot-Fisk, 1998, Gruell, 2001), or reconstructed demographic patterns in modern stands (e.g., Vankat, 1977, Vankat and Major, 1978, Kilgore and Taylor, 1979, Parsons and DeBenedetti, 1979, Bonnicksen and Stone, 1982, Krofta, 1995), as well as by directly quantifying long-term landscape-scale change using repeat measurements (Minnich et al., 1995, Albright, 1998, Bouldin, 1999, Stephenson and Calcarone, 1999). The availability of long-term data at CRSP provided unique opportunity to directly measure changes in forest structure and composition that culminated in stand-replacement wildfire.
We utilized historical data collected for the Vegetation Type Map (VTM) survey of California (Weislander, 1935a, Weislander, 1935b; after Minnich et al., 1995) and aerial photographs taken in 1928 as reference conditions comparable with modern repeat measurements, to quantify twentieth century changes in tree density, stem diameter class distribution, canopy cover, and forest composition. We also calibrate observations of forest change against data obtained from a relatively unaltered MCF landscape (∼40 600 ha) where fire suppression had not been practiced, located 200 km S at the Sierra San Pedro Mártir (SSPM) in Baja California, Mexico (Minnich et al., 1997, Minnich and Franco-Vizcaíno, 1998, Minnich et al., 2000, Stephens and Gill, 2005, Stephens et al., 2007). Forest successional trajectories (i.e., probable long-term changes) after stand-replacement wildfire are analyzed on the basis of regeneration at CRSP, and trends observed in MCF elsewhere in California. We hypothesize that (1) fire suppression management in the twentieth century caused increased tree density in old-growth MCF stands, (2) stand-densification broadens the size of crown fire from a process that historically cleared small forest gaps, to stand-replacement of entire landscapes in modern conditions, and (3) such large crown fires extirpate stands of non-serotinous conifers composing this ecosystem.
Section snippets
Study area
The CRSP is located about 65 km east of San Diego, California, and 30 km north of the USA–Mexico international border (116°35′W, 32°57′N). Cuyamaca Mountain is uplifted Cretaceous intrusive igneous batholith within the broader Peninsular Range, which extends from Baja California, Mexico, north to the San Jacinto Mountains in southern California, USA. Soils are derived from gabbro colluvium at the study sites, and have sandy-loam to loam textures in the A horizon (Goforth et al., 2005). The
Stand-replacement wildfire
The CRSP was burned at the end of a week-long episode of dry easterly winds moving from the Great Basin to the Pacific Ocean (i.e., “Santa Ana” winds). From October 24 to 28, an upper level ridge with a closed center of 6000 m at 500 mb steered dry northeast flow over southern California (NOAA Daily Weather Maps, available online at http://www.hpc.ncep.noaa.gov/dailywxmap/). The winds weakened on October 28 as the high pressure ridge axis shifted northward. Westerly anabatic winds along the
Forest densification
Replication of VTM forest plots and repeat aerial photography of CRSP document significant densification of stems, similar to trends reported in other studies of MCF throughout California (Vankat, 1977, Vankat and Major, 1978, Kilgore and Taylor, 1979, Parsons and DeBenedetti, 1979, Bonnicksen and Stone, 1982; McKelvey and Johnston, 1992, Minnich et al., 1995, Albright, 1998, Ansley and Battles, 1998, Bouldin, 1999, Barbour et al., 2002). Suppression of wildfire in the twentieth century
Conclusions
Fire suppression destabilizes MCF by allowing widespread accumulation of fuel (live and dead biomass) in the form of leaf litter accumulations, coarse woody debris, and understory growth of shrubs and conifer regeneration. Fire suppression also selects for wildfires during extreme weather states when ignitions are least efficiently extinguished as in summer “heat waves” or dry easterly “Santa Ana” winds in autumn that pushed the October 2003 firestorm across southern California (Minnich, 2006).
Acknowledgements
This research was funded by California State Parks Interagency Agreement C0343027. We thank Jim Dice, Mike Wells, Kim Marsden, and Bob Hillis for permitting our research in CRSP, as well as Ayzik Solomeshch and Michael Barbour at the University of California, Davis, for providing the archived VTM plot data, and Ken-ichi Ueda and Maggi Kelly at UC Berkeley, for their assistance in obtaining a digitized map of the Cuyamaca quadrangle. The lead author and Walter Boyce at UC Davis, prepared a
References (70)
- et al.
Impact of a high-intensity fire on mixed evergreen and mixed conifer forests in the Peninsular ranges of southern California, USA
Forest Ecology and Management
(2006) Snow levels and amounts in the mountains of southern California
Journal of Hydrology
(1986)- et al.
Impact of fire suppression on a mixed conifer forest
Forest Ecology and Management
(1979) - et al.
Forest structure and mortality in an old-growth Jeffrey pine-mixed conifer forest in northwestern Mexico
Forest Ecology and Management
(2005) - et al.
Coarse woody debris and canopy cover in an old-growth Jeffrey pine-mixed conifer forest from the Sierra San Pedro Martír, Mexico
Forest Ecology and Management
(2007) - Albright, D., 1998. Vegetation change in second-growth mixed conifer forests of western San Bernardino Mountains....
- et al.
Forest composition, structure, and change in an old-growth mixed conifer forest in the northern Sierra Nevada
Journal of the Torrey Botanical Society
(1998) - et al.
Introduction
- et al.
Present and past old-growth forests of the Lake Tahoe Basin, Sierra Nevada
Journal of Vegetation Science
(2002) The statistical analysis of Chaparral and other plant communities by means of transect samples
Ecology
(1943)