Densification, stand-replacement wildfire, and extirpation of mixed conifer forest in Cuyamaca Rancho State Park, southern California

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Abstract

A century of fire suppression culminated in wildfire on 28 October 2003 that stand-replaced nearly an entire 4000 ha “sky island” of mixed conifer forest (MCF) on Cuyamaca Mountain in the Peninsular Range of southern California. We studied the fire affected Cuyamaca Rancho State Park (CRSP), which represents a microcosm of the MCF covering approximately 5.5 × 106 ha (14%) of California, to evaluate how fire suppression unintentionally destabilizes this ecosystem. We document significant changes in forest composition, tree density, and stem diameter class distribution over a 75-year period at CRSP by replicating ground-based measurements sampled in 1932 for the Weislander Vegetation Type Map (VTM) survey. Average conifer density more than doubled, from 271 ± 82 trees ha−1 (standard error) to 716 ± 79 ha−1. Repeat aerial photographs for 1928 and 1995 also show significant increase in canopy cover from 47 ± 2% to 89 ± 1%. Changes comprise mostly ingrowth of shade-tolerant Calocedrus decurrens [Torr.] Floren. in the smallest stem diameter class (10–29.9 cm dbh). The 1932 density of overstory conifer trees (>60 cm dbh) and 1928 canopy cover at CRSP were similar to modern MCF in the Sierra San Pedro Mártir (SSPM), ∼200 km S in Baja California, Mexico, where fire suppression had not been practiced, verifying that the historical data from the early twentieth century represent a valid “baseline” for evaluating changes in forest structure. Forest successions after modern crown fires in southern California demonstrate that MCF is replaced by oak woodlands and shrubs. Post-fire regeneration in severely burned stands at CRSP includes abundant basal sprouting of Quercus chrysolepis Liebm. and Quercus kelloggii Newb., but only few seedlings of Abies concolor [Gord. and Glend.] Lindl (average 16 ± 14 ha−1), while whole stands of C. decurrens, Pinus lambertiana Dougl., and Pinus ponderosa Laws. were extirpated. Prescribed burning failed to mitigate the crown fire hazard in MCF at CRSP because the low-intensity surface fires were small relative to the overall forest area, and did not thin the dense understory of sapling and pole-size trees. We propose that larger, more intense prescribed understory burns are needed to conserve California's MCF.

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

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