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

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 × 10 6 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.