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Wildfire and Oak Regeneration at the Urban Fringe

In July 1992, wildfire burned 500 acres of rural lands owned by Stanford University. Within the fire zone are five plots, ranging in size from 0.1 acre to more than 1 acre, on which nearly 600 naturally established juvenile California oaks (Q. agrifolia, Q. douglasii, and Q. lobata) have been monitored since 1990. Surveys following the fire revealed that although 32 percent of the oaks up to 10 inches tall died, only 9 percent of the oaks taller than 10 inches died. Twenty-two percent of all juvenile oaks were topkilled, but 22 percent of those resprouted in years following the fire. Both naturally occurring and human-induced fires have been important elements of California native oak ecology. In recent decades, people have prevented or rapidly suppressed many fires in oak habitat. As humans have built and paved in and near oak habitat, we have extended the urban-open space interface, and fire management has become an increasing concern. Land managers and restoration ecologists are accordingly interested in the effects of fire and its suppression on native oak populations. However, only a few studies have explored how fires—whether uncontrolled or managed—affect California oaks (Allen-Diaz and Bartolome 1992, Haggerty 1991). This paper describes the effects of a 1992 wildfire on young, naturally occurring oaks located within a 500-acre oak woodland at the urban interface. Site Description and Methods Stanford University lands include approximately 1,600 acres adjacent to the central campus that remain rural in character. This property, denoted “Academic Reserve,” is currently used for recreation, livestock grazing, and a handful of academic programs, and is adjoined by housing developments. Located at 38° north latitude, 20 miles inland from the Pacific Ocean and 3 miles from the San Francisco Bay, Stanford enjoys a Mediterranean climate, with mild winters and warm, dry summers. Average annual rainfall is 14.8 inches, almost all of which falls between October and May. The Reserve is divided by a prominent, flattopped southeast-northwest ridge with relatively steep (sometimes >30°) slopes. Soils vary and may be loosely classified as loams, clays, and stony clays with zones of admixture. Oak woodlands cover many northern exposures on the ridge; grassland and oak savanna dominate southern slopes. A few areas of chaparral persist, and native woody plants like toyon (Heteromeles arbutifolia [Lindley] Roemer), coyote brush (Baccharis pilularis DC.), chamise (Adenostoma fasciculatum Hook. & Arn.), and poison oak (Toxicodendron diversiloba [Torrey & A. Gray] E. Greene) occur widely. Native grasses and forbs have been largely supplanted by exotic annuals like wild oat (Avena fatua L.), rye grass (Lolium multiflorum Lam.), nonnative mustard (Brassica spp.), and milk thistle (Carduus nutans L.). The oak population consists of coast live (Quercus agrifolia Née), blue (Q. douglasii Hook. & Arn.), and valley (Q. lobata Née) oaks. In the early 1980’s, University land managers became concerned that too few seedlings were surviving to maintain stable oak populations. Severely skewed age 1An abbreviated version of this paper was presented at the Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues, March 19-22, 1996, San Luis Obispo, Calif. 2Volunteer staff at Magic, P. O. Box 5894, Stanford, CA 94309. 3Manager, Grounds Department, Stanford University, 315 Bonair Siding, Stanford, CA 94305-7270.