The Distribution of Engelmann Oak (Quercus engelmannii) in California

Engelmann oaks (Quercus engelmannii) only occur in the foothills of San Diego (93 pct of extant stands), Riverside (6 pct), Orange (0.5 pct), and Los Angeles (<0.1 pct) counties, covering the smallest range of any oak species in California. The overall distribution of the species covers approximately 31,500 hectares of woodlands, although they are subdominant (contrib­ uting <50 pct of canopy area) to coast live oak (Quercus agrifolia) over about 52 pct of that area. Individual stands across the species range were mapped at 1:24,000 scale into a geo­ graphic information system (using 1:20,000 scale aerial photographs). Stands were separated into 6 classes of Engelmann oak canopy dominance: (one) 0 to 5 pct of canopy area; (two) >5 to ≤25 pct; (three) >25 to ≤50 pct; (four) >50 to ≤75 pct; (five) >75 to ≤95 pct; and (six) >95 to ≤100 pct. All areas were field checked for accuracy in boundary and canopy classification. There are approximately 7,300 ha of woodlands in categories five and six; 14,000 ha in categories three and four; and 9,200 ha in categories one and two. Combining these data with USGS Digital Elevation Models suggests that Engelmann oaks are most concentrated on 0° to 10o slopes with southwestern aspects between the elevations of 700 m to 1250 m above sea level. They tend to occur at higher elevations and slightly steeper slopes (5° to 10°) than coast live oaks, but there are no differences in the distribution of the two species relative to slope-aspect. The largest landholder of Engelmann oak stands is the Cleveland National Forest (24 pct of all stands), followed by Spanish Land Grants (29 pct; unbroken large land holdings), Native Ameri­ cans (7 pct; on Indian Reservations), and the US Marine Corps (6 pct; Camp Pendleton). A large number of small private parcels control the remaining 31 pct of Engelmann oak stands. This study was undertaken to define the distribution of the Engelmann oak (Quercus engelmannii). It is the first step in conserving and managing this oak resource in a rapidly urban­ izing part of California. Wieslander and Jensen (1946) mapped part of the Engelmann oak range in the 1940's; if it were not for the rapid conversion of wildland habitats and the increased interest in woodland, these maps probably would have been sufficient to typify the species distribution. The U.S. Forest Service maps (Anderson 1969) are more up to date and at a finer Presented at the Symposium on California's Oak Woodlands and Hardwood Rangeland, October 30 November 1, 1990, University of California, Davis. Natural Resource Specialist, Integrated Hardwoods Management Program, Department of Forestry and Resource Management, University of Cali­ fornia, Berkeley, and Assistant Adjunct Professor, Department of Earth Sciences, University of California, Riverside. scale than the Wieslander maps, but still did not provide the data for private lands outside the Cleveland National Forest. Bolsinger (1986) provided the best estimates of Engelmann oak area, but these data were neither location specific nor retrievable by geographic unit. The goals of this paper are threefold: (1) to define the extent of Engelmann oak woodlands, (2) to make broadscale predictions on the occurrence of the species relative to topographic features, and (3) to outline the ownership, administration, and manage­ ment control of Engelmann oak woodlands.