Size Class Distribution of Quercus engelmannii

Abstract: Size class distribution of Quercus engelmannii (Engelmann oak) on the Santa Rosa Plateau was studied to understand whether current recruitment of young oaks is suffi cient to maintain the population in spite of high natural mortality and impacts of development in some portions of the plateau woodland. Sapling-size oaks (1-10 cm dbh) made up 5.56 pct, 4.17 pct and 6.58 pct of total trees sampled at hill (N=252), mesa (N=216) and valley (N=228) sites respectively where cattle were removed in 1986. Approximately 22.5 pct out of 200 trees sampled in an adjacent oak woodland (estimated to have been non-grazed for 20 years) were of sapling size. Peak abundance of Quercus engelmannii was in the 46-50 and 51-55 cm dbh size classes on hill, valley and mesa sites and in the 36-40 and 46-50 cm dbh size classes in an adjacent oak woodland. Trees in size classes larger than the 46-50 cm dbh increment declined in abundance at all four sites. Only 17.1 pct of total trees (N=896) were in sizes classes beyond 66-70 cm dbh and only two trees in size class 131-135 cm dbh. Despite the high numbers of young oaks at one of the four sites sampled, current stand size structure of Quercus engelmannii on the Santa Rosa Plateau indicates that reproduction is insufficient to maintain the present occurence and abundance of older age classes in the future. Quercus engelmannii Greene (Engelmann oak) and Q. agrifolia Née (coast live oak) are characteristic of the southern oak woodland (Thorne 1976). Griffin (1977) divides the south-ern oak woodland of California into two phases: 1) the coast live oak phase, which occurs between Orange and Santa Barbara counties where Juglans california S. Wats (California walnut) is often co-dominant with Quercus agrifolia; and 2) the En gelmann oak phase, which mostly occurs in Riverside and San Diego counties, completes the southern oak woodland. Quercus engelmannii has a small total range compared to Q. agrifolia (Plumb and Gomez 1983) and occurs only in the southern part of the woodland range where it dominates on drier slopes and savanna-type habitats. It ranges, in dense to scattered stands