Growth of Blue Oak on California's Hardwood Rangelands1

An individual tree basal area increment model was developed for blue oak (Quercus douglasii Hook. & Arn.) in California, an important woodland tree for wildlife habitat. Box-Cox regression based on 895 increment cores that showed current basal area and the relative size of the tree crown were significant variables. Models to link basal area changes over time with tree height and crown cover were developed to provide initial estimates of stand structure dynamics. Relative tree crown characteristics, site index, and diameter at breast height (DBH) were significant variables in predicting tree height. Individual tree basal area, basal area competition in trees larger than the subject tree, and tree height were significant in predicting crown cover. Use of these models for assessing projected changes in stand structure under different levels of management is demonstrated. Blue oak (Quercus douglasii Hook. & Arn.) is a widely distributed tree species on California rangelands, occurring in more or less pure woodland or savanna stands on 1.2 million ha of California's 3.0 million ha of hardwood rangelands, and in mixed stands on another 360,000 hectares (Bolsinger 1988). Blue oak woodlands have an understory of annual grasses and occasional native perennial grasses, and are found in association with foothill pine (Pinus sabiniana Dougl.), coast live oak (Quercus agrifolia Nee), and interior live oak (Quercus wislizenii A. DC.). Good ecological descriptions of these areas are found in Bartolome (1987), Griffin (1977), and Holmes (1990). Livestock grazing is the predominant land use on these areas, 75 percent of which are privately owned (Bolsinger 1988). Historically, little attention was given to tree growth on blue oak woodlands because they are classed as "noncommercial" by USDA Forest Service standards (average annual growth less than 1.4 cubic m per ha per year), they have little commercial use other than for firewood, and they inhibit forage production for livestock on some areas. In recent years, however, it has been recognized that blue oak rangelands are a rich source of biological diversity. The valley-foothill hardwood and valley-foothill hardwood conifer habitat types, of which blue oak woodlands are a principal component (Mayer and Laudenslayer 1988), have 278 vertebrate species of wildlife, which rely on these lands for at least part of their habitat needs (Airola 1988). Watershed protection and esthetics are other important public values supplied by blue oak woodlands. Concern about longterm sustainability of blue oak woodlands has been expressed by policy makers, resource mangers, and the general public because regeneration failure has been documented in some areas (Bolsinger 1988, Muick and Bartolome 1987, Standiford and others 1991). To evaluate blue oak woodland environmental values, managers need to be able to assess changes in tree cover and stand structure over time. Blue oak stand structure is closely correlated with the quality of wildlife habitat (Block and Morrison 1991, Wilson and others 1991). To date, the only information on growth and yield available for assessing blue oak stand dynamics is a whole stand model of volume, basal area, and crown cover developed for use in economic optimization of multiple resource management on blue oak woodlands (Standiford and Howitt 1993). However, this is not adequate for assessing stand structure changes over time and their effects on habitat values. The objective of this study is to develop preliminary individual blue oak tree models to allow managers to assess stand structure changes over time. 1An abbreviated form 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. Research was funded in part by Integrated Hardwood Range Management Program Grant 86-4. 2Forest Management Specialist, Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720.