Patterns and Processes of Adaptation in Blue Oak Seedlings1

Reciprocal transplant studies examined the contribution of genetic differentiation and phenotypic plasticity to intraspecific variation in blue oak (Quercus douglasii) seedling survival and growth. A nested, mixed model design partitioned seedling survival and growth responses into between-population effects, within-population (among family) effects, and environmental effects. Significant between-population differences in seedling survivorship and growth were observed at both sites. Interactions between population source and planting block suggest local scale adaptation. Differences among maternal families in survival and growth were significant. Phenotypic variation in seedling performance may be related to indices of acorn quality such as embryo dry weight. Although blue oak (Quercus douglasii Hook. & Arn.) is found only in California, it has a wide geographic distribution within the state. In wideranging plant species, it is often assumed that natural selection has resulted in the formation of genetically differentiated populations adapted to local environmental conditions. For example, provenance testing in many conifer species has indicated that there is often adaptive differentiation in response to environmental clines existing along elevational gradients (Campbell 1979, Conkle 1973, Libby and others 1969). Although it is true that natural selection can be a very powerful force in shaping the genetic structure of a species, it has also been demonstrated that gene flow, if of sufficient magnitude, can overwhelm the force of natural selection. Thus, even fairly strong natural selection may not create locally adapted populations if gene flow is very high. Gene flow is typically higher in wind pollinated, outcrossing species like blue oak. The genetic structure of a wind pollinated, outcrossing plant species is usually characterized by relatively large amounts of genetic variation within populations but relatively little genetic differentiation among populations (Hamrick 1979, 1983). Thus, all else being equal, one would expect that local adaptation would be less likely to occur in an outcrossing species. Previous electrophoretic surveys of blue oak populations at various locations in the State have confirmed expectations that blue oak is highly outcrossing (Riggs and others 1991). In general, the allozyme markers indicated that there was little electrophoretic differentiation among populations and no evidence for the formation of geographically distinct subpopulations. If the allozyme variation described by Riggs and others (1991) also reflects patterns of genetic variation in traits with potential adaptive significance (i.e., quantitative traits), then one might argue that there is little indication of local adaptation in blue oak. This conclusion would have obvious implications on strategies for restoring blue oak populations through planting efforts. If local adaptation is not occurring in blue oak, then one would not need to be concerned about the source used in planting projects. In a very real sense one might conclude that “When you have seen one blue oak population, you have seen them all!” However, a problem arises in trying to use patterns of variation in allozyme markers to predict patterns of variation in traits of ecological and adaptive significance. Allozyme markers are excellent markers for measuring evolutionary processes such as drift and gene flow because these markers are generally thought to be neutral to the effects of selection. Thus, patterns of allozyme 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. 2Associate professor and graduate research assistant, respectively, Department of Agronomy and Range Science, University of California, Davis, CA 95616 3Associate professor, Department of Land, Air, and Water Resources, University of California, Davis, CA 95616