Volume 53, No. 2 29 Improving Root Growth and Morphology of Containerized Oregon White Oak Seedlings Warren D. Devine, Constance A. Harrington, and Darlene Southworth Research Forester, Forest Service, Pacific Northwest Research Station, Olympia, WA; Research Forester, Forest Service, Pacific Northwest Research Station, Olympia, WA; Professor Emerita, Department of Biology, Southern Oregon University, Ashland, OR Abstract We conducted four trials to determine if we could alter root morphology of containerized Oregon white oak seed- lings in order to potentially improve their performance in restoration plantings. Early pruning of the radicle produced a branched taproot, though with fewer branches than reported for other oak species. Pruning the taproot at 15 cm (6 in) promoted greater taproot branching than radicle pruning but did not increase formation of lateral roots. Nontrans- planted seedlings grown in tall containers (2.83 L; 0.75 gal) responded to air-pruning with increased lateral root growth and minimal circling of roots. Inoculation with soil contain- ing ectomycorrhizal fungi substantially improved shoot- and root-growth response to fertilization. Introduction The range of Oregon white oak (Quercus garryana Dougl. ex Hook.) extends from southern California to southern British Columbia. It is the only species of Quercus native to northern Oregon, Washington, and British Columbia. Oregon white oak occupies sites ranging from rock outcrops to riparian zones, although its extent has been reduced signiicantly during the past 150 yr by agriculture, urbanization, and lack of regular ire. In recent years, inter- est in restoring oak-associated plant communities and in landscaping with native species has resulted in the planting of Oregon white oak seedlings on a variety of private and public lands. Bareroot and container-grown Oregon white oak seedlings are produced by numerous commercial nurseries in California, at least 30 in Oregon, and several in Washington and British Columbia. Many native Oregon white oak sites are relatively harsh, with shallow or coarse-textured soils, dense vegetative competition, and limited soil water availability during the dry summers characteristic of its range. Access to soil water on these sites is related to early growth of planted seedlings (Devine and others 2007); thus, development and morphology of seedling roots will likely inluence performance. The extra costs associated with container- ized seedlings (compared to bareroot seedlings) might be justiied on these dry sites, as survival rates may be higher and the window of time for planting may be greater than for bareroot seedlings (Wilson and others 2007). Addition- ally, restorationists or landowners planting relatively few oak seedlings or planting at low density may be willing to accept the higher per-seedling cost for the beneits of container stock. Container-grown Oregon white oak seedlings ideally have root systems with many ibrous lateral roots and a straight taproot that ends at the base of the container. The lateral roots increase the total root surface area, which, in turn, is related to greater potential for water and nutrient uptake, reduced planting stress, and improved post-planting survival and growth (Schultz and Thompson 1996, Gross- nickle 2005). The taproot grows rapidly following germi- nation in the fall. Given the droughty growing-season soil conditions in much of the species’ range, this taproot may be an evolved strategy for acquiring water from deeper soils with relatively more available moisture in summer. In a standard pot, this taproot grows straight to the bottom and circles repeatedly, creating a ‘pot-bound’ seedling. Oak seedlings planted with this root morphology perform poorly (McCreary 1996) and may never fully recover. To prevent this and other root deformities, techniques such as air-pruning or chemical pruning often are used for tree seedlings grown in containers. In several oak species, pruning the newly emerged radicle promotes branching of the taproot (Barden and Bowersox 1989, McCreary 1996, Tilki and Alptekin 2006). These branches grow down, with the appearance of multiple taproots of similar size and morphology. We refer to these branches as multiple taproots in this report. If we assume that lateral roots origi- nate from each taproot at a similar frequency regardless of