Aspects of water relations in coast live oaks and valley oaks subjected to root damage from land development operations
- Stephen W. Roberts, Robin I. Smith
- In: Plumb, Timothy R., technical coordinator…
Twenty-five coast live oaks (Quercus agrifolia) ranging in size from 15 to 100 cm (diameter at breast height), transplanted to accommodate housing developments at three different sites in Calabasas, Calif., were studied for 3 to 4 years after boxing. Transplanted trees, plus 15 native control trees, were monitored quarterly. Water potential, shoot and root growth, and visual condition were measured. Although all 15 controls remained healthy, 16 percent of the transplanted trees died, 20 percent were nearly dead, 24 percent were in decline, 32 percent were stable, and 8 percent were improving. If declining transplants fail to stabilize, then the projected long-term survival rate would be approximately 10 to 40 percent. Transplantation of mature coast live oak trees (Quercus agrifolia) as mitigation for loss due to development has become increasingly controversial as the extent of oak woodlands in Southern California decreases. In addition to concern over the protection of one species while ignoring the complex associated community, there are also questions of cost effectiveness and long-term tree survival. The cost of moving an oak tree varies with box size and site accessibility, ranging from around $1,000 to more than $100,000. To date, few studies have examined transplantation or the physiological consequences of root injury. Roberts and Smith (1980) did a one-year study of water potential and stomatal conductances of oak trees impacted by root removal due to trenching and terracing associated with development. Scott and Pratini (1992) followed 593 transplanted trees in Orange County, Calif., for more than 4 years. However, their observations did not include quantitative physiological evaluation. Our study used both quantitative and qualitative evaluation to assess establishment of transplanted oaks in landscaped settings. Calabasas Transplant Study The City of Calabasas (Los Angeles County), California, Oak Tree Protection Ordinance discourages transplanting and requires mitigation for tree removal. In addition, monitoring of trees that are moved was required for 5 years. In January 1992, monitoring of transplanted trees began at Site 1, followed by the addition of two more sites in April 1993, either as the trees were being boxed, or immediately afterward. All portions of the sites to which trees were moved experienced extensive grading and drainage changes before replanting. Sites 1 and 2 were originally north-facing hillside drainages with intermittent streams, clay soil, and mixed chaparral vegetation. Site 3 was a level riparian area. The perimeter of all three sites had been affected by previous development. Trees were selected for transplanting by the tree-moving company and their associated arborists. Concurrent with root pruning and side boxing, the canopies of the selected trees were pruned, removing 30 to 70 percent of living tissues. Deadwood, inner foliage, and terminal buds were trimmed, leaving a thin shell of foliage on the perimeter of the canopy. A backhoe was used to trench all four sides around each tree at once. Box sizes ranged from 1.5 × 1.5 × 1 m to 8.5 × 8 × 2.5 m. Bottom boxing was completed 3 to 6 months later. Irrigation while trees were boxed was carried out weekly or 1An abbreviated version of this paper was presented at the Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues, 19-22 March 1996, San Luis Obispo, Calif. 2Conservation biologist, Resource Conservation District of the Santa Monica Mountains, 122 N. Topanga Canyon Blvd., Topanga, CA 90290. 3Farm advisor, University of California Cooperative Extension, 669 County Square Dr., Suite 100, Ventura, CA 93003.