To Prune or Not to Prune: Responses of Coast Live Oaks (Quercus agrifolia) to Canopy Retention during Transplanting

A total of 62 coast live oaks (Quercus agrifolia) were monitored since they were initially boxed for transplantation in 1993. At that time, only branches injured during the moving process and deadwood were removed, leaving the entire canopy intact. This was a departure from the usual transplanting methodology that traditionally removes up to 70 percent of the canopy in order to compensate for the massive root loss incurred during boxing. To date, survival of non-pruned trees has exceeded that of a cohort of 25 transplanted oaks that received the standard canopy reduction. A discussion of the impacts of pruning and transplanting on diameter growth, canopy condition and overall health and vigor of the transplanted oak trees is provided. Introduction Transplantation of mature coast live oak (Quercus agrifolia) as mitigation for development of oak woodlands has been a common practice in southern California. Typically, tree canopies are severely pruned (up to 70 percent loss) at the time of root pruning, which usually takes place in the extreme heat of late summer/early fall. The justification for this is to reduce transpirational stress and “balance” the root:shoot ratio. Since 1992, the City of Calabasas, California, has required monitoring of 4 development sites where coast live oaks were transplanted as part of a mitigation effort to compensate for the loss of oak woodland resources to development. At Sites 1-3, a total of 30 mature coast live oaks with severe root and canopy pruning were monitored following boxing in January 1992 to October 1997. At Site 4, only the roots of 100 mature coast live oaks were pruned, leaving the canopies intact, except for removal of deadwood and any branches damaged during the moving process. Monitoring of the 4th site began in October 1996 and concluded in April 2001. Comparing the condition and survival rate of the trees that retained their natural canopies to the severely pruned trees provides important data on the effects of extensive canopy loss on mature transplanted coast live oaks. To date, few studies have evaluated the success of transplanting mature coast live oaks, the physiological 1 An abbreviated version of this paper was presented at the Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape, October 22-25, 2001, San Diego, California. 2 Senior Conservation Biologist, Resource Conservation District of the Santa Monica Mountains, 122 N. Topanga Canyon Blvd., Topanga, CA 90290 (e-mail: [email protected]) 3 Farm Advisor/Environmental Horticulture, University of California Cooperative Extension, 669 County Square Dr., Suite 100, Ventura, CA 93003 (e-mail: [email protected]) USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 369 To Prune or Not To Prune: Responses of Coast Live Oaks (Quercus agrifolia) to Canopy Retention During Transplanting—Dagit and Downer responses of the trees to the extensive root loss, or the cost effectiveness of moving trees considering the need for long-term maintenance and poor survival rates. Roberts and Smith (1980) did a 1-year-study of water potential and stomatal conductance of oak trees impacted by root injury from trenching and found that mortality increased when more than half of the root system was impacted. Scott and Pratini (1992) followed the health and vigor of 593 transplanted coast live oaks in Orange County, California, for more than 4 years and found that following high initial mortality, trees that began to decline were unable to regain lost vigor and subsequently died. Dagit and Downer (1996) reported on the survival status of the 30 pruned trees at Sites 1-3, finding that 16 percent of the trees died immediately following boxing. Of those that survived the initial trauma, 28 percent of the trees were dead, 24 percent were nearly dead, 32 percent were in decline and only 16 percent were stable 5 years after transplanting. This study compares canopy recovery, leaf tissue conditions, growth and condition of the pruned to the non-pruned transplanted coast live oak trees. Transplantation Methodology All portions of the sites from which trees were moved experienced extensive grading and drainage changes before replanting. Sites 1, 2 and 4 were originally north-facing hillside drainages with intermittent streams, clay soil, and mixed chaparral/oak woodland vegetation. Following grading, the sites consisted of 95 percent compacted cut and fill pads which maximized development opportunities. Site 3 was initially a level riparian area that was transformed into a freeway interchange, with limited area for planting. Trees were selected for transplanting by the developer’s tree-moving company and their arborists. Trees selected ranged in size from 15 to 175 cm DBH and most were multi-stemmed. Tree height ranged from 4-15 meters. Crown diameter varied as well, from 4 to 25 meters. Concurrent with root pruning and side boxing, the canopies of the selected trees were pruned at Sites 1-3, removing up to 70 percent of foliage bearing branches. Deadwood, nearly all inner foliage, and many terminal buds were trimmed, leaving a thin shell of foliage on the perimeter of the canopy. At Site 4, the tree canopy was left intact, with only dead branches or damaged branches removed (fig. 1). A backhoe was used to trench all four sides around each tree, resulting in the loss of approximately 90 percent of the root system. Plywood box sizes ranged from 1.5 to 8.5 meters wide, and 1 to 2.5 meters deep. Bottom boxing was completed 3 to 6 months later. After boxing, trees were irrigated weekly by water trucks, as directed by the tree-moving company. The trees were planted in a hole dug by backhoe, usually 1 to 2 meters wider than the box and approximately the same depth as the root ball. The plywood box bottoms were left in place, the sides removed, and backfilling done by backhoe and hand tools. Sprinklers were installed at Sites 2 and 4, and irrigation was automated and poorly overseen. The other two sites continued to be watered by truck one to three times weekly. By coincidence, 3 of the trees were planted in the same orientation as they had originally grown. The situation at Site 4 is unusual, in that 31 trees have been planted, while the remainder are still boxed. A wildfire swept through the property in October 1996, burning many of the boxed trees, but none of the planted trees. The 31 burned, boxed USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 370 To Prune or Not To Prune: Responses of Coast Live Oaks (Quercus agrifolia) to Canopy Retention During Transplanting—Dagit and Downer trees were thus removed from further consideration in this study. Despite inoculation with mycorrhizae, mulching and irrigation, these boxed trees continue to decline. Those planted in the landscape have been subjected to a variety of irrigation and drainage problems. The majority of the transplants have been installed in common open space areas, except at Site 4, where they have also been placed on private property, adjacent to driveways and street intersections. Pruned coast live oak 1993 2001 Original condition After pruning Present condition Non-pruned coast live oak