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6.1 C.1 Page 6 of the tree root pruned five years ago. It could not be determined that the decay was the result of the root pruning as the decay extended well into the trunk of the tree. If basal decay was observed to be present at the time of construction, root pruning would not have been performed. On all four trees, staff found large lateral roots that had been left intact and were either torn or pulled from the soil as the tree failed. Conclusion Root pruning has served as an alternative to tree removal during the 19 year progression of the sidewalk repair program. As the program has evolved, there are many streets where the trees continue to thrive and provide the canopy desired in the neighborhoods. On some streets, no trees were removed and the neighborhood has retained the original character with more space between the sidewalks and trees. On some streets, as follow-up tree removal has been required, the impact has been spread out over several years, creating a transition to a more mixed-aged tree population which is the most sustainable tree canopy for a City. There is a variable balance between tree removal and root pruning. If the goal is to have no trees fail, in order to achieve that goal with no risk, the project would require removal of all the trees that are in conflict with sidewalks. Staffs goal has been to practice root pruning to emulate a "normal" failure rate - failures that would randomly occur under normal to exceptional circumstances. There is some risk associated with root pruning. A portion of the root system of the tree will be removed or compromised. Staff has designed the project specifications to minimize the risk of tree failure, with the following steps: . Include Certified Arborists in the project plans, tree decisions, and inspections. . Evaluate the tree condition first to validate the tree should be retained. . The target distance from the tree trunk that the root pruning is performed to was increased from 18 to 24 inches in the early project years to 30 inches in the mid 1990's. The target distance was increased to 36 inches following the 1999 Task Force recommendations. . The target depth to which root pruning is performed was decreased from 18 inches in the early project years to twelve inches in the mid 1990's. Following the 1999 Task Force recommendations, the target depth was reduced to eight inches - the minimum depth needed for concrete construction. . Relocate curbs to avoid root pruning. . Develop species profiles to avoid root pruning less stable species. . Prune trees to reduce the canopy "sail" when too dense. . Test trees suspected of instability to determine if the tree should be removed. There are some other impacts to trees following root pruning. Some branch and twig dieback occurs in most trees following root pruning. Supplemental irrigation does have a beneficial affect on an individual tree's response to root pruning. The adjacent resident's care can provide a beneficial or detrimental effect. The City has provided deep watering to root pruned trees during exceptional circumstances such as during the drought. TreFFailure Data Analuysis Page 5 of 6 .' ~ ,.- ____H. u._...._.._. --.--'-,._--,'._---.,--"._---,-------_.__..~._._--~--~----.-.---.,-------