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6.1C.1 Page 4 The Tree Preservation and Sidewalk Repair Program (100% Project) has completed sidewalk repairs following a block by block format through the City, by geographic areas, on a scheduled basis. Root pruning has been performed on 138 to 354 trees per project year, depending on the scope of work. The Cost Shared Sidewalk Repair Project (50/50 Project) was introduced in response to property owner requests for sidewalk repairs outside of the block by block area. Root pruning has been performed on 21 to 68 trees per project year, depending on the scope of work. The 100% Project has completed 17 construction contracts over the 20 years of the program. The 50/50 Project has completed 14 construction contracts. An average of 250 trees were root pruned per year on the 100 % project and an average of 41 trees were root pruned per year on the 50/50 project for a historic total root pruning of approximately 4,790 trees. which is 27% of a total population of 18.000 existing trees. Historv of Tree Failures Most arborists can agree on one thing: Any tree will fail given enough time. Inspection and observations are often focused on how to quantify the likely timing of failure so trees can be removed prior to failure. but not too prematurely. In Redwood City, there have been tree failures due to many factors. The failures can be separated into two types - branch or limb failures and whole tree failures. Branch or limb failures occur when: . The wood structure of the crotch or attachment point is weakened due to decay or abnormal growth. . The weight of foliage on the limb or pruning performed results in foliage growth that exceeds the strength of the attachment. . Summer branch drop - an "unpredictable" limb failure that occurs under normal weather conditions with no apparent decay or other obvious causal factor. Whole tree failure occurs when the root system can no longer support the tree. This can be the result of: . Outside forces - a storm or high wind exceed the holding capacity of the tree's root system. . Root decay that reduces the root system necessary to support the tree. . Excessive soil moisture that reduces the ability of the roots to hold onto the soil. . Excessive mechanical damage or severance of roots PWS staff monitors the tree failures that occur on City-owned trees and some private trees as observed through the Tree Preservation Ordinance permit process. This approach provides continued learning about external signs and symptoms of intemal or underground degradation. Staff also participates in the University of California Cooperative Extension California Tree Failure Reporting Program (in the process of shifting to an international database). Annually, staff attends the Cooperative Extension tree failure update meetings which provide information about tree failure and information to improve prevention. TreFFailure Data Analuysis Page 30f6 "' -.---~...'-'-'- -- .. ~..- .._...._... -_......~",._._.-.....- - ._,"'_._-~._.._._-,._... ....~_...'----- _.,~_._-------, .__....__..._.~-,.~----_...-- -