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Creating more fire -resilient trees involves a three-part process (USFS 2013): 1) Reducing surface fuels 2) Reducing ladder fuels 3) Reducing tree crown density through crown thinning For the majority of wildfires, fire behavior, or the intensity and rate of spread, is determined by the volume and arrangement of vegetation on the surface. Surface and ladder fuels should be the highest priority for management in order to reduce fire intensity, rate of spread, and crown fire potential. Active crown fires are initiated with individual tree crowns igniting but are ultimately sustained by the density of the overstory crowns. Reduction in potential surface fire behavior plus an increase in canopy base height minimizes torching potential (Agee & Skinner, 2005). The tree maintenance standards presented in this section are intended to reduce fire hazards by rearranging and maintaining the fuels' spatial distribution. All vegetation, including trees, will burn, given the right conditions (Reinhardt et al., 2008). Therefore, the goal of fuel treatment is not to remove all trees or vegetation but to minimize the potential for ignitions, crown fires, and extreme fire behavior by reducingfuel loads and altering the retained vegetation structure, composition, and spacing (horizontal and vertical). These recommendations should be conducted for all trees within 100 feet of the structure, and those that overhang or are within 20 feet of a road or driveway. The Livermore -Pleasanton Fire Department has published a vegetation management standard that details how to properly maintain other types of vegetation, such as grass, brush, and other types of vegetation (LPFD 2021). SubAppendix B contains the LPFD's Vegetation Management Standard. Tree maintenance recommendations and vegetation management to meet defensible space requirements and reduce wildfire risk generally do not change depending on who performs the work or where the tree is located. In the WUI areas of the City, trees within 100 feet of a building are recommended to be maintained to meet the recommendations in section 1.3 and appendices D, E, and F of the UFMP and the guidelines published by the Livermore -Pleasanton Fire Department. The two exceptions to this rule are: 1) Trees along public roads should be maintained so that vehicles can travel unobstructed 2) Vegetation clearance around high-voltage electrical equipment is generally the responsibility of the electric utility, which is PG&E in the City. Tree Arrangement Fire spreads horizontally (from shrub to shrub or treetop to treetop) and vertically (from understory vegetation to tree). A more significantfire hazard exists where the spacing between trees and between shrubs and lower branches is close enough for the fire to preheat vegetation and ignite across the gap, either horizontally or vertically. If a ground fire transitions into a tree canopy fire, it will quickly spread from one tree to closely adjacent trees. The primary goal of tree arrangement is to break up the continuity of the flammable materials in the tree canopy through proper tree spacing. To accomplish this, the following recommendations are provided in the LPFD vegetation management standard (Livermore -Pleasanton Fire Department, 2021). It is important to note that while the standards below should be followed for new tree plantings, it may not always be possible or advisable to prune existing trees to provide 10, 20, or 30 feet of clearance between tree branches, so existing trees should be pruned as feasible while still within ISA standards: • For slopes less than 20 percent: trim or space trees to provide 10 feet of space between the tips of tree limbs. Trees should be limited to groupings of 2-3 trees, with each grouping separated horizontally as described herein. • For slopes that are 20 to 40 percent: increase spacing to 20 feet. APPENDIX A -WILDFIRE 19 of 219