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1. The trunks on live, healthy trees take much longer to ignite than the leaves and small branches in the crown. Most wildfires don’t burn in an area long enough to ignite the trunk. 16 Tree Management in the Wildland Urban Interface Area The behavior of a wildfire in the City’s WUI areas, as described in Chapter 2 of this section, is influenced by several factors, including the arrangement and volume of vegetation present. The maintenance of this vegetation can significantly impact the spread and intensity of a wildfire. This includes the maintenance of trees on private and public property. Proper tree care in the WUI areas can reduce the risk of extreme fire behavior, including torching or crown fires. The Role of Tree Maintenance in Managing Wildfire Risk Wildfire spread and intensity are dictated by the surface vegetation volume and arrangement. Trees and the tree canopy influence the fire's capacity to transition into and sustain torching or crown fires. Continuous vegetation from the surface to the tree canopy creates a path for fire to spread into the tree canopy. Tree litter indirectly contributes to the spread of wildfire on the surface by adding to the fuel volume on the ground. Therefore, the main fire prevention role of tree maintenance in Pleasanton’s WUI areas is minimizing the risk that a wildfire on the surface can spread into the tree canopy. A secondary role would be reducing surface fuel volumes by maintaining leaf litter. Tree maintenance recommendations intended to reduce the risk of a wildfire spreading into the tree crowns focus on separating the lighter, more readily ignited leaves and small branches in the tree crown from the surface vegetation that can sustain a wildfire. Vertical separation is created by reducing surface vegetation and raising the height of the lowest branches in the tree crown, which creates space to prevent direct flame contact and allows radiant and convective heat to dissipate.1Creating horizontal space between tree crowns also reduces the risk of a fire transitioning into them by allowing heat and hot gases to vent into the atmosphere and by reducing the risk that flames from one tree torching can come into contact with another tree crown. Tree maintenance recommendations for reducing tree litter’s contribution to the surface fuel load focus on maintaining the level of accumulated litter at a minimum so that leaf litter cannot contribute to the spread or intensity of a fire. In areas with minimal surface vegetation, particularly areas that serve as a barrier to fire spread( e.g., roads), the focus is on removing any accumulations of litter. Tree Maintenance Standards Trees in the WUI area require maintenance practices to increase horizontal spacing between retained trees to reduc the potential for crown fire spread. These practices remove ladder fuels (i.e., fuel that can facilitate fire spread fro ground fuels into tree crowns) by increasing the vertical spacing between surface fuels (shrubs, grasses) and tree crown to reduce the potential for surface-to-crown fire transition. Creating more fire-resilient trees involves a three-part proces 1) reducing surface fuels, 2) reducing ladder fuels, and 3) reducing tree crown density through crown thinning (USF 2013). For the majority of wildfires, the fire behavior, that is, the intensity and rate of spread, is determined by the volum and arrangement of vegetation on the surface. Surface and ladder fuels should be the highest priority for managemen in order to reduce fire intensity, rate of spread, and crown fire potential. Active crown fires are initiated with individu tree crowns igniting but are ultimately sustained by the density of the overstory crowns. Reduction in potential surfac 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