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Trees

veg above and below

Trees have many beneficial qualities, including their ability to absorb solar radiation and provide shade. From a fire safety perspective, trees overhanging a home can cause physical damage to the house from branches rubbing on the roof or walls, but more importantly, they produce leaves and debris that accumulate on the roof, in gutters, or the surrounding landscape. For these reasons, it is recommended pruning trees or branches that overhang any roof or deck. A healthy and lush green tree canopy itself is not a concern.

The table below describes the intent behind each zone as well as recommended strategies for the placement and maintenance of trees within each zone.

 

Zone

Purpose

Recommendations

Zone 0 (0-5 ft)

"The Ember Resistant Zone"

Zone 0 can reduce the likelihood of structure ignition by reducing the potential for:

  • Direct ignition of the structure from flame contact
  • Embers that accumulate at the base of a wall
  • Indirect ignitions when embers ignite vegetation or other combustible materials located close to the structure that result in either a radiant heat and/or a direct flame contact exposure to the structure.
  • Rake, sweep, or dispose of leaves and other debris that accumulate throughout the year.
  • Remove tree limbs that overhang structures. These limbs can lead to greater accumulation of vegetative debris on and near the home. 
Zone 1 (5-30 ft)

"The Lean, Clean, and Green Zone"

Zone 1 emphasizes fuel modification such that there is a disconnection between planting groups and trees so that fire does not travel directly to the structure. Short-statured trees are appropriate for Zone 1.
  • Remove and dispose of all fallen leaves, needles, twigs, bark, cones, and small branches surrounding the vegetation.
  • Remove and dispose of dead and dying trees.
  • Trim tall trees to remove limbs from 6 feet to 10 feet off the ground.
    • For younger and smaller trees, limbing lower branches over time but maintaining at least two-thirds of the total height in foliage (that is, only removing branches in the lower one-third of the tree).
  • Remove branches that overhang the roof of the home or come within 10 feet of chimneys.
Zone 2 (30-100 ft)

"The Reduced Fuel Zone"

Zone 2 is focused on changing the behavior of fire to help personnel defend a structure safely. Reducing the densities of trees can slow fire spread and reduce flame height. Trees do not have to be eliminated, but should  be managed.
  • Remove and dispose of all fallen leaves, needles, twigs, bark, cones, and small branches surrounding the vegetation.
  • Remove and dispose of dead and dying trees.
  • Thin trees so the branches that extend between trees, or groupings of trees, are separated by at least 10 feet. 
    • Plan for the growth of retained trees.
  • For taller trees, limb the lower branches up to a height of at least 10 feet to reduce connectivity between fuels on the ground and the tree canopies. 
  • Remove of all piles of dead vegetation.

 

Thinning for Defensible Space: Tree Selection Considerations

Before deciding which trees to cut, it’s important to start with determining the goals for your property and setting objectives to accomplish your land management goals.  In addition to prioritizing the modification of vegetation structure and composition to reduce anticipated fire behavior, landowners might also want to consider secondary goals like safety, tree and forest health (i.e. improve tree growth and resistance to drought, fire, insects and diseases), and/or aesthetics.   

 

Set Your Objectives

Objectives should be goal-oriented and based on how you change existing conditions towards the desired conditions representative of your goals. Below is a table outlining examples of goals and associated potential objectives.

Goal
Common Potential Objectives
Defensible Space
  • Reduce surface fuels (e.g. leaves, tall grasses, shrubs) and ladder fuels (e.g. large shrubs and small trees).
  • Break up the vertical and horizontal continuity of surface and ladder fuels with the overstory tree canopy.
Safety
  • Remove dead and dying trees within striking distance of structures and roads.
Tree and Forest Health
  • Retain healthy and large and medium sized trees.
  • Thin/remove less unhealthy or diseased medium and large trees to reduce overall tree density and capture future mortality.
  • Maintain species diversity.
Aesthetics
  • Promote visual depth, diversity of textures, or fall colors.
  • Maintain and highlight high value trees or landscape features.

 

Develop a "Prescription" and Guidelines to Meet Your Objectives

A prescription is developed to describe the treatment, or a series of treatments, designed to modify existing forest structure and composition to one that meets the landowners goals and objectives.  The prescription typically includes tree designation guidelines with a general order of priority to provide direction on how to select trees to retain or remove in order to meet the various landowner goals and objectives. 

Consider the following prescription criteria with higher priority criteria generally taking precedence over the next higher criteria:  

  1. Tree Size and Species: Prefer to retain healthy, large diameter trees. Prioritize the removal of shrubs and medium to small trees that have connectivity with surface fuels.
  2. Tree Crown Characteristics: Retain trees with full healthy tree crowns.
  3. Disease or Damage: Remove trees that display signs of insects, disease, dead tops, excessive limb dieback, or other damage (e.g., forked stems, crooks, broken tops).
  4. Tree Spacing: Prioritize the retention and tree spacing for the largest, healthiest, most desirable trees.
  5. Desired Tree Species: Retention of various species will depend on your goals and objectives.

 

Additional Resources

Check out these trees and defensible space-related resources from the UC ANR Fire Network and our colleagues!