Will Zone Zero regulations require removing trees?
- Trees do not reduce home ignition risks, so removing trees is an expensive waste of trees and loss of shade, beauty and urban nature.
- Start by watching two “shorts,” each two-minute: How come some trees don’t burn in urban fires? and Why trees aren’t burning in urban wildfires?
- Wait for Board of Forestry to revise Zone Zero regulations to require the following: clear needles and leaves from roofs, remove “ladder” branches, trim canopy 10 feet away from chimneys and 5 feet away from roofs.
- Scroll down to read about what homeowners can do and what happens when trees are removed needlessly.
How do trees ignite? How do houses ignite?
What does tree biology tell us about how trees ignite? (or not)
- Tree trunks have high moisture content, covered with protective bark, and often 6 to 20 inches in diameter (dbh).
- Primary branches are 2 to 6 inches in dbh and are generally smooth or lightly textured.
- The trunk and branches are like big green firewood logs, which ignite only after burning a lot of kindling and smaller logs. Leaves are green and well hydrated, and in a wildfire would be blowing wind, embers blowing through them without landing or igniting them.
- Current regulations require branches 6 to 12 feet up from the ground to be removed, so they aren’t “ladder fuel” that could be ignited from burning ground plants and then spread to upper branches.
How can trees ignite houses? (or not)
- If the trees are not ignited (trunk, branches or leaves), they can’t and won’t ignite the structure.
- Leaves and needles on a roof can ignite from embers during a wildfire, which then ignites the roof. Existing regulations require that they be removed, and that tree branches growing over the roof be trimmed, as they will drop leaves and needles.
- Palms are a different story. Embers land in the dry fronds at the top of untrimmed palms and ignite the fronds. Winds can break off and blow the burning fronds onto other houses or palms, known as “Roman candles.”
What do studies tell us?
- In cities, houses and structures burn from ember ignitions and adjacent burning structures. Many post-fire photos show living trees among burned houses.
- Scientific evidence from wildfire research and post-fire assessments demonstrates that mature, hydrated trees do not pose an inherent ignition risk to structures. References and links here.
- Large-diameter trunks and primary branches contain substantial internal moisture and require prolonged, high-intensity heat to ignite, conditions not typically produced by wind-borne embers alone.
- Observations from recent California wildfire events consistently show that trees are more often damaged by burning structures rather than serving as ignition sources for homes.
- Some studies suggest that tree canopies may sometimes serve as screens to intercept or deflect embers and reducing ember deposition on roofs, vents, and other vulnerable building components.
What can homeowners do to protect trees and structures?
- Clear needles and leaves from roofs, remove lower branches of trees (“ladder fuels”) and trim canopy 10 feet away from chimneys and 5 feet away from roofs.
- Watch this 2.5-minute video from California Office of the State Fire Marshal, “Wildfire prepared home, incorporating Zone Zero.”
- Remove leaves and needles from roofs. Both of these are in current PRC 4231 and outlined in CalFire’s Defensible space info.
- Learn about the Safer from Wildfires cooperative effort of CalFire and California Department of Insurance, with Ten Steps that will qualify properties for an insurance discount. Step 2 needs to be clarified, to retain trees that are trimmed, maintained and hydrated.
- Take this Ready for Wildfire Self-assessment that guides homeowners into identifying risks and specific steps to reduce those risks in defensible space, home hardening and firesafe landscaping.
- Take care of your trees. Keep trees hydrated (before, during and after wildfires), info at Save our Water and Trees.
- Maintain trees property. Hire an arborist.
- Refer to San Diego County information on defensible space, and the City of San Diego’s guide to defensible space.
What happens if trees are removed needlessly?
- At risk is the expensive wasteful removal of trees that don’t ignite in urban ember-driven wildfires and won’t reduce structure ignition risks.
- These trees provide many valued and familiar benefits of trees, including shade, cooling, lower energy use, aesthetics, social pleasures, and canopy goals in Climate Action Plans.
- Local implementation is critical, as fire departments interpret, advise, and enforce tree retention/removal.
- Tree care companies are obligated to prune but not remove trees.
- There is great risk and already evidence that insurance companies become “de factor” interpreters and enforcers of Zone Zero regulations.
- Urban tree losses can have broad county- and state-wide unintended consequences in tree canopy cover, public health, and sense of community.