Southern California’s Palisades and Eaton wildfires killed 31 people, burned 37,000 acres of land, and destroyed more than 16,000 structures.
A new study by a Central Coast research team looked at a question surrounding the fires: Did neighborhood trees play a major role in the loss of homes, or was the density of homes a bigger issue?
"Our study found that once these fires were established in the neighborhood, the really big drivers were whether there was a structure close to a neighboring structure, in general, how many homes were packed tightly together, and not so much whether there was a lot of urban vegetation."
Reed Kenny is a Cal Poly San Luis Obispo biological sciences lecturer and the lead author of a new study looking at the question.
"My group, the Urban Forest Institute, and Cal Poly were getting a lot of inquiries about species of trees that might be particularly hazardous to grow in your neighborhood, or near your house," said Kenny. "That's because we run a website that helps homeowners select the right kinds of trees to plant in their neighborhood. We really want to help homeowners make more informed decisions."
Kenny says they used different data sets to compare the before and after of the Eaton and Palisade Fires. The research included 15,000 structures and more than 52,000 tree canopies in the footprints of both fires.
"We did some extensive touring of the fire scars and thought about how we could help generate data to make better decisions about these processes in the future," said the researcher. "One of the things we do at the institute is we make maps of where urban tree canopy is. It's going to be difficult to make any species-specific determinations, but we decided we could generate some data on whether having trees (more urban vegetation) in a neighborhood had an effect on whether individual structures were more likely to burn."
Kenny said the findings showed that houses and other structures were the primary fuel source, and not trees.
"Structure density was the strong driver of home loss across both fires. It's a very extreme situation once multiple homes in a neighborhood start burning. The fire can often then go between buildings, and one structure lights another on fire. That, combined with the really extreme wind events that were happening, meant that the homes that ignited also started generating embers, which spread extremely rapidly."
The researcher said the study is important because there are moves by the insurance industry and through state law to restrict urban canopies in the name of fire prevention.
"There's been a lot of attention paid to trees close to structures, and trees in neighborhoods which previously would not have had a lot of attention in terms of fire risk," said Kenny.
Kenny added that the Cal Poly team is continuing its research, with one important effort focused on helping to define types of trees that could be planted, which are more fire-resistant.
"Are the species of trees that are safer to plant in these high fire severity zones?" he asked. "We're going to do some lab experiments on different tree species that are commonly grown in California's urban forests."
Kenny believes the answer to improving safety is prioritizing home hardening, with steps like making vents and roofs ember-resistant. He thinks removal of properly maintained trees is not the answer, and that we need to remember the many environmental benefits trees bring to a community.
"We don't want to lose valuable urban canopy if we don't have to. We know there are benefits, and we know there are risks, and we need to accurately weigh those against each other to make informed decisions."
