Public Safety

Measuring and Managing Fire Risk in the Backcountry

CAL FIRE Forester Eric Just demonstrates how he clips and dries vegetation to check for fuel moisture content.

After five years of prolonged drought, San Diego County native plants, trees and grasses are either extremely dry, dying out or dead, potentially making for more volatile fire conditions and a longer peak fire season.

How volatile exactly? Firefighters in CAL FIRE’s Pre-Fire/Resource Management Office go out twice a month from May to October, to check on fuel moisture content to gauge how dire conditions are in the backcountry areas.

“Five years of below average rainfall is definitely increasing die-off of our shrubs in the foothills and mountains,” said Eric Just, a CAL FIRE forester, who also serves San Diego County Fire. “When you have dead fuels, it will give us more of a fire issue.”

Firefighters deal with two types of fuels: the living portion of the vegetation, or live fuels, and the portions that have died over the years, or dead fuels. One of the biggest factors affecting fuel flammability is fuel moisture content. Live fuel moisture depends on the plant species, time of year, and annual rainfall, said Pete Scully, who works in the unit with Just. The less moist it is, the more likely to burn.

Three pine trees in the Mount Laguna area show the various stages of the drought.
This line of bedraggled pine trees in the Mount Laguna area show the various stages of the drought.

Currently, San Diego County is about 10 percent below average for live fuel moisture — not quite a historic low, but on track to get there. Once plants get below 65 percent of fuel moisture in the live portions of the plant, they are in a critical stage and are more likely to burn easily. San Diego vegetation is holding fuel moisture now in the high 40s, low 50s, said Just.

Dead fuel moisture depends on the current weather conditions and can range between 1 and more than 20 percent. Due to this much lower moisture content, dead fuels are primarily what carry a fire from one point to the next under normal conditions, he said.

Just and his staff check on sentinel sites to test the moisture content in a native plant called chamise, which is abundant throughout the state. The results are then entered into a national database, which allows firefighters to compare conditions and give a regional picture of the fire risk.

To determine fuel moisture content, they clip and collect established growth from live plants and then put them in a live fuel moisture analyzer machine. It weighs the sample at the beginning of the process, then heats up and evaporates all the moisture out of the sample and then weighs it again. The weight difference is the percentage of moisture that was in the sample.

“It tells you what you can expect in terms of relative fire behavior of what you could expect if a wildland fire were to ignite,” said Just.

As for our trees, much of our forestland in San Diego County has been consumed in past fires and not a lot of it is left with the exception of just a few unburned islands of trees, said Just. The County as a whole is still trying to heal from the 2003 and 2007 firestorms, he said.

A nearly dead oak tree in the Mount Laguna area is under attack by drought and a beetle infestation.
A nearly dead oak tree in the Mount Laguna area is under attack by drought and a beetle infestation.

Oaks that survived those fires have come under attack from a different source: the goldspotted oak borer. The beetle has devastated oak woodlands, which typically surround communities like Ramona and Julian. Since the insect has no known predators here, the infestation has grown and the problem has largely gone unabated.

“We estimate at least 75- to 95,000 acres have been lost,” said Just. “It’s completely altered our oak woodland forest that San Diegans are so in love with.”

Just said there has been no funding for research into scientific means of combating the beetle, such as disrupting its reproduction cycle. For now, they rely on best management practices for containing the beetle.

“As far as affecting fire behavior, whenever you have dead trees, fires are going to burn much hotter and more intensely in those particular areas,” Just said. “When we are fighting a fire, oak woodlands are actually seen as a place where you can expect fire behavior to moderate because oaks are resistant to burning, humidity is higher and there’s not a lot of surface growth, just grass. But if you remove a woodland area and replace it with brush, then you completely alter the fire ecology of that area.”

Just said there is debate over whether removing dead trees truly reduces the fire risk or whether it has little influence at all from a fire behavior perspective.

The biggest concern for Just is what grows in the place of the oaklands. The shade from the canopy of the oaks keeps brush from growing underneath, but if the tree dies and native vegetation and grasses move in, then the area is more susceptible to fire.

“Our native vegetation, the shrubs, are the most volatile vegetation you can have, so it’s good having a little break in that with oak woodland,” Just said. “So, that component of not having it will add a lot to our concerns.”

The fuel moisture program is just one component of the work the Pre-Fire office is tasked with to help manage fire risk. He and the other forester in his office plan and carry out prescribed burns, create fuel breaks, cut defensible space around communities, and oversee fuel reduction projects and grant management.

On the personal preparedness side, property owners can also help the fire service manage live and dead vegetation by creating and maintaining a minimum of 100 feet of defensible space around their homes to reduce the fire risk. Defensible space includes cutting out and removing all dead fuels and thinning out the surrounding vegetation. To learn more, visit

Yvette Urrea Moe is a communications specialist with the County of San Diego Communications Office. Contact