Nearly a century of fire suppression and the preferential logging of large-diameter trees, which are better able to withstand forest fires, have left forests vulnerable to more destructive, albeit less frequent, wildfires, the researchers said. In addition, the lack of fire has hindered nutrient cycling in forests and the proliferation of certain plant species, such as the sequoia, that rely upon fire to promote seed dispersal.

This realization led to the gradual re-emergence during the past 20 years of fuel-reduction as a forest management tool. The goal is simple: Thin or remove dense stands of trees, ground vegetation and downed woody debris in a carefully controlled way before they become fuel for a raging wildfire. When low- or moderate-intensity controlled burns are not an option, fire-prone trees are mechanically removed or shredded on site.

Such techniques are an attempt to emulate the frequent fires common in California for thousands of years. Before 1800, Stephens said, an estimated 1.1 million acres of forest burned annually in California, including wildfires ignited by lightning and other natural sources, and blazes set intentionally by Native Americans as a way to manage or alter landscapes. Most were blazes of low-to-moderate intensity that more than 80 percent of the trees could survive, unlike the catastrophic wildfires of modern times.

“Today, the combination of wildfires and fuel-reducing treatments only touch 6-8 percent of the land that used to burn annually before 1800, and fuel-reducing treatments alone only affect 1 percent,” said Stephens. “That’s a pittance. At that level, it’s just triage rather than fire prevention.”

To approach levels that have a chance of reducing wildfire risk in the long term, he said, the amount of land to be treated in a year would need to increase by 2-4 percent — still low compared to historical levels.

Stephens noted that two-thirds of the fuel-reduction treatments in the western United States rely upon mechanical thinning, which would be much more costly than prescribed burns to scale up. In the southeast region, the use of prescribed fire dominates.

In the West, particularly in California, the biggest challenge to expanding controlled burns is the potential reduction in air quality during treatment, said Stephens.

“We have a choice,” he said, “of dealing with lower levels of smoke from prescribed fires that may only be needed every 15 years or so, and which can be timed for optimum wind conditions, or acute levels of smoke from catastrophic fires that can last for months when they hit.”

The U.S. Department of Agriculture-U.S. Department of the Interior Joint Fire Science Program helped support this research.