Â鶹ÒùÔº

On the shores of Lake Tahoe at Emerald Bay State Park grows what some consider to be the most iconic old-growth forest in the Lake Tahoe Basin. Giant ponderosa pines—some of the last remaining in the area—share space with at least 13 other tree species.

Yet despite its high conservation value and proximity to severely burned forests, the Emerald Point stand has not been managed to reduce its risk of drought or catastrophic wildfire. The fire-adapted forest has also not experienced fire for at least 120 years. This has led to massive increases in forest density, fuels, and insect- and drought-driven mortality.

A fire modeling study conducted by the University of California, Davis, and the University of Nevada, Reno, found that forest thinning followed by a prescribed burn could greatly improve the stand's resistance to catastrophic fire. The study, in the journal Fire, indicates that such treatments could also help other seasonally dry, mature, old-growth forests in North America.

"I know it sounds cliché, but we need to fight fire with fire," said lead author JonahMaria Weeks, a recent Ph.D. graduate from the UC Davis Department of Environmental Science and Policy. "When it comes to the conservation of old growth stands like the one at Emerald Point, prescribed fire is an essential management tool in reducing the risk of complete loss due to catastrophic wildfire."

Big, dense and dry

The Emerald Point stand supports the largest remaining ponderosa pines in the Lake Tahoe Basin. Some trees are more than 200 centimeters, or 6.5 feet, in diameter. Other sizable residents of the stand include Jeffrey pines and California incense cedar.

Old forests like this used to dominate California's mountain landscapes. Frequent, low-severity fire was critical to their long-term persistence in the Sierra Nevada. It removed fuels, knocked back competitive but fire-intolerant tree species, and drove the evolutionary selection of traits that protected the pines from most fire damage. But the arrival of Euro-American and other settlers in the late 19th century brought with it a fear of fire and more than a century of fire exclusion.

Most large ponderosa pines at Lake Tahoe were logged in the 1800s to support silver mining. Although the Emerald Point stand was spared, a lack of low-severity fire has made the stand far more dense, as historical photos and accounts indicate. Surface fuels and tree deaths have also increased, the latter driven by water stress and insect outbreaks linked to the forest stand's high density.

Modeling fire behavior

Several severe wildfires have burned in the southern Lake Tahoe Basin over the past two decades. The 2018 Emerald Fire burned just 1.25 miles south of the study site, and the 2021 Caldor Fire damaged or destroyed numerous old-growth forest stands.

To explore wildfire risk at the Emerald Point stand, the authors modeled potential fire behavior under severe fire weather conditions using plot data collected at the site. They simulated four fuels management scenarios to test the efficiency of each in reducing fire risk:

• The most conservative scenario included no thinning or fuel removal.
• The most intensive scenario used historical, pre-1850s forest conditions as a target. It removed most trees between 8 to 32 inches in diameter at breast height, followed by a fall prescribed fire.
• A third scenario included hand thinning followed by pile burning.
• The final scenario was a spring prescribed fire treatment without thinning.

Two scenarios—no management and the spring prescribed fire—resulted in complete stand mortality from the simulated wildfire. The hand thinning plus pile burning scenario and the historically based thinning plus prescribed fire scenario resulted in only minimal losses.

The authors determined that the management scenario based on historical conditions was most likely to help old trees at Emerald Point persist. The paper poignantly acknowledges the loss of the Beaver Creek Pinery old-growth forest in the Lassen National Forest to the 2024 Park Fire. Plans to reduce fuels in that stand had been discussed for years but were never implemented.

Conservation alone won't protect forests

"The conservation of old growth in dry conifer forests of the American West is impossible without due consideration and mitigation of wildfire risk," said senior author Hugh Safford, research faculty in the UC Davis Environmental Science and Policy department. "After 100-plus years of fire suppression and the loss of most of the old trees on our landscapes, it is reckless and short-sighted to think that mere protection of old growth in fire-prone landscapes will conserve it."

Instead, Safford said, conserving old-growth forest at Lake Tahoe and other fire-dependent ecosystems means actively managing the forest in ways that replicate the essential ecological roles of fire.

Additional co-authors include Bryant Nagelson and Sarah Bisbing with the University of Nevada, Reno.