In 1972, Yosemite National Park began allowing lightning fires to burn within special fire management zones, resulting in some of the park’s most beautiful landscapes.

During the drought conditions in the summer of 2021, wildfires raged throughout the western U.S. For example, as of September 12, 2021, the Dixie Fire is the second-largest wildfire in California history.

It might seem ironic, then, to choose this time to talk about wildfires in a positive light. But a new study published in the science journal Environmental Research Communications on August 6, 2021, states that half a century of allowing lightning fires to burn in Yosemite National Park’s Illilouette Creek Basin has created a “lost” forest ecosystem that is far more resilient to the impacts of drought, wildfires and climate change.

Of course, there is risk in allowing lightning fires to burn. And detecting lingering smoke over iconic sights, such as Yosemite’s Half Dome, isn’t something tourists especially want to see. But in the Illilouette Creek Basin, such fires have brought undeniable ecological benefits, including boosting plant and pollinator biodiversity, limiting the severity of other wildfires and increasing water availability during times of drought. What’s more, these benefits are likely to make forests in the Sierra Nevada more resilient to the warmer, drier conditions now being caused by climate change.

Public Domain

During the 2021 Dixie Fire in California’s Lassen National Forest, the Ruby Mountain Interagency Hotshot Crew conducted burn-out operations.

Back in time

Recently, scientists from the University of California, Berkeley, gathered decades of research documenting how the return of wildfire has shaped the ecology of Yosemite National Park’s Illilouette Creek Basin and Sequoia and Kings Canyon National Parks’ Sugarloaf Creek Basin since the parks adopted policies allowing lightning-ignited fires to burn—at Illilouette Creek in 1972 and Sugarloaf Creek in 1968. For almost 50 years, then, lightning-sparked blazes have rippled across those landscapes. They were closely monitored, but largely unchecked, whether their flames exploded into plumes of heat that burned whole hillsides at once or sat smoldering in the underbrush for months.

What resulted, say the scientists, is approximately 60 square miles of forest in the Illilouette Creek Basin that look remarkably different from other parts of the Sierra Nevada. Instead of dense, wall-to-wall tree cover—the outcome of more than a century of fire suppression—the landscape is broken up by patches of grasslands; shrublands; wet meadows filled with wildflowers more abundant than in other parts of the forest; spots with young, regenerating trees from a fire 15 years ago; and areas where a classic understory burn has resulted in big, old, widely-spaced trees. These gaps in the canopy are often punctuated by the blackened husks of burned trunks or the fresh green of young pines. There are even areas untouched by fire because of more moisture, such as those adjacent to creeks or on the edge of meadows. It’s a glimpse into what the Sierra Nevada was like 200 years ago.

The fires-allowed era

For millennia, wildfires sparked by lightning or lit by Native Americans regularly shaped the Western landscape, triggering the necessary cycles of rebirth and regeneration. However, the arrival of European colonists in the late 1800s—followed by the formation of the U.S. Forest Service in 1905—ushered in an era in which fire was viewed as the enemy of forests and humans alike, and the vast majority of wildfires were quickly extinguished.

Half Dome obscured in smoke from a wildfire is not what most visitors hope to see when they arrive at Yosemite National Park. ©John Leszczynski, flickr

In the 1940s and 1950s, though, a number of ecologists and forest managers began to question the wisdom of fire suppression, noting that the practice was eliminating valuable wildlife habitats and increasing the severity of fires by allowing decades of fuel to build up. They realized that fire was an integral piece of these ecosystems.

Finally, in 1968, the U.S. National Park Service changed its policy to allow lightning fires to burn within special fire management zones, usually remote regions at high elevations, where danger to human settlements was low. Sequoia and Kings Canyon National Parks established the first fire management zone that year, followed by Yosemite National Park in 1972.

Wildfire benefits already present

Illilouette Creek Basin experienced 21 fires between 1973 and 2016 that were larger than 100 acres—approximately equal to 75 football fields—while Sugarloaf experienced 10 fires of that size. When we picture a forest, we tend to think that they should be green all the time and made up only of big trees. But every forest needs to grow young trees and regenerate. Fire is the way to do that, but it isn’t always clean and nice-looking. In Illilouette, the forest may appear a bit messy, but it holds a lot of resilience.

Smoke from a forest fire makes an eerie but ethereal backdrop for Yosemite’s iconic Vernal Fall. ©Jeremy, flickr

The detailed history of wildfires in Illilouette has also provided foresters with valuable information on how the impact of one wildfire on the landscape and its vegetation can influence the trajectory of the next wildfire. Since fires are generally allowed to burn freely here, researchers can look at what happens when two fires have burned close to each other: when does the second fire burn into the area that was burned by the first fire, and when does it stop at the previous perimeter? The researchers discovered that it depended on the amount of time that had passed since the first fire. If it had been nine years or under, fires almost never burned into a previous fire’s perimeter.

Illilouette has also given forest managers a unique opportunity to study how wildfires behave under a variety of conditions, rather than only at their most dire. Under the fire suppression policy, huge fires are allowed to burn when we can’t put them out. But if a fire is more mellow, it’s extinguished. By leaving fires alone in Illilouette, they burn under a full range of weather conditions, making for varied effects.

A key finding was that as California endures yet another year of extreme drought, returning fire to Illilouette has had the somewhat counterintuitive impact of increasing the availability of water in the basin. By using measurements and simulations, environmental engineering experts found indications that small gaps in the tree canopy created by wildfires have allowed more water from rainfall and snow to reach the ground, while also reducing the number of trees competing for water resources. As a result, soil moisture in some locations in Illilouette increased as much as 30 percent between 1969 and 2012, which likely contributed to very low tree mortality in the basin during the drought years of 2014 and 2015.

A helicopter drops water from a Bambi Bucket (which has a release valve on the bottom controlled by the helicopter crew) on the 2013 Rim Fire near Yosemite National Park. ©California National Guard, flickr

Measurements also indicate that streamflow out of Illilouette Creek Basin has increased slightly since the managed wildfire program began, while streamflow out of other similar watersheds in the Sierra Nevada have all decreased. Boosting the amount of water that flows downstream benefits both aquatic ecosystems and humans.

Accepting future risks

Most U.S. national parks now practice some form of fire use, rather than full fire suppression; and in 1974, the National Forest Service also changed its policy to allow some fires to burn on its lands. However, these federal fire use policies have struggled to gain a foothold, largely because of the inherent risks involved in managing wildfire.

The study’s authors acknowledge that the current fuel density in much of the Sierra Nevada—mixed with the hotter, drier conditions triggered by climate change—has made managing wildfire even riskier than it was when forest managers started allowing fires to burn in Yosemite in 1972. However, they argue, fire suppression will never succeed in the long term, because the longer that forest fuel sources are allowed to build up, the more likely it becomes that wildfires will turn catastrophic when they are finally sparked.

Researchers are in favor of more aggressive prescribed burning in places such as Yosemite National Park so that lightning-sparked fires can be allowed to burn more safely. ©Rennett Stowe, flickr

For expanded fire use to happen, however, forest managers and the public will need to be willing to accommodate unpredictability and risk. Some fires will be large and more severe, burning in places that have had very little fire for a century or more. And there’s no guarantee that future outcomes will look exactly like what’s happened in Illilouette, since the effects of climate change loom larger every year.

The researchers also advocate for more aggressive prescribed burning and restoration thinning throughout the Sierra Nevada to help get the forests to a place where lightning-sparked fires can burn more safely.

A moment with the monuments

Yosemite National Park’s wooded areas are certainly places of beauty. We can thank wildfires for some of that and appreciate the work that they do by watching this 4K-resolution video of Yosemite, shown below. It features iconic scenery from across the park, including Bridalveil Fall, El Capitan, Half Dome, Tunnel View and the gorgeous Mariposa Grove of Giant Sequoias. Recorded in June 2015, it predates the Mariposa Grove’s restoration of 2018, but the trees themselves haven’t changed.


Geological uplift and glaciers carved Yosemite, but wildfires decorated the scenery.

What may have undergone a transformation, I hope, is how we look at them—and the fires that restore them.

Here’s to finding your true places and natural habitats,