By Cristina Santiestevan

Little Elden Mountain during Arizona’s Schultz Fire in 2010

Air support flies in to drop fire retardant on the mountain range just north of Little Elden Mountain during Arizona’s Schultz Fire in 2010. Credit: Brady Smith/USDA Forest Service, Coconino National Forest

On Saturday, February 19, 2011, a low-burning forest fire pushed up the western slope of Virginia’s Shenandoah National Park, toward Jenkins Gap. The fire would become known as the Smith Run Fire, and it was an accident. Improperly disposed ashes from a wood stove ignited in the wind and set the dry undergrowth aflame. By the end of that first day, the fire had crested the ridge and jumped the park’s famous Skyline Drive. The road and many surrounding trails were closed. Eight days later, after burning through 1,712 acres, the fire was extinguished and the area was reopened to visitors.

After the fire cleared, the air still tasted of smoke. The forest was blackened, and many trees were charred. In several places, the forest floor was a moonscape. The leaf litter — a near-constant presence in deciduous forests — was burned away. A spongy layer of dirt and ash had taken its place.

Despite all these signs of fire, the trees did not burn to the ground. In fact, it appeared that the flames rarely reached more than a few feet in height, and the burn marks mostly ended within a foot of the ground. None of this looked like the work of the roaring forest fires of our imaginations. The aftermath of the Smith Run Fire looked nothing like the photos and videos of just-burned western landscapes that have dominated the news cycles in recent summers. How could things that share the same name — forest fires — be so starkly different?

Forest-fire Factors
It’s simple, really: different forests.

Forests vary by species composition, density of growth, elevation, geology and climate, including temperature and rainfall patterns. Every one of these factors influences the nature of fire in a forest. For example, in one 1999 study, researchers discovered that fires tend to spread faster and further in white-fir forests than in adjacent red-fir forests in the Sierra Nevada. The difference has to do with needle size. Red firs produce smaller needles than white firs. The small needles create a dense mat on the forest floor, which burns less readily than the looser mat of needles found in white-fir forests. Because fire needs oxygen, dense mats of needles offer less access to oxygen than loose mats.

2007 Georgia wildfires

In 2007, Georgia experienced its largest fire in its history due to extreme drought conditions and the presence of excess forest fuels. Credit: National Interagency Fire Center Archive,

Fire also needs burnable, dry fuel. Wood, leaves, needles and other plant matter burn more readily than damp or wet fuel. This helps explain the fire season in California and other western states. Many western forests receive most of their moisture in the form of winter snow. As the snowpack melts, it saturates the soil, feeds streams and refills natural water reserves. By late summer, much of this moisture is gone, leaving the forests relatively dry and, as a result, more flammable. Eastern forests, by contrast, generally receive rainfall year round.

It should be no surprise that drought conditions can make fire worse. In the West, a winter drought will result in less snow, which means that snowpack-dependent ecosystems will dry earlier in the season. The result may be an earlier and more intense fire season. But drought affects eastern states as well. Moderate to severe drought is currently contributing to an increased number of wildfires in Florida, Georgia and the Carolinas.

Temperature, too, contributes to fire formation. Hot air accelerates evaporation, pulling moisture from soil and plants, and sometimes worsens drought conditions. East of the Rockies, heat waves tend to be associated with high humidity, which may help suppress fires. West of the Rockies, however, humidity is less frequently associated with high temperatures. The drier heat typical of most western states may help fuel fires by further drying forests.

Forest-fire Regimes
All of these ecological characteristics — climate, elevation, species composition and even needle size — combine to determine the type and frequency of fires for a particular forest or region, as forests fires are measured in two ways: severity and frequency. These two traits determine the fire regime — or pattern of fires — for a particular forest or region. In very general terms, fire severity tends to increase as fire frequency decreases. Fire Regime I, for example, describes an area where fires burn with low severity, but relatively high frequency — in a cycle of zero to 35 years. This regime covers much of the United States east of the Great Plains and south of New England, including Shenandoah National Park. Fire Regime II, which includes the Great Plains and other grasslands, features high-frequency and high-severity, stand-replacing fires — even a relatively minor fire will replace a “stand” of grass. West of the Rocky Mountains, however, Fire Regimes III and IV are more common. In these areas, fires are less frequent — operating on a 35 to 100-year cycle — but are more severe. For example, Fire Regime IV is characterized by severe, stand-replacing fires every 35 to 100-plus years. This regime is almost entirely absent from the eastern two-thirds of the continental United States, but dominates the southern third of California, an area that includes San Diego, Los Angeles and Santa Barbara. But, historic natural fire regimes can actually be misleading. A century of fire suppression has left a tinderbox of fuel in many forests.

Historical Natural Fire Regimes

Credit: Fire Sciences Laboratory, Rocky Mountain Research Station, USDA Forest Service

Less than half of all forests within the continental United States are actually burning like they should. Nearly 40 percent of forests are currently considered to be Condition Class 2, as determined by the USDA Fire Service. Forests in Condition Class 2 have experienced a moderate departure from their historic fire regime — they are now burning more or less frequently than they should or more intensely than historic records suggest is typical. Fifteen percent of our forests are in Condition Class 3. These forests are significantly altered from their historic fire regime — perhaps through fire suppression or other causes — and carry a high risk of fires intense enough to fully alter the ecosystem.

Forests in Condition Classes 2 and 3 are found across the United States, which suggests that many of our forests are burning more frequently or more severely than is natural. The impacts of this may be compounded in western forests, since many of these forests already belong to fire regimes that naturally burn more severely than others.

Fire Regime Current Condition Classes

Credit: Fire Sciences Laboratory, Rocky Mountain Research Station, USDA Forest Service

Fire suppression is not the only possible factor in why so many forests have departed from their historic fire regime. In some areas, invasive plants and invasive or over-abundant pests contribute to the problem. For example, some non-native plants — especially grasses — may help fuel fire by creating a flammable layer of dead or dry plant matter. And, because many invasive plants are opportunistic and fast growers, they are sometimes able to recover faster than native vegetation in post-fire landscapes. For example, fountaingrass — a non-native invasive in southern California — was one of the first plants to re-sprout following the San Diego wildfires of 2003. Invasive pests, such as bark beetles, may also increase the severity of forest fires by weakening or killing trees, which effectively increases the amount of flammable fuel in a forest.

Ultimately, however, climate change may have the largest impact on forest fire frequency and severity. One 2006 study found that “large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons.” Although the study’s authors found some link between wildfire frequency and historic fire suppression, they determined that rising temperatures, earlier snowmelt and more frequent droughts all play a much larger role. Ultimately, the paper concludes “the broad-scale increase in wildfire frequency across the western United States has been driven primarily by sensitivity of fire regimes to recent changes in climate over a relatively large area.” Climate change may also increase the frequency or severity of fires in eastern states, where hotter temperatures and more frequent droughts could contribute to future fires.

Re-growth in Shenandoah

Shenandoah National Park three months after the Smith Run Fire

Shenandoah National Park three months after the Smith Run Fire. Credit: Cristina Santiestevan

Shenandoah National Park immediately after the Smith Run Fire

Shenandoah National Park immediately after the Smith Run Fire. Credit: Cristina Santiestevan

By June 2011 — only four months after the Smith Run Fire — Shenandoah National Park’s forest is alive with fresh green growth. Young sassafras, pokeweed, aster and other opportunistic plants carpet the ashen forest floor. By October, those sassafras saplings are chest-high.

This is a typical response to a low-intensity surface fire. And, for much of the United States, this is what a post-fire forest should look like: a scattering of fire-killed trees and shrubs, engulfed by lush new growth and overshadowed by towering, fire-resistant trees. A mixed-severity fire would show more damage, including moderate effects on the canopy. These two post-fire scenarios should dominate the United States: low-severity fires east of the Rockies and a blend of low- and mixed-severity to the west. But, as a century of fire suppression combines with the impacts of climate change and invasive pests, these low-intensity burns are being replaced by powerful and destructive forest fires that have the ability to wipe out entire stands of trees.

It is too soon to know the full consequences of these changes to historic fire patterns, but current research suggests we can be confident they will continue. This winter’s unusually severe drought across the western U.S. already has experts wondering what the fire season will bring. Ultimately, we may need to replace the Historic Natural Fire Regimes map with an updated version.

Action Alert: Proper ecosystem management is essential to the survival of all species — plant and animal — within forests. Help urge USDA Forest Service Chief Tom Tidwell to shift post-fire forest management practices towards an ecosystem-level approach that considers and balances the needs of all species by sending him a letter with our easy-to-use, letter-writing form.