Study finds severe blazes less common in today’s forests

By Phil Taylor | 09/11/2015 01:17 PM EDT

Severe wildfires were less common over the past three decades than they were prior to the 20th century, according to a peer-reviewed study that challenges popular views that such wildfires are on the rise.

Severe wildfires were less common over the past three decades than they were prior to the 20th century, according to a peer-reviewed study that challenges popular views that such wildfires are on the rise.

The study published this week in PLOS ONE found that Forest Service efforts to lower fire intensity actually harm wildlife that depend on severely burned patches of woods. Such "restoration" efforts can also create uniform tree stands that are less resilient to climate change, insects and other disturbances, it found.

"Infrequent severe fires are major ecosystem renewal events that maintain biological diversity, provide essential habitat for wildlife and diversify forest landscapes so they are more resilient to future disturbances," said study author William Baker of the University of Wyoming. "Recent severe fires have not increased because of mismanagement of dry forests or unusual fuel buildup, since these fires overall are occurring at lower rates than they did before 1900."


But the assumptions underlying Baker’s study have been challenged by more than a dozen scientists, and his policy recommendations yesterday were criticized by a Nature Conservancy official as unrealistic in the modern West.

Calls for more severe wildfires are politically unpopular in the West, particularly in a wildfire year that has burned 8.7 million acres — just shy of the most acres burned to date in the past decade.

The Forest Service yesterday declined to comment on Baker’s work.

A spokesman cited Chief Tom Tidwell’s testimony to a House committee in April that forests are "at risk due to uncharacteristically severe wildfires" and must be thinned to reintroduce low-intensity fire. The agency believes 58 million acres of national forests are at "high or very high risk" of severe wildfire.

"The Forest Service position is based on extensive research by hundreds of experts in their field from throughout the country and is not impacted by the assertions in the study you mention," said spokesman John Haynes. In addition, he said, the increase in homes in the wildland-urban interface (WUI) — where houses and other structures mix with undeveloped areas — "creates a unique set of challenges for limiting severe wildfire activity."

Baker’s study found the agency’s position flawed.

It reconstructed pre-1900 fire rates using thousands of surveys conducted by the General Land Office (GLO), early government reports and old aerial photographs. He confirmed his findings by sampling charcoal sediments in rivers in Arizona, Idaho, New Mexico and Oregon that are indicators of past severe fires.

Baker’s study covered roughly 63 million acres, an area about the size of Oregon that accounts for about 20 percent of the conifer forests in the West. A severe fire was defined as one that kills at least 75 percent of the area of tree stems in a forest stand.

Baker concluded that, prior to 1900, it took between 217 and 849 years for severe fires to burn through all of the dry forests in the West. That’s faster than the rate from 1984 to 2012, when it would take more than 875 years for severe fires to burn across the same area, he found.

That challenges the belief held by politicians and the Forest Service that the West is experiencing an unnatural burst in uncharacteristic wildfires as a result of a century of wildfire suppression.

Baker’s study further concluded that even if severe wildfires increase by midcentury, as predicted, most regions in the West would still see a lower frequency of severe fires than pre-1900 levels.

While earlier springs, warmer temperatures, decreased precipitation and increased drought have fueled an increase in fires since the 1980s, land managers should welcome more severe fire onto the Western landscape, rather than work against it, Baker wrote.

"High-severity fires are infrequent powerful events, not unlike volcanic eruptions, tornadoes or large floods that are almost beyond control or management," Baker wrote in the study. "Yet, the ecological importance of large, infrequent, and often severe natural disturbances in structuring historical landscapes and maintaining their biological diversity is well established."

While severe fires can bring catastrophe to Western homeowners, they’re a welcome sight to species like the black-backed woodpecker, which comes to feast on wood-boring beetles, Baker noted. Such fires also create sunny patches for native shrubs and foster a new generation of trees that are more resistant to insect outbreaks and drought.

The Forest Service currently squelches about 98 percent of fires that ignite, including many that would become severe, to protect assets like homes, power lines and water supplies.

Baker said it’s all right to reduce fire risk near houses and critical infrastructure, but the dry forests in the WUI are "not a safe place to live." Communities can promote more fire on the landscape by adopting growth limits, his study said.

"The people-fire problem is complex," he wrote. "But if expansion of housing and infrastructure into dangerous dry forests were redirected into safer settings, both people and nature would benefit."

So far, homebuilders do not appear to be heeding that call.

A report released yesterday by the Forest Service found that the percentage of homes in the WUI increased by 5 percent from 2000 to 2010. The estimated 44 million homes in the WUI — about 1 in every 3 homes in the Lower 48 states — are a significant driver for increased wildfire fighting costs, the Forest Service warned.

Such growth pressures the Forest Service to aggressively attack newly sparked blazes and pursue restoration projects designed to reduce the intensity of wildfires.

Such efforts include the Healthy Forests Restoration Act and Collaborative Forest Landscape Restoration Program, which enjoy broad support from the logging industry and many conservation groups in the West. But Baker said they are misguided.

Study draws skeptics

The assumptions in Baker’s research have earned him many skeptics.

Baker used similar historical GLO survey data to conclude in a 2012 study with Mark Williams, a recent Ph.D. graduate of the University of Wyoming’s ecology program, that dry forests were historically more densely packed with understory trees and shrubs and experienced significantly more severe fires than many believed. It challenged earlier studies that found dry forests were predominantly "park-like" and experienced frequent, low-severity fires that stayed low to the ground.

The study also challenged the ecological premise of a major Forest Service restoration project in northern Arizona known as the Four Forest Restoration Initiative, which aims to thin 2.4 million acres of what are believed to be overgrown ponderosa pine forests. The Williams-Baker study said much of those forests were historically dense to begin with.

As of last year, the Forest Service was pursuing nearly two dozen landscape-scale collaborative forest restoration projects that seek to "re-establish natural fire regimes and reduce the risk of uncharacteristic wildfire," according to Tidwell.

Other forestry scientists don’t buy Baker’s findings.

A group of 18 scientists led by Pete Fulé of Northern Arizona University published a rebuttal paper in Global Ecology and Biogeography questioning the Baker-Williams methodology.

"The inferences drawn by W&B about past forest ecology contrast sharply with those reported by numerous previous researchers, who used tree-ring and historical data to show that dry forests had predominantly surface fire regimes with relatively open, uneven-aged forests," Fulé wrote in a fact sheet accompanying the rebuttal.

Fulé and company took issue with the scientists’ use of GLO surveys — which provide clues on forest density, species and tree sizes at the time of the survey — to make conclusions about the era’s fire regime. Specifically, they challenged the scientists’ assumption that the presence of small trees was an indication of recent severe wildfire activity.

The dissenting scientists also question whether the classification of severe fires using GLO surveys can be compared to modern definitions of severe fires, which rely on more than just tree mortality.

"The weight of scientific evidence indicates that conserving these ecosystems and the valuable services they provide to society is not consistent with the modern pattern of increasingly larger and more severe wildfires," Fulé wrote. "Following the recommendations of W&B would be an experiment we cannot afford to conduct."

Fulé was in the field yesterday and unavailable to comment on Baker’s new study.

Baker responded yesterday that critics of his methods are ignoring steps he took to validate the fire frequencies he observed in the GLO surveys. One method was to carbon-date sediments in riverbeds that are believed to have been washed out by rains following severe wildfire events.

"These reconstructions are calibrated, validated, and corroborated," he wrote in his study. "There is calibration and validation with tree-ring reconstructions, there is corroboration by early historical records and maps, and there is congruence between the findings of GLO-based, aerial-photo-based, and paleo-fire based methods."

Baker’s study also tracks with the findings of a peer-reviewed study last year by nearly a dozen scientists that similarly found Western forests today experience fewer high-severity wildfires than they did more than a century ago (Greenwire, Feb. 14, 2014).

Chris Topik, a forest specialist at the Nature Conservancy, said he could not comment on the accuracy of Baker’s work. But he said he was concerned with the "vigor with which he jumps from his science to policy."

"The presumption is that historic forest conditions are the goal for future forest conditions," he said. "There are many more future needs of society and nature to consider."

For one, people have built homes and livelihoods in and around Western forests since the 1800s, forcing land managers to make tough decisions about where and when fire should be allowed to burn, Topik said. Moreover, roughly half of the water Americans use comes from forests, Topik added. Severe wildfires can create erosion that clogs critical water facilities, costing communities millions of dollars to fix.