How does tree mortality caused by drought and insects affect forests accustomed to frequent fire?

Research results were published January 17, 2018

This week a group of nine scientists and researchers published the results of their work considering how unusually high tree mortality affects wildfires in California’s Sierra Nevada forests that over thousands of years have adapted to frequent fire. They point out that fire suppression-caused forest densification has increased competition among trees for water and other resources, destabilizing many frequent fire forests by making them prone to mortality from other agents such as bark beetles.


Scott L. Stephens, Brandon M. Collins, Christopher J. Fettig, Mark A. Finney, Chad M. Hoffman, Eric E. Knapp, Malcolm P. North, Hugh Safford, Rebecca B. Wayman

The abstract and conclusions are below. The entire paper can be accessed at BioScience. The illustrations are from the document.


Massive tree mortality has occurred rapidly in frequent-fire-adapted forests of the Sierra Nevada, California. This mortality is a product of acute drought compounded by the long-established removal of a key ecosystem process: frequent, low- to moderate-intensity fire. The recent tree mortality has many implications for the future of these forests and the ecological goods and services they provide to society. Future wildfire hazard following this mortality can be generally characterized by decreased crown fire potential and increased surface fire intensity in the short to intermediate term. The scale of present tree mortality is so large that greater potential for “mass fire” exists in the coming decades, driven by the amount and continuity of dry, combustible, large woody material that could produce large, severe fires. For long-term adaptation to climate change, we highlight the importance of moving beyond triage of dead and dying trees to making “green” (live) forests more resilient.

Fire Responses Post Drought Beetle
A conceptual diagram showing fuel load and expected fire behavior in a mixed-conifer forest prior to and following a major bark-beetle-caused tree-mortality episode, with either (a) no follow-up-fuels treatment or (b) periodic prescribed fire to consume fuels. Surface-fire intensity is expected to roughly follow surface fuel load, whereas crown-fire potential is regulated by the amount of surface fuel (necessary to heat and dry live fuels to the point of combustion), as well as crown bulk density.


Forest Responses Severe Drought
Forest responses following a severe drought (1999–2002) in the Sierra de San Pedro Mártir (SSPM), Baja California, Mexico (a, drought and bark-beetle-caused tree mortality followed by wildfire; b, drought- and bark-beetle-caused tree mortality only) and in the southern California mountains (SCM), California, United States (c, drought- and bark-beetle-caused tree mortality at larger scales; d, drought and bark-beetle-caused tree mortality at stand scale. Note no wildfire in either SCM area). The SSPM and SCM photos were taken in 2004 and 2003, respectively. The SSPM site experienced a wildfire immediately following the multiyear drought (picture from 2003), with the photos capturing effects of both drought- and wildfire-related tree mortality. Pictures (a), (b), and (d) from SLS, (c) from G. Barley.



Unprecedented Sierra Nevada tree mortality has rapidly occurred after a severe drought with effects compounded by forest densification from decades of fire suppression. In the central and southern Sierra Nevada some areas have experienced more than 90% tree mortality, producing extensive landscapes of standing dead trees. This differs from mortality resulting from stand-replacing wildfire because bark beetles do not reduce surface fuels or jumpstart succession of shade-intolerant, fire-resistant pines. Forest managers have been struggling to determine whether these new postmortality conditions will increase wildfire intensity and/or severity, what the near- and long-term effects on forest communities will be, and what the appropriate intervention measures are.

In the first decade, wildfire severity in bark beetle killed frequent fire (FF) forests may be little affected over current conditions. Other than a brief increase during the “red phase” when most dead needles are still on recently killed trees, the reduction in canopy fuels is counterbalanced by an increase in surface fuels (figure 2). However, these are no grounds for complacency because current conditions in the majority of mixed-conifer and yellow pine forests in California already consist of unnaturally high surface fuel loads and corresponding elevated fire hazards (figure 2; Lydersen et al. 2014, Stephens et al. 2015).

The more troubling projection is how extensive loading of large-sized woody fuels in future decades may contribute to dangerous mass fires beyond the predictive capacity of current fire models. These fires can generate their own wind and weather conditions and create extensive spotting, making fire behavior and its impact on structures and public safety difficult to manage and predict. In addition, such intense fires could prevent forests from becoming re-established. Lacking the legacy of live trees that historic FF would have left (Stephens et al. 2008), large unburned areas of dead trees may also produce unusual forest succession patterns. These patterns will likely favor shade-tolerant and hardwood tree regeneration, limited shrub growth, and accumulating large woody fuels that would likely kill regenerating forests when wildfire inevitably occurs. The scale of contiguous tree mortality entrenches the homogeneity produced by fire suppression, reducing the fine-scale heterogeneity of forest conditions that contributes to resilience and biodiversity. Management could enhance adaptation to climate-change-induced stress if it focused more of its resources on creating spatially and temporally variable patterns in green FF forests that are better aligned with local moisture availability and fire patterns (North et al. 2009).

Many of our FF forests have failed to receive the very management that could increase resilience to disturbances exacerbated by climate change, such as the application of prescribed fire and mechanical restoration treatments (Stephens et al. 2016). Recent tree mortality raises serious questions about our willingness to address the underlying causes. If our society doesn’t like the outcomes from recent fires and extensive drought-induced tree mortality in FF forests, then we collectively need to move beyond the status quo. Working to increase the pace and scale of beneficial fire and mechanical treatments rather than focusing on continued fire suppression would be an important step forward.

More evidence of the “fireproofing effect” of insect outbreaks in a forest

Additional research finds evidence of a “fireproofing” effect on host trees from defoliation due to western spruce budworm outbreaks.

Above: Grand fir in Oregon defoliated by western spruce budworms. William M. Ciesla.

There is no dispute that severe outbreaks of western spruce budworm (WSB) and mountain pine beetle (MPB) in a forest have huge visual impacts. Many land managers have worried about more, larger wildfires and politicians have used it as an excuse for more logging.

But the commonly held belief that the effects will lead to higher intensity, more rapidly spreading wildfires has been disproven many times in the last eight years by scientists.

We first wrote about this issue in 2010 (Firefighters should calm down about beetle-killed forests) when some early research started to bring the facts to light.

The WSB and MPB attack trees very differently. The WSB defoliates the tree, consuming the needles and removing fuel from the canopy relatively quickly. The MPB kills the tree from the inside, leaving the dying “red” needles on the tree until they fall off in one to two years. The possibility of crown fires may increase during that red needle period, but it makes sense that fewer fine fuels in the canopy would reduce the fire intensity and make it less prone to transition from a ground fire to a crown fire. Both types of attacks eventually produce more course fuel on the forest floor as the branches break off and  the trees eventually fall over.

Research conducted by Daniel G. Gavin, Aquila Flower, Greg M. Cohn, Russell A. Parsons, and Emily K. Heyerdahl found evidence of a “fire proofing effect” for outbreaks of WSB. It does not address the MPB.

Below is an excerpt from their work:

“Extrapolating Results: Reduced Tree Mortality

“Taken together, the tree-ring and modeling studies suggest a lack of synergism between WSB outbreaks and wildland fires. However, a different kind of synergism may exist: Defoliation might dampen the severity of a subsequent wildfire. To explore this possibility, we used existing empirical equations that show the probability of mortality due to defoliation (fig. 3A) and the probability of mortality due to crown scorch (fig. 3B), combined with the simulated results of canopy consumption at different levels of defoliation (fig. 3C), to extrapolate the summed probability of mortality under a range of surface fire intensities and defoliation levels (fig. 3D). The results suggested a distinct “fireproofing” effect of defoliation: The increased risk of mortality by WSB is more than compensated for by reduced foliage consumption during moderate surface fire intensities. For example, trees with 50-percent defoliation have a distinctly lower probability of mortality when surface fires are less than about 74 kilowatts per square foot (800 kW/m2 ).

“However, we considered only the partial effect of defoliation on fire occurrence; we did not take into account other effects of WSB outbreaks, such as mortality of small trees. Of course, field observations are required to test our prediction. Remotely sensed burn severity maps, in combination with prior surveys of insect effects, could address this issue. One such study of the 2003 B&B Complex Fire in Oregon showed that prior defoliation had a marginal effect on reducing fire severity that was not statistically significant (Crickmore 2011). However, an analysis by Meigs and others (2016) of all post-WSB fires in Washington and Oregon from 1987 to 2011 showed that there is a statistically significant reduction in fire severity that persists for up to 20 years following an outbreak. Thus, the effect of defoliation on crown fire behavior modeled by Cohn and others (2014) appears to be confirmed by the analysis of burn severity data by Meigs and others (2016).

“Fireproofing Effect?

“It may seem reasonable to assume that extensive defoliation, causing sustained low levels of tree mortality in mature trees, should have a measurable effect on wildfire occurrence. However, fire is a highly variable disturbance in itself, and it is highly sensitive to specific climate and winds during the fire event. The scale of fuel changes wrought by WSB may be too small to affect subsequent fire probability in ecosystems where fire is limited by fuel moisture and ignition sources rather than fuel availability. Our data show that these two disturbance types do not share similar histories, despite a common link to drought events.

“Nevertheless, we hypothesize a “fireproofing” effect on host trees from defoliation due to WSB outbreaks. Although such an effect has been detected statistically from recent fire events (Preisler and others 2010; Meigs and others 2016), the inferred processes at play remain to be studied in detail at the site scale.”

UPDATE January 5, 2018: These researchers are not the only ones with similar findings on this subject. If you would like to read more, scroll through the articles on Wildfire Today tagged “beetles”.

Wildfire news, September 14, 2016

Highlights of recent news about wildland fire.

California has fewer inmates available for fighting wildfires

With fewer inmates available for fighting fires, the state of California is turning to civilian crews within their Conservation Corps.

From KCRA:

…But the number of available inmates is declining because counties now oversee most lower-level felons under a law aimed at easing prison overcrowding. In addition, there are fewer incentives for inmates to risk their lives since a federal court broadened an early release program for firefighters to include other inmates.

The state is about 600 inmates short of the 4,300 prisoners who could be available for fire lines. So this year, the California Conservation Corps reopened a camp to train three crews of young civilians to do the same backbreaking work as the inmates. Corps Director Bruce Saito expects to create at least four more fire crews with roughly 15 members each by next summer and a half-dozen new crews during each of the next two years.

The corps has more than 1,400 members, but fewer than 200 currently work alongside local, state and federal firefighters battling blazes in rural areas.

The members include both men and women and range in age from 18 to 25. They enlist for one year and earn the state’s minimum wage of $10 an hour. Military veterans can enroll until they turn 30…

Oregon sues 3 people responsible for starting the Ferguson Fire

Oregon hopes to recover $892,082 from three individuals who they say are responsible for starting the Ferguson Fire that burned 200 acres and destroyed two structures in Klamath County in July 2014.

The suit alleges that Joe Askins started a campfire, then took a nap. When he awoke, the campfire had escaped. Askins also said “I’ll take all the blame for the fire,” according to the lawsuit.

More evidence that beetle-killed forests do not increase the risk of catastrophic wildfires.

An article at News Deeply summarizes several research studies which mostly concluded that beetle-killed forests do not burn more severely than forests that have not been attacked by the insects. This is in spite of statements to the contrary by the Secretary of Agriculture, a spokesperson for CAL FIRE, and media stories about trees that are now part of a “tinder box”.

Air tanker 132 starts contract in Australia

air tanker 132 australia
Air Tanker 132 is reintroduced to the media in New South Wales, Australia. Photo by Sgt. Brett Sherriff, Royal Australian Air Force.

Fire Aviation reports that Coulson’s Air Tanker 132 started its contract with New South Wales on September 6, helping to provide air support for wildland firefighters in Australia. This is the second year in a row that the L-382G, a variant of the C-130 platform, has worked down under during their summer bushfire season.

Cheyenne is concerned about the effects of the Snake Fire on their water system

“The location of the fire is close proximity to our major watershed collection area for the Hog Park Reservoir” said Dena Egenhoff, the Board of Public Utilities’ (BOPU) Water Conservation Manager. “We are unable to know the impact of the Snake Fire at this time, but the location suggests there may be some adverse impacts to the City of Cheyenne’s water collection system.” As of September 11, 2016, the Hog Park Reservoir is 91.8% full

For Cheyenne, BOPU collects water in the Little Snake River drainage from snow melt and streams and transports it under a mountain by a tunnel to the eastside of the Continental Divide. That water is then stored in Hog Park Reservoir. From there, the collected water from Hog Park Reservoir is traded for water in Rob Roy Reservoir which can more easily be transported without pumping to Cheyenne. “In this way, the amount of water can be exchanged between the two different Mountain Ranges with all water rights being satisfied,” said Dena Egenhoff.

The Snake fire is in south-central Wyoming just north of the Colorado border. It is 115 air miles miles west of Cheyenne, and 20 miles west of the 38,000-acre Beaver Creek Fire that has been burning in Colorado and Wyoming since July 19, 2016.

Colorado’s Beaver Creek fire expected to burn into October

Firefighters are anticipating that it will take them until late October to contain the Beaver Creek fire, which is burning in one of the forests hardest hit by mountain pine beetle.

Tactics being used to contain the blaze have already emerged as a case study in how to suppress fire in an environment transfigured by thousands of dead trees.

Beetle-kill trees in the area thwarted firefighters’ attempts at a direct attack — downed trees made building a fireline difficult and gusts from helicopter rotors only caused more trees to fall, according to a lessons learned report published on July 27.

An indirect approach containing the fire became essential when initial attack crews felt radiant heat from flames a half a mile away:

Because of the extreme fire behavior exhibited early on in the Beaver Creek Fire, firefighters knew a direct attack would be both dangerous and ineffective…Firefighters removed fuels, wrapped buildings, laid hoses and sprinklers around the structures, and strategically burned out around buildings in advance of the fire.

The conditions in the Routt National Forest, along the Colorado-Wyoming border, also proved challenging to firefighter safety, according to a post from the incident management team on InciWeb.

The fire is burning in heavy beetle killed timber. The infested trees are subject to blowing over contributing large amounts of down timber and providing fuel for extreme fire behavior when strong winds and terrain features are in alignment, making the timbered areas unsafe for firefighters.

The fire, which started on June 19 in north-central Colorado, spread by several hundred acres during a hot, windy and humid day this week and forced firefighters to pull back to safety zones, The Denver Post reported.

As of July 29, the fire had burned 30,137 acres and is 12 percent contained.

Thanks to some northwesterly winds, Colorado residents can expect to see smoke from the Beaver Creek fire and other western wildfires this weekend, according to an update from the Colorado Department of Public Health and Environment.

Thanks and a tip of the hat go out to Allen.

The spread of the Beaver Creek Fire in northern Colorado slows

Above: Varying burn intensities on the Beaver Creek Fire.

The spread of the Beaver Creek Fire in northern Colorado one mile south of the Wyoming border has slowed over the last week. It has been listed at 13,275 acres since June 30 and according to the incident commander is 5 percent conplete after burning for 18 days. The strategy is not to put it out, but to manage it for “multiple objectives”.

The fire is 17 miles northwest of Walden, Colorado and 52 miles southwest of Laramie, Wyoming.

Within the last 48 hours the fire received about 0.2 inches of rain but the fuels should dry out today, aided by a 9 mph southwest wind gusting up to 23 mph.

Beaver Creek Fire beetles intensity
Photo of a portion of the Beaver Creek Fire in an area with heavy beetle kill. One smoke is visible. USFS photo by Andrea Holland.

People who are extremely worried about forests attacked by beetles and assume fire intensity will be greatly enhanced in those areas, should examine the photo above that was taken within the fire area.

Jay Esperance’s Type 2 incident management team will transition to the West Slope Type 3 Team B on Thursday.

The photos were provided by the Medicine Bow-Routt National Forests and Thunder Basin National Grassland. Except as noted the photographer and dates taken were not given.

More information about the Beaver Creek Fire.

Beaver Creek Fire map
Map of the Beaver Creek Fire July 3, 2016. USFS.
Beaver Creek Fire moose
A moose and her calf investigate evidence of firefighter activity on the Beaver Creek Fire.
Beaver Creek Fire sprinklers
Sprinklers are set up on an ATV bridge near the Beaver Creek Fire.
Beaver Creek Fire Chinook
A Chinook helicopter uses its snorkle to refill its internal water tank while working on the Beaver Creek Fire.
Beaver Creek Fire
AN area of high burn intensity on the Beaver Creek Fire.

Do California’s beetle-killed trees constitute an emergency?

Western Pine BeetleSome of the forests in California are experiencing a natural phenomenon that other areas in the Rocky Mountains, the Pacific Northwest, and British Columbia have been dealing with for years. Pine beetles, in this case Western Pine Beetles (WPB), are attacking and killing millions of trees. These things run in cycles and in this case the extended severe drought in the state has stressed the trees making it more difficult for them to fend off insects.

Politicians, residents, and even some individuals in fire organizations look at the hillsides with numerous dead or dying trees and intuitively think — dead vegetation — increased wildfire hazard.

Here are examples from the California Department of Forestry and Fire Protection (CAL FIRE):

From a 2015 news release:

These dead and dying trees create an environment more readily susceptible to dangerous and destructive wildfires.

In a video on YouTube the narrator says when referring to a beetle-attacked stand of trees:

…an increase in extremely flammable vegetation which could lead to larger, more intense and damaging wildfires.

SFGate quoted spokesperson Daniel Berlant:

“No level of rain is going to bring the dead trees back,” Berlant said. “We’re talking trees that are decades old that are now dead. Those larger trees are going to burn a lot hotter and a lot faster. We’re talking huge trees in mass quantity surrounding homes.”

A phone call to Mr. Berlant was not returned.

Those warnings are not 100 percent accurate. In increasing numbers, scientists are determining that generally, insect damage reduces burn severity. In one of the more recent studies, researchers from the University of Vermont and Oregon State University investigated 81 Pacific Northwest fires that burned in areas affected by infestations of two prevalent bark beetle and defoliator species, mountain pine beetle (Dendroctonus ponderosae) and western spruce budworm (Choristoneura freemani). The fires spanned the years 1987 to 2011.

Pine trees killed by bark beetles
Pine trees killed by bark beetles. Photo by Ethan Miller.

Few of the 81 fires occurred in forests while the needles were still on the trees in the red highly flammable stage of the outbreak shortly after the trees were killed by mountain pine beetles, so more research is needed about this phase. Aside from the one to two year red stage, the burn severity decreased for more than 20 years following a MPB attack. It makes sense that fewer fine fuels in the canopy would reduce the fire intensity and make it less prone to transition from a ground fire to a crown fire. This data was derived from fire behavior and data on actual fires, not laboratory experiments.

We contacted one of the researchers that conducted the study in the Pacific Northwest, Garrett Meigs, a Postdoctoral Research Associate at the University of Vermont, and asked him if their conclusions about reduced fire severity following a Mountain Pine Beetle attack in the Northwest could be compared to California’s situation — a drought combined with a Western Pine Beetle attack:

I am aware of the impressive amount of tree mortality in California but have not seen it with my own eyes. As such, I am hesitant to comment on the current conditions in California forests, which are beyond the scope of our recent studies in Oregon and Washington. My understanding is that most of the dying/dead trees are ponderosa pines, which have been affected by intensive drought and the western bark beetle (whereas in the PNW, we studied lodgepole pines affected by mountain pine beetle and mixed-conifers affected by western spruce budworm).

Another thing that is a bit different in California is that many of these forests are generally closer to large human populations, so there are more human values/resources at risk…and these forests at the wildland-urban interface have elevated fuel/fire hazard with or without dead trees (whether caused by insects or drought).

Regarding your specific questions, I would expect that fire behavior and effects would be similar in forests with similar amounts of dead trees, whether the tree mortality was caused by bark beetles or drought (or some combination).

This does not mean that residents near insect-damaged forests can ignore the dead trees. There is legitimate cause to be concerned about fires during the one or two year red needle stage after a pine beetle attack when fire intensity may be temporarily increased, although more research studying actual fires is needed in this area. And there is danger from falling snags (dead trees) 5 to 20 years after an attack. Snags are dangerous for firefighters and any structures, hikers, traffic on roads, and any improvements that could be damaged by the falling trees.

In a fire prone environment, residents should remove any dead vegetation within 100 feet of structures. If there are numerous trees near homes, thinning them so that the crowns are at least 10 feet apart will not only reduce the intensity of an approaching wildfire, but will make more water and nutrients available to the remaining trees, giving the them a better chance of fighting off an insect attack.