Drought, Tree Mortality, and Wildfire in Forests Adapted to Frequent Fire

Observations of total burned area and remotely sensed fire severity from fires across the western US do not universally support the assertion that bark beetle outbreaks lead to increased fire severity.

“However, observations of total burned area and remotely sensed fire severity from fires across the western US do not universally support the assertion that bark beetle outbreaks lead to increased fire severity (e.g., Meigs et al. 2016).”

• cites Meigs et al. 2016 sourcePartly supported

  The Figure 2 caption reports lower remotely sensed burn severity where prior insect damage was greater, supporting the claim that bark beetle outbreaks do not invariably increase fire severity. The Introduction limits this analysis to Oregon and Washington, however, and the paper does not report whether outbreaks changed total burned area across the western US. (LLM, conf=0.87)

  Experimental — accuracy not yet measured

In historically infrequent, high-severity adapted forests, beyond early stages of mortality, fire behavior could be reduced because of thinning of the canopy and associated reduction and continuity of canopy fuels.

“In the latter forest types, it is likely that beyond early stages of mortality, fire behavior could be reduced because of the “thinning” of the canopy and associated reduction and continuity of canopy fuels (Hicke et al. 2012, Meigs et al. 2016).”

• cites Hicke et al. 2012 sourceNot checked — no full text of the source
• cites Meigs et al. 2016 sourcePartly supported

  The Discussion reports that canopy thinning after mountain pine beetle outbreaks is associated with a lasting reduction in fire impacts, and describes lower initial burn severity after spruce budworm defoliation as consistent with reduced potential fire behavior. It does not establish the claim specifically for historically infrequent, high-severity fire-adapted forests; the Discussion also says its burn-severity measure cannot directly assess fire behavior. The later trend differs by insect: the thinning effect is less persistent after spruce budworm outbreaks. (LLM, conf=0.84)

  Experimental — accuracy not yet measured

Areas that burn under wildfire conditions within 15-25 years after extensive mortality are prone to long-term conversion to shrub fields because of the potential loss of both established tree regeneration and seed source for postfire conifer regeneration.

“Areas that do burn under wildfire conditions (as opposed to prescribed fire) within 15-25 years after extensive mortality are prone to long-term conversion to shrub fields because of the potential loss of both established tree regeneration and seed source for postfire conifer regeneration (Coppoletta et al. 2016).”

• cites Coppoletta et al. 2016 sourcePartly supported

  Split into 2 atoms — 1 confirmed, 1 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  Experimental — accuracy not yet measured

Well-recognized tools and tactics are available for increasing the resilience of fire-frequent forests to disturbances.

“Well-recognized tools and tactics are available for increasing the resilience of FF forests to disturbances (Stephens et al. 2012, Collins et al. 2014).”

• cites Stephens et al. 2012 sourceSupported
• cites Collins et al. 2014 sourceNot checked — no full text of the source

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.

“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).”

• cites Lydersen et al. 2014 sourcePartly supported

  The Introduction reports that fire suppression has led to a buildup of surface fuels in forests historically shaped by frequent, low-severity fire, and that low- and mid-elevation fires now burn at higher severity. The Field data section identifies mixed-conifer and some ponderosa pine sites, but the paper does not establish that a majority of California’s mixed-conifer and yellow pine forests currently have high surface fuel loads and elevated fire hazards. (LLM, conf=0.86)

  Experimental — accuracy not yet measured
• cites Stephens et al. 2015 sourcePartly supported

  The Introduction (page not provided) says many contemporary ponderosa pine and mixed-conifer forests have become more vulnerable to uncharacteristically severe fire. The Discussion (page not provided) also documents substantial changes in forest structure. Neither passage establishes unnaturally high surface fuel loads or that the claim applies to a majority of these forests across California. (LLM, conf=0.86)

  Experimental — accuracy not yet measured

Many fire-frequent forests have failed to receive management that could increase resilience to disturbances exacerbated by climate change, such as prescribed fire and mechanical restoration treatments.

“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).”

• cites Stephens et al. 2016 sourceSupported

Large unburned areas of dead trees may produce unusual forest succession patterns.

“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.”

• cites Stephens et al. 2008 sourceSupported

The percentage of live tree basal area killed by fire was associated most strongly with the percentage of basal area in the red phase prefire, followed by stand basal area, percentage of trees in red phase prefire, and prefire shrub cover.

“The percentage of live tree basal area killed by fire was associated most strongly with the percentage of basal area in the red phase prefire, followed by stand basal area, percentage of trees in red phase prefire, and prefire shrub cover.”

No single variable was identified as influential to all three measures of fire severity.

“No single variable was identified as influential to all three measures of fire severity.”

Torch percentage decreased with increases in TRMI up to an index value of approximately 30, which is midway between xeric and mesic.

“Torch percentage decreased with increases in TRMI up to an index value of approximately 30, which is midway between xeric and mesic.”

The most important predictors for RdNBR were temperature, percentage of trees in the red phase prefire, percentage of tree basal area in the red phase prefire, elevation, and dominant tree genus.

“The most important predictors for RdNBR were temperature, percentage of trees in the red phase prefire, percentage of tree basal area in the red phase prefire, elevation, and dominant tree genus.”

Increases in fire-caused mortality corresponded to increases in total stand basal area up to approximately 60 m2 per ha.

“Increases in fire-caused mortality corresponded to increases in total stand basal area up to approximately 60m 2 per ha.”

Topographic, weather, vegetation, and prefire tree mortality variables were identified as influential to the study's measures of fire severity.

“Topographic, weather, vegetation, and prefire tree mortality variables were identified as influential to our measures of fire severity.”

Fire-caused mortality was mildly negatively associated with estimated prefire shrub cover at low shrub cover levels.

“Fire caused mortality was mildly negatively associated with estimated prefire shrub cover at low shrub cover levels.”

A similar relationship between prefire mortality measures and RdNBR was observed.

“A similar relationship between prefire mortality measures and RdNBR, the remotely sensed fire severity metric, was observed.”

Torch percentage was best predicted by TRMI.

“Torch percentage was best predicted by TRMI.”

Aggressive and largely successful fire suppression has left frequent-fire forests increasingly vulnerable to the negative effects of fire and other tree mortality agents.

“Paradoxically, aggressive and largely successful fire suppression has left FF forests increasingly vulnerable to the negative effects of fire and other tree mortality agents (Young et al. 2017).”

• cites Young et al. 2017 sourceNot checked — no full text of the source

The recent massive tree mortality in Sierra Nevada frequent-fire-adapted forests is a product of acute drought compounded by the long-established removal of frequent, low-to-moderate-intensity fire.

“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.”

As a result of aggressive fire suppression in the early 1900s, frequent-fire forests experienced prolific tree regeneration.

“As a result of aggressive fire suppression promulgated by land managers in the early 1900s, FF forests soon experienced prolific tree regeneration (Show andKotok 1924, Covington et al. 1997).”

• cites Show andKotok 1924Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Covington et al. 1997 sourceNot checked — no full text of the source

The distribution and structure of western US fire-dependent ecosystems were sustained by fire up until Euro-American settlement, particularly in low-to-mid-elevation yellow pine and mixed-conifer forests where fires recurred at intervals of a few years to several decades.

“The distribution and structure of these ecosystems were sustained by fire up until Euro-American settlement in the late nineteenth century, particularly in the low-to mid-elevation yellow pine and mixed-conifer forests where fires recurred at intervals of a few years to several decades (Safford and Stevens 2017).”

• cites Safford and Stevens 2017Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

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.

“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.”

To the authors' knowledge, no studies using field or remote data have documented the relationship between high levels of tree mortality and wildfire severity in the Sierra Nevada.

“To our knowledge, no studies using field or remote data have documented the relationship between high levels of tree mortality and wildfire severity in the Sierra Nevada.”

Other studies have revealed that fire spread and fireline intensity may decrease during the red phase, especially when tree mortality was gradual and resulted in reduced canopy fuel loads.

“Other studies have revealed that there may be a decrease in these metrics during this time period especially in cases in which tree mortality was gradual and resulted in reduced canopy fuel loads (Simard et al. 2011).”

• cites Simard et al. 2011 sourceNot checked — no full text of the source

Mass fire behavior and firestorm conditions are strongly dependent on high loads of long-burning fuels similar to those observed now in frequent-fire-adapted forests that have experienced severe mortality from drought and bark beetles.

“The implication is that mass fire behavior and firestorm conditions are strongly dependent on high loads of long-burning fuels-similar, but not restricted, to those observed now in FF adapted forests that have experienced severe mortality from drought and bark beetles.”

Although decade-length impacts of dead tree biomass on fire behavior have not been investigated, the potential for crown fire in short-stature regenerating forest would be expected to increase during this time period.

“Although decade-length impacts of dead tree biomass on fire behavior at any scale have not been investigated, the potential for crown fire (passive and active) in the short-stature regenerating forest would be expected to increase during this time period (figure 2a).”

Fire departments, communities, and forest managers likely will underestimate the wildfire threat posed to people, homes, and natural resources following severe tree mortality in frequent-fire-adapted forests.

“Thus, fire departments, communities, and forest managers likely will underestimate the wildfire threat posed to people, homes, and natural resources following severe tree mortality in forests adapted to FF.”

Over time, live fuels may increasingly influence fire behavior as regenerating saplings and small trees increase canopy bulk density and reduce average canopy base heights.

“Over time, live fuels may increasingly influence fire behavior as regenerating saplings and small trees increase canopy bulk density and reduce average canopy base heights (e.g., Simard et al. 2011).”

• cites Simard et al. 2011 sourceNot checked — no full text of the source

The consequences of the increase in total energy to wildfire behavior cannot be determined by today's operational fire behavior models, which were designed to predict the forward spread rate of thin linear flame zones.

“Unfortunately, the consequences of this increase in total energy to wildfire behavior cannot be determined by today's operational fire behavior models, which were designed to predict the forward spread rate of thin linear flame zones.”

The fuel impacts of large-scale forest mortality suggest this could lead to a greater incidence of mass fire behavior.

“The fuel impacts of large-scale forest mortality suggest this could lead to a greater incidence of mass fire behavior.”

Exacerbated fire effects on live trees would be expected following large-scale tree mortality because of neighborhood effects of torching dead trees.

“Similarly, exacerbated fire effects on live trees would be expected following large-scale tree mortality because of the “neighborhood” effects of torching dead trees.”

Converting live trees into dead trees increases the amount and continuity of dry, combustible large-woody material.

“One thing is certain: converting live trees into dead trees increases the amount and continuity of dry, combustible large-woody material.”

Hoffman and colleagues (2015) found that during the near term (1-3 years following mortality), gradual and rapid tree-mortality scenarios increased fire rates of spread by 1.2- to 2.7-fold.

“Hoffman and colleagues (2015) used a physics-based model to compare the effects of a rapid and gradual tree-mortality event on simulated fire behavior and found that during the near term (1-3 years following mortality), the gradual and rapid scenarios increased fire rates of spread by 1.2-to 2.7-fold, suggesting that conversion of live needles to dead is an important characteristic driving near-”

• cites Hoffman et al. 2015 sourceNot checked — no full text of the source

During the red phase, conversion of live to dead canopy fuels reduces foliar moisture content and alters foliar chemistry, contributing to increased flammability within the tree by decreasing heat requirements for ignition.

“During the initial phase, often termed the red phase, the conversion of live to dead canopy fuels reduces foliar moisture content and alters foliar chemistry.Both of these contribute to increased flammability within the tree by decreasing the heat requirements for ignition (Jolly et al. 2012, Page et al. 2012).”

• cites Jolly et al. 2012 sourceNot checked — no full text of the source
• cites Page et al. 2012 sourceNot checked — no full text of the source

Outcomes from mesic forest types are not directly applicable to xeric frequent-fire forest types because their historical fire regimes differ.

“Outcomes from the mesic forest types, which historically experienced infrequent (75-300 years), generally more intense fires are, in many cases, not directly applicable to the xeric forest types that historically experienced frequent (every 5-25 years), generally low-to moderate-intensity fire, and where the effects of fire exclusion on stand density are most pronounced.”

A different and dangerous class of fire behaviors emerges at large scales and depends on the combination of high dead surface fuel loads and long burning times extended across a large area (mass fire).

“In fact, a different and dangerous class of fire behaviors emerges at large scales and depends on the combination of high dead surface fuel loads and long burning times extended across a large area (mass fire; see below).”

The shift from live to dead biomass has the potential to significantly alter fire behavior and forest succession in frequent-fire-adapted forests.

“This shift has the potential to significantly alter fire behavior and forest succession in FF-adapted forests.”

Increased connectivity of heavy dead fuels over hundreds to thousands of hectares associated with rapid and extensive tree mortality events likely will increase future fire spread rates in frequent-fire forests.

“Although there is little landscape-level fire behavior research to draw on, increased connectivity of heavy dead fuels over hundreds to thousands of hectares associated with rapid and extensive tree mortality events likely will increase future fire spread rates in FF forests.”

Unless dead biomass is removed, recent and current bark-beetle-caused tree mortality in the Sierra Nevada could add tens to hundreds of Mg per hectare of dry woody fuel to the wildland fuel complex.

“Unless some of this dead biomass is removed, either mechanically or by fire, recent and current bark-beetle-caused tree mortality in the Sierra Nevada could add tens to hundreds of Mg per hectare (ha) of dry woody fuel to the wildland fuel complex (John J. Battles, Department of Environmental Science, Policy and Management, University of California, Berkeley, personal communication, 20 March 2017)”

• cites John J. Battles, personal communication, 20 March 2017Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Tree mortality alters fuel dynamics by reducing live canopy fuels and increasing dead fuels, altering the arrangement, composition, and quantity of fuel available for combustion.

“The process of reducing live canopy fuels and subsequently increasing dead fuels alters the arrangement, composition, and quantity of fuel available for combustion.”

Discrepancies in findings on bark beetle outbreaks and fire severity are likely due to differences in timing and intensity of tree mortality, forest type, and spatial scales at which studies were conducted.

“Such discrepancies are likely due to differences in the timing and intensity of tree mortality, forest type, and the very different spatial scales at which studies were conducted.”

In areas not yet burned at uncharacteristic severity, 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 bark beetles.

“In areas that have not yet burned at uncharacteristic severity, fire suppression-caused forest densification has increased competition among trees for water and other resources, destabilizing many FF forests by making them prone to mortality from other agents such as bark beetles (Dendroctonus, Ips, Scolytus spp.; Kolb et al. 2016).”

• cites Kolb et al. 2016 sourceNot checked — no full text of the source

High climatic water deficit was associated with tree mortality in the Sierra Nevada.

“A recent summary of tree mortality patterns in the Sierra Nevada showed that high climatic water deficit was associated with mortality (Young et al. 2017).”

• cites Young et al. 2017 sourceNot checked — no full text of the source

Oak seedlings may benefit from the increase in light at ground level and the absence of enhanced shrub cover that usually occurs after fire.

“Oak seedlings, which survive relatively well but grow slowly under the shade of a dense overstory, may also benefit from the increase in light at ground level and the absence of enhanced shrub cover that usually occurs after fire.”

Where current conditions remain suitable for conifers, restoration of drought-affected forests could be challenging because the resulting composition, structure, and successional patterns will differ depending on whether relatively frequent fire is also returned at meaningful scales.

“Where current conditions remain suitable for conifers, restoration of drought-affected forests could be challenging because the resulting composition, structure, and successional patterns will differ depending on whether relatively frequent fire, the dominant historic disturbance, is also returned at meaningful scales.”

Bark-beetle-caused tree mortality is unlikely to stimulate as much shrub cover in the earlier years (1-5 years following overstory mortality) because response will mainly be limited to expansion of existing shrubs.

“Bark-beetle-caused tree mortality is unlikely to stimulate as much shrub cover in the earlier years (i.e., 1-5 years following overstory mortality) because response will mainly be limited to the expansion of existing shrubs (figure 1).”

In unsalvaged areas, accumulated dead fuels will place any naturally recruited or planted trees at risk of complete loss in the event of a wildfire.

“otherwise, the accumulated dead fuels will place any naturally recruited or planted trees at risk of complete loss in the event of a wildfire (McGinnis et al. 2010).”

• cites McGinnis et al. 2010 sourceNot checked — no full text of the source

Hardwoods such as California black oak may achieve a larger size and more quickly dominate the overstory following bark-beetle-caused tree mortality because they are not killed.

“On the other hand, hardwoods such as California black oak (Quercus kelloggii) may achieve a larger size and more quickly dominate the overstory following bark-beetle caused tree mortality because they are not killed (figure 1).”

Proactively treating forests to reduce density prior to wildfires, droughts, and bark beetle outbreaks is increasingly constrained.

“Unfortunately, proactively treating forests to reduce density prior to wildfires, droughts, and bark beetle outbreaks is increasingly constrained.”

• AssumptionRests on an assumption: Public land agencies such as the USDA Forest Service have focused considerable staff time toward managing fire-frequent forest areas affected by large, high-severity wildfires.

Effective use of prescribed or managed fire without killing young regenerated or planted trees can be difficult in fire-frequent forests, in part because surface fuels have become heavy and continuous in the long-term absence of fire.

“Effective use of prescribed or managed fire without killing the young regenerated or planted trees can be difficult in FF forests (Bellows et al. 2016), in part because surface fuels have become so heavy and continuous in the longterm absence of fire.”

• cites Bellows et al. 2016 sourceNot checked — no full text of the source

This difference following bark-beetle-caused mortality relative to fire may significantly limit habitat for many small mammals and bird species that benefit from patchy distribution of shrubs.

“This difference following bark-beetle-caused mortality of the overstory, relative to what might be expected after a fire, may significantly limit habitat for many small mammals and bird species that benefit from patchy distribution of shrubs (Coppeto et al. 2006, White et al. 2015).”

• cites Coppeto et al. 2006 sourceNot checked — no full text of the source
• cites White et al. 2015 sourceNot checked — no full text of the source

Prescribed fire and mechanical thinning of small-medium diameter trees are well documented to increase resilience in fire-frequent forests.

“Treatments such as prescribed fire and mechanical thinning of small-medium diameter trees are well documented to increase resilience in FF forests (Collins et al. 2014).”

• cites Collins et al. 2014 sourceNot checked — no full text of the source

From 1999 to 2002, forests in the southern California mountains experienced a mortality event similar to that observed in the central and southern Sierra Nevada today.

“From 1999 to 2002, forests in the southern California mountains experienced a mortality event similar to that observed in the central and southern Sierra Nevada today (Preisler et al. 2017).”

• cites Preisler et al. 2017 sourceNot checked — no full text of the source

Treatments would enhance adaptation to climate change by increasing the vigor, resistance, or resilience of remaining trees to multiple stressors, buying time for forest ecosystems to respond more incrementally to changing environmental drivers.

“Treatments would also enhance adaptation to climate change by increasing the vigor, resistance, or resilience of the remaining trees to multiple stressors, buying time for forest ecosystems to respond more incrementally to changing environmental drivers.”

Therefore, Sierra de San Pedro Mártir forests have not seen the dramatic tree densification that occurred in California frequent-fire forests from fire suppression and logging.

“Therefore, SSPM forests have not seen the dramatic tree densification as occurred in California FF forests from fire suppression and logging (Dunbar-Irwin and Safford 2016).”

• cites Dunbar-Irwin and Safford 2016 sourceNot checked — no full text of the source

Fire under more moderate fire weather conditions can create considerable heterogeneity in vegetation composition and structure.

“One advantage of including fire in management plans, besides the obvious fuel reduction to mitigate the intensity of subsequent wildfires, is that fire under more moderate fire weather conditions can create considerable heterogeneity in vegetation composition and structure (Collins et al. 2016).”

• cites Collins et al. 2016 sourceNot checked — no full text of the source

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.

“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.”

In the first decade, wildfire severity in bark beetle killed fire-frequent forests may be little affected over current conditions.

“In the first decade, wildfire severity in bark beetle killed FF forests may be little affected over current conditions.”

Such intense fires could prevent forests from becoming re-established.

“In addition, such intense fires could prevent forests from becoming re-established.”

Extensive loading of large-sized woody fuels in future decades may contribute to dangerous mass fires beyond the predictive capacity of current fire models.

“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.”