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Article
Beyond Fuel Treatment Effectiveness: Characterizing
Interactions between Fire and Treatments in the US
Kevin Barnett 1, *, Sean A. Parks 2, Carol Miller 2and Helen T. Naughton 1
1Department of Economics, The University of Montana, Missoula, MT 59701, USA;
helen.naughton@mso.umt.edu
2Aldo Leopold Wilderness Research Institute, Rocky Mountain Research Station, USDA Forest Service,
Missoula, MT 59801, USA; sean_parks@fs.fed.us (S.A.P.); cmiller04@fs.fed.us (C.M.)
*Correspondence: kevin.barnett@umontana.edu; Tel.: +1-406-830-0130
Academic Editors: Michael C. Stambaugh and Timothy A. Martin
Received: 13 August 2016; Accepted: 4 October 2016; Published: 14 October 2016
Abstract:
In the United States, fuel reduction treatments are a standard land management tool to
restore the structure and composition of forests that have been degraded by past management.
Although treatments can have multiple purposes, their principal objective is to create landscape
conditions where wildland fire can be safely managed to help achieve long-term land management
goals. One critique is that fuel treatment benefits are unlikely to transpire due to the low probability
that treated areas will be burned by a subsequent fire within a treatment’s lifespan, but little
quantitative information exists to corroborate this argument. We summarized the frequency,
extent, and geographic variation of fire and fuel treatment interactions on federal lands within the
conterminous United States (CONUS). We also assessed how the encounters between fuel treatments
and fires varied with treatment size, treatment age, and number of times treated. Overall, 6.8% of
treatment units evaluated were encountered by a subsequent fire during the study period, though
this rate varied among ecoregions across the CONUS. Larger treatment units were more likely to be
encountered by a fire, and treatment units were most frequently burned within one year of the most
recent treatment, the latter of which is likely because of ongoing maintenance of existing treatments.
Our results highlight the need to identify and prioritize additional opportunities to reduce fuel
loading and fire risk on the millions of hectares of federal lands in the CONUS that are in need
of restoration.
Keywords:
encounter rate; treatment maintenance; treatment longevity; MTBS; LANDFIRE;
wildland fire
1. Introduction
Interactions between historical fire exclusion, land use changes, and a warming climate have
increased fuel loading and fire hazard across millions of hectares of federal forested lands in the
United States [
1
]. Fuel reduction treatments, whereby surface and canopy fuels are removed through
mechanical thinning and/or prescribed fire, are a standard management tool to reduce fire risk and
restore the vegetative structure of ecosystems that have been degraded by past management and
fire suppression [
2
,
3
]. Fuel treatments can moderate subsequent fire behavior [
4
,
5
], mitigate fire
severity [
6
,
7
], and increase forest resilience to subsequent disturbances [
8
,
9
]. At the stand level, fuel
treatment effects vary according to treatment type, size, and age [
10
], while their spatial arrangement
and rate of implementation can affect outcomes at the landscape level [
11
,
12
]. One principal critique of
fuel treatments is that their benefits are rarely realized because of the low likelihood that an unplanned
fire will encounter a previously treated area during its effective lifespan [
13
–
15
], though the rate and
extent to which this occurs remains largely unknown.
Forests 2016,7, 237; doi:10.3390/f7100237 www.mdpi.com/journal/forests
Forests 2016,7, 237 2 of 12
Myriad economic and operational constraints to fuel treatment implementation on federal lands in
the United States make it unlikely that treatments alone can achieve forest restoration goals at landscape
scales [
16
]. Recognizing this limitation, several calls have been made to expand the use of unplanned
fire to accelerate the pace of forest restoration [
17
,
18
]. Managing fire in fire-adapted ecosystems
is challenging given the current social and institutional constraints to managing fire for resource
benefits [
19
,
20
]. However, low-risk opportunities to use unplanned fire to achieve land management
goals can be expanded when tied into existing fuel treatment networks or previously burned areas [
21
].
Treated areas can serve as “anchor points” [
2
] during incident management to facilitate indirect
suppression strategies that allow fires to burn inside large areas buffered by treatments, previously
burned areas, or other terrain features that limit fire spread [
22
] or facilitate suppression efforts [
23
].
Indeed, leveraging treated areas to support the use of fire is a principal objective of fuel treatment
strategies [21], yet little information exists to evaluate its successes or failures.
Recognizing that the successful use of wildland fire is a necessary component of long-term fire
risk management, the National Cohesive Wildland Fire Management Strategy identified priority
areas where fuel treatments might be used as a precursor to a safer and expanded use of wildland
fire [
24
]. Successfully implementing this aspect of the Cohesive Strategy requires a programmatic
and strategic alignment of resources and management objectives from the national to local level;
national resources are allocated across agencies and geographical regions where the need to reduce
fuel loadings is most critical, and local managers respond by capitalizing on low-risk opportunities to
manage unplanned ignitions to achieve long-term fire and land management objectives. Disconnects
within this management framework will result in inefficiencies and help to reinforce the current fire and
land management paradigm [
25
]. For example, suppressing wildland fire within a matrix of previously
treated areas, especially during moderate weather conditions, forgoes a low-risk opportunity to capture
the fuel treatment benefits provided and maintained by wildland fire [
22
,
26
]. In turn, managers will be
forced to use their limited resources to retreat previously treated areas to maintain low fire hazard rather
than expand treatment networks. Quantifying interactions between fuel treatments and subsequent
fire at large spatial extents provides managers and policy makers with a means to track their successes
and may also reveal where progress towards achieving the goals of the Cohesive Strategy is lacking.
Due to data limitations, previous attempts to characterize fire and fuel treatment interactions in the
United States made broad assumptions when estimating the probability that treated areas would burn
by unplanned fire [
14
], most notably the assumption that fire and fuel treatments are randomly located.
Findings based on such assumptions may have limited ability to inform contemporary fire and fuels
management strategies because the likelihood of fire occurrence and spread is known to exhibit spatial
patterning and be highly variable across large landscapes [
27
,
28
]. The advent of modern datasets
containing spatially referenced fire and fuel treatment data [
29
,
30
] enables a more refined assessment
of fire and fuel treatment interactions that accounts for fire’s natural variability and improves our
ability to assess fuel treatment efficacy.
In this study, we used spatially-explicit, standardized datasets of fuel treatments and wildland
fires that occurred between 1999 and 2013 on federal lands to summarize the frequency, extent, and
geographic variation of recent fire and fuel treatment interactions across the conterminous United States
(CONUS). We focused on fire and fuel treatment interactions outside of the wildland–urban interface
(WUI), where forest restoration goals are assumed to supersede other potential fuel treatment objectives
(i.e., fire-mitigation) [
31
]. We quantified the percentage of fuel treatments that were encountered by
subsequent fire during the study period in terms of ecoregion, treatment size, treatment regime (i.e.,
number of times treated), and treatment age. Our findings are discussed in the broader context of
potential implications for fire and fuel management strategies.
Forests 2016,7, 237 3 of 12
2. Materials and Methods
2.1. Study Area
We evaluated fire and fuel treatment interactions on federal lands in the CONUS. Federal lands
were identified from the Protected Areas Database (Version 1.3, United States Geological Survey Gap
Analysis Program, USA) [
32
] (Figure 1). We restricted our analysis to fuel treatments located >2.5 km
outside the WUI [
3
,
31
] (Figure 2). This distance threshold has been suggested as an appropriate buffer
around WUI communities for community wildfire protection zones where fire-mitigation treatments
are prioritized [
33
,
34
]. The WUI was defined as both the ‘interface’, where housing is in the vicinity of
contiguous vegetation, and the ‘intermix’, where housing and vegetation intermingle. A spatial data
layer of both the interface and intermix was obtained from the SILVIS lab [
35
] and was developed
following federal definitions of the WUI [
36
]. For clarity, we refer to the WUI and its 2.5 km buffer
as WUI2.5.
Forests2016,7,237 3of12
2.MaterialsandMethods
2.1.StudyArea
WeevaluatedfireandfueltreatmentinteractionsonfederallandsintheCONUS.Federallands
wereidentifiedfromtheProtectedAreasDatabase(Version1.3,UnitedStatesGeologicalSurvey
GapAnalysisProgram,USA)[32](Figure1).Werestrictedouranalysistofueltreatmentslocated
>2.5kmoutsidetheWUI[3,31](Figure2).Thisdistancethresholdhasbeensuggestedasan
appropriatebufferaroundWUIcommunitiesforcommunitywildfireprotectionzoneswhere
fire‐mitigationtreatmentsareprioritized[33,34].TheWUIwasdefinedasboththe‘interface’,where
housingisinthevicinityofcontiguousvegetation,andthe‘intermix’,wherehousingandvegetation
intermingle.AspatialdatalayerofboththeinterfaceandintermixwasobtainedfromtheSILVISlab
[35]andwasdevelopedfollowingfederaldefinitionsoftheWUI[36].Forclarity,werefertothe
WUIandits2.5kmbufferasWUI
2.5
.
Figure1.MapoffederallandsacrosstheconterminousUnitedStates(CONUS).
Figure2.Distributionofwildland–urbaninterface(WUI)landsincluding2.5kmbuffer(gray)
amongregionsandecoregionsoftheCONUS.SeeFigureS1forcorrespondingecoregionnames.
Figure 1. Map of federal lands across the conterminous United States (CONUS).
Forests2016,7,237 3of12
2.MaterialsandMethods
2.1.StudyArea
WeevaluatedfireandfueltreatmentinteractionsonfederallandsintheCONUS.Federallands
wereidentifiedfromtheProtectedAreasDatabase(Version1.3,UnitedStatesGeologicalSurvey
GapAnalysisProgram,USA)[32](Figure1).Werestrictedouranalysistofueltreatmentslocated
>2.5kmoutsidetheWUI[3,31](Figure2).Thisdistancethresholdhasbeensuggestedasan
appropriatebufferaroundWUIcommunitiesforcommunitywildfireprotectionzoneswhere
fire‐mitigationtreatmentsareprioritized[33,34].TheWUIwasdefinedasboththe‘interface’,where
housingisinthevicinityofcontiguousvegetation,andthe‘intermix’,wherehousingandvegetation
intermingle.AspatialdatalayerofboththeinterfaceandintermixwasobtainedfromtheSILVISlab
[35]andwasdevelopedfollowingfederaldefinitionsoftheWUI[36].Forclarity,werefertothe
WUIandits2.5kmbufferasWUI
2.5
.
Figure1.MapoffederallandsacrosstheconterminousUnitedStates(CONUS).
Figure2.Distributionofwildland–urbaninterface(WUI)landsincluding2.5kmbuffer(gray)
amongregionsandecoregionsoftheCONUS.SeeFigureS1forcorrespondingecoregionnames.
Figure 2.
Distribution of wildland–urban interface (WUI) lands including 2.5 km buffer (gray) among
regions and ecoregions of the CONUS. See Figure S1 for corresponding ecoregion names.
Forests 2016,7, 237 4 of 12
2.2. Data Background
Our primary datasets were obtained from the LANDFIRE program [
29
] and the Monitoring
Trends in Burn Severity (MTBS) project [
30
]. The LANDFIRE program produces geospatial datasets
(e.g., historical fire regime, existing vegetation type, and recent fuel treatments) to support strategic fire
and resource management and planning. The LANDFIRE fuel treatment dataset comprises treatment
events that occurred between 1999 and 2012. Each fuel treatment event is a spatial polygon representing
a treatment boundary and is attributed by year and type of treatment (Table 1).
Table 1. Description of treatment types from the LANDFIRE Public Events Data Dictionary.
Treatment Type Description
Clearcut The cutting of essentially all trees, producing a fully exposed microclimate for
the development of a new age class
Harvest
A general term for the cutting, felling, and gathering of forest timber. The term
harvest was assigned to events where there was not enough information
available to call them one of the two distinct types, clearcut or thinning
Mastication Means by which vegetation is mechanically “mowed“ or “chipped“ into small
pieces and changed from a vertical to a horizontal arrangement
Other mechanical
Catch all term for a variety of forest and rangeland mechanical activities related
to fuels reduction and site preparation including: piling of fuels, chaining, lop
and scatter, thinning of fuels, Dixies harrow, etc.
Prescribed fire
Any fire ignited by management actions to meet specific objectives. A written,
approved prescribed fire plan must exist, and NEPA requirements (where
applicable) must be met prior to ignition.
Thinning
A tree removal practice that reduces tree density and competition between trees
in a stand. Thinning concentrates growth on fewer, high-quality trees, provides
periodic income, and generally enhances tree vigor
MTBS data are derived from Landsat TM, ETM+, and OLI imagery and include perimeters for
fires greater than 200 ha in the eastern US and greater than 405 ha in the western US since 1984.
Although these perimeter data are not without error [
37
], the consistent mapping methodologies and
comprehensive coverage reduce potential data bias over time and space relative to other potential data
sources; these data have been successfully used to investigate fire frequency, severity, and size over
significant geographic and temporal extents [38–40].
Fires labeled by MTBS as ’prescribed’ or ‘unknown origin’ were removed. Prescribed fires from
the MTBS dataset that occurred between 1999 and 2012 (n= 4543) were added to the LANDFIRE fuel
treatment dataset. Duplicate prescribed fire records between the LANDFIRE and MTBS datasets were
subsequently removed.
2.3. Assessing Fuel Treatment Regimes
Many treated areas received several treatments throughout the study period, presumably for
treatment maintenance purposes. For example, an area might first be mechanically thinned to reduce
vertical and horizontal fuel connectivity, and then treated with prescribed fire the next year to remove
residual surface fuels. In such cases of multiple treatments, we identified and delineated all sets of
overlapping fuel treatment polygons that constituted a treatment ‘unit’ and used the most recent
treatment type when summarizing interactions between treatments and subsequent fires. In the case
where the two most recent treatment types comprised a mechanical treatment (i.e., clearcut, thinning,
harvest, mastication, or other mechanical) followed by prescribed fire, we assigned a new treatment
type, ‘thin-and-burn’. To quantify treatment maintenance and summarize the overall treatment regime
for a treatment unit, we recorded the number of original treatment polygons that intersected each
treatment unit. Inconsistent digitizing of original treatment boundaries resulted in the creation of many
Forests 2016,7, 237 5 of 12
‘sliver’ treatment units, so all treatment units less than 415 m
2
were removed (the 1st percentile in the
treatment size distribution). A total of 136,107 treatment unit polygons were identified and analyzed.
2.4. Deriving Encounter Rates
All treatment units that occurred on federal land from 1999 to 2012 that were encountered by a
subsequent wildland fire between 2000 and 2013 were identified; by definition, treatment units could
not be encountered by a fire that occurred in the same year or previous to the treatment. We calculated
the encounter rate as the percentage of treatment unit polygons that were intersected by wildland fires
and summarized this rate across four variables: ecoregion, treatment size class, treatment regime (i.e.,
number of times treated), and time-since-treatment. Sixty seven ecoregions were determined from a
spatial layer obtained from The Nature Conservancy [
41
] which is loosely based on Bailey’s ecoregion
delineation [42].
Calculating encounter rates in terms of treatment age was a two-step process. First, for treatment
units encountered by a subsequent fire, we calculated the time-since-treatment as the difference
between the years of the fire and treatment. Where multiple treatments occurred within a treatment
unit, we used the most recent treatment year before the fire occurred, and when a treatment was
encountered by multiple subsequent fires, we used the earliest fire date. Second, we normalized
the number of treatments within each time-since-treatment interval to remove the bias introduced
by a truncated fire record. For example, only treatment units installed in 1999 were evaluated for
the 14 years-since-treatment interval because treatment units installed after 1999 did not have the
opportunity to be burned by a fire 14 years later. Conversely, all treatments were evaluated for the one
year-since-treatment interval because treatments from each year had the opportunity to be encountered
by a fire the next year. We derived encounter rate within each time-since-treatment interval as the
number of treatments encountered by a subsequent fire divided by the total number of treatments
within each time interval.
3. Results
Our final sample of 3908 unique fire events that occurred between 2000 and 2013 on federal
lands in the CONUS burned a total of 18,851,801 ha. Total treated area between 1999 and 2012 was
2,804,850 ha. A total of 9249 of the 136,483 treatment units were encountered by subsequent fire,
resulting in an overall encounter rate of 6.8% (Table S1). Of the total treated area, 216,287 ha (7.7%)
burned by subsequent fire.
The number of treatments and area treated varied widely among the treatment types (Table 2).
Prescribed fire was the most commonly observed fuel treatment fuel treatment type and comprised
more area than all other treatment types combined. Thin-and-burn units were more frequent and
comprised a larger area compared to clearcut, harvest, or mastication units.
Table 2. Summary statistics for all fuel treatment units. All areal units are in ha.
Treatment Unit Type Number of
Treatment Units
Total Treatment
Unit Area
Mean Treatment Unit Size
(25th, 75th Percentiles)
Clearcut 2847 29,729 10.44 (1.94, 12.47)
Harvest 7929 92,432 11.66 (1.50, 13.59)
Mastication 2209 38,465 17.41 (0.49, 14.73)
Other mechanical 29,173 473,957 16.25 (0.40, 9.50)
Prescribed fire 47,261 1,631,087 34.51 (0.29, 11.20)
Thin-and-burn 9397 107,311 11.42 (0.72, 12.36)
Thinning 37,667 431,869 11.47 (1.74, 13.13)
Treated area and area burned varied among ecoregions (Figure 3). Treated area was greatest in the
Cascade Mountain Range (303,731 ha), Blue Mountain Region of the Columbia Plateau (252,501 ha),
Forests 2016,7, 237 6 of 12
and Floridian Coastal Plain (229,163 ha) (Figure 3a). The highest area burned by wildland fires on
federal lands occurred in the western United States (Blue Mountain Region of the Columbia Plateau,
Snake River Plain, and Northwestern Rocky Mountains ecoregions) (Figure 3b). In the eastern CONUS,
area burned was greatest in the Southeastern Coastal Plain ecoregion. Five ecoregions contained zero
wildland fires on federal lands during the study period.
Treated area burned tended to exhibit similar spatial patterns to treated area, although some
ecoregions of the interior western United States with relatively high treated area had relatively low
treated area burned (e.g., Wyoming Basin, Middle Rocky Mountains) (Figure 3c). The encounter rate
substantially varied among ecoregions (Figure 3d). The highest encounter rates across the CONUS
were observed in the Southern California, Mogollon Rim, and Snake River Plains ecoregions. Encounter
rate was less than 5% in 19 of the 25 westernmost ecoregions. During the study period, there were
23 ecoregions with a 0% encounter rate.
Forests2016,7,237 6of12
easternCONUS,areaburnedwasgreatestintheSoutheasternCoastalPlainecoregion.Five
ecoregionscontainedzerowildlandfiresonfederallandsduringthestudyperiod.
Treatedareaburnedtendedtoexhibitsimilarspatialpatternstotreatedarea,althoughsome
ecoregionsoftheinteriorwesternUnitedStateswithrelativelyhightreatedareahadrelativelylow
treatedareaburned(e.g.,WyomingBasin,MiddleRockyMountains)(Figure3c).Theencounterrate
substantiallyvariedamongecoregions(Figure3d).ThehighestencounterratesacrosstheCONUS
wereobservedintheSouthernCalifornia,MogollonRim,andSnakeRiverPlainsecoregions.
Encounterratewaslessthan5%in19ofthe25westernmostecoregions.Duringthestudyperiod,
therewere23ecoregionswitha0%encounterrate.
Figure3.Distributionof(a)areaburned,(b)treatedarea,(c)treatedareaburned,and(d)the
encounterratebetweenfueltreatmentsandfiresonfederallands,summarizedforeachof67
ecoregionsacrosstheCONUS.
Theencounterrateincreasedwithtreatmentsize,especiallywhentreatmentswerelargerthan
200ha(Table3).However,only1.4%ofalltreatmentunitsevaluatedweregreaterthan200ha.
Aboutone‐thirdofallfueltreatmentunitsreceivedatleasttwotreatmentsduringthestudyperiod
(Table4).Thevastmajorityoftreatedarea(77.6%)andtreatedareathatwassubsequentlyburnedby
fire(70.5%),however,wasattributabletotreatmentunitsthatonlyreceivedonetreatmentduring
thestudyperiod.Encounterratesbetweentreatmentsandsubsequentfiresincreasedwithnumber
oftimestreated(Table4).
Figure 3.
Distribution of (
A
) area burned; (
B
) treated area; (
C
) treated area burned; and (
D
) the
encounter rate between fuel treatments and fires on federal lands, summarized for each of 67 ecoregions
across the CONUS.
The encounter rate increased with treatment size, especially when treatments were larger than
200 ha (Table 3). However, only 1.4% of all treatment units evaluated were greater than 200 ha. About
one-third of all fuel treatment units received at least two treatments during the study period (Table 4).
The vast majority of treated area (77.6%) and treated area that was subsequently burned by fire (70.5%),
however, was attributable to treatment units that only received one treatment during the study period.
Encounter rates between treatments and subsequent fires increased with number of times treated
(Table 4).
Encounter rates were highest within one year of the most recent treatment and tended to decline
with time since treatment (Figure 4).
Forests 2016,7, 237 7 of 12
Table 3.
Summary statistics of frequency, area treated, treated area burned by wildland fire, and
encounter rate by treatment unit size class.
Treatment Unit
Size Class (ha)
Number of
Treatments
Area Treated
(ha)
Treated Area
Burned (ha)
Encounter Rate
(%)
0–5 74,966 99,547 6331 6.8
5–10 21,809 158,899 9718 6.5
10–25 24,156 374,289 21,107 6.2
25–50 8125 281,081 15,543 6.8
50–100 3755 259,466 13,981 7.2
100–200 1753 244,308 11,783 8.1
200–500 1122 352,008 23,844 10.9
500–1000 503 352,731 23,907 15.5
1000–5000 276 498,034 61,382 21.4
>5000 18 184,486 28,690 50.0
Table 4.
Summary statistics of frequency, area treated, treated area burned by wildland fire, and
encounter rate by treatment regime.
Number of
Times Treated
Number of
Treatments
Area Treated
(ha)
Treated Area
Burned (ha)
Encounter Rate
(%)
1 85,337 2,178,223 152,405 5.2
2 32,955 461,365 42,889 7.9
3 12,143 126,897 17,985 11.3
4 3992 25,021 2206 13.3
≥5 2056 13,344 802 15.7
Forests2016,7,237 7of12
Table3.Summarystatisticsoffrequency,areatreated,treatedareaburnedbywildlandfire,and
encounterratebytreatmentunitsizeclass.
Treatmentunitsize
class(ha)
Numberof
treatments
Areatreated
(ha)
Treatedarea
burned(ha)
Encounterrate
(%)
0–574,96699,54763316.8
5–1021,809 158,899 9718 6.5
10–2524,156374,28921,1076.2
25–508125281,08115,5436.8
50–1003755 259,466 13,981 7.2
100–2001753244,30811,7838.1
200–5001122352,00823,84410.9
500–1000503 352,731 23,907 15.5
1000–5000276 498,034 61,382 21.4
500018184,48628,69050.0
Table4.Summarystatisticsoffrequency,areatreated,treatedareaburnedbywildlandfire,and
encounterratebytreatmentregime.
Numberof
timestreated
Numberof
treatments
Area
treated(ha)
Treatedarea
burned(ha)
Encounter
rate(%)
185,3372,178,223152,4055.2
232,955461,36542,8897.9
312,143 126,897 17,985 11.3
4399225,021220613.3
5205613,34480215.7
Encounterrateswerehighestwithinoneyearofthemostrecenttreatmentandtendedto
declinewithtimesincetreatment(Figure4).
Figure4.Encounterrateasafunctionoftimesincemostrecenttreatmentandtreatmentregime.
Numberoftreatmentsrepresentsthenumberoftimesanareawastreatedbeforebeingencountered
byasubsequentfire.
Figure 4.
Encounter rate as a function of time since most recent treatment and treatment regime.
Number of treatments represents the number of times an area was treated before being encountered by
a subsequent fire.
4. Discussion
Characterizing interactions among fuel treatments and wildland fires at broad spatial and
temporal scales is an important step to track investments made in fuels reduction programs. Prior
efforts have quantified interactions between certain types of fuel treatments and subsequent fire.
Rhodes and Baker [
14
] estimated that between 7.2% and 16.5% of treated areas in ponderosa pine
Forests 2016,7, 237 8 of 12
forests of the western United States are encountered by fire within 20 years of treatment assuming
random locations of fire and fuel treatments. An empirical study in southeastern Australia found that
22.5% of all prescribed fire patches were subsequently burned by unplanned fire within five years [
43
].
Our more comprehensive CONUS-wide analysis examined additional fuel treatment types and we
observed similar, though somewhat lower encounter rates overall. We found that 6.8% of treatment
units created between 1999 and 2012 on federal lands outside of the WUI
2.5
were encountered by a
subsequent fire by 2013.
The Cohesive Strategy identified portions of both the western and southeastern United States
as priority areas for active restoration where wildland fire can be more safely used to help achieve
long-term land management objectives [
24
]. In the southeastern United States, treated area was
relatively high in four ecoregions (Ouachita Hills, Ozark Highlands, Southeastern Coastal Plain, and
Floridian Coastal Plain), and their associated encounter rates were slightly higher than those found in
much of the western US (Figure 3). Although western ecoregions contained the highest area burned
and treated area during the study period, only six ecoregions experienced encounter rates greater than
5%. Treated area was relatively high across the western CONUS but did not correlate to encounter
rates (Spearman’s r= 0.12); several western ecoregions had high treated area but a low encounter
rate (e.g., Northwestern Rocky Mountains, Cascade Mountain Range, and Blue Mountain Region of
the Columbia Plateau). This finding has implications for fuels treatment planning in the western US
because simply treating more area may not help to achieve long-term fire and land management goals if
wildland fire cannot be safely managed. Strategically placing fuel treatments to create conditions where
wildland fire can occur without negative consequences [
21
] and leveraging low-risk opportunities to
manage wildland fire will remain critical factors to successful implementation of the Cohesive Strategy.
Not surprisingly, we found that the encounter rate increased with treatment unit size (Table 4).
In addition to being more likely to be encountered, larger fuel treatments can be more effective at
moderating fire behavior relative to smaller treatments because they contain more interior area and less
edge [
7
,
44
,
45
]. Implementing large fuel reduction treatments in fire-excluded forests on federal lands,
however, is challenging due to regulatory and funding constraints [
46
]. Indeed, our fuel treatment
data suggest that 55% of all fuel treatment units on federal lands were less than 5 ha, while only 2.7%
of treatment units were greater than 100 ha. These large fuel treatment units (i.e., >100 ha) comprised
a significant amount of the total treated area burned; 149,606 ha out of the 216,287 ha (69.2%) of
treated area burned occurred within large treatment units. A large portion of this (59,324 ha) occurred
inside large treatment units in three ecoregions in southeastern United States where large tracts of
federal lands are regularly treated with prescribed fire (i.e., Ouachita Hills, Floridian Coastal Plains,
Southeastern Coastal Plain) (Figure S1) [
47
]. For comparison, 72,447 ha of treated area burned within
large treatment units in the ten most treated ecoregions of the western CONUS combined, with over
half (37,420 ha) attributable to the Snake River Plain ecoregion alone. Because many of the regulatory,
institutional, and social barriers to large scale fuel treatment implementation are likely to remain in
place in the near future, alternative solutions to reducing fuel loads across millions of hectares of
federal lands, especially in dry forests of the western CONUS, are needed [16].
Fuel treatment longevity is influenced by several factors, including treatment type, vegetation,
and fuel decomposition and accumulation rates [
10
]. Treatment longevity can be extended by applying
prescribed or managed fire within the temporal window that fuel treatments remain effective to
consume surface fuels and regenerating vegetation that increase fire hazard [
48
]. In general, treatments
have been found to be most effective at moderating fire behavior within the first few years of
treatment [
49
], though in less productive forest types with low fuel accumulation rates, treatments
can moderate burn severity for up to 20 years post-treatment [
7
]. In this study, encounters of fuel
treatment units with a subsequent fire occurred most frequently within one year of the most recent
treatment (Figure 4). However, nearly half of the treatment units encountered by a fire within one
year of treatment had received at least two treatments during our study period. This finding reveals
the tradeoff that exists between management of existing treatments to maintain low fire hazard and
Forests 2016,7, 237 9 of 12
implementation of additional treatments to reduce fire risk at larger spatial extents [
48
]. Treatment
maintenance is a necessary component of fuel management [
2
], but maintenance comes at the expense
of restoring additional forested lands. One option to extend the longevity of existing treatments is to
leverage treated areas during incident management to encourage the use of unplanned fire to maintain
and create low fire hazard conditions [
17
,
48
]. Wildland fire can rapidly change landscape structure
and successional pathways at much larger spatial extents than restoration treatments [
18
]. Indeed,
our data show that the ratio of area burned by wildland fire to treated area exceeds 5:1 for most of
the western CONUS ecoregions (Figure S2). The long-term success of fuels management programs
depends upon the successful use of fire to achieve land management goals [
21
], but with only 7.8%
of the total treated area in the CONUS burned by a subsequent fire, our results suggest that existing
treatments are not being sufficiently exploited to accelerate the pace of forest restoration.
Even though we used the best spatial datasets available to quantify encounters between treatments
and subsequent fires, these estimates cannot be used to formally evaluate the success of fuels
management at the programmatic level without additional context. Comparing these encounter
rates with what might occur under random chance may highlight where in the CONUS they are lower
or higher than their expected value. Such an analysis could address whether or not treatments are
being strategically placed across large landscapes. Geospatial decision support tools can prioritize
treatment locations to establish large, contiguous tracts of land where managed fire can occur without
loss of important ecological functions, such as those provided by old growth stands of a fire-resistant
species [
50
]. Implementing such treatment regimens could potentially increase encounter rates and
help expedite restoration of forest ecosystems. In addition, risk-based decision support tools are
being developed to identify low-risk opportunities for the management of unplanned ignitions [
51
,
52
].
Integrating these two approaches could aid local fuel treatment planning efforts by identifying priority
areas for active restoration where managed fire can occur without posing an excessive risk to resources,
assets, and ecological values.
Although we used the most comprehensive, standardized datasets of fire and fuel treatments
available, our analysis was limited by the length and completeness of the data records. While we
observed relatively low encounter rates, it’s expected they will increase as time goes on, especially
if projections of increasing fire activity in North America are accurate [
53
,
54
]. Continued efforts to
maintain and distribute spatial databases of fire and fuel treatments will aid future investigations of fuel
treatment and fire interactions. We focused on treatments and encounter rates occurring outside of the
WUI
2.5
because treatments in these areas are more likely to have had the goal of forest restoration [
31
].
However, we recognize that these treatments may have included other fire and land management
objectives, including WUI protection [
31
], and may have helped to achieve important land management
goals unrelated to forest restoration and independent of being encountered by a wildland fire. Future
research can evaluate fire and fuel treatment interactions with respect to treatment objectives when
such data become available. MTBS fire perimeters can fail to detect unburned islands and oversimplify
complex polygon geometries [
55
]; these limitations are unlikely to affect the interpretation of our
results due to the spatial scale of our analysis and the metrics we summarize. Even though large,
recently treated areas can mitigate fire spread [
56
] and therefore affect future encounter rates, we
did not explicitly evaluate fire sizes. This is likely to have a negligible effect on our results because
98.6% of treatments in our dataset were less than 200 ha and the average fire size was 4824 ha. Lastly,
the LANDFIRE fuel treatment dataset is by no means a complete record of all treatments implemented
on federal lands, and its accuracy is likely to vary among the agencies and groups who contributed
their data. Nonetheless, we found it useful in this broad scale analysis as a first approximation of fuel
treatment and fire interactions across the CONUS.
5. Conclusions
In this study, we used standardized spatial datasets of fire and fuel treatments to systematically
quantify the frequency, extent, and geographic variation of fire and fuel treatment interactions on
Forests 2016,7, 237 10 of 12
federal lands across the CONUS. Overall, we found that 6.8% of treatment units between 1999 and
2012 were encountered by a subsequent fire through 2013, with significant geographic variability
among ecoregions. Identifying opportunities to jointly reduce fuel loadings on federal lands and safely
reintroduce wildland fire will likely remain a priority into the near future. Continued maintenance
and distribution of standardized spatial datasets of fire and fuel treatments will allow researchers
to monitor interactions among fuel treatments and fires over space and time, hopefully exposing
opportunities to improve both fire and fuel treatment planning and management to expedite forest
restoration on federal lands.
Supplementary Materials:
The following are available online at http://www.mdpi.com/1999-4907/7/10/237/s1,
Table S1: Summary statistics of wildland fires, fuel treatments, and their interactions across ecoregions of the
CONUS, Figure S1: Distribution of WUI lands including 2.5 km buffer (gray) among regions and ecoregions of
the CONUS, Figure S2: Map showing the ratio of area burned to area treated across ecoregions of the CONUS.
Acknowledgments:
We thank two anonymous reviewers for thoughtful comments that significantly improved
the manuscript. Funding for this research was provided by the Joint Fire Science Program under Project 14-5-01-25.
Author Contributions:
K.B., S.A.P., C.M., and H.T.N. conceived and designed the experiments; K.B. performed
the experiments; K.B. analyzed the data; K.B., S.A.P., C.M., and H.T.N. wrote the paper.
Conflicts of Interest:
The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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