Forest Ecology and Management 334 (2014) 43–52

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Forest Ecology and Management

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Effectiveness of fuel reduction treatments: Assessing metrics of forestresiliency and wildfire severity after the Wallow Fire, AZ

http://dx.doi.org/10.1016/j.foreco.2014.08.0260378-1127/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.:+1 (928) 523 8991.E-mail address: Amy.Waltz@nau.edu (A.E.M. Waltz).

Amy E.M. Waltz a,⇑, Michael T. Stoddard a, Elizabeth L. Kalies a, Judith D. Springer a, David W. Huffman a,Andrew Sánchez Meador a,b

a Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011-5017, United Statesb School of Forestry, Northern Arizona University, Flagstaff, AZ 86011-5018, United States

a r t i c l e i n f o

Article history:Received 25 June 2014Received in revised form 15 August 2014Accepted 16 August 2014

Keywords:Treatment effectivenessWildfireMega-fireRestorationReference conditionsHerbaceous community

a b s t r a c t

Landscape-scale wildfire has occurred in higher frequencies across the planet. Fuel reduction treatmentsto fire-adapted systems have been shown to reduce the impact to human values-at-risk. However, fewstudies have examined if these treatments contribute to ecosystem resilience, or the capacity of a systemto absorb perturbation and return to a similar set of structures or processes. We defined short-term met-rics of resiliency to test the hypothesis that fuel reduction treatments in mixed conifer forests increased afire-adapted system’s resiliency to uncharacteristically severe wildfire. In addition, we tested the hypoth-esis that fuel reduction treatments reduced burn severity, thereby increasing protection for adjacenthuman communities. We examined a mixed conifer forested landscape in the southwestern U.S. thatwas burned by a landscape-scale ‘‘mega-fire’’ in 2011; fuel reduction treatments had been establishedaround communities in the 10 years prior to the fire. Fire effects were highly variable in both treatedand untreated forests. However, analysis of resiliency metrics showed that: (a) treated units retained ahigher proportion of large trees and had post-fire tree densities within the natural range of variability;(b) the understory herbaceous community had significantly higher cover of native grasses in the treatedunits, but no significant differences in nonnative cover between treated and untreated units; and (c) high-severity patch sizes were significantly larger in untreated stands and covered a larger proportion of thelandscape than historical reference conditions. Fire severity, as defined by overstory mortality and basalarea loss, was significantly lower in treated units; on average, trees killed per hectare in untreated unitswas six times the number of trees killed in treated units. Fuel reduction treatments simultaneouslyreduced fire severity and enhanced short-term metrics of ecosystem resiliency to uncharacteristicallysevere fire.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent decades, larger and more severe wildfires haveerupted with increasing frequency in fire-prone systems aroundthe world. While fire is a natural disturbance process in 46% ofthe global area of major habitat types, some recent fires have beendetrimental both to human communities, human-valued resourcesas well as to ecological integrity (Westerling et al., 2006; ClimateCentral, 2012; Attiwill and Binkley, 2013). To further challengeresource and fire management, human communities continue toexpand into fire-adapted and/or fire-prone systems. In the U.S.,44 million homes are located in these Wildland Urban Interface(WUI) environments (Radeloff et al., 2005). In the last two decades,

uncharacteristically severe fire events resulted in increased loss ofstructures and damaging impacts to ecosystem services and water-sheds (Savage and Mast, 2005; Allen et al., 2011). Full cost account-ing of these events incorporates socio-economic loss and can bemore than twice total costs that include mitigation activities, andcan be up to 15 times the wildfire suppression costs alone(Combrink et al., 2013). Ecological impacts include ecosystemand disturbance regime shifts in response to these novel distur-bances (Folke et al., 2004; Buma et al., 2013). While studies haveassessed the effectiveness of preventive fire hazard reductiontreatment in protecting society’s priority resources (Pollet andOmi, 2002; Fulé et al., 2012; Martinson and Omi, 2013), little isdone to assess treatment effectiveness at protecting ecological val-ues and maintaining ecosystem integrity (Ager et al., 2010;Cochrane et al., 2012). In this study, we examined the potentialfor fire hazard reduction treatments to meet human-value

44 A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52

protection goals and increase ecosystem resiliency to large-scalenovel disturbances.

Ecosystem resiliency is defined (sensu Holling, 1973) as theability of a system to absorb a perturbation or novel disturbanceand return to a similar set of structures or processes. Assessingand measuring resiliency in natural systems is challenging (Allenet al., 2011; Benson and Garmestani, 2011), but initially requiresstating parameters, including clearly defining the ‘‘resilience ofwhat [variable] to what [disturbance]’’ (Carpenter et al., 2001;Benson and Garmestani, 2011). Resiliency can be impacted bythe scale and novelty of landscape-scale disturbance events likewildfire (Folke et al., 2005; Savage and Mast, 2005; Buma et al.,2013). While fire is an integral process in the evolution of vegeta-tive communities across the globe (Meyers, 2006), recent large,uncontrollable wildfires can exhibit fire effects well outside thenatural range of variability for some systems, including uncharac-teristic severity or patch size (Savage and Mast, 2005; Bowmanet al., 2009; Buma et al., 2013; Larson et al., 2013). The occurrencesof landscape-scale, high-severity wildfires correlate with regionalweather patterns such as increased temperatures and drought(Seager et al., 2007); however, contributing factors in the westernU.S. also include millions of hectares of forested stands withuncharacteristically high tree densities and fuel loads (Covingtonand Moore, 1994; Hessburg et al., 2005; Everett et al., 2007; Fuléet al., 2012). These conditions will be exacerbated by furtherdrought and temperature increase associated with climate change;fire modeling under conservative climate change scenarios showsincreasing probability of occurrence of these uncharacteristicallylarge and severe fires, also called ‘‘mega-fires’’ (Freid et al., 2004;Westerling et al., 2006; Karl et al., 2009; Littell et al., 2009; Liuet al., 2010). Detrimental ecological impacts from these novel dis-turbances include forest plant and animal species compositionchanges, hydrologic changes, increased vulnerability to invasiveplant species, disturbance regime shifts and/or ecosystem shiftsfrom forest to non-forest (Beisner et al., 2003; Buma et al., 2013;Bowman et al., 2013). Continued departures from keystonedisturbances may result in less resilient systems, including forestsunable to support all associated successional stages or forests tran-sitioning to shrub–grass systems (Beisner et al., 2003; Bowmanet al., 2013; Stephens et al., 2013).

While resiliency metrics encompass a broader perspective thanstrict restoration, the key to resiliency thinking is to integrate mul-tiple ecological variables to identify ecosystem thresholds, adap-tive cycles and potential regime shifts (Carpenter et al., 2001;Benson and Garmestani, 2011; Rist and Moen, 2013). In thisrespect, historic reference conditions from time periods with intactecosystem structures and disturbance processes can be used todevelop resiliency benchmarks. Forests that evolved with low-severity, frequent-fire that today are susceptible to high-severityfires, can cross thresholds to novel systems, and therefore lackresilience.

In the western U.S., mixed conifer forests encompass a diverseassemblage of fire-adapted pine and fir and sometimes hardwoodspecies (Agee, 1993). Historic fire regimes in mixed conifer foresthistorically changed along an elevational gradient, with frequentsurface fires often found at lower elevations and south-facingaspects (warm/dry mixed conifer), and infrequent, stand-replacing fires found at higher elevations and north-facing, moremesic aspects (cool/moist mixed conifer) (Fulé et al., 2003;Hessburg et al., 2005; Margolis and Balmat, 2009; Romme et al.,2009). Warm/dry forests were historically less dense andwere dominated by fire-resistant tree species compared to cool/moist forests. Today, warm/dry mixed conifer forests are denserthan they were historically and include more fire-sensitivespecies due to fire exclusion (Evans et al., 2011; Margolis et al.,2013).

In the U.S., efforts to mitigate the effects of large, severe wild-fires do not focus on ecological or resiliency goals, but includeprioritized funding for hazardous fuel reduction treatments on fed-eral, state and private lands to protect communities and otherhuman values (US Congress, 2003). The priority locations for thesefuel treatments focus on the WUI, where relatively small treatmentunits are concentrated around towns and dispersed settlements(Ager et al., 2010). The 2011 fire season in the southwestern U.S.produced some of the largest and most severe wildfires on record.Arizona’s largest wildfire to date, the Wallow Fire, was a human-caused fire ignited May 29, 2011. By early July, the Wallow Firehad burned across 217,741 ha of forest and montane grasslandsin eastern Arizona and western New Mexico, with fire managersreporting unusually extreme fire behavior including crowningpatch sizes as large as 10,000 ha and spotting distances greaterthan 5 km (Wadleigh, 2011). This fire burned primarily on theApache-Sitgreaves National Forests (ASNF), also known for the first10-year stewardship contract on federal land awarded in 2004under the then-pilot Stewardship Contracting Authority. Theobjectives of the White Mountain Stewardship Project (WMSP)were to thin forests to reduce wildfire risk to local communitiesand grow local industry to process the large amounts of smalldiameter wood removed through thinning efforts (USFS WhiteMountain Stewardship, 2013). Restoration and increasing resil-iency of the ecosystem were secondary goals.

Forest fuel reduction treatments and the Wallow Fire occurredin a mix of ponderosa pine (Pinus ponderosa) and mixed conifer for-est types. Both fuel reduction and restoration objectives in mixedconifer forests can be met by thinning out small diameterfire-intolerant species (removing ladder fuels) and shifting foreststructure and composition to larger-diameter, fire-resistant trees(Evans et al., 2011), moving systems toward historic reference con-ditions Reintroducing fire consumes accumulated surface fuel andcontributes significantly to nutrient cycling, vegetation composi-tion, and quality wildlife habitat while also increasing resiliencyto wildfire (Franklin et al., 2008; North et al., 2009; Evans et al.,2011). Several empirical studies and systematic reviews haveshown fuel reduction treatments reduce wildfire severity (Fuléet al., 2012; Stephens et al., 2012; Martinson and Omi, 2013). Wehypothesize fuel reduction treatments not only reduce fire burnseverity, but increase ecosystem resiliency to landscape-scale,high-severity fire, and that post-fire systems remain similar ormove closer toward the structure and composition conditionsfound under intact disturbances and climate regimes (i.e., historicreference conditions).

We tested the hypotheses that wildfire hazard reduction treat-ments increase ecological resiliency to uncharacteristically severefire and that fuel reduction treatments reduce wildfire severity intreated compared with untreated stands. Our specific questionswere: (1) Do mixed conifer forests with fuel reduction treatmentsshow higher resiliency than those without treatment as measuredby the following three metrics: (a) tree survivorship, especiallylarge mature trees; (b) nonnative/native herbaceous understorycover ratio; and (c) high severity patch size?; and (2) Are fuelreduction treatments effective in mitigating burn severity undersevere wildfire conditions?

2. Methods

2.1. Study area

Study sites were located on the Apache-Sitgreaves NF, near theeastern Arizona communities of Greer, Alpine and Nutrioso andwhere multiple WUI-associated fuel reduction treatmentsoccurred (Fig. 1). These sites were on the northern extent of the

Fig. 1. Location of study sites and Wallow Fire in eastern Arizona, ASNF, U.S.

A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52 45

Wallow Fire and burned between June 4 and June 9, 2011. Ourstudy sites were in warm/dry mixed conifer ecosystems with ele-vations ranging from 2469 to 2838 m. Sites were identified usingTerrestrial Ecosystem Units (TES) and then confirmed in the fieldby the following criteria: codominance of ponderosa pine,Douglas-fir (Pseudotsuga mensiesii) and Gambel oak (Quercus gam-belii) (Romme et al., 2009). Forest composition in all size classesincluded ponderosa pine, white-fir (Abies concolor), Douglas-firand dispersed patches of quaking aspen (Populus tremuloides) andGambel oak. On the days our sites burned, the fire made consider-able runs, consuming 16,250–30,850 ha daily (Inciweb, 2012). Theclosest Remote Automated Weather Station (Alpine and Greer;Western Regional Climate Center, www.wrcc.dri.edu; accessed 8March 2013) recorded maximum temperatures between 23.3 and25.0 �C, minimum relative humidity of 6–15%, 1000-h fuel mois-ture of 5.7–6.9%, and maximum wind speeds of 38.5–67.7 km h�1

out of the southwest. Average total annual precipitation from2001 to 2011 was 399 mm. The total annual precipitation in2011, the year of the Wallow Fire, was close to average (90% ofthe decadal average). However, the majority of the precipitationfell with an exceptionally wet monsoon season following theWallow Fire; winter precipitation prior to the fire only accountedfor 8% of the year’s total precipitation, yielding the second driestwinter in a decade.

2.2. Experimental design

In order to evaluate whether fuel reduction treatments demon-strated differences in measures of resiliency, and were effective inreducing burn severity, we utilized a quasi-experimental design(Shadish et al., 2002) and sampled nine pairs of treated–untreatedunits (Fig. 1). Treated units were selected based on the followingset of criteria: (1) warm/dry mixed conifer forest type; (2) treatedwithin 10 years; (3) greater than 12 ha in size; and (4) burned as aresult of the Wallow Fire. The untreated paired units were geo-graphically near treated units (but independent of or not affectedby them), had similar topography and were within the same fireprogression day (24-h period). We avoided selecting sites withroads or other potential firebreaks between the treated anduntreated area. Treatments studied included non-commercial or

pre-commercial mechanical thinning followed by residual fuelremoval (mechanically or broadcast burned) prior to wildfire inci-dent. The highest priority for fuel reduction in the last 10 years wasin the WUI, thus the treatment units coincided with areas adjacentto communities. Treatments were identified using the U.S. ForestService (USFS) Activity Tracking System (FACTS) database, GoogleEarth and consultation with local USFS staff. A geographic informa-tion system (GIS) was used to overlay systematic point grids(100 � 100 m) on treated and untreated site polygons. Ten pointswere randomly selected at each site for the establishment of fieldsample plots (18 units � 10 plots = 180 plots). To address thesequestions, we avoided edge effects with a 100-m buffer betweenplots and treatment perimeter.

We defined ecosystem resiliency metrics for three short-termecosystem variables. We utilized known reference conditions(Stoddard, 2011) for mixed-conifer overstory composition andstructure and high severity patch size as resiliency benchmarks(Margolis et al., 2007, 2011; Roccaforte et al., 2012). We utilizedcover of nonnative and native species and overall functional groupcover to assess understory resiliency. To address the secondhypothesis, whether fuel reduction treatments reduced fire sever-ity, this study utilized both basal area (BA) loss and tree mortalityas measures of relative burn severity.

2.3. Field methods and statistical analysis

Sample plots were measured between June and August 2012,one year after the Wallow Fire. Plot centers were permanentlymarked with steel stakes, and all trees were tagged to ensure exactrelocation for potential re-sampling in subsequent years. Trees tal-ler than breast height (137 cm) were measured on a 400-m2

(11.28-m radius) circular plot. Tree measurements includedspecies, condition (living or snag/log classes sensu Thomas et al.,1979), diameter at breast height (dbh), and bole char height(minimum and maximum). Fifty-meter transects were aligned per-pendicular to the general slope of each plot and end points weremarked with pieces of steel rebar. Understory measurements wereassessed along the 50-m line transect; species richness was col-lected in a 500-m2 (10 � 50-m) belt transect; cover of vascularplants was measured in five 1-m2 (0.5 � 2-m) quadrats systemati-cally located at 10-m intervals along the transect. Foliar cover byspecies (%) was visually estimated and recorded for each quadrat.

Forest structure conditions prior to the Wallow Fire (2011)were reconstructed by categorizing the observed dead trees mea-sured in 2012. Dead trees were determined to be killed by the fireusing the following characteristics: absence of green needles; pres-ence of scorched (red) or burned (black) needles and fine twigs.Tree mortality based on these criteria is conservative; it can takeup to five years for trees killed by fire to lose needles.Reconstructed pre-fire forest structure (average trees per hectare(TPH) and BA) were used to compare differences between treat-ments by year — the reconstructed pre-fire (2011) and the post-fire(2012) — with a two-tailed Wilcoxon signed-rank test. Wilcoxonsigned-rank tests were also used to quantify forest structurechanges over time (pre-fire to post-fire) within treatment. Canopybulk density was estimated using equations found in Cruz et al.(2003) and analyzed with a Wilcoxon signed-rank test. Differencesin pre- and post-fire tree composition were analyzed using a per-mutational multivariate analysis (PERMANOVA, Anderson, 2001)with Bray–Curtis dissimilarity distance measure (Faith et al.,1987). Differences in composition were tested in terms of basalarea and trees ha�1 (PC-ORD software version 5.10, McCune andMefford, 2006). Understory richness and cover were notreconstructed and were compared between treatments post-fire(2012) using a Wilcoxon signed-rank test.

46 A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52

To assess patch size differences among treated and untreatedstands, we used the remotely sensed Rapid Assessment ofVegetation condition after wildfire (RAVG, 2011) layer in ARCGISto identify high severity patches, defined by the RAVG as pixelswith 75–100% BA loss (RAVG, 2011; Miller and Thode, 2007). Weused FRAGSTATS (McGarigal et al., 2012) to delineate high severitypatches, and their sizes within each treated and untreated unit. Thearea-weighted mean (see Turner et al., 2001) was used tostatistically test differences in patch size between high severitypatches in treated and untreated units. In all cases P < 0.05 wasused as a threshold for statistical significance (a = 0.05).

3. Results

3.1. Treatment effectiveness: reconstructed forest structure, fire effectsand burn severity

Treatment differences before the wallow fire: Treated units weredesigned to meet fuel reduction objectives by removing less fire-resilient species, resulting in stand conditions that favored ponder-osa pine and Douglas-fir. Forest fuel reduction treatments resultedin significantly lower basal area and tree density than untreatedunits, prior to the Wallow Fire (P = 0.004 for both variables, Table 1,significance denoted by a,b). Reconstructed pre-fire basal area waslower by an average of 57% and tree density was lower by an aver-age of 80% in treated units than untreated units (Table 1, signifi-cance denoted by a,b). Effects of fuel reduction treatmentsresulted in significantly lower canopy bulk density in treated units(P = 0.004), averaging 0.05 kg/m3 compared to 0.26 kg/m3 inuntreated units. Diameter distributions pre-fire in the treated areawere unimodal with a peak at the 15-cm and 25-cm diameter class(Fig. 2). Diameter distributions in the untreated units followed areverse j-distribution, indicating strong dominance by small trees,most of them Douglas-fir and white fir (Fig. 2). Forest compositionwas also different (PERMANOVA P = 0.002) between treated anduntreated units prior to the Wallow Fire, reflecting the dominanceof fire-adapted species in treated units and of white-fir in theuntreated units.

Wallow fire effects and burn severity: Comparisons within treatedand untreated units from pre-fire reconstructed data showed thatthe Wallow Fire significantly reduced basal area and tree density(P = 0.004, for all variables, Table 1, significance denoted by boldedtext) from pre-fire conditions for both treated and untreated units.In treated units, basal area and tree loss ranged from 0.6 to

Table 1Forest structure of reconstructed pre-fire stand condition and post wildfire stand conditioTreated and untreated unit differences (within year) indicated by letters (a, b for 2011 andtreated and untreated units indicated by bolded text. Significant at a = 0.05 (N = 9). Spec(Pinus strobiformis) POTR (Populus tremuloides), PSME (Pseudotsuga menziesii), and QUGA (

Treatment Total ABCO PIEN PIPO

Density (trees ha�1)Treated2011 219.2 (46.5)a 16.1 (7.8)a 6.4 (5.8) 44.2 (5.1)2012 83.3 (15.1)x 5.8 (4.7) 1.4 (1.1) 35.6 (4.8)

Untreated2011 1093.1 (73.4)b 163.9 (67.4)b 12.8 (5.6) 331.4 (742012 292.2 (77.9)y 30.6 (13.6) 3.9 (2.6) 138.9 (35

Basal area (m2 ha�1)Treated2011 14.3 (1.8)a 1.8 (1.3) 0.4 (0.3) 6.7 (1.2)a2012 9.8 (1.4) 1.1 (1.0) 0.2 (0.1) 6.2 (1.2)

Untreated2011 33.4 (1.9)b 3.5 (1.5) 0.3 (0.1) 16.6 (3.4)2012 15.3 (2.9) 1.1 (0.4) 0.1 (0.1) 9.5 (2.3)

8.9 m2 ha�1 and from 20 to 375 trees ha�1, respectively. Inuntreated units, burn severity metrics were more extreme; sevenof nine units had BA losses >10 m2 ha�1 and density losses>600 trees ha�1 due to the Wallow Fire. Comparisons of severitymetrics between treatment units showed that treated areas hadlower tree basal area loss (P = 0.04) and lower tree mortality(P = 0.05) when compared to untreated units (Fig. 3). In treatedunits, basal area was reduced by 29%, compared to a 55% BA reduc-tion in untreated areas; tree density reduced by 53% (�140 TPHkilled) in treated areas compared to a 74% reduction (�600 TPHkilled) in untreated areas. Maximum bole char height showed nosignificant differences among treatments; treated units averaged3.5 ± 1.9 m and untreated units averaged 5.4 ± 2.2 m (P = 0.13).

These reductions in basal area and tree mortality were highlyvariable in both treated and untreated units. Basal area reductionsvaried from 6.7% to 57.1% with a coefficient of variation (CV) of67.5% in treated units and 19.0–90.7% (CV = 41.1%) in untreatedunits. Tree mortality ranged from 25.0% to 82.4% (CV = 41.5%) intreated units compared to 42.4–98.6% (CV 25.7%) in untreatedunits. In treated units, >90% tree mortality occurred in 28% of thesample plots, and 27% of the plots had <10% mortality, comparedto 52% and 3%, respectively, in untreated plots.

Treatment differences after the wallow fire: While post-fire live-tree density remained significantly lower (P = 0.01) in treated unitsthan untreated units, basal area (P = 0.20) and forest composition(PERMANOVA P = 0.65) were statistically similar between treatedand untreated units following the Wallow Fire (Table 1, signifi-cance denoted by x,y). Tree mortality in both treated and untreatedunits occurred predominately in the smaller size classes, shiftingthe peak distribution to the 45-cm class in treated units and the15-cm class in untreated units. Diameter distributions post-firein treated units and untreated units both resemble an unimodaldistribution (Fig. 2).

Mortality of large trees (defined as P37.5 cm dbh for conifersand P20 cm dbh for aspen and Gambel oaks) was lower, but notsignificant (P = 0.05) in treated units compared to untreated units(Table 2). Mortality of large ponderosa pine, the most fire-resistantspecies in the forest was significantly greater (P = 0.008, Table 2,significance denoted by a,b) in untreated units, averaging 42%(range 10–100%) compared to a 6% average mortality in treatedstands (range from 0% to 20%). Mortality was dominated by largeaspen in the treated units (15.3 tree ha�1). Large ponderosa pineexperienced the highest species mortality in untreated units(15.8 tree ha�1), followed by aspen (14.7 tree ha�1) and Douglas-

ns between treated and untreated areas (means with standard error in parentheses).x,y for 2012). Pre-fire (2011) and post-fire (2012) forest structure differences within

ies code: ABCO (Abies concolor), PIEN (Picea englmannii), PIPO (Pinus ponderosa), PISTQuercus gambelii).

PIST POTR PSME QUGA

a 12.5 (4.6)a 45.0 (15.8) 23.3 (4.4)a 70.6 (46.9)x 3.9 (1.7) 8.6 (4.2) 11.1 (2.6)x 16.9 (12.5)

.9)b 66.9 (19.0)b 133.3 (75.6) 298.1 (59.3)b 83.6 (51.3)

.7)y 12.2 (6.3) 13.6 (6.4) 79. 4 (38.4)y 13.1 (7.1)

0.5 (0.2)a 1.9 (0.7) 2.1 (0.5)a 0.9 (0.6)0.2 (0.1) 0.5 (0.3) 1.3 (0.4) 0.4 (0.2)

b 1.4 (0.4)b 2.3 (1.1) 8.1 (1.5)b 1.1 (0.6)0.5 (0.2) 0.4 (0.2) 3.4 (1.3) 0.2 (0.1)

Fig. 2. Changes in diameter distribution by species for treated and untreated units. Diameter class midpoints are shown on x-axis. Tree survival is shown before and after theWallow Fire.

Fig. 3. Basal area and trees per hectare percent change relative to pre-fire forest structure in treated and untreated units. Box plots represent minimum, 25% quantile, median,75% quantile, and maximum values. Dotted lines represent the means.

A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52 47

fir (10.3 tree ha�1). Species with little resiliency to fire showed nosignificant differences in mortality rates between treated anduntreated units (aspen, white fir, Gambel oak, Table 2). The firesignificantly lowered canopy bulk density in the untreated unitsto below thresholds identified by Cram et al. (2006) and Cruzet al. (2003) for torching and active crown fire. However, treatedunit average canopy bulk density remained significantly less(P = 0.01) post-fire, averaging 0.02 kg/m3 compared to 0.07 kg/m3

in the untreated units.

3.2. Treatment effectiveness: herbaceous understory response

Total herbaceous understory plant cover was, on average, 1.5times greater (P = 0.008) in treated units compared to untreatedunits, one year following the Wallow Fire (Table 3). Native plantcover dominated both treated and untreated units composing89% and 84% of the post-fire vegetation, respectively (Fig. 4). Nativeplant cover was significantly greater (P = 0.008) in treated unitsthan untreated units. No distinguishable differences were found

Table 2Large (P37.5 cm dbh for conifers and P20 cm dbh for aspen and oaks) tree density prior to fire (tree ha�1), followed by percent mortality for treated and untreated units (meanswith standard error in parentheses). Species code: ABCO (Abies concolor), PIEN (Picea englmannii), PIPO (Pinus ponderosa), PIST (Pinus strobiformis) POTR (Populus tremuloides),PSME (Pseudotsuga menziesii), and QUGA (Quercus gambelii). Data comparisons from paired sites are for percentage mortality as measured after wildfire in 2012. Bolded textindicates significantly different means a = 0.05, denoted by letters (N = 9).

Treatment Total ABCO PIEN PIPO PIST POTR PSME QUGA

Treated (pre-Wallow) 70.8 (8.7) 7.8 (6.0) 0.8 (0.6) 27.5 (5.5) 0.6 (0.4) 20.6 (6.3) 6.9 (2.2) 5.8 (4.3)% Mortality 32 53 0 6a 50 75 22 47Untreated (pre-Wallow) 91.7 (10.5) 5.3 (2.4) 0.3 (0.3) 40.8 (9.5) 1.4 (0.8) 19.7 (8.2) 17.2 (4.2) 6.9 (4.1)% Mortality 56 57 100 42b 22 73 55 65

Table 3Average total cover, species richness and native cover and richness by Functionalgroups (±SE) for 2012 one-year post-wildfire. Significantly different means areindicated in bold; a is different from b; x is different from y at a = 0.05.

Treated Untreated

% Cover (m2)Total cover 12.2a (1.6) 8.1b (1.3)Annual forb 1.9 (0.7) 0.9 (0.1)Ferns 0.4 (0.2) 0.3 (0.2)Graminoid 3.8x (1.0) 1.8y (0.4)Perennial forb 3.0 (0.6) 2.5 (0.5)Shrub 0.3 (0.1) 0.3 (0.3)Tree 1.6 (0.5) 1.0 (0.4)

Richness (500 m2)Total richness 50.3 (3.5) 48.3 (3.8)Annual forb 7.2 (0.7) 7.4 (0.6)Fern 0.3 (0.1) 0.2 (0.1)Graminoid 9.4 (0.4) 8.4 (0.5)Perennial forb 36.5 (2.3) 34.9 (2.5)Shrub 1.8 (0.3) 2.6 (0.5)Tree 1.4 (0.3) 1.7 (0.3)

Fig. 4. Average native and nonnative cover within treated and untreated units.Nonnative seeded species as a percentage of total nonnative cover is noted inparentheses. Different letters index a statistical difference at a = 0.05. Bar represent1 standard error of the means (N = 9).

Fig. 5. Area-weighted mean patch size in treated and untreated units. Box plotsrepresent minimum, 25% quantile, median, 75% quantile, and maximum values.Dotted lines represent the means.

48 A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52

in nonnative cover between treated and untreated units (Fig. 4).The most common nonnative species were Taraxacum officinale(common dandelion) and two species intentionally seededfollowing the fire: Triticum aestivum (common wheat) andHordeum vulgare (common barley). The invasive Bromus tectorum(cheatgrass) was recorded on 27 plots (15% of all plots). Meannative graminoid foliar cover was on average twice as high(P = 0.02) in treated units than untreated units (Table 3). Nativeperennials and graminoids were the dominant functional group,representing 42% and 31% of the total cover across treatments.Total richness was primarily comprised of native perennial forbsacross both treated and untreated units.

3.3. Treatment effectiveness: high severity patches

High severity patches were significantly smaller (P = 0.008) intreated units compared to untreated units (Fig. 5). In treated units,31 high severity patches were identified ranging from 0.09 to7.9 ha, whereas 47 individual patches ranging from 0.09 to64.2 ha were identified in untreated units. Area-weighted meanfor high severity patches was 0.9 ha in treated units and 13.9 hain untreated units. High severity patches accounted for 3% of thetreated units and 34% of the untreated sampled units. As shownin Fig. 5, both the median and the maximum weighted area patchsize are an order of magnitude larger in untreated units than intreated units.

4. Discussion

We tested the hypotheses that wildfire hazard reduction treat-ments increased ecological resiliency to uncharacteristically severefire and that fuel reduction treatments reduced wildfire severity intreated compared with untreated stands.

4.1. Ecosystem resiliency

In this study, we defined ecosystem resiliency for three ecosys-tem metrics using a combination of known historic reference con-ditions and empirical data from systems with observed typechanges. Results from this study suggest fuel reduction treatmentsincrease resiliency to uncharacteristically severe fire as measuredby short-term forest structure attributes. Treated units showedlow tree mortality and small high severity patch sizes, maintaininga similar ecosystem structure to that reconstructed prior to theWallow Fire. The treated units also supported significantly higherunderstory herbaceous cover; however, nonnative cover metrics of

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resiliency showed no differences among treatments one year post-fire.

Forest structure: We utilized the historic range of variability(HRV) as a system reference condition to parameterize and charac-terize overstory metrics of ecosystem resilience (Carpenter et al.,2001; Rist and Moen, 2013). The use of HRV provides a valid metricof forest resiliency in the southwestern U.S.A. Adaptive character-istics of the historic climate, including fire resilient species and for-est structure, may help retain native species on the landscape withclimate change projections of increased fire probability(Westerling et al., 2006). A compilation of forest reconstructionanalysis and historical surveys across southwestern warm/drymixed conifer forest indicated that historical forest densities fellbetween 52 and 255 tree ha�1 with stand basal area rangingbetween 9.1 and 28.4 m2 ha�1 (as summarized in Stoddard, 2011;Reynolds et al., 2013). In treated stands, reconstructed forest struc-ture data used to depict pre-fire conditions showed seven of ninetreated units fell within HRV for tree densities and eight treatedunits were within the range for basal area. Following the WallowFire, all nine treated units were within HRV for tree density,whereas five units fell slightly below HRV for basal area. Untreatedunits prior to the fire were on average four times the upper end ofHRV for tree density and 1.2 times above the basal area range. Fol-lowing the fire, three untreated units were reduced to tree densi-ties within HRV and seven were within the range for basal area.

These ranges suggest the Wallow Fire resulted in restorationbenefits, even in untreated forests. In fact, the Wallow Fireappeared to have a thinning effect across treated and untreatedunits in which smaller (<40 cm DBH for conifer, <20 cm DBH fordeciduous) and fire-susceptible species (accounting for 78% of alltree mortality) were killed, similar to Fulé and Laughlin’s (2007)post-fire findings in the Grand Canyon, AZ. Reduction in both basalarea and tree density across treated and untreated units werelargely due to decreases in white fir and aspen, tree species lessresilient to fire. These results were reflected by the non-significantPERMANOVA analysis post-fire, suggesting the fire had restorationbenefits in untreated units and moved tree composition closer tohistorical ranges. In treated units, HRV conditions for forestcomposition were maintained following the fire.

Although the Wallow Fire moved density and species composi-tion towards HRV conditions, other metrics of resiliency showeddifferential results between treated and untreated units followingthe fire. Large trees are often a metric of forested ecosystem resil-ience. Large, old trees are the most fire-resistant trees in the stand,because they have the tallest crown, the thickest bark and are resil-ient to low-severity fires, which are characteristic of historic fireregime. Conservation of large, old trees is a priority for restorationobjectives because they represent a rare structural legacy that pro-vides numerous ecosystem services (Franklin et al., 2000; Abellaet al., 2007). Retaining large trees following a large fire event con-tributes to the overall ability of the stand to regenerate, maintainsstructural components of the forest that contribute to habitatdiversity for dependent wildlife, and contributes to overall standresiliency to fire. In untreated units, more than half of large trees(all species) were killed, compared to a 32% mortality rate in trea-ted stands (Table 2). Although structurally the BA of seven post-fireuntreated stands fell within the HRV, the high mortality of largetrees in untreated stands resulted in a lack of the large-tree compo-nent of a reference-condition mixed conifer forest diameter distri-bution (Fulé et al., 2009).

Understory herbaceous response: Increases in nonnative plantcover have consequences for native cover and fire return intervalsand can suggest a loss of ecosystem resiliency. For example,increases in cheatgrass (Bromus tectorum) in the Great Basin desertsystems have modified the fire regime from infrequent fires to aregime with frequent fire (D’Antonio et al., 2000). Our study

showed nonnatives contributed to just 5% of the total cover onboth treated and untreated sites and were found on 73% of all sam-pling plots. The understory herbaceous community in the Apache-Sitgreaves NF mixed conifer forests was uniquely dominated bygrass, unlike shrub-dominated mixed conifer understory commu-nities more commonly observed in both the intermountain andPacific Northwest mixed conifer systems (Agee, 1993; Korb et al.,2012).

Native understory plant cover was significantly greater in trea-ted units than untreated units one year following the Wallow Fire.These differences were driven by native perennial and graminoidspecies. Post-fire vegetation differences between treated anduntreated units may be due to differences in the pre-fire plantcommunity; surveys near our study units indicated treated areashad approximately twice the cover than untreated areas (Sitkoand Hurteau, 2010). These results support observations by Voseand White (1987) that post-fire understory responses werestrongly linked to the pre-fire community. However, our initialtreatment differences following severe wildfire were inconsistentwith other research, which indicated untreated sites were consis-tently higher in total plant cover than treated sites (Shive et al.,2013b). Several studies indicate rapid plant recolonization andincreased understory cover in severely burned areas (Crawfordet al., 2001; Griffis et al., 2001). Kuenzi et al. (2008) found that highburn severity played a greater role in the post-wildfire plant com-munity than fuel treatment. In this study area, the high nativecover in treated units may contribute to rapid soil stabilizationand continued resistance to nonnative species. Future studies arenecessary to determine if herbaceous cover differences remain sig-nificant through time.

Fire severity and patch size: The warm/dry mixed conifer studiedhere can be classified as frequent fire regimes (Hessburg et al.,2005; Battaglia and Shepperd, 2007; Margolis et al., 2007, 2013;Evans et al., 2011), and historically were dominated by surface firewith small portions of high severity fire (Hessburg et al., 2007;Collins and Stephens, 2010). In the Wallow Fire, high severity fireburned over a significantly smaller portion of treated units thanin untreated units. The largest high severity patch in the treatedunits was just under 8 ha while in untreated units, 75% of highseverity patches were larger than 8 ha. The magnitude of highseverity or stand replacing patches compromises forest ecosystemresiliency by decreased probability of recovery with similar vegeta-tion types. If high severity patches are too large, increased dis-tances to seed sources can impede establishment (Turner et al.,1998). Long-term studies in ponderosa pine ecosystems haveshown that some areas in high severity burn patches are convert-ing toward a shrub dominated community with little pine regener-ation (Savage and Mast, 2005; Roccaforte et al., 2012; Shive et al.,2013a, but see Haire and MGarigal, 2010), suggesting low resil-iency of the pre-fire ecosystem to type change. However, long-termmonitoring is needed to assess impacts in mixed conifer.

We found fuel reduction treatments did increase mixed coniferforest resiliency specifically for overstory structure metrics, includ-ing patch size, forest composition and survivorship of large trees.We also observed significantly higher native understory cover intreated units; however, both native and nonnative cover measuredone year post-fire have too much uncertainty to contribute to thefull understanding of ecosystem resiliency.

4.2. Burn severity

Results from this study support existing empirical evidence andthe hypothesis that fuel treatments reduce fire effects even inextreme conditions (Raymond and Peterson, 2005; Strom andFulé, 2007; Prichard et al., 2010). Wildfire burn severity is the mea-sure of the ecological effect a fire has on a system. Classifications of

50 A.E.M. Waltz et al. / Forest Ecology and Management 334 (2014) 43–52

severity in forested systems have long been based on mortality oftrees, measured as the percentage of basal area lost (Agee, 1993);greater than 70% basal area loss is classified as high severity; RAVGdata used in this study classified 75% basal area loss as the highseverity threshold. Treated mixed conifer forests within the Wal-low Fire perimeter had significantly less tree mortality and lowerBA loss than paired untreated units. High severity fire burned overa significantly smaller portion (3% vs. 34%) and with smaller aver-age patch sizes in the treated units than in the untreated units.

The fuel treatments implemented prior to the 2011 wildfiredecreased tree densities by 74–93% and canopy bulk density by65–93% compared to untreated units, significantly reducing thetree canopy fuel that is necessary component of active crown fire.The fire hazard reduction treatments accomplished under theWhite Mountain Stewardship Project can be considered broadlysuccessful at reducing fire severity.

4.3. Additional metrics of resilience and study constraints

Not all metrics of resiliency we examined are appropriate for allsystems, and additional metrics are needed for southwestern U.S.forested systems. Nonnative and native cover proportions may bemore appropriate for systems where invasive nonnatives are pres-ent and colonization risks are higher. Time of monitoring andrepeated monitoring is important, as long-term understoryresponses can take several decades before patterns of establish-ment and succession become clear. Additional metrics importantto consider in southwestern forests include tree regeneration andshrub establishment. Reference conditions for understory (herba-ceous and shrub) composition and abundance and tree regenera-tion are less available from empirical studies. However, wesuggest these variables would add important long-term resiliencymetrics because some post-burn assessments of frequent-fire sys-tems have shown type changes from forested systems to grass orshrub-dominated systems (Savage and Mast, 2005; Roccaforteet al., 2012; Buma et al., 2013). In large overstory mortalitypatches, lack of overstory seed source and subsequent shrub andgrass fire regime effects can maintain the system in a shrub–grasscondition.

This study benefited from but also was constrained by measur-ing ecosystem conditions one year post-fire. The measurementsallowed the reconstruction of overstory stand conditions prior tothe fire; often, opportunistic studies of landscape-scale distur-bances are often constrained by a lack of pre-treatment data. Thisallowed a better understanding of the post-fire effects on foreststructure. Limitations of the study included potential underesti-mates of overstory mortality and uncertainty in understoryresponse. Although future tree mortality changes may alter finaltrees per hectare killed, the mortality differences among treat-ments are of the scale that changes in overall patterns are notexpected. In addition, the authors were fortunate that some under-story data were available adjacent to burned units to more fullyunderstand understory responses.

The USFS methods of reporting accomplishments can lead touncertainties about the actual on-the-ground treatment conditionimmediately prior to the fire. It was unclear for some treatmentpolygons if final fuel treatments, such as slash removal or post-thinning prescribed fire, were completed. Thinning treatments thatlack residual fuel removal have been shown to contribute to severefire effects and overstory mortality (Hudak et al., 2011; Martinsonand Omi, 2013). Another constraint is the limited knowledge of firesuppression activities. That is, treatments placed around commu-nities can serve as staging efforts for suppression activities, includ-ing backburning. However, records and spatial data of these firingtechniques are incomplete in the Wallow Fire (as in many wildland

fires); some treatment fire effects may have been due to suppres-sion tactics, not the wildfire.

4.4. Conclusions and management implications

This study defined three short-term metrics of resiliency for fre-quent-fire adapted conifer forests and tests the effectiveness offuels reduction treatments at maintaining resiliency. This is impor-tant to federal land managers that currently work under nationalpolicy to ‘‘make our NFS [National Forest Systems] lands moreresilient to climate change. . .’’ (National Forest System, 2012),but lack any accompanying definitions of resiliency for their land-scapes (USDA, 2013). In western U.S. forested systems, treatmentsthat aid protection of human values and resources will always befirst priority for federal land managers. However, this paper showsfuel reduction treatments can additionally increase specific resil-iency metrics of forested ecosystems, even within short-term time-frames. Not only are Western landscapes seeing increasing risksfrom fire to human communities, but landscape-scale fire impactsthe ecological integrity across larger landscapes. This study sug-gests broader landscape restoration and fire hazard reductionefforts may be justified to increase resilience across the landscape.

Acknowledgements

This research was funded by a Grant from the USDA Forest Ser-vice. The authors thank the Apache-Sitgreaves National Forestsstaff for treatment information, Wallow Fire information and per-mitting. Field work was conducted by Ecological Restoration Insti-tute undergraduate students and seasonal field assistants. Theauthors thank Peter Fulé for reviews of early drafts and fouranonymous reviewers for reviews and suggested improvementsto the manuscript. NAU is an equal opportunity provider.

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