FIRE SCIENCE AND MANAGEMENT (M ALEXANDER, SECTION EDITOR)

Wildland Fire: Understanding and Maintainingan Ecological Baseline

Lee E. Frelich1

# Springer International Publishing AG 2017

AbstractPurpose of Review The goal was to synthesize the literatureon wildland fires, how they create resilient landscape mosaicsthat affect ecosystem function and maintenance of biodiversi-ty, and how the fires themselves are affected by wildernessedge effects and climate change. The emphasis is on cold-temperate and boreal forests.Recent Findings The interactions of fires with landforms cre-ate large-magnitude spatial and temporal heterogeneity in fireseverity that cannot be seen in smaller natural areas. Patternsof live and dead biomass and characteristic syndromes of fire-species interactions determine future successional trajectoriesand spatial-temporal dynamics of landscape mosaics.Therefore, wildlands with freely occurring fires provide a sci-entific baseline for complexity of vegetation structure andmaintenance of biodiversity and ecosystem processes on timescales from years to centuries and millennia. Although wild-lands are impacted by climate change, they may have consid-erable resilience, partly due to fire occurrence, and may stillserve as a moving baseline. Thus, the patterns observed inwildlands can be used as blueprints for restoration of land-scape structure, biodiversity, and ecosystem function inhuman-dominated ecosystems.Summary Wildland fires play an important role in mainte-nance of ecological function and biodiversity, even on land-scapes where fire is considered to be rare. At time scales of

centuries, fires in large wilderness areas maintain a balanceamong successional stages, and although early and late-successional stages are rarely absent, their occurrences changein space and time, possibly leading to metapopulation dynam-ics for species that depend on certain successional stages.Over thousands of years, fires influence the trajectory of eco-system retrogression, and in cold climates, fires can preventecosystem acidification that reduces forest productivity. Fireregimes within large wildlands are subject to change causedby fragmentation effects at large spatial extents; this can resultin increased or reduced fire frequencies (disturbance dilutioneffect) within wildlands. Wildland managers need to thinkabout how changes in the surrounding landscape influencethe integrity of the natural disturbance baseline, while forestmanagers need to think about how harvesting compares to thebaseline with respect to maintenance of productivity andbiodiversity.

Keywords Disturbance dilution . Forest fire . Landscapemosaic . Fragmentation .Wildland fire

Introduction

Wildland fires (the term used here as synonymous with wil-derness fire as in [1••]) have played a pivotal role in ecosystemdynamics on the Earth. Fires have acted as a major selectiveevolutionary force and as important regulators of the balanceamong biomes, such as forests versus grasslands, and are ca-pable of creating grasslands or savannas in locations where theclimate and soils might otherwise support forest vegetation[2]. Most pertinent to this review, fires create landscape pat-terns that are connected to ecosystem function and biodiver-sity [3]. The myriad mechanisms by which this force operatesare now visible in wildlands that occupy an increasingly small

This article is part of the Topical Collection on Fire Science andManagement

* Lee E. Frelichfreli001@umn.edu

1 Department of Forest Resources, The University of MinnesotaCenter for Forest Ecology, 1530 Cleveland Ave. N., St.Paul, MN 55108, USA

Curr Forestry RepDOI 10.1007/s40725-017-0062-3

proportion of the Earth’s surface. Inasmuch as these mecha-nisms are key to our understanding and maintenance of bio-diversity, the ecology of wildland fires is an important topic ofscientific research [4].

At the time scale of decades to centuries—and workingwith the suite of available species that fire helped to createover evolutionary time—fires are a major factor in successionand landscape patch dynamics across a wide spectrum of spa-tial and temporal scales, leading to the coexistence of manyspecies with different adaptations to fire [5, 6]. Patterns suchas the juxtaposition of heavily burned and unburned or lightlyburned patches of all shapes and sizes are important structuralfeatures of landscapes [7, 8], as are the full suite of biologicallegacies including coarse woody debris and other complexstructures left after wildfires [9, 10]. Large wildlands exhibitpatterns that cannot be seen at smaller spatial extents of mostnatural areas; the more one samples, the more variety of suc-cessional trajectories and other effects are found after fire,with a variety of interactions with topography, and variabilityin severity [11, 12]. Even in ecosystems where fires are rare,they still play a surprisingly important role in maintainingbiodiversity [13]. Like herbivores, fire also alters the plantspecies composition and structure at the base of trophic pyra-mid, and therefore, it affects all levels above. Furthermore, thisregulation of the base of the trophic pyramid by fires occurs ina spatially patchy and highly variable form over time, creatinggreat complexity in ecosystem structure and function [14] andcomplex metapopulation dynamics for many species [15].

A baseline or control is the most basic principle of science.Wildlands where fires occur freely provide baselines, withcertain limitations explained below, for how disturbancesmaintain biodiversity and ecosystem function [1••, 16].Wildland fires exhibit ecosystem response to fire in the ab-sence of post-fire salvaging and fire behavior after insect out-breaks and windstorms in the absence of pre-fire salvage.Baselines can represent a Blet nature take its course^ orBessentially natural^ ecosystem state, or in some cases, suchas North America, could represent a pre-European settlementecosystem state, which may also include Native American useof fire. Wildland baselines are useful for restoration forestryon the majority of the landscape matrix, via concepts such asBclose-to-nature forestry^ [17]. Forest health—defined mostsimply as maintenance of productivity and biodiversity overtime—was maintained by natural disturbance for thousands ofyears prior to human use of forests for commercial products.In a world with increasing human demands on forests, it isessential that we understand how natural disturbance, and firein particular, contributed to the long-term maintenance of for-est health.

At this time, the true presettlement baseline is fading aswildlands are being affected indirectly by human activitiessuch as fragmentation, climate change, and invasive species.In the last few decades, most scientists and managers have

acknowledged that indirect human influences such as climatechange are pervasive and wildlands are headed towards beinga moving baseline, although the main difference of the pres-ence or absence of active harvesting of commercial productsand purposeful alteration of the landscape still exists [16].Fires can still have natural behavior and spatial-temporal pat-terns within wildlands, even though the regime and occur-rence of fire is no longer the same as it would have beenwithout human influence [5, 7].

The major focus of this paper is to review how wildlandfires provide a scientific baseline for ecosystem processes andlandscape mosaics and how those impact biodiversity andresilience. However, the resulting Bblueprint^ for restorationof fire on the landscape obtained from knowledge of wildlandfire is affected by surprisingly large fragmentation effects onfire frequency that occur in addition to fire suppression effects.Furthermore, the baseline itself is a moving target as the cli-mate changes. These difficulties indicate that careful monitor-ing and restoration of fires within wildlands is necessary insome cases—to keep such wildlands as a fire-based ecologicalbaseline—that remains informative for restoration of smallernatural areas and for comparison to the effects of commercialforest management. Boreal and temperate forests are empha-sized in the review, and my experiences and previous work intemperate and boreal forest ecosystems of central NorthAmerica are woven into the review to provide illustrativeexamples.

Fire, Ecosystem Processes, and Biodiversity

Forests evolved without harvesting. All wood was eithercombusted in fires, or went into the dead wood pool, whichincluded standing snags or coarse woody debris (CWD) onthe forest floor, resulting from trees that were killed by fire,wind, disease, or insect infestation. Thus, landscapes withnatural disturbance regimes, including wildland fires, provideopportunities to observe dead wood dynamics for cases wherethe woody material is not harvested. Although fires often con-sumemost of the duff and twigs, generally 10% or less of bolewood and large branches are consumed, leaving enormoustonnages of very large snags and coarse woody debris(Fig. 1) that are the base of a complex food web for manyspecies from many taxonomic groups—especially fungi, li-chens, mosses, insects, cavity nesting birds, and amphibianshave been well studied [18, 19]. This CWD is also substratefor tree seedlings and other vascular plant species once thewood has reached a high stage of decay; often these are non-fire adapted species in late-successional forests that benefitfrom CWD substrate created by fires. CWD can also protectpost-fire plant growth from herbivores [20]. This large volumeof CWD has immense impacts on biodiversity and the light,water, and nutrient environments that exist in the forest [18].

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The comparison of post-fire ecosystem dynamics in wildlandswith that in commercial forests is immensely valuable forassessing the sustainability of harvesting and other ecosystemmanipulations [14, 21].

Post-fire environments, especially in dense conifer forestswith infrequent crown fire regimes, are extremely differentfrom closed-canopy conditions (Figs. 1 and 2). These pulsedvariations from conditions that exist most of the time (i.e.,after post-fire revegetation until the next fire) have many im-portant differences, including reduced duff thickness, relative-ly high soil temperature, low surface soil moisture due toincreased runoff, but sometimes increased deep soil moisturedue to death of trees that transpire water, high light levels, highcation availability, and high pH [22]. All of these allow forgrowth of nutrient-rich herbaceous vegetation within reach ofground-based herbivores (e.g., deer, moose, and elk) and re-production of shade-intolerant fire-dependent tree species.Tree species can become established that persist for a longtime in the absence of fire [11, 13]. These pulses of changedenvironment on the forest floor are essential to maintenance ofbiodiversity and tree species composition of the forest over

vast regions, along with the important ecological cascade ef-fects on wildlife species.

Fires operating over centuries to millennia can have largeimpacts on chemistry and ecological function of the entireecosystem, including soil and lake chemistry. Fires play variedroles in the rate of ecosystem retrogression; they can causerock and atmosphere derived nutrients (especially cations) toerode out of the terrestrial ecosystem, possibly acceleratingretrogression towards a nutrient-poor state of the forest unlessweathering replaces the losses [23]. Alternatively, fires canprevent retrogression caused by development of Sphagnummosses on the forest floor that acidify ecosystems, eventuallyleading to nutrient-poor ecosystems in a process known in theboreal forest region as paludification. In the Canadian borealforest fires every one-to-two centuries prevent paludificationby keeping forests at early-successional stages, while

Fig. 1 Within-stand, post-fire structural complexity and coarse woodydebris in an uneven-aged stand (upper, large logs are red pine, smallerdead trees are jack pine, black spruce, balsam fir, paper birch and aspen)and an even-aged stand (lower, mostly jack pine), in the BoundaryWatersCanoe Area Wilderness (BWCAW), MN. Photos by Dave Hansen,University of Minnesota

Fig. 2 Post-fire structural complexity of the landscape mosaic in theBWCAW, MN. Upper, stands after mixed severity fire (Ham Lake FireofMay 2007, picture taken 3 months later) has live trees (green) and treeskilled by scorch (brown) or direct contact with fire (charred trees mostlyfallen by the time the picture was taken) in this boreal forest composedmainly of jack pine and black spruce. Some late-successional white cedarand balsam fir trees along the lakeshore in the photo are also still alive.Lower, stands after high-severity fire (Cavity Lake Fire of 2006, picturetaken 1 year later), are totally killed and much of the duff and forest floormoss layer was also consumed. The green vegetation is composed mainlyof Bicknell’s geranium that germinated from buried seeds. Photos byDave Hansen, University of Minnesota

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cessation of fires leads to progressive paludification starting afew centuries after the last fire [24, 25]. These processes areespecially important in boreal ecosystems located on glaciatedgranitic bedrock which cannot supply cations to the ecosystemduring the bedrock weathering process, such as parts of theBoundary Waters Canoe Area Wilderness (BWCAW) innorthern MN and northern Sweden [12, 26]. Pockets of acid-ification can occur on fire-prone landscapes in areas with lowfire occurrence associated with certain landforms, or changingfire frequency associated with changing climate can lead toshifts in paludification. Thus, spatial and temporal variation infire regimes causes alternate successional and ecosystem de-velopmental trajectories on time scales of centuries tomillennia and at vary large spatial extents [27].

Release of cations from vegetation and forest floors duringand after fires also affects lake chemistry [23]. In the borealforest, lakes can acquire cations that are important to main-taining buffer systems and relatively high pH (e.g., 6.5–7.5 innorthern MN [12]) as long as fires continue to occur everycentury and avoid acidification that could occur over severalcenturies to a few millennia due to natural acidification pro-cesses or acid deposition [12, 28, 29].

Fire, Landscape Mosaics, and Biodiversity

Wildlands with free-ranging fires can exhibit all of the varia-tions in fire severity (Fig. 1 and Fig. 2) and successional pat-terns that no longer exist on most of the landscape wherelogging or other direct human influences occur. Variabilityin fire effects, landform interactions with fire, and frequencyand size of unburned patches are all visible after wildlandfires. Fragmentation patterns caused by natural disturbanceare quite different than human fragmentation patterns [30],but species have evolved in response to the natural patterns,and we can learn about these differences through study ofpatterns created by wildland fire.

Perhaps most importantly, large wildland fires exhibit pat-terns visible only at large spatial extents (Fig. 3 and Fig. 4) [5,31]. Very large fires that can occur in large wildlands producea structurally different landscape than small fires [32].Unburned remnants (or islands) within wildland fires tend tohave more variation in size than remnants left after harvesting[7], and shapes and locations are also variable, including low-land patches that were too wet to burn, rocky or sandy areaswith too little fuel to burn intensely, and stringers—long, thinremnant strips of live vegetation that occur in the middle ofareas with flat topography and uniform forest due to the inter-nal dynamics of large, high-intensity fires [8, 12].

Often, there are several coexisting disturbance regimes onlarger landscapes, as characterized by frequency, intensity, andtype of disturbance, which are associated with landform var-iation (cf Fig. 3 and Fig. 4) [13]. Sometimes, this type of

heterogeneity is ignored and instead land managers pick upon one regime (by studying a small natural area remnant oronly a small portion of a large wildland) and then use only thatfor restoration elsewhere, thus unknowingly homogenizingthe landscape. Fire managers also tend to homogenize the fireregimewith respect to season of burn, whereas large wildlandsexhibit the effects of varied seasons of burning.

Only by studying the occurrences of fires within large wild-lands could we have learned classic concepts of landscapeecology such as the post-fire distribution of distances fromlive trees that is surprisingly short compared to the total areawithin the fire perimeter, so that relatively few species expe-rience lack of regeneration due to long dispersal distances[33], landscape stand age-class distributions created by fireand how historic changes in fire frequency are locked intothe age distribution of stands [31], and the different modesof landscape stability that result from the relationships amonglandscape size, disturbance size distribution, and disturbancefrequency [34, 35].

Fires, Landscape Size, and Stability

How large does a landscape need to be to support a stablemosaic to accommodate all successional stages? A variety ofanswers exist, and I will show two here. First, it has beenproposed that in order to avoid having one large disturbancedisrupt landscape structure, a landscape should be 2× the max-imum fire size [31] or 50× the mean fire size [36], although[13, 35] point out that it is also necessary to assume thatdisturbances occur at a constant rate, and an index combiningdisturbance size relative to landscape size and disturbance raterelative to recovery rate of the vegetation are needed to trulyassess stability. Assuming constant disturbance rate and goodrecovery abilities for vegetation, we can use the 2× maximumsize and 50× mean fire size criteria. For the North Americanboreal forest, mean and maximum fire sizes are on the order of5000–10,000 and 100,000–600,000 ha, respectively [12, 37],and a wildland would have to be from 200,000 to1,200,000 ha to meet the previously mentioned criteria forstability.

A second way to look at the issue is that a wildland shouldspan several different landscape types, such as land type as-sociations (LTAs), landscape units that have similarities inclimate, geology, and vegetation, often with repeated patternsof landforms [38]. Examples include repeated patterns of for-ested valleys separated by rocky ridges within a mountainrange, pitted outwash plains with kettle-hole lakes spreadthroughout, or moraine complexes with repeated patterns ofhills and valleys. Land type associations have a general sizerange from 104 to 105 ha [38]. Wildlands that span severalland type associations exhibit a large range of landform-fireinteractions, which vary among LTAs and are informative for

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a baseline on how natural fire regimes create landscapepatterns.

Therefore, given these two analyses of wildland and firesize, I propose that wildlands of 105 to 106 ha, with fires of 103

to 105 ha (i.e., landscapes 1–2 orders of magnitude larger thanfire sizes, and fires large enough to have multiple interactionswith landforms within), may be adequate to exhibit the rangeof variability of landscape interactions with fire. Such wild-lands are likely to exhibit a type of resilience not found insmaller natural areas, because they may show the full suiteof post-fire successional stages, have habitat refuges even af-ter the most extreme fire events, and can accommodate spatialand temporal variability in rate of disturbance without losingspecialized habitats.

Smaller wildlands can also be important, but are more like-ly to have smaller fires, to only represent part of the range ofpossible fire-landform interactions, and several small wild-lands may have to be studied to get a complete picture offire-landform relationships and their impacts on forest compo-sition and biodiversity. In the central and northern boreal re-gions, the fire-related landscape patterns, ecosystem process-es, and biodiversity dependent on them are largely intact with-in very large extant wildlands, while in southern boreal andtemperate forest regions, much of the landscape has been al-tered, hugely elevating the importance of even small wild-lands in those regions.

Fire, Landscape Mosaics, and Species

Many species only occur after fires and are able to move aroundthe landscape to take advantage of recently burned areas, creatingcomplex metapopulation dynamics for species with several life-history traits, which may or may not have evolved in response tofire, but which still provide the needed function in a fire context[2, 6]. Examples from the BWCAW landscape of northern MNinclude the sprouters aspen (Populus tremuloides), paper birch(Betula papyrifera), and beaked hazel (Corylus cornuta); plumedseeders capable of finding recently burned areas, aspen and fire-weed (Chamerion angustifolium); small seeders that can disperseinto large burns (paper birch); seeds with symbiotic relationshipssuch as zoochory that aid dispersal into burned areas such asnorthern red oak (Quercus rubra), which is moved up to 2 kmby blue jays (Cyanocitta cristata) [39]; and species with buriedseed strategies with seeds that survive until the next fire occurs,pin cherry (Prunus pennsylvanica), and Bicknell’s geranium(Geranium bicknelii). This geranium species is often the firstevidence of recovery after severe fires (Fig. 2).

The neighborhood-effect theory of forest dynamics pre-dicts three different categories of landscape dynamics, eachwith a distinct syndrome of species interactions that interactwith fire severity to create patterns of dominance and patchdynamics on the landscape [13, 40]. Note that four landscapedynamic categories exist (A-D), but that C is not relevant here

Fig. 3 Area burn map for 1863–1864 for the BWCAW, MN, by M.L.Heinselman. This map illustrates the potential disturbance dilution effectof fragmentation in the BWCAW. The large fire in the central portion ofthis map alone burned ca 112,000 ha or 28% of the BWCAW forests. Thisfire was ignited to the south of the wilderness boundary (mapped as theblack line) and later burned into and through it; it is unlikely that the fire

would have occurred if the forests outside the wilderness area had beenlogged prior to 1864. The potential number and area of fires that wouldhave ignited south of the wilderness and burned into it after that area wascleared for settlement during 1895–1920 can never be known, but likelyadded to the impacts of fire suppression and climate effects, to reduce firefrequency by an order of magnitude during the twentieth century

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and not further discussed. Categories A, B, and D are definedand discussed individually in the following three paragraphs.In each case, the traits of dominant trees and most commontypes and severities of disturbances are in synch. Neigh-borhood effects are defined as interaction between adjacenttrees that can reinforce (positive), have no effect on (neutral),or reduce (negative) post-disturbance replacement by the samespecies of tree that died. Although neighborhood effects occurat small spatial extents of ca 0.1 ha, they lead to emergentproperties at the landscape scale [40].

First, category A landscapes are dominated by late-succes-sional, shade-tolerant tree species, and with positive

neighborhood effects in synch with low-to-moderate-severitywind regimes and gap dynamics that comprise the majority ofthe disturbance regime, allowing shade-tolerant advance re-generation to be released and replace dying canopy trees ofthe same species. Rare combinations of disturbance of unusu-ally high severity—windthrow followed by fire—knock outthe positive neighborhood effects and allow patches of shade-intolerant, early-successional species to establish on the land-scape, and these early-successional species occur as isolatedpopulations connected via metapopulation dynamics. Theremust be enough of these early-successional patches so thatinteractions by pollination and seed exchange can occur, or

Fig. 4 Stand origin map for theColeman Island Quadrangle,BWCAW, MN, by M.L.Heinselman. This illustrates themixed severity fire regime createdby topographic complexity thatbreaks up the flow of large, high-intensity fires across thelandscape. Note that many standshave trees with 2–4 dates of originafter medium-severity firesburned those stands, causingpartial mortality and post-firerecruitment. There are also someeven-aged stands that originatedafter the exceptionally large, high-severity 1864 fire (see Fig. 3),which was unable to burn manyolder stands at this location due tofire interactions with landform

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the early-successional plant species will go regionally extinct,along with the insects and wildlife species that depend onthem. For example, in the hemlock (Tsuga Canadensis) andsugar maple (Acer saccharum) forests of Upper MI USA,burned patches support white pine (Pinus strobus), northernred oak, paper birch, quaking aspen, and bigtooth aspen(Populus grandidentata), all of which are able to colonizerather isolated burned patches due to long-distance dispersalcapabilities. Even if disturbances severe enough to propagatethese species only occur with rotations of 1000 years, thespecies can dominate post-fire patches for 100–200 years,and a few individuals can persist for 200–300 years, so thatthey are present in 10–30% of the stands that compose thelandscape [13]. Thus, even in ecosystems where fires are rare,fires are still important for maintaining biodiversity, as firecauses ca 20–25% of the tree species to be moderately fre-quent rather than rare or absent across the landscape.

Second, category B landscapes are the opposite of categoryA, dominated by early-successional, shade-intolerant tree spe-cies that have disturbance-activated neighborhood effectssuch as sprouting or serotinous seeds. The dominant speciesare adapted to, and favor the occurrence of, high-severity fireswith large spatial extents. On these landscapes, the late-successional species are restricted to refuges from high-severity disturbance, which must be common enough on thelandscape to allow metapopulation dynamics among the late-successional species, or those species will be lost. The borealforests of MN’s BWCAW historically provided an example,with the landscape matrix of young, even-aged post-firestands of serotinous jack pine (Pinus banksiana) and blackspruce (Picea mariana), and the long-distance seeder/sprouter quaking aspen, and small inclusions of late-succes-sional, fire-sensitive balsam fir (Abies balsamea) and northernwhite cedar (Thuja occidentalis) in places that are skipped byfire by chance or due to landform effects on fire behavior. Along period of reduced fire occurrence from 1910 to 1990(caused by combined effects of climate relatively unconduciveto fire [31], fire suppression [12] and fragmentation effectsdescribed below in the BEdge Effects, Fragmentation, andDisturbance Dilution^ section) allowed balsam fir to greatlyincrease its abundance across the landscape, so that the histor-ical pattern of tree abundances is barely visible today.

Third, when the landscape is dominated by tree specieswith neutral to negative neighborhood effects (category Dlandscapes), then mixed fire regimes are common [13].These landscapes have a fine-grained, patchy distribution oflow, and moderate and high-severity fire that maintain irregu-lar mosaics with coexistence at stand and landscape scales oftree species with a variety of traits, including serotinous pinespecies that survive fire as seeds, tall thick-barked pine speciesthat resist fire and survive as adults, non-fire adapted speciesthat creep out from refuges on complex landforms, and possi-bly also sprouters. Two prominent examples from North

America include jack pine, spruce, fir, white and red pine(Pinus resinosa) mixtures in parts of MN’s BWCAW, andlodgepole pine (Pinus contorta) , whitebark pine(P. albacaulis) and subalpine fir (Abies lasiocarpa), and/orEngelmann spruce (Picea engelmannii) mixtures in theRocky Mountains.

These mixed regimes require fine-grained variation in fuelstructure to cause equally fine-grained variation in fire inten-sity [7]. Probably this is tied to landform, with rocky sites orsites with intermittent lakes and swamps leading to changes infire severity and vegetation. Areas within wildlands with rel-atively uniform topography tend to have large high-intensityfires. For example, Heinselman’s fire history maps of theBWCAW clearly show a region in the south-central part ofthe wilderness with repeated very large high-severity fires andeven-aged stands (Fig. 3), and other parts of the wildernesswith complex landforms (Fig. 4), mixed severity fire, and theBgrand mix^ of tree species as he referred to it [5],Heinselman, personal communication.

Maintaining a mosaic on the landscape where both fire-dependent and fire-sensitive plant species and all of the otherspecies that depend on them were maintained was often ac-complished on these category D landscapes—which are verycommon around the world [13]—by natural fire regimes forthousands of years, due to the large spatial and temporal var-iability of fire occurrence and its interaction with landforms. Ifthe time between fires becomes too long, or distance betweenburned areas too large, then metapopulation dynamics for fire-dependent species can fail, and if the fire intervals become tooshort or the fires too large, then metapopulation dynamics forthe fire-sensitive species can fail. Metapopulations dynamicsfor either the fire-dependent group, or the fire-sensitive groupsof species, can fail due to the natural fire regime or prescribedburning regimes that are not carefully planned with regard tospatial pattern, mean fire interval, and season of burning, asinformed by the study of wildland fires.

Landforms that have rocky/sandy soil refuges allow early-to mid-successional species to persist [41] on category A land-scapes, and seepages, sheltered valleys, lakeshores, islands,and peninsulas as landforms can also allow late-successionalspecies to persist on categories B and D landscapes [5]. Thus,landform works with the fire regime to maintain diversity onlandscapes in all types of dynamic categories. Harvesting re-gimes that remove trees in refuge locations from competitionwith late-successional species on type A and D landscapes orrefuge locations from fires on type B and D landscapes canalso disrupt the ability of species to persist on the landscapeand reduce tree diversity [3, 7, 14].

Note that high-intensity fire regimes with random burningwith respect to stand age (category B landscapes) can create anegative exponential stand age distribution [31], and the ran-dom timing of fires can create a variable landscape mosaic,even without landform refuges. These regimes feature tri-

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partite patches of (1) a landscape matrix of stands burned attypical intervals, (2) stands burned twice in a short time, and(3) stands skipped for a relatively long time. Again, the largeBWCAW in northern MN provides an example, where thelandscape matrix featured early-successional jack pine withserotinous cones, while areas burned twice in less than 15–20 years had pine exterminated and replacement by also early-successional aspen or birch, and areas skipped by fire bychance for a long time had late-successional patches of fir,spruce, and cedar. Thus, there was coexistence of sprouters,long-distance seeders, and serotinous and non-fire adaptedspecies.

Edge Effects, Fragmentation, and DisturbanceDilution

We usually think of fragmentation as affecting small remnantwoodlands or grasslands of 10s to 100s of hectares, embeddedin an agricultural or urban landscape matrix, with large swathsof the eastern USA and Europe as important examples. Edgeeffects from such fragmentation consist of higher light levels,lower humidity, and higher wind speeds that penetrate tens ofmeters into the forest, depending to some degree on canopyheight [30]. However, fragmentation and edge effects can alsooperate at surprisingly large spatial extents that affect manywildlands and their fire regimes. The natural vegetation mo-saic created by fire and landform interactions often abruptlyends at wildland edges, and due to fragmentation and edgeeffects, this cutting off of the mosaic can be reflected deeperand deeper into the wildland over time. Whether the edgeeffects increase or decrease fire frequency within a remnantwildland depends on the balance between passive and activefragmentation effects [42]. Passive effects tend to reduce firefrequency because of discontinuity in vegetation which re-duces access to external fires and has been termed the distur-bance dilution effect of fragmentation [13]. Active effects caneither reduce fire frequency because of increased access byfire fighters to suppress ignitions or increase fire frequencybecause of increased access for human ignitions [42]. Activeeffects that increase fire frequency predominate in regionswithout a cultural history of suppression. An example is inthe Amazon rainforest where fragmentation caused by newroads and land clearing leads to more ignitions that spreadfrom edges into dense interior forest [43], influencing compo-sition of the forest in core areas up to several hundred thou-sand hectares in area [44].

The opposite situation occurs in boreal forests, where dis-turbance dilution effects of fragmentation can lead to verylarge decreases in fire frequency deep into the cores of somewildlands. Fire frequency is reduced in a buffer zone withspatial extent equal to the size (lengths) of ecologically signif-icant fires—those few fires that account for most of the area

burned [31]. For example, if ecologically significant firesrange from 10 to 100 km in length, then many fires that burnpart of a wildland will start outside of it, perhaps quite faroutside, and later burn into or through it [12]. If these firesdisappear from the landscape mosaic outside the wildland,then fire frequency can be reduced 10–100 km inside thewildland boundary, which in many cases could include mostor all of a given wildland. Such effects are heavily influencedby the dominant direction of movement of fire and shape ofthe wildland [42], with the greatest disturbance dilution effectson the side of a wildland from which fires usually approach(commonly the southwest side in North America). The land-scape mosaic outside a given wildland may become non-conducive to spread of fire due to suppression, or change invegetation type caused by humans [45]. The latter could bereplacement of grassland or forest by nonflammable annualcrops, exposure of mineral soil by tillage, or harvesting (thusremoving contiguous flammable fuel) when crops turn brownin the late season. Alternatively, replacement of conifer forestoutside a wildland by deciduous forest (e.g., aspen or birch)after logging could reduce fire penetration into a wildland,since these forests can carry fire, but at a much lower intensitythan conifer forests, so that it can be easily suppressed. Theoverall effect is the dilution effect on fire frequency withinwildlands, although in some cases, the dilution occurs after aperiod of higher fire frequency during land clearing and set-tlement of the surrounding landscape matrix [5]. This type offire exclusion occurs in addition to any effects of direct firesuppression or climate change [45, 46]. The BWCAW ofnorthernMN has sustained large disturbance dilution impacts.The three largest fires in the 400-year record of fire occur-rence, which collectively burned three-fourths of the400,000 ha wilderness area, started outside the current wilder-ness boundary (e.g., the 1864 fires, Fig. 3). After road buildingand conversion of conifer forests to aspen and birch outsidethe wilderness in the early 1900s, the only fires that occurredwithin the wilderness were also ignited within. Disturbancedilution effects of fragmentation combined with direct sup-pression of ignitions within the wilderness increased the meanfire return intervals from 50 to 100 years during the 1700s and1800s to about 2000 years from 1910 to 1987, leading toexpansion of late-successional spruce and fir forest at the ex-pense of the original pine-dominated landscape mosaic [12].Over time, the natural fire mosaic of a large wildland canchange and gradually disappear due to the disturbance dilutioneffect of fragmentation. As Heinselman [12] pointed out, onlyat the time that he was collecting data on the landscape mosaicand fire history in northern MN’s 400,000 ha BWCAW—during the 1960s and 1970s—was it possible to characterizethe original, natural mosaic created by historic fires that oc-curred from 1595 to ca 1910. Prior to that, methods forreconstructing disturbance history were not well developed,and now, 40–50 years later, the evidence of that mosaic has

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started to fade due to death and decay of trees with fire scarsand loss of cohorts of trees of fire-dependent species that ger-minated after fires. The pattern of new fires over the last cen-tury since the forest mosaic surrounding the BWCAWchanged has been very different from the mosaic thatHeinselman characterized in his classic publications [5, 12].

Only in the middle of very large wildlands can there be anatural mosaic similar to that before the twentieth centuryexpansion of human impacts. One such place may be theinterior of the North American boreal forest, more than100 km from the edge of the forest-grassland boundary inthe prairie provinces of Canada (i.e., Saskatchewan andManitoba) or north of the temperate-boreal forest ecotone inOntario and Quebec. In the prairie provinces, exclusion ofgrassland fires that formerly burned into the boreal forest hasreduced fire frequencies in the southernmost 50–100 km ofthe boreal forest, so that the part of the forest that formerly hadthe highest fire frequency and parkland or savanna-like vege-tation structure, now has the lowest fire frequency in the ab-sence of supplementation by prescribed burning, while mod-erately frequent high-intensity crown fires continue to occurfurther north in the interior of the boreal forest [45]. It isinteresting to note that changes in fire frequency have oc-curred in these interior boreal forest regions over the last fewcenturies, but in the absence of disturbance dilution effects offragmentation and suppression, these changes are attributed toclimate change [31].

Disturbance dilution effects are minimal under two circum-stances. One is in wildlands that are embedded in a mostlynatural landscape matrix (e.g., the interior of the boreal forestin the previous paragraph). In such cases, the size of a desig-nated wildland relative to fire size matters little to the long-term average rate of burning within the wildland, althoughunstable age structure within the wildland as discussed abovein the BFires, landscape size, and stability^ sub section maystill be a concern. The second circumstance is in regionswhere landforms with natural fragmentation limit fire sizes[47] so that fires are small relative to the size of a wildland.Examples include boreal forests with large lakes andmountainranges where patch size of burns is controlled by the sizes ofvegetated valleys that are isolated from each other by rockyterrain of large enough extent to prevent fire spread from onevalley to another. Conversely, disturbance dilution effects aremaximized in wildlands which are not embedded within nat-ural landscapes, and that are not large relative to historic (inNorth America, pre-European settlement) fire sizes. This is thecircumstance under which the ratio of wildland size to fire sizematters the most for maintaining a natural rate of burningwithout supplementation by prescribed fire. Examples withhigh disturbance dilution effects include the aforementionedBWCAW, as well as numerous small (10–1000 ha) prairie andpine or oak forest remnants embedded within agriculturallandscapes in the Midwestern USA.

On landscapes where disturbance dilution occurs, one ofthe consequences (in addition to previously mentioned longermean fire intervals and a landscape dominated by older for-ests) is that a finer-grained disturbance mosaic, limited by thespatial extent of the wildland, may develop. The new fireregime consists of smaller fires that can be visualized as frag-ments of fires that ignited within the wildland and then burnedto its edge, without attaining the sizes that fires would havehad in the landscape prior to fragmentation. This pattern ofsmaller fires can be exaggerated even more if prescribed fires,which tend to be relatively small, are substituted for wildfiresto compensate for the dilution effect. Such wildlands couldplausibly have the same age-class distribution of stands as thehistoric landscape, but the landscape mosaic could still bedifferent, being composed of smaller patches. Of course, smallwildlands may end up burning almost entirely each time thereis a fire, leading to a very unstable age distribution and alter-nating lack of early or late-successional habitats, dependingon the time since the last fire. In any case, development of afiner-grained fire mosaic than existed previously may be abetter outcome for maintenance of biodiversity than havingall old forest due to lower fire return intervals or all youngforest after wildfire in a small wildland, since it is rare forspecies to be adapted to a certain large fire size, but commonfor species to specialize in habitat of a given successionalstage [12, 19]. Species that do require large habitats (e.g., highlevel predators like wolves) can survive with a mosaic com-posed of patches smaller than their territory, since it is theresponse of herbivores (their prey, moose, or deer) to the veg-etation mosaic that is the most important to them [12].

Restoration of Wildland Fire

There is a paradoxical relationship between using wildlands toinform the science of restoration via fires and restoring fire inthem. The patterns of size, intensity, and seasonality of wild-fires in wildlands are hard to duplicate through restoration.However, fire suppression and exclusion via the dilution effectof large-scale fragmentation discussed above leads to situa-tions where restoration of fire should sometimes be consid-ered, even in wildland ecosystems with large high-intensitycrown fire regimes or mixed severity regimes. In addition,former savannas or open woodlands that now have densewoody vegetation might benefit from restoration of fire. Inthese cases, knowledge of the former landscape structuregleaned from age structure of fire-dependent trees, fire scars,and paleoecological evidence might help inform restoration offire [12, 48, 49]. In large, complex wildland landscapes, onepart of a wildland might serve as a baseline at the same time asanother part needs restoration.

Prescribed fires can be very useful in some cases such asremoving woody cover from former savannas and prairies or

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reversing mesophication by removing maple from oak andpine forests in the eastern USA [50]. Long-term experimentsindicate that it takes many years of low-intensity prescribedburning to undo the effects of decades of fire exclusion andsuppression in ecosystems that historically had frequent, low-severity fire regimes. If it took a long time to get into anecologically degraded condition, it will probably take a longtime to get back out. For example, the Cedar Creek EcosystemScience Reserve shows that 40+ years of almost annual burn-ing was needed to restore oak woodland to oak savanna [51].Essentially, fires kept oak regeneration from becomingestablished, but in the absence of mechanical removal, it wasnecessary to wait for the existing mature oak trees to die, andmany did as a result of rot around basal fire scars followed bywindthrow, a process that took several decades.

One of the advantages of wildlands is that large-scale, var-iable intensity prescribed burns might be plausible (althoughfear of lighting these fires might prevent such action). Therecan be effective landscape-based fire breaks such as lakes,river valleys, and mountain ranges that are remote from thewildland urban interface. Large-scale prescribed fires can ap-proach the spatial variability of wild fires. However, relativelyfew such prescribed fires have been done. One exception wasa series of prescribed fires in the BWCAW in northern MN,which were done after a major windthrow event in 1999, toreduce extreme fuel accumulations and reduce the chances oflarge uncontrollable wildfires escaping the wilderness andburning private property. In this case, large lakes were usedas fire breaks, which were supplemented with sections of fire-break created by clearing woody fuels, and burns were done infall due to concerns that spring fires would burn all summer ifthey escaped. Fires with flame lengths ranging from <1 to20 m occurred in parts of these burns, which were allowedto burn on their own until they reached fire breaks, reachingsizes of up to 4800 ha [52]. The efficacy of these prescribedburns was tested when a large, high-intensity wildfire ignitedby lightning occurred during July 2006 (The Cavity Lake Fire,Fig. 2 lower), which although it burned during an extremedrought in windfall slash fuels with very high loadings, wasprevented from moving towards the nearby Gunflint Trail byprescribed burns with low fuel loadings.

The relative contribution of natural fire that is not sup-pressed, variously termed Blet burn,^ Bwildland fire use,^ orBprescribed natural fire^ versus purposeful ignition, is a hardbalance to establish [53]. Fire managers do not know when alarge natural ignition will occur nor whether the political sit-uation will allow such an ignition to burn in conditions wheresuppression is possible or whether suppression will be impos-sible, but managers will still be blamed for any damage the firecauses. However, well parameterized and validated simula-tions by fire behavior models such as FARSITE andLANDFIRE and landscape models such as LANDIS thatshow the likely ecological results of fires can help inform

difficult decisions, such as when or how to respond to naturalignitions and when to supplement natural ignitions with pre-scribed fires (see the review by [1••]). Prescribed fires andnatural fires are probably not completely additive in their ef-fects, since prescribed fires are unlikely to be undertaken inareas that have had recent natural burns, and natural burns willburn recent prescribed fire areas with lower intensity.However, there is some additivity between natural and pre-scribed fires, and therefore, some chance of ending up withmore fire than desired if large prescribed fires are completedand then a severe fire year yields another round of high-intensity wild fires.

Climate Change and Resilience

Wildlands are subject to climate change, but not direct humanmanipulation such as harvesting for commercial products.Thus, they can still function as a type of moving baseline asthe climate changes. They can still answer the question ofwhether commercial forests maintain the same level of pro-ductivity and biodiversity, given that both the wildland and theadjacent commercial forest are physiographically similar andsubject to the same climate change.

Are landscapes with natural fire frequenciesmore resilient toclimate change than those where fire is partially or whollyexcluded? Although increased fire frequencies expected to ac-company a warming climate are often viewed as a negativefactor that will add stress to ecosystems already under stressfrom changing climate and harvesting, fires may also help eco-systems adjust to a changing climate by providing opportuni-ties for existing species to move to new sites, undergo naturalselection, or for new (neo-native) species adapted to thewarmerclimate to reproduce [54, 55]. Species with relatively largepopulations, long dispersal distances, and short lifecycles havethe best chances of responding to climate change through eithermigrating or adapting [56, 57]. However, fires that removecompeting species and prepare good seedbed conditions mayalso be required to take advantage of these traits. Perhaps jackpine in the North American boreal forest could adapt to a newclimate if stands burned every few decades and could undergoseveral rounds of selection among large cohorts of seedlings asthe climate warms. Indeed, jack pine has evolved many eco-types across its range with different growth forms and serotinylevels [58]. However, many tree species have slower reproduc-tive cycles and are incapable of responding this way [56, 57].Also note that at this point, we do not know if any tree speciescan fully adapt to the likely magnitude of climate changeprojected to occur by the end of the twenty-first century, andresearch on this topic is needed.

The ability of fires to confer resilience to climate changedoes have limits, and in certain circumstances, it could befutile to resist change [59]. Changing fire frequency along

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with climate can trip a disturbance frequency threshold, takingforests out of safe operating space [60]. This could lead todevelopment of a completely different type of vegetation;however, this brings up the question of whether such transi-tions are in themselves a form of resilience, since moving to anew basin of attraction in ecosystem properties in synch withthe new climate may be unavoidable and more desirable thanvegetation that is continuously in transition [61•]. Assistingthe development of the new vegetation type may a good op-tion, helping to make a graceful transition, and fires may helpshorten the lag time for adjustment by vegetation to the newclimate and reinforce the placements of vegetation types ontothe available physiographical settings.

The interaction of fires and topographic variability in largewildlands is probably a key factor determining whether firescan help the vegetation to adjust to a warming climate. Avegetation type that occupies a small proportion of the land-scape now (e.g., a type confined to small relatively dry inclu-sions on sun-exposed slopes) could creep onto the rest of thelandscape and become the dominant type as the climatewarms. Therefore, large wildlands with inclusions of currentlyrather odd vegetation types could be more resilient to climatechange, and fires could facilitate the expansion of these veg-etation types. There are many such examples of fire and/ordrought adapted species expanding in abundance in responseto warming climate in the paleoecological literature [62, 63,64•]. A current example is tiny inclusions (much less than 1%of the landscape) of bur oak and northern red oak embeddedwithin boreal forests, which, facilitated by the future occur-rence of fires, are likely to become the dominant vegetation ofnorthern MN’s BWCAW for a business as usual climatechange scenario [12, 59].

Many wildlands with dense conifer-dominated forests andinfrequent crown fire or moderately frequent mixed fire re-gimes would logically (in a warming climate) transition tolower density woodlands or savannas with more frequent,lower intensity fire regimes [55]. Dense mesic forests wherefires are currently rare could also transition to savannas. Basedon paleoecological reconstruction of past changes in climate,this is one expected outcome when northern or high-elevationforests transition to a warmer climate [63]. However, idiosyn-cratic variations are likely to occur. Although we often pre-sume that a warmer climate will lead to more frequent fire dueto warmer, drier conditions, we need to be cautious, because itis also possible that warmer, wetter conditions will prevail insome regions, and fire frequency could go down, as happenedwhen the Little Ice Age ended in the boreal forests of easternCanada [65]. Black spruce forests in interior Alaska couldburn more frequently as climate warms, due to drying of wet-lands and increases in vegetation/fuel density, but this may beonly a transitional disturbance regime, since short fire inter-vals could cause conversion to deciduous birch forests thatthen disfavor high-intensity fires [64•].

In any case, we know little about how such transitions invegetation would happen, but it is reasonable to assume suchtransitions would be messy from the points of view of firesuppression, ecological continuity, and habitat maintenancefor the suite of wildlife species that are present on a givenlandscape. The fact that disturbance dilution effects and directsuppression have also happened over the last century in wild-lands that are close to human-settled areas, and have causeddominance by late-successional species with high tonnages offuel per hectare in some wildlands, could make such transi-tions doubly messy, since unusually high-intensity fires couldoccur during the transition, making prediction of the outcomedifficult.

Finally, note that fires in a changing climate can be a doubleedged sword—invasive species are as good, or better than,native species at responding to changing environments andcould get into the ecosystem and establish irreversible domi-nance after burns [55, 60]. This latter problem ismore likely inwildlands next to human-dominated agricultural or urbanareas and less likely in remote areas where propagules ofinvasive species are absent, thus allowing some leeway inthe time it takes for the neo-native vegetation to becomeestablished in those more remote landscapes.

Conclusions

Physiographic variability in a wilderness area and its interac-tions with fire are key to maintaining a diversity of plant spe-cies and habitats for wildlife and also inform the extent towhich fires may help in the adaptation to a warming climate,since species currently confined to rare landforms may be thewinners in the future and have a much broader spatial niche.Fires provide opportunities for these species to increase thespatial extent of their habitat and may increase resilience ofwildlands in a changing world. Ecologists have a tendency toignore natural processes perceived as rare on a given land-scape (as opposed to rare species), and yet one major lessonfrom wildland fire is that rare processes can have very largeimpacts on landscape-level diversity. For example, it is correctto say that fire is just as important for maintaining biodiversityon landscapes where fire is rare as on landscapes where it isthe dominant form of disturbance. These observations fromwildlands lead to the questions of whether timber harvestingpractices do as well to promote resilience to changing climateand a variety of ecological processes or whether harvestingpractices should be modified or supplemented with fire.

Ecosystem processes that interact with fire at large spatialextents and long time periods can be pervasive in the ecosys-tem and yet invisible without the help of paleoecological stud-ies. Knowing where forest stands and landscapes sit withinecosystem developmental processes that occur at multipletime scales can create important contexts for fire and forest

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management options. Knowing how post-fire successionaltrajectories that occur on time scales of decades to centurieswork, and how fire affects the trajectory of ecosystem retro-gression on time scales of centuries to millennia, and howthese vary over the landscape allow for a baseline comparisonof whether commercial timber harvesting is accelerating ordecelerating ecosystem retrogression and whether it is main-taining the wide natural variety of successional stages presenton natural fire-dominated landscapes.

Management zones around wilderness areas give the illu-sion of seamless linkages to the surrounding landscape.However, increases or dilution effects on fire frequencycaused by unrecognized fragmentation effects that occur atsurprisingly large spatial extents can be profound in certainwildlands. These effects can be propagated deep into wild-lands, causing the fire-dependent landscape mosaic to change.After several decades, the original mosaic may only remain asa vague shadow. Researchers also tend to detect spatial pat-terns that occur at the spatial extents of their study areas at thetime of the study, rather than the original mosaic. Therefore,caution should be used in the interpretation of landscape datato be used for restoration purposes. Ironically, mixed fire re-gimes, although perceived as difficult to emulate, may be rel-atively easy to restore, since fine-grained variation in intensityacross large spatial extents was the rule in natural landscapeswith mixed fire regimes. The ultimate area within the perim-eters of mixed intensity fires is likely less important than thefine-grained variability in intensity within.

Wildlands are useful as baselines for maintaining foresthealth over long time periods. However, some wildlandsthemselves require restoration of fire to continue to exhibitvariation of the interactions among fire, landform, ecosystemprocesses, and biodiversity. Is it essential to restore the origi-nal mosaic within a wildland if it can still be ascertained, or ifit was ascertained from paleoecological techniques, or is itsufficient just to have all of the successional and developmen-tal stages present? It seems likely that letting nature take itscourse to a large extent, with judicious supplementation of firewhen needed to maintain variability in successional stages, isa sound fire management strategy. In smaller wildlands, it maynot be feasible to represent all successional stages via thelarge-scale patches that formerly existed, and it may be nec-essary to accept development of a finer-grained mosaic ofsuccessional stages.

There is much that we do not know about wildland fire andit roles in maintaining ecosystem function and biodiversity.Therefore, wildlands with intact spatial and temporal patternsof fire and landform interactions can be seen as a library offuture knowledge waiting to be extracted, as techniques to doso advance along with the frontier of scientific knowledge thatconstantly leads to new questions that cannot be foreseen at agiven time. Patterns created by fire differ within each of theland type associations embedded within each ecoregion,

embedded within each biome, and the most difficult of alltasks given to fire scientists is to assess the relative contribu-tion of generalities and individualistic effects that explain howfires maintain ecosystem function and diversity. Such pursuitof knowledge will help to maintain intact ecosystems, to re-store degraded ecosystems, and to develop adaptation strate-gies to a changing climate, while maintaining biodiversity andproductivity of the forest.

Compliance with Ethical Standards

Conflict of Interest Dr. Frelich has no conflicts to declare.

Human and Animal Rights and Informed Consent This article con-tains no studies with human or animal subjects performed by the author.

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