Disturbance ecology and forest management : a review of the literature

22 Pages·1996·1.5 MB·English

by  Rogers, Paul

Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.

United States Disturbance Ecology
Sfnd
Department
ofAgriculture
Managementl^^
ForestService Forest
Intermountain
Research Station Review of the Literatqr|
General Technical
Report INT-GTR-336
May 1996
Paul Rogers
Received by: J ^-"""^^/^
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IndexingBrancH t
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11
The Author Forest Service has adopted a policy of ecosystem management
thatemphasizesmaintainingthevaluesofsustainability,biodiver-
Paul Rogersisan ecologistinthe InteriorWest Resource Inven- sity,productivity,andforesthealthratherthanfocusingonparticu-
tory, Monitoring, and Evaluation Program at the Intermountain lardeliverable products.
ResearchStation.Hisprimaryresponsibilityistoconductfieldwork In the attempt to implement ecosystem management, land
andreportonForestHealthMonitoringeffortsintheInteriorWest. managersareaskingbasicscientificquestionsthatgofarbeyond
He holds a B.S. degree in geographyfrom Utah State University the scope of historical Forest Service research. Concurrent with
andanM.S.degreeingeographyfromtheUniversityofWisconsin- this demand are budgets being slashed and research positions
Madison. Since beginning his career with the Forest Service in being eliminated. Land managers are being asked to implement
1987, Rogers hasspecialized inconducting large-scalefield and newknowledge-intensiveprogramsatatimewhen resourcesare
remotevegetativesurveys. being dramatically reduced. To address this significantdilemma,
we need to re-evaluate how research is conducted within the
Research Summary ForestServiceand how more effectivelyto involve partnersfrom
the researchcommunity.
Land managersare incorporating ecosystem perspectivesinto As a part of this effort, we have established an Intermountain
theirlocalandregionalmanagementdecisions.Thisreviewofthe Centerfor research on Disturbance Ecology at the Logan For-
daigsetmuernbta,nciesaecroelsooguyrcleitfeorratfuorree,stanmdanhaogweritspearntdairnessetoarfcohreersstminatner-- teshtoruyghSctiheenocbejsecLtaibvoersaotfortyh,ecIenntteerrmoaurnetmaionreRefuslelyadrecshcrSitbateidoni.ntAhle-
estedindisturbancetheory,specificdisturbanceagents,theirinter- memorandumofunderstandingthatcreatedit,thedefiningobjec-
actions,andappropriatemethodsofinquiryforspecificgeographic tivesofthecenterare:
regions. The approach is broadly interdisciplinary and includes • Createanenvironmentthatismoreresponsivetotheresearch
effortsfromecologists,biologists,geographers,historians,wildlife needsofthe National Forest System
scientists,foresters,entomologists,pathologists,hydrologists,and • More effectively utilize information from ongoing Forest Ser-
modelers. Theauthorbroadlydefinesdisturbanceecologyasthe vice research projectstoaddressdisturbance issues
studyofanydistincteventsthatdisruptthefunctionofecosystems. • Facilitate effective collaborations with universities and other
Thesedisruptionsmayoccuroverwidelyvaryingscalesoftimeand extramural research partners.
space.Greaterunderstandingofmultipledisturbancemechanisms,
and howtheyinteractwithinforests,willcontributesignificantlyto An important way in which the Intermountain Center for Re-
land managers' ability to work with natural systems, rather than search on Disturbance Ecologyhopestoencouragecollaborative
battlingagainstindividualdisturbanceagents. Implicationsforthe research, both within the Forest Service community and the re-
futureofdisturbanceecologybased managementarediscussed. search community at large, is through visiting collaborator posi-
Additionally,thispaperintroduceslandmanagerstoawidebody tions. Working a few months to perhaps a few years in the
ofliteraturepertainingtodisturbanceecologyandforestmanage- center,visitingcollaboratorswillreaptheintellectualandpractical
ment. The Referencessection is recommended asa resource. wbietnhefdiitvserfsreomwoirnkteerxacpteiroinenbceetsweaenndpsrcoiefnetsissitosnaalnbdaclkangdromuanndsa.geWres
anticipatethatthe resulting insightswill be difficultto gain in any
Contents page other way. We anticipate additional material benefits (products)
that will advance the objectives of the center and facilitate the
Introduction 1 practicalapplicationofdisturbanceecologyprinciplestomanage-
Defining disturbance ecology
1 mentpracticeson publiclands.
Equilibrium and nonequilibrium 2 Paul Rogerswasthefirstcollaboratoratthe center. Paul is an
Forestdynamicsanddisturbanceagents 3 ecologist with the Interior West Inventory Project, Intermountain
Abioticdisturbance agents 3 Station, Ogden, UT. He received his B.S. degree in geography
Bioticdisturbanceagents 7 fromUtahStateUniversityandanM.S.degreeingeographyfrom
Disturbance interactionsonthe landscape: the University of Wisconsin-Madison. He has wide ranging inter-
twocasestudies 10 ests in landscapeanddisturbanceecology. Paul spent 2 months
Established methods indisturbanceecology 10 on detail as a visiting collaborator, and given his relatively short
Historical sources 1 tenure, we decided that a review of the disturbance ecology
Field ecological methods 1 literature pertainingto forest management issues would provide
Modeling 12 forboth accomplishing his goals and furthering the interests of
Toward managementwith disturbanceecology 13 thecenter. This review isthefinal productof Paul'sefforts.
References 14 Ratherthanprovidinganexhaustivelistingofeveryreferenceon
disturbance ecology, the objectives were to focus specifically on
Foreword the aspects ofdisturbance ecologythat mostdirectly impinge on
management of public lands and to identify key references that
Land managers in the National Forest System are currently provide an entry intothe literature on these important issues. As
facedwithsomeofthemostseriouschallengesinthehistoryofthe such,thispublicationissimilarinscopeandobjectivesofanAnnual
ForestService, U.S. DepartmentofAgriculture. Nolongerdoesa Reviewjournal article. Paul also initiated creation of a computer
careerinthe ForestService result in an idylliclifeofafree roving databaseondisturbanceecology. Referencesinthisreviewarea
ranger. Increasingly it has become a life spent in front of aword subsetofthatdatabase.
processor working on public law documents or in courtrooms The intention ofthecenteristocontinuedeveloping thedistur-
facing litigation. The land managementdecisionsonourNational banceecologydatabaseforprovidinganinformationresourcefor
Forests are subjected to excruciating scrutiny, not only from the research community at large.
traditional resource-basedindustries,butalsofromcitizengroups JesseA. Logan
representingbroadlybased interestsinthemultifacetedvaluesof Acting Director, Intermountain Centerfor
our forested lands. In response to these societal pressures, the Research on Disturbance Ecology
March 1996
Disturbance Ecology and Forest
Management: a Review of the
Literature
Paul Rogers
—
Then thereare insects moths, weevils, cater- understand their role at larger temporal and spatial
pillars, worms, beetles, sawflys, termites, and scales.
borers. They march, dig, fly, bore, crawl, repro- Ifnatural disturbance is fundamental tothe devel-
duce within trees, and nourish themselves and opmentofforestecosystems,thenourmanagementof
theiryoung. Some, like the bark beetles, are the naturalareasshouldbebasedonanunderstandingof
mortalfoe ofconifers. disturbance processes (Attiwill 1994; Gordon 1993;
Grumbine 1994; LorimerandFrelich 1994;Malanson
Insects are the worst ofall enemies oftrees.
Theycausetwiceasmuchdamageasdisease,and and Butler 1984; Pfister 1993). Now, as Federal land
— managementagencies adoptecosystem management
seventimesmoredamagethanfire. Frome(1962,
philosophies, it becomes critical that disturbance is
p. 240).
viewedascomplementary,ratherthansolelydeleteri-
Naturaldisturbancesarequitecommoninall ous, to human and forest functions (Grumbine 1994;
regions ofNorth America and the world when Monnig and Byler 1992; Pfister 1993).
viewedfrom a long-termperspective. Each area Thisreviewhighlightsrecentworkinabroadarray
hascharacteristicfrequenciesandtypesofdistur- ofstudiesrelatedtodisturbanceecology.Thispublica-
bances based on its climate, soils, vegetation, tionis aresourceformanagersandresearchersinter-
animals, and other factors. Fires, windstorms, estedindisturbancetheory,specificdisturbanceagents,
and other disturbances have specific behaviors their interactions, and appropriate methods
—
andleavecertainconditionsforgrowth. Oliver ofinquiry for specific geographic regions.
and Larson (1990, p. 92-93).
Previous to settlement of the country, fires Defining Disturbance
started by lightningandIndians kept the brush
Ecology
thin,keptthejuniperandotherwoodlandspecies
decimated, and gave the grass the upper hand Disturbanceecologyencompassesthestudyofinter-
with respect topossession ofthe soil. In spite of relationships between biotic and abiotic components
periodicfires, thegrasspreventederosion.... The ofanenvironment. White and Pickett(1985) give the
removal of the grass [by settlers] relieved the most widely quoted definition of disturbance: "Any
brushspeciesofrootcompetitionandoffiredam- relativelydiscrete eventintimethatdisrupts ecosys-
ageandtherebycausedthemtospreadand"take tems,community,orpopulationstructureandchanges
the couAiiry "—Leopold (1924, p. 2-3). resources,substrateavailability, orthephysicalenvi-
ronment."No definitionofdisturbance will satisfyall
Introduction ecologists, butWhite andPickettspeaktothe critical
elements ofdisruption and changed resource alloca-
How land stewards view ecological disturbance in tion.Amorethoroughdiscussionofalternativedefini-
anecosystemisanindicatorofhowagivenlandscape tions maybe found in a recent publication by Glenn-
will be managed. The first two quotes above typify Lewin and van der Maarel (1992). Though both
differentphilosophies in different eras offorestman- "disturbance" and "ecology" are defined somewhat
agement. The third quote is an exceptional view of loosely,togethertheyfocusprimarilyondistinctevents
managementbyanearlyforestranger. Furtherscru- that disrupt the function ofecosystems. That broad
tinyrevealsaparadigmaticshiftfromthatofhumans statement will be used as a working definition of
havingaantagonisticrelationshipwithnature,toone disturbance ecology forthe purposes ofthis review.
of acceptance of nature as a regulator with which A further consideration is scale. Definitions ofdis-
humans must work in tandem. Leopold illustrated turbance cannotbe limitedbysize ortiming, asthese
that our capacity to work with natural systems, in- factors are relative to the systems being evaluated.
cluding disturbance, is only limited by our ability to Forexample,disturbanceinafungalcommunitymay
1
take place several times a year and at a scale meas- outbreaks, which tend to kill more selectively, are
ured in square feet, while other disruptions, such as spoken ofin terms ofpatch disturbance, rather than
tropicalcyclones,maycoverhundredsofsquaremiles gaps (Amman 1978; Castello and others 1995).
and occur, in the same place, on a temporal scale of Apossibleexceptionisastudy(Canhamandothers
centuries. 1990) that compared light falling through "tree-faU
White and Pickett (1985) also clarify the use of gaps"inforesttypesoftheEasttotherelativelymoist
"perturbation"and"catastrophe"ascoveringtherare PacificNorthwest.Theconclusionwasthatthegap-to-
small and large events, respectively. Perturbation height ratio ofthe Western forest precluded signifi-
refers primarilyto specific alteration ofsystems that cantresourceavailabilityfromgapoccurrence. Inthis
areclearlyandnarrowlydefined.Mostoftenperturba- particularDouglas-fir/hemlockforesttype, onemight
tionsarepurposefulhumanmanipulationsthatcanbe concludethatotherenvironmentalfactors,mostlikely
measuredintotality.Catastrophes,ontheotherhand, large-scale disturbance or nutrient availability, take
are rare events, especially destructive ones, and are precedence over light allocation from single tree-gap
unlikelytoberepeatedwithregularity.Thoughmany formation. The Eastern forests, on the other hand,
have used these terms interchangeably (such as Fos- immediately took advantage of the light resulting
ter 1988; Odum 1985), forthisreviewIwilluseWhite from gap formation by filling in the space with new
and Pickett's narrower definitions, which rely pre- treegrowth. Still, itappearslikelythatPacificNorth-
dominantly on the term "disturbance" to describe all westernforestsdotakeadvantageofgapsbyreleasing
but the rarest events. otherresourcestoshade-toleranttreespecies,thereby
Discussionofscaleoftencentersaroundthemedian promoting an overall patchiness in the absence of
area and timing of disturbances for specific land- large-scale disturbance.
scapes, otherwise known as disturbance regimes. These examples illustrate a basic dichotomy in the
Understanding disturbance regimes for a particular scale ofprevailing disturbance mechanisms. Further
landscape is fundamentaltothe studyofdisturbance discussion of disturbance processes and scale often
ecology. In NorthAmerica, the scale ofthe dominant centersonthequestionofsystemstability, orequilib-
force ofvegetation change for a given landscape de- rium(Botkin 1993;Glenn-LewinandvanderMaartel
fines study parameters (Glenn-Lewin and van der 1992; Veblen 1992).
Maarel 1992; O'Neill and others 1986; Shugart and
West 1981; Sprugel 1991). Single plant mortality Equilibrium and
forms"gaps,"groupsofplantsaffectedbydisturbance
Nonequilibrium
form"patches,"andwholelandscapesarediscussedin
termsof"community"or"standreplacement."Gener-
Spatial and temporal scale are pivotal to any dia-
ally, in studies of forest disturbance in the Eastern
logue regarding equilibrium versus nonequilibrium
United States and Eastern Canada where mortality
systems.Atissue is whethersystems are maintained
occurs most frequently at the single tree or small-
in some equilibrium by disturbance, or whether the
groupscale,thefocusisongap-phasedynamics.Though ubiquity ofdisturbance prevents systems from ever
large-scaleeventssuchasfireandhurricanesdooccur reaching "steady state." If one makes the case for
inthisregion(HenryandSwan1974),itisthetree-by- nonequilibrium at one scale, then critics may charge
tree replacement over time that controls the overall
that at a larger scale ecological "balance" can be
structureandfunctionofthesesystems(Bormannand
Likens 1979; Busing and White 1993; Canham and reached. Inotherwords,ifthingsappearunbalanced,
just expand the scale ofinterest until equilibrium is
others 1990; Lorimer and Frelich 1994; Payette and
reached. Forexample, Botkin (1993) describes recent
others 1990). These gap disturbances often result in
usageoftheterm'landscape-level"stabihtyasacondi-
an uneven-aged vegetation structure that is largely
tionwheresmalldisturbancesoccurringcontinuously
dependentuponmaximumattainableageofthe dom-
across a landscape "average" each other out at a
inant species being shorter than the stand-replacing chosenscale.Thisargumentcaneffectivelybemadeto
disturbance interval (Oliver 1980-1981).
encompass several eco-regions, orevenup to a global
IntheWesternUnited States andWestern Canada
scale (Prentice 1992).
where long-lived species and mostly dry conditions
On a temporal scale. Vale (1988) presents argu-
prevail, disturbance regimes are almost universally
ments for employment ofvarious equilibrium view-
studiedatthestandorlandscapelevel(Covingtonand
points to justify or vilify clearcut logging simply by
Moore 1994; Johnson and Larsen 1991; Keane and
adopting an appropriate time frame. For example, if
others 1990). When disturbance intensity is high, for we look at logging (as a disturbance) as it affects a
example severe wildfires in portions ofYellowstone foreston a 100-yearscale, we can imagine a clearcut,
NationalParkin 1988, even-agestand establishment
andthenadevelopmentalpathtowardanequilibrium
follows (Romme and Despain 1989). Even lower in-
composed of vegetation similar to the original site.
tensity events in the West, such as insect or disease
2
From a 1,000-year view, several logging events may systemsinstatesofcontinualchange(Sprugel 1991)?
take place at regular intervals never allowing the In any event, it seems prudent to use caution when
forestto reach a balance. Or at an even longer scale, applyingthe equilibrium and nonequilibriumlabels.
vegetationcovermaymovecompletelyawayfromthe
concept ofbalance due to climatic changes toward a Forest Dynamics and Disturbance
totally new landscape dynamic.
Agents
Ingeneral, as we expemd spatial ortemporal scale,
explanations ofequilibriumbecome eithermorecom-
Most land managers are aware of the variety of
plex—forexample,"quasi-equilibrium,""shifting-mo-
disturbance agents but have less knowledge ofhow
saic steady state," "near-equilibrium,"—or in other
theyinteractonalandscapetomoldpast, present, or
cases,nonfunctional,nonequilibrium,"non-linear,"or
future ecological systems. This section focuses on
even "chaotic."
forms of disturbance and the dynamics that bind
Current thought appears to favor an overall
them.
nonequilibrium perspective, although significant ex-
Disturbances can be both endogenous (internal) or
ceptions do exist (Botkin 1993; Peet 1992; Veblen
exogenous (external) to the ecosystem; they can be
1992). Similar to the earlier discussion ofthe East-
WestdichotomyinNorthAmericaexhibitedbyscaleof biotic (such as insects, disease, animal damage) or
abiotic (such as wind, flood, fire). They can be large
disturbance, a roughly parallel argimient could be
madeforequilibriumandnonequilibriumecosystems. (measuredinhectares)orsmall(measuredinmeters).
As Sprugel (1991, p. 6) points out: "Whenever indi- T(shuecyhacsancrebeepiinngtegnrsoeun(dsufcirhesa)s.Ocnreowthninfigrmeso)stordiwsetuark-
vidual disturbances are so large that a single distur-
bance agents have in common, however, is that they
bance event can affect a relatively large proportion
rarely act alone. Agents such as drought and fire or
ofthe landscape, achievement ofan equilibrium be-
disease and insects most often act in concert across
comes unlikely."
both time and space in shapingthe landscape. While
Where large-scale fire, windstorms, disease, or in-
sect infestations occurregularly, forests are unlikely itistruethatadiscreteevent,forinstancealandslide,
drastically alters a successional course, recentheavy
toreachsteady-state(BergeronandDansereau 1993;
Botkin 1993; Foster 1988; Prentice 1992; Romme precipitationand possiblyprevious loss ofvegetation
1982;Rommeandothers 1986).Manywesternforests cover from road building, fire, orgrazing, aU contrib-
ute significantly to that event. So most disturbance
are dominated by such large-scale disturbance dy-
bum eventsareaninteractionofmanydisturbanceagents.
namics. IntheNorth,boreallandscapes atlarge
scales (Hunter 1993), and in the East, near-coastal
Abiotic Disturbance Agents
forests often experience large cyclonic winds (Foster
1988;HenryandSwan1974;Wyantandothers1991). Drought—Concerns about drought are closely
InlanddeciduousforestsoftheEasternUnitedStates linked to discussions ofglobal warming and climate
seemtobethemajorexceptiontothetrend,exhibiting change in general (Ojima and others 1991; Overpeck
infrequentoralmostnolarge disturbances, andthus, and others 1990; Prentice 1992). However, not all
near equilibrium conditions (Bormann and Likens climate change is toward drought. Witness, for in-
1979; Busing and White 1993; Lorimer and Frelich stance, the "little ice age" ofthe mid-1400's to early
1994;Payetteandothers 1990;Prentice 1992;Runkle 1800's.Whendroughtdoesoccur,ahostofdisturbance
1985). Busing and White (1993) conclude, however, processes are likelyto acttogether. It maybe helpful
thatequilibriummaybebasedonavarietyoffactors, to view drought as an indirect, or secondary, distur-
suchasbiomassorcomposition, andthatbychanging
bance agentasopposedto discretelyaffectingsucces-
such factors, not unlike changing scales, different sionaldevelopmentalone.Anarrowdistinctionexists
conclusions about equilibrium may result. betweenagentsthathavethepotentialtodirectlykill
Inabroadercontext,weshouldaUcriticallyevaluate perennialplantsandthose,suchasdrought,thatoften
the use ofequilibrium and nonequilibrium labels. If do not lead to mortality. Drought overall is clearly a
these terms are so dependent on scale (Botkin 1993; significant factor invegetation change overtime.
Vale 1988;Veblen 1992)andotherselectfactors(Bus- Severalauthorshavelinkeddroughttoincidentsof
ing and White 1993), can they be manipulated to fit insectinfestations(HadleyandVeblen1993;Mitchell
whateverneeds an authorintends? Ordoes the exer- and others 1983), disease (Baker 1988; Castello and
cise ofdemonstratedequilibrium(ornonequilibrium) others 1995), and fire (Amo 1980; Callaway and
further our understanding ofdisturbance and land- Davis 1993; Johnson and Larsen 1991; Romme and
scape processes? Are these concepts merely human Despain 1989; Weaver 1951). Moreover, George and
constructs that, forwhateverreason, become psycho-
others(1992)describethe effectsthatsevere drought
logically attractive in their effort to impose order on
has on wildlife, in their case grassland birds ofthe
3
Fires that burn at high intensity leave
forests that recover more slowly than
after less severe burns. This charred
landscapeisaresultofasevereportion
oftheYellowstonefiresof 1988. Less
intense burns were visible from the
samevantagepoint.Somewereevenly
burned ground fires, while others
seemedtoburnmoreintenselyinsmall
patches of tree crowns.
northern plains, in terms of short- and long-term Views ofwildland fire onthis continenthave evolved
population dynamics. Currently, many disturbance from Native Americans' purposeful use of fire, to
agents are experiencing near epidemic levels in the Europeanimmigrants'fearof,disdainfor,andcontrol
Western United States because ofprolonged drought offire,totheappreciationforfireinitsvariedecologi-
incombinationwithotherhuman-inducedfactorssuch cal guises. Rather than tracking the entire history,
as fire suppression, historic forest cutting practices, and the myriad ofpapers on this subject, I will focus
andurbanencroachment.Furthermore,Overpeckand on the current understanding offire in disturbance
others (1990) predict that these conditions will be ecology.
amplified in the coming decades as result ofhuman- Whilefireiscommonlycorrelatedwithdrought,many
induced global warming. authors have highlighted other environmental stres-
Fire—Fire is probablythe most widelyknown and sorswhichpromoteignitionconditions, suchasinsect
infestations (Amman 1978; Baker and Veblen 1990;
extensively studied disturbance, possibly due to its
spectacular appearance or innate human attraction. Gara and others 1984; Knight 1987; Schowalter and
others 1981),diseaseoutbreaks(Baker 1988; Castello
and others 1995), windthrow (Lorimer and Frelich
1994), and global climate change (Overpeck and oth-
Table 1—Examplesoffireregimestudiesbyregion. ers 1990). Many studies have focused on establishing
Region Authors Year historical fire regimes to gain some understanding of
how fire shapes vegetation development. Examples
PacificNorthwest, U.S.A. Garaandothers 1984
Swansonandothers 1992 canbe found fornearlyeveryecologicalregion across
PacificSouthwest, U.S.A. Callawayand Davis 1993 North America and internationally(see table 1).
Southwest, U.S.A. Weaver 1951 Studies document changes in fire regimes that are
Covington and Moore 1992, 1994 attributed to specific human actions, such as fire sup-
Intermountain, U.S.A. Romme 1982 pression, land clearing, logging, orgrazing. Ifhuman
Arno 1980
Rockies,Canada Johnson andLarsen 1991 disruptions of"natural" fire regimes can be gleaned
Reed 1994 from the historical data, then, presumably, a better
Boreal,Canada Bergeron and Dansereau 1993 understandingoftheroleoffireindeterminingforest
Hunter 1993 structure and function can be achieved. Problems
UpperMidwest, U.S.A. Lorimerand Frelich 1994 arise, however, when we attempt to distinguish be-
Baker 1992
Northeast, U.S.A. Bormannand Likens 1979 tweenhowmuchhumaninterventionis"natural"and
HenryandSwan 1974 how much is not "natural." Determining that might
Sweden Bradshawand Hannon 1992 lead to management practices that emulate the dis-
Australia Attiwell 1994 turbanceregimesbeforehumanschangedtheircourse.
Argentina Veblen andothers 1992 Unfortunately, separating out "the human factor" is
4
not so easy. As a related question, should fire sup- "industrial scale" then possibly some comparisons
pression,orthepreventionofanynaturaldisturbance, couldbemade,andconsequently,"values"of"natural-
be considered a disturbance process initself?The act ness" might be assigned. This loose approach shows
ofprevention certainly "disrupts ecosystem, commu- that human intervention in fire regimes is probably
nity, or population structure," as White and Pickett as much a philosophical dilemma as it is a scientific
(1985, p. 7) have defined it. This places the focus of one. Arehuman actions partofnature? Ifso, atwhat
the question onthe "discreteness" ofthe disturbance. level do they become "unnatural?" However, manag-
Other disturbance types, such as livestock grazing, ers and scientists must address these difficiilt ques-
affectlandscapesoverlongperiodsinasimilarfashion tions or risk aiming at constantly moving ecosystem
tofiresuppression. Ifviewedfromalongertimescale, targets in their management objectives.
disruptions in natural processes that occur over de- Wind—Intense wind storms, often ofcyclonic ori-
cades may appearvery discrete. gin, maydestroyhealthyvegetation withoutemploy-
Aboriginaluseoffireandtheirsettlementpractices ingotherdisturbanceagentsfromwithinasystem. In
hadsomeeffectonnature'sownfireregimes(Attiwell the Teton Wilderness in Wyoming, an unusual high
1994; Bradshaw and Hannon 1992; Denevan 1992;
elevationcyclonicallydrivenstormflattenedastripof
Hammett 1992). Evidently Native American popula-
trees 15 miles long and up to 1 mile wide (see photo).
tions were at 500-year lows during the 19th century
Thebulkofthesetrees werevigorous, large diameter
(Denevan 1992; Sprugel 1991), a time commonly re- coniferswithoutanyobvioussignofpreviousdamage.
ferred to by ecologists as "presettlement." Does that
Otherresearchershavedocumentedlong-term"wind
meanthis period was characterized by"unnaturally"
regimes" resulting fi'om occasional hurricanes along
low levels ofhuman-caused fires, and therefore, his- the East Coast (Bormann and Likens 1979; Foster
toricallyunusualvegetation patterns? 1988;HenryandSwan1974;Wyantandothers 1991).
One approach to finding answers mightbeto sepa- Both intense storms and smaller wind events are
rate, by an order ofmagnitude, the scale ofhuman importantdisturbanceagents.Thesmallerblowdowns
intervention between, say, agricultural and indus- usuallythinforests oftrees already damaged byfire,
trial-level societies. Ifwe could define some average
insects,ordisease,creatinggapsinthecanopy(Castello
levelofinterventionatthe"agriculturalscale"andthe
An unusual, high elevation windstorm in northwestern Wyoming caused
large-scaledamagein1987.Damagetoapparentlyhealthytreescameintwo
forms:eitherwholetreesuprooted,ortreessnappedoffatthemainboleabout
15feetfrom the ground. In thefollowing yearportions ofthis 15-mile swath
were burned in theYellowstonefires.
5
and others 1995; Lorimer and Frelich 1994; Payette Veblen and others (1992) measured the effects ofa
and others 1990; Runkle 1985). geomorphic agent in the Argentine Andes, including
Wind may increase the severity of fire at several treemortalityfi"omearthquakes,fire,andwindthrow.
scales. At its minimum, wind acts as a drying agent, Apparently, sometrees died and others showed signs
increasingthecombustibilityofforestfuels.Moderate ofreducedgrowthfi-omthe"intense shaking."Inthis
windspushafiresteadilyfromoneflammableplantto sameregion,butontheChileanslopeoftheAndes,one
the next. Highwinds, incombinationwith severecon- large earthquake triggered several thousand land-
ditions, can drive fire at a furious pace,jumping any slides (Veblen and others 1992).
human-constructedbarriers. These events, knownas Unlike the above processes, snow avalanches nor-
"firestorms,"canconsumehugeacreagesofforestina mallydo not remove soillayers. Snowavalanches act
single day (Romme and Despain 1989). as thinning agents within forests and along forest
Geomorphology and Gravity—Included in this margins.Treesaffectedbyavalanchesaretypicallyat
least4to5inchesdiameteratbreastheightandfound
category are earth-shapingprocesses such asvolcan-
on slopes ofgreater than 25 degrees (Johnson 1987;
ism, subduction, earthquakes, andglacialmovement.
Potter1969).Areasmeetingtheseconditions,withopen
Also included are disturbance mechanisms governed
snowaccumulationzonesabovethem,mayhaveaver-
bygravity, suchasmasswasting(landsUdesandmud-
age avalanche frequencies of3 to 10 years (Carrara
flows), fluvial processes, and avalanches, both snow
1979; Johnson 1987). Areas in mountainous terrain
and rock. Ingeneral, these disturbancestendtohave
with heavy snow cover may be affected by unusually
longlastingeffectsbecausetheyofteninvolveremoval
ordestructionoftheuppersoilhorizons.Asaresult,it largeavalancheseveniftheydon'tmeettAheslopeand
may take many years before any plants can become treesizerequirementsmentioned above. large ava-
lanchemayhavearunoutzoneonflatterrain,oreven
established, thereby beginning the successional pro-
up opposing slopes.
cess. Swansonandothers(1992)lookedatavarietyof
In areas where avalanches run fi-equently, non-
geomorphicdisturbancesintwodifferingforestt5T)es
inNewMexicoand Oregon. Inbothareastopography forest conditions will persist in "avalanche tracks."
Youngertrees,ormoreflexiblespecies(suchasaspen),
was a key factor influencing "potential disturbance
energy."Whensteepareaswerecombinedwithhuman will surviveinthese tracks, butmore commonlythey
are dominated by shrub or forb layers. Avalanches
impacts,suchasloggingorroadbuilding,debrisflows
were common (see £dso, Olsen 1996). in mountainous zones create a patchy or striped mo-
saic running parallel to steeper slopes (Patten and
6