Disturbance Dynamics anil

Ecological Response: The

Contribution of Long-Term

Ecological Research

MONICA G. TURNER, SCOTT L, COLLINS, ARIEL E, LUSQ, J&T. SCOTT RU.PP, AND FREDERICK J. SWANSON

j:. MAGNUSON,

itt m.a-wgional context, We highlighted lime-axe studies, from Img Term Ecological Research' fLYER) Network shet-ihiu have &w-fcf1«ffi'<rw MwrW^rtw^^ily la fnidy(lifti.i!:t!(irif(' by fd) -e-iiaimpttismgccpfy&ctns subject-to a:wif&.-Ktn^<tf&$nirkan£®>, (b)-ffoviding.a long-Unit baseline •againstwhich la deled change- and '-measure ecosystem respojisi-s to disivrba-mx, {•f)'''perftt!ttif.ig.obs'wiiticin ofsloW.ar inffeiptenS fct'tris-, $•') facilitatingthc-ase t:f multiple research approaches,. (si pt'ovhling.itfpcsiffar modeling i&tin'biitife dynamics, sadijj contributing to litndami resouxcmanfigctnmi. Lofig-ientrressaiTeh if cm'cial to understanding JMKI, prrsefil, arid future djsiyrbam't ''ifytufmicSj sttid'tiie LTER Nel'wnii; is. poisedto ntakt continuing. ceniribittimts to

: scale, heleragvnwty,. hwitf,. invasion, mmiawe, jm

The important role of disturbances in shaping land-scapes and influencing ecosystems is ww well recog-

nized in ecology. Fires,-.stotms, floods, pest outbreaks,-speciesinvasions* and resource eArarticm, to name btit a/few typesof disturbance,.affect many ecosystems over ft range of spa-tral and. tei-nporal scales..Dlstutbance is defined as "any rela-tively discrete even! in. thn« that disrupts eeosystem, com-mimity, or popuiaiibn^struefureaM changes resource poo!ss

substrate availabilityvpr the physical ;enviroameiH" 1 WH3te.andPickett 1985)i it niay be natural or anthropogenic in origin,This general but flexible definition requires that the spatial andtemporal scales of .the system, iiind disturbance be specified.Disturbances span a broad range of sizes and frequenciesand can be characterised by their extents spatial distribu-tion, IVequency-or return interval, predictability, and'magni-tude, including [>c>th intensity -and severity (White and Pick-et! 1985.;).

Increased understanding- of disturbance has cont ribufeci i<oseveral important conceptual developmertfs in ecology. Ac-ceptance of the nonequilibrial stave of many ecosystemsmeans recognizing- that ecological systems are often in st>mestate of .reeovetv from: prior disturbance tWu and Louefcs1995), Spatial heterogeneity is i-ecognixed as a central struc-turing force in ecosystems;because disturbances both createand respond to .spatial heterogeneity, flinch understanding ofeeolosfical mosaics has resulted from research iirdisturba.nce-

prene ecosystems .Recognition of seale-dependent dynamics,:and of the importance .of understanding the regional contextof disturbance regimes as they influence the structure andSanction of .local ec<isystetm,;has:akobeenenharK:edby ourunderstanding of tiatu:ral:and human- induced1 disturbajices.The importance of natural disturbance regimes, in deter-mining the historic range of variability in ecosystems (e.g.,Landreset al. 19^9) and. iiiaking -ecologically infQB'ned man-agement decisions Jias been catalysed by our improved u.n-derstand:ing of disturbance. The iii-fiuen.ce of human so-ciopolitical structures .on Undseape structure has beenelucidated in part fey studies of disturbance dynamics onlands under different ownerships or management (e.g., Spieset al, 1994). Disturbance .continues to have centra! role in.ecological studies.

tuKirt af Zoology, liirgc Hall, Vtiivenuy <if Wneemiiii Modaim, H-7(!6. Sfoxi L Colliiis. isiin'iu'fjliiust-prtil&s'ortifbiologyat KiimafSMK tini-i)1. Manhattan, -KS'6'65W>. ariclm iHn Unh'ff&y ifMniylaml, Coilege f-Vjfiti

forestry, USIM fimyl Sen-let, Rla Ki'dnis. Pstrnv Rice Mm-$8(M. 'Mm I

'tmishi, Madison, 'Wl 5)7i>&- 1C "Sail .Rupp w tiii {ixisttun }>ro(fsinf in tin: l:of-isst Sfinncef OttWiHifiit at fi'ie Vs/versitytfAliAt, .Rurbanks. AK 99775. frcA-sxiek I, Swattssii is-u pA-iwrkft gAihifi'ii tit ttiv fJSDA FD'OSSI .SVmc'tv.-ttw^ftN'arfhwest R'esearck Slatkmi, forestry Sciences 1Mb, (JervaiKf, .OR 9?3fl. :<ti2003 American tnflitu

46 BloScfencc • fantuify 2803 ' Vol. 5.5 iS'o. 1

Studying disfurbance is challenging. Disturbances are un-predictable and usually unplanned; they vary across a widerange of-spatial and temporal-scales; they may leave persis-tent legacies; and there may be interact ions between multi-ple disturbances. Ecosystem-.responses, to' disturbance are.similarly complex.The Long Term Ecological Research (O'ER)Network offers an excellent setting for studying such episodicevents in ecosystems.(Bobbie etail..20(33). The sustained*multi'decade, multisite character of the LTER program per-mits gradual, .thoughtful development of complementarylines of inquiry on .the: problem lofundevstanding.ecologicalconsequences :of disturbance processes. The duration andnature of this prograntlnerease the prospects for observingeffects of a major, infrequent disturbance where predistur-bance conditions are well:known and the infrastructure to sup-portpostdisfurbanee measurements is. largely in. place.

Research at LTER sites has contributed substantively to.o.urimproved understandiiigdf theeauses and consequences ofdisturbance :in /ecological syslems^at a; variety of .spatial andtemporal scales. In this article, we first .highlightthree casestudies from LTM sitesjhat tiave contributed to under-standmgdisturbariees and. their ecological effects. These ex-amples are not exhaustive, but rather illustrate a range of dis-turbance types and scales in an array of different ecosystems1.We then discuss the unique ways in which-'the LTEB Networkcontributes tO:Studiesof disturbance. Hinally, we conclude bydiscussing the opportunities for future research within or incollaboration with the LTER Network on causes;and conse-quences of disturbance.

LTER contributions to understanding disturbanceThe LTER Network encompasses ecosystems that experiericea wide array of disturbances: across a range of spatial and tem-poral scales. Many studies of disturbance atLTERsites have

-taken advantage of natural events and :studied them from anexperimental perspective using varied methods. Under-standing has often been .enhanced by the existence of pre-disturbance data, .and some events have been deteeteilbeeauseof the regular long-term measurements made in the sys-tems: White arid lentsch (20915 recently implored ecblogiststo seek generality in studies of disturbance. They emphasizedthe need for dioosiug a focus of interest and level of resolu-tion for study; for establishing the spatial and temporal frameof reference; for documeritmg disturbance duration,inten-sity,,and..magnitude; and for determining patterns of distur-ba nee regimes. The LTER .program- is uniquely positioned toaddress these needs and contribute to our general .under-standing of disturbance dynamics. Here, we illustrate thecontributions of long-term research to general understand-ing ofdisturban.ee by highlighting exampfeffrom three LTERsites: a temperate lake ecosystem that experienced invasion ofex'otic species^a tropical-for^sfaffected by a hurricane, and atemperate grassland, influenced, by fire.

Long-term research can detect, monitor, and understandunanticipated events. Long-term measurement of ecosys-

tem and community .structure makes possible the detectionand .analysis of infrequent events that, produce long-termecological changes, for example, the introduction of an: ex-otic species or sa new but systemic stressor like acid rain maylead to. fundamental and persistent, rather than episodic,changes in system structure and -function. The disturbance andits: initial effects may be subtle, local, and ditlcultto: detect,yet its long-term influence may be profound and extensive(Hra'bik and Magnuson 1999, Lodge et al. 2000),

The LTER program provides a framework designed, to de-tect, understand, and generalize from .infrequent .and unex-pected events using site-based .research. Long- term researchat the North Temperate Lakes LTER site in northern Wisconsindetected the initial in vision of the exotic rainbow smelt ('O5>-iiiernsmai-dtixj in Crystal a-ndvSparkling lakes and the earlycolonization .of the^xotic rusty crayfish (<QKfftteete$:ni$tims)in Trout Ijake (figure 1L The LTER program allowed obser-vation and analysis1 of etimecouKe of changes in ecosys-tem structure -and function following these two :toasions,im-ibqdied in these studies of changing biodiversity werefundamental ecological questions- about disturbance; dis-persal; ecosystem tf ansformatio;n, resistanee, and resilience;and' the;re5toration otecosystem structure :and fcnction.

Smelt were not found in Crystal and Sparkling Lakes, andthe rusty crayfish had just colonized the:littoral zone of TroutLake,:whenthe;North Temperate Lakes LTER:si:te was estab-lished in 1983 fHrabik et aL 1998, Lodge Et al. 19;86),Intro-dueing species was not part: of designed . research; rather, re-search took advantage of the unjanticipated arrival of newspeeies in the El'ER lakes. Research at the Horth Temperatelakes LTER site documented the arrival, dispersal, and effectsof the^exotics; the meeh<misms:t>y which the rbioticcommu^nity was transfornjed.; and the. density thresholds for re-

The incisions profoundly altered the LTER lake ecosystems.Alter the rainbow smeltlnvaded Sparkling Lake in 1:982» tiepreviously. abundant Cisco {Coregontis artedii) no longt»rsuc-cessfclly-reproduced..!HcLain 19ii;,Hrabiket-al. 1998). Ciscocontinued to prod uce v table eggs, but virtually none of theyaung-6f-year cisco survived theirflrst -summer of life, CMlyone.lar.ge:cisco was observed by 1998,ln Crystal Lake, yellowperch (Pewa flavescemj] declined in abundance and bodycondition (an index of growth ;rate} following invasion ofsmelt, in 1 985 (Hrab1k:et^l. 1998), The young-of-year perchgrew more, slowly, and decimsing numbers survived their -firstsummer. No young'o! year have been caught in late suminerduring recent years, and tiieadu.lt population numbers onlya feW: In Trout Lake, rusty crayfish were first observed in1979 near -a popular boat landing (Lodge etal. 1986). The cray^fish dispersed slowly along the inshore 'waters of the lake, and18 years later in 1997 occupied the littoral zone of the entirelake f Wilson: 2QQ) ). As the crayfish dispersed, more than'80% ©f the aquatic macrophyte species disappeared; atsarrj-ple sites, two resident species of crayfish f 0.. virilus and 0.prapinqtiiis) nearly, disappeared, and soine fishes and benthieinvertebrate -groups declined in abuilclancc (Wilson 2001).

Jainmry 200 / Vol. 53 No, I • BioSciencc 47

Figure I. Studies of the invasion of rusty crayfish andrainbow smdt in rfre North Temperate Lakes HER sitedocumented profound changes in lake ecosystems.

Mechanisms of community change yaned^greatlyamonglakes, depending oil differences in the initial, bip.tic coraaiui.'-nily and in the invading .species. In. Sparkling Lake, predat.ionby smelt on young-of-year cisco was 1Jie inechanisrn of change(McLaiii 1991,:Hrahik etal,.J:99$:. PredatiOn wasso.intensethat survival of young cisco declined1 essentially to-zap; In olig-otrophic Crystal lake, intense competuiQn .for food betweensmelt and.perch was the agent of;change. InlUoutLake, titedispersing; riisty crayfish "eliminated their preferred foods(vnacTOphyifis, fish eggs, and Maiiy benthic invertebrates)while. iraalnMning high population densities by .using alter-nate prey ('Wilson 2001). Declines in the native crayfishspecies resulted from complex mechanisms iiickiding.coni-petitionj predittiDn,:differences:in abilities to e.v;ade predatiqii,and reproductive interference or hybridization {Lodge etal.2000);

These data -'have been used to enhance understanding ofwhat may happen in other lakes of the region if they are col-onized by these, exstic species, A model of smelt dispersalamong lakes in: the Bear 'River ••"Watershed' near the Traut 1-alffiStation .( Hrabik andMagausoM£99i suggested tin at suitablehabitatifpr smelt is present only in some .lakes* and only someof (hose lakes are connected by surface waters allowing smeltdispersal from adjacent lakes. Model simiilatiom suggested thatonly 25% of the inhabitable lakes, would be ifjvaded..by smeltevenater 1000 years, if invasion occurred only through con-.neeting streams in this region with high proportions of land-locked lakes (figure 2i However, if the observed, rate of in-vasion :into isolated lakes via human transport, as peopleimdverteritly move oi'ganisrns around in their boats and baitbuckets, was included in the, model, then 50% of thelakes were•projected to be invaded after .only 2QD years, 75% after 30.0years, and almost ail afer 1000 wars. This result hasarnpor-tanivimplieations for the long-term use .of biological reservesand; the significance of people in dispersal.

Invasions of island -like environments sue)] as lakes "are aunique, type :of disturbariee. The dense populations of rain-bow smelt a:nd::rusty crayfish originated, from a small num-ber of colonists, likely transported by well-itttentiqned erurtinformed humans. After such an invasion, the resulting dis-turbance frequency becomes continuous or annual throughreprod:!icti:o:n. of the invader, makiiig it continuous stress. IBthat sense* it fc not like the more periodic disturbance ofprairie:tires;Qr:flood pu.lses of large river systems, v«here thedistuf baticevisiincorporated into system behavifliv As a lakee£osysie,m:: loses species, re.titrn. to <m original state is inrpos-sibleina sttkt: sense; the system hasten ;:transforniecl bothstructurally and functionally, for the foreseeable future, anyresilience exhibited by a lake.ecosystem wouldjcoine frcto a.complex process invokiBg human managenieHtresponse asa part ;of the system behavior. Human reactions to taaclesir-able invaders eatildinclude,:andihave .included, thereductianoneliniination of the invaders and reintroduction of some-efthe lost species. Such ideas are now being explored in Sparkling.Lake, where experiffien tal removal of both the rusty crayfishand the rainbow smelt will 'be conducted.

Long-term research has changed understanding of tropicalforest dynamics. Paradigms for understanding the structureand function of tropical, forests in the Luquillo CTER site,Puerto Rico, had evolved from a long tradition of research that

Hugo, a category IV' hurricane with sustained winds ..over166 kilometers per hour, passed .over the northeast corner ofthe island of Puerto Rico on IS September 1989, one year af-ter EFBR funding began. Hurrieaoes are an important forceinfluencing forest composition arid structure on -numerousislands and coastal regions; 15 hurricanes haKepassedsver theisland of Puerto Rico in the past 300 years (Scatena andLarsen. 1993 ), I JER studies of the effects of Hurricane Hugobenefited, tremendously frorat he extensrve preciisturbance datafor the site and resulted in clramatic: changes to our under-

48 BioScicnce-« )nnin^2003 / Vdl. 53 Nit. I

Articles

500Years

tooo

species turnover Followed: by sSlf-organixalion of newconimtinities (LugQ and. .Scatena 1:99.5),

figure 2. Simulated percentage of lakes invaded by rain-bow smelt in the Bear River watershed in North- CentralWisconsin under two conditions: (1) no further introduc-tion by humans aiid (2) at the current rate of human in*-troductmn (modified from Hr<ibik and Magnuson 1999:),

standing of the role &f disturbanee in the X-uquillt* Forest Forexample:

* MSK of fragility were replacedly ideas of resilience.Alt b«wghC0nndl;p: roposed In 1WS: fiat: rateferestswere resilient^ the prevailing-view 0f;these:ee0systemsremmned%M; of fragiJiWj as reflected intteybool^byFarnsworth andGolley (1974) entitled Fra'gile Ecosys-tems. The UniR study in Luquillp after Hurricane Hugowas the first to provide^mpiriea] evidence for tropicalforest resilience (Wafer mil. .1996),

•. The rates of ecosystem processes were reassessed. Over aperiod of:more than 20 Deal's,, rates af Btterfallrin thekiqdillo Forest did not deviate significantly ''from ab6i.it5 megagramsper hectare per year (Mg • ha"1 * j?r'), andabovegrmindibioniass aceumulatioh did hot exceed 5Mg *h,r! «y-r"r.'the estimated iiet.abcwegroti.nd pri-marfprodUGtioTT:(NA:PPl. ranged Frani 10 to 12 iMg«tar^yr1. These results suggested- that rate processeswere constant iii these forests (Lugo and Scatena 1995).However, in the .five years after Hurricane Hugo, Scate-na .and colleagues (1996) observed a wide fluctuation inthese rates,.and average rates <;>f NPP that:,exceeded anymeasured, before in. mattir.e natural forests:(21.6 Mg»ha".vt

The::ehar.acteristics of ecosystems that lead to TesitiencevvereJidertitTed, iricluding.mpid hirnoyer rate:of :nutri:L

ents, nrass, and. p()pii,latio,ns;,bi(iti.c tonttol of fluxes;high species richness at .ecosystem interfaces (redun-dancy); high nutrient arid carbon storage in soils; nega-tive feedback at all levels of bi.otic interaction; and:high.

disturbances were documented and compared. Thesestudies deino;nstraited;that:siia;essit>n: wasdower altisfan'throp&gentc disturfeances than?after iiatirral ones

200(1, Zimnierman etaL 1995}.

» DiFlerences' between ecplogicatand geographic spacewere identified. After the hurricane, a draina tic geo-graphic restructuring of environmentai gradients: w.asubservedy resulting ftom the opening of the forestcanopy. Animal popvilations'thcn shifted in spacelqea-,tion in response to changes, in envircmmental :co.Hdi-tions (%ide I9ftl, Willig <md Gamite 1991),. Ecologicalgradients are not spatially feed, and organisms move inresponse to the. distribution of suitable conditionsregard less of geegraphiea]. space.

Four ittew -ways of analyzin§.tlte forest at-Luquitloienifirged.First, the relationship bet vveen turnover of btbfic cempart-ments,, includin.§:the length of succession and time requiredto: restore bifttriass, and the recurrence interval of distur-bances was. vreeognfeed. For exafflple, a tabo'nueo (Bacry6d.e$.axcelsa) stand inatui'fes in 60 yedrs, eOincidiitgwitJvtne re-eurrehteHriterval of eaKgpry.?V-V liurrieanes iri the region(Scatena 1:095:),

Seecmcly mseatehsrs, realized that Carabbean feasts sharea conimcm cadre of physiognomic and. functional charac-teristics that are huirf i'cscri'e-'inducssd a.ryd occur regardless oflocation.. That is, ;dry, irioist, wet, or: rain forests sh are simi-lar .characteristics, that are traceable to hurricane responses.Examples of these are canopy structure, species dominance*size class dominance, forest height | absence of Emergentt:rces),.lb.w bidmass, and. capacity to sprp.ut. These character-istics are-assoefeted with resistance and resilience to tarrieanes,

Tllird, th:eWiTi|)lexitf :of:the-eifocts of a: large-scak dis-turbance stuch as a hiirrieane was recogiiized |Boose eti al.19S4}, Like other large, infrefnignt disturbaneesrhurricanesproduce a spatial mosaic of varying: disturbance .severitiesacross the landseajSf thiit.affect different ecosystem compo-nents (turner et al 1997, Foster et al, 1998)l For^xample,;^hurricane has wind and rain components that change inmagnitude as thef pass over the landscape and prodLiee sub-stantial heterogeneity in effects. Moreover, each of theseforces affects a different eomponentof the forestvWind in-teraits mostly, with the Canopy, whereas rairi lar|ely affects tliesdilaiid st.reain ecpsystenrs. Ecosystem response depends1 on.the type of disturbance-ferce that; prevailed at the time of hur-ricane passage. By isolating thedtstiirbance ferce fbrfee af-feeted ecosystem component/feedbacks operating at a land-sea pescaksaii bewAserved.

Eotrrth,iif^pite of recurrent hurricanes, ecological legaciesaetjuired through anthropogenic changes persisted. Forests re-covering from prior land use usually developed a structure,

January 2Q03/ VW. 53 No. I • BioScienec 49

Mtides

plwsiognonn', and function similar to that of forests, not; in-fluenced by human, land use. However, species coin positionand dominance we.re different between sites that had or hadnot experienced land use. These differences, which change. veryslowly, p.ensistech even when hurricanes occurred (Zimmer-man e.t ai. 1995, Wyiig.etal. .1996).

Hurricanes produce sudden* striking changes in forestecosystems, including massive tree niortal.it>' (Lugo and•Seatena 1996} and cransifir of materials from the canopy tothe forest floor '(.Lodge et al 199,1 ), Hurricanes shape foreststructure and function and initiate a slow but continuous tes~

state is unlikely. Only through Mug-term ecological researchcan such continuous: but slow change in ecosystems be as-sessed.

Long-term research provides understanding of disturbancescale and heterogeneity. Disturbances occttr at dtteedt spa-

. Hal and temporal, scafes, and many ecosystems experience avariety of different disturbances. Gonsecfu.ei-Uivvanalj'sisof asingfo typei'of disturbance niaybe;misteading!because differentkinds of disturbance may:mte:raetin space aiid time. Ratherthan focusing on a single disturbance type or eveiitjloiig-termresearch has the luxury of focusing on feedisturbajice regirne,the sum; of all. disturbances affecting an ecosystem in space andtime (White and Jenlsch 2001 }. Grassland ecosystems, for ex-ample, are subject to fire, periodic drought, add in tense pe-riods ofherbivory by ungulates, invertebrates, and. numerousspecies of ,sraalJ;mammaIs,:Qlher common disturbances ingrasslands include soil mounds', fecal deposition, animal bur-rows* and bison wallows. Together these disturbances createspatial and temporal heterogeneity to resource availability,community structure, and ecosystem processes,

Researcbat grassland LTERsites has provided insight intohow .different disturbances affect these ecosystems, /ft theKonza Prairie ITER. site, 4 tallgrass prairie in northeasternKansas, the interactive effects ot" tire and .grazing have' beeninvestigated experimentally (Knapp;erjtl- 1998, 1.999). Repli-.cate watersheds at Koma Prairie have been burned at K 2~,4-, and 2Q-VT frequencies since 1972, anda free-ranging herd:pf bison vvus; rejntroduced to a subset of the area in 1987, Un-like hurricanes er iBvasiye species, fire and grazing ereateadis-turbance regime in which large- and small-sealedistatbaneesrecur at -intervals mat are mash shorter thaw the life.span. ofthe dominant native species. These species thus exhibit nu-merous adaptations that allow them to. survive and thrive: un-der air intensive disturbance .regime.

Early studies at Konza 'Prairie focused primarily Q:H fire fre-quency. Long-term research has shown that as fire frequencyincreases iri' tallgrass prairie, plant spcd.es diversity, cominu-nity heterogeneity, and nitrogen. (N) availability decline,while annual net primary production increases. Burning re-sults in N losses uvthe atrnospherevthus, N availability dediuesas lire frequency increases. Nitrogen is a .key limiting re-source in tallgrass prairie, :and the negative relationship be-tween N' availability and production is highly unusual (rig-

800 '

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Qc

Unfertilized, « A

\r °•"-fL^ __ A

A

A^ ^ A A

9^ *•Is - 0,1?p = 0.047

3 1 2 :3' *

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• *...„"-- . ,__

A"--- :oA r --,

O "v--.^. A* Burner] Mam} A >'t*v.

!:Jk ByrneEl uplaR^ ^ = 049

^ Unburned upland • ' - ' • • * •

t 2 3 4 f

\

:W mineralized (Q N/rn2)

Figure 3. Animal abovegroitnd net primary production(ANPP) tis a function of cumulative nitrogen mineralized(Nm!ll) in unfertilized and fertilized, burned and as-burned ttiltgntss prairie, Konza Prairie IT£R site. Wnfer-mheA:-ANPP = 3542 4 54.9/{M .J, JT = Q.17,¥ =0.047.Fertilized: ANPP =-116.2 (NiitJ'*767,f* = OA9,P = 0,012 (redrawn from Turner et at 1997).

UK 3; Turner et at 1.997). At Koitea Prairie, increases in. pro-duction following fire result .from die I'eJiic.n'iil ;of the litter laprjwhich allows grovvth of the dominant:C, grasses to:begia ear-lier in the growing seassn compared with unburned sites(Kuapp aii.d:.Seasted.t ] 98i6j. Work-atthe Cedar Q-eek LTERsitehas shown tliat the dowifiant C,j grasses are powerful com-petitors under low-.N conditions (Tilman and Wedin 1991).This increase in';-production.b.y the pergnaial 'grasses resultsin developmerit ef a dense- canopy and intense shading, lead-ing to the .campetith'e excl:usioi] of many fotbiaud to a re-duction in plant species divsrsitv'.CTilnian 19S4, Collins. etal.

Although spring burning results in relatively clear re-sponses at Kenzii Prairie, these responses do not reflect thefull impact of the disturbance regime that now characterizesnatural or managed tallgrass ecosystems. Further research atKonza Prairie and .elsewhere has shown that light to mod;er-.ate grazing by ungulates, either bison or domestic cattle, al-ters many pf the patterns produced byburningfKnapp etal.1 999) . Ni trogen :a vailab ility increases in grazed areas becausenuttiest cycles are more rapid, and less abpveground .pr©-ductioji is test during fire in grazed areas than in ungraded

•50 BwScteiice * Quinary 2003 / IW. 53 No.

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areas. Gracing reduces the dominance and, competitive abil-ity of the C.i( grasses, which Leads to higher plunt.speeies di-versity and greaterspatial heterogeneity (Collins: et al. 1998),Iiiadditiony ungulates enhancespafialheterogeneity by wal-lowing and by^creating gracing lawns within sire-as of ungraded:vegetation. These grazing .lawns form around: mine deposi-tion sites,which are small patches ofhigh productivity (Steiii-auer«n:d Collins. l99S,:20uL). These small patchesservea^ fe-cal sites, tor the formation of larger grassing lawns. Thus,small nutrient-rich patches Create brger-seaie heterogeneityin grazed:ian:dscapes.

The role of disturba;iree.s like 'fire <vnd! grazing varies re-gionally. Although burning & common in eastern taUgrass.prairie, it is less cOniinomin:westem shortgrass steppe, whereless abcweground production limits fuel loads that ean:sus-tatij fire, Water constrains production in 'tjie.se systems morethan N availability ftauenroth 1979),More0ver1in:$h0rtgrasssteppe, grazing .seems to have little 'impact en ecosystemstructure and function (Milchunas and Lauenroth 1993,Mikbu.nas:etaL 1998)..Thus, sbortgrass steppe is highly re-sistantto disturbance, whei^but perhaps more resilient {i.e., better able to weaver frprn agiven disturbance) (Coffin et al. 1996, Baer et al, 2000).

Resilieneeiiasbeeh well demonstrated in/efforts to .reestab-lish native vegetation, through ecological restoration. TheCurtis Prairie at -the-University of Wisconsin Arboretumserves as the prototype systenxfer ecological restoration 0fgrasslands (Jordan et al, 1987)'. Many.gr;assland restoratioiKhave been conducted, but one d ifticulty .has bee it the main-tenance: ef native species diyersity in restored prairie (Kind-scher and: Tiesztm 1998 j. At Kon/a Prairie, an experiment wasinitiated to determine whether resource heterogeneity efl-.haTic.es species diversity during ecological restoration. Re-sults thusfar show that-native speeiesJiave beenrriofeabun-dant and diversity higher on: restoration plqts where N:avMability was reduced by carbon (C) :»iMendments. Tliws,Imig^terni integrated research.on grassland ecosysterns haspjOvided:firadaraea8rl understanding needed tor the restora-4on:<gf hiotk diyersity as -regibnai patterns of land use changefro:rn production agriculture:te-conservation.

The value of long-term research forunderstanding disturbance dynamicsThe IXF.K Network enhances understanding.of the impor-tance and role of natural dlsturbanees in ways that arequal-i tatively different froin sites or programs that lack a king-termperspective and funding base. The preceding examples il-lustrated scientific contributions from site-based, research,togeneral understanding. Here we discuss several key ways inwMchthe overall LTER progranv. significantly enhances theability to study disturbance.

The LTER Network encompasses ecosystems subject to a widerange of disturbances. Only a: subset:ofsitesisOTsceptible:toany particular disturbanee type, but collectively many im-portant -ecological disturbances occur across the network of

sites (table 1;:)> For exam pie, 'twcjuillo, .Harvard Forest, andCoweeta;are ferested:Sites that hav^ experieitced^ hurricanes;Konza^Prairie, Jorwada, Sevilleta, and Shortgrass Steppe areall grazed by ungulates; SoManza Creek experienees crdwntires; and :H.J, Andrews, Hubbard Brook, an.d:Coweeta havetill been subjected. to..forest harvest. Various sites experiencefloods in forested streams, many sites have exotic speciespresent or likely :toinwide.rn the near future, and many bear:the legacies of :pfi(ar human land:use.:The;pEftIS!:etworkeri-hances opportariities for samp] i nginajor disturbances, and

ine the same event in different ecosystems. For example,Hurrieane:Hugo affected1 1-iaquilloi iiarviird Forest, and NorthWet :(anffistuarine site in South Carolina, fprrnerly an IXERsite); iTER. seiaitists engaged in productive collaborativestudy o.f these disturbances :.( e.g., Boose et al. 1994).

Long-term data provide a valuable baseline against which todetect change and measure ecosystem responses to distur-bance. Even dramatic ecological changes might go unde-tected without long-term studies. In many cases, extensive pre-disturbance data on ecosystem stracture or function arelacking when a disturbance occurs (e.g., the 1 988 Yellow-stone fires, the Exxon Valdez oil spill). Long-term data havebeen crucial to understanding the response of tropical andtemperate forests to hurricanes, floods, and droughts. Long-term observations at Bonanza Creek have identified the im-portance of annual sedimentation to floodplain successionaldynamics (Van Cleve et al- 1991 , 1996) and the importanceof major flooding events (Yarie et al, 1998). Understandingof the effects of the 1996 flood at H. T. Andrews was en-hanced by pretlood data on channel geometry, riparian veg-etation, and aquatic biota (Swanson et al. 1998). Detection andmonitoring of the exotic species invading the North TemperateLakes emerged directly from long-term observations. Long-term studies at the Shortgrass Steppe site documented plantcommunity changes in response to changing climate (Al-ward et al. 1999). LTER data provide a rich resource for in-terpretation of system response following a disturbance.

Long-term research permits observation of slow or infre-quent events that are only apparent over longer time scales.As the number of sitesln the -Lite Network has grown andthe : duration of study lias lengthened,: the opportunityipr di-

increased. For example, hurricanes only rarely affect inlandregions of the Appalachian Mountains. The cen ter of Hurri-cane ©pal passed Over southeastern Tennessee on 5 Octoberl::995,.,how.eyer, causing,high winds and1 heavy rainfall inmuch of the southern Appalachians, ineludmg the CoweetaLTER basin. 'FMuiiMSuabixe andstretigth of Opal producedbroad-scale tree damage to inland forests, followed by sub-sequent salvage logging. Although forest disturbance hasbeen a main. facj.is.of research UK Goweeta tor .decades, this wasthe firstopportunity to study the effects :-<jf catastrophic winddamage on vegetation dynamicsln theibasinvThe long- term

ol '53 No. 1 • BmScience 51:

Articles

Table I. Examples of disturbance processes that are represented within the long '[erniEcoh&ctirReseanh(n%K) Networkfor selected ecosystem types,

Ecosystem type

Forest :onlyForest and stream

Forest and lakeI Grassland, shrub, and tundra,i Grassland, shrub, tundra,

and streamGrassland, shrub, tundra,

and take-: lakes

jV»(i.¥.- Site abbreviations anHa mix! Port's!; JR-N, Jornada:

Abbreviations us«l for biu:

ITER site Fire

HRVAND, Bm, XGWT, HBR, ,LUQNTLJRN

KNZ X

ARC ?NTl

Flood or drought Hurricane insect pests :E)((>tic species Land use

X X X feFX F, RX X X X F, RX F,R:X A v G:X G

y. x

as Follows: ARC Aa lie Tundra;- AMD, H, !. Ami revs; KNZ, BoiBiis Qwek; CWT,-'Gowe«ia; 1-JBR Flulibatd. Brook; HRV,<-NX", K«is>.:!.P.rairir, NTL, Noah 'Ifc.m'pei".Us,' Lakes,usg effect* are A, agrii'tihurc; l.v fevssiTy; H, -roads; G, gracing,

ppedisturhance data, on forest structure and fimcticui atCovseeta provided, a valuable baseline for tracking the re-sponse of the ecosystem, tp .Miirrican Opal Insights gainedfrom the hurricane studies at'IuqitiUe and Harvardlbrestalsoin form.ed.'hypot.hesis-:-t£&ting at Govveeta.

.As UK* period of Study eontimies to grow across the O'ERNetwork, weean expect -additional opportunities to detect arid,study infrequent events; for example, fire is the key disturbancethat lias shaped ecosystems of the Bonanza Creek LTEE sitein libe boreal forests of Alaska. With a.fi-.iacreasing duralfoitof study, long-term ecological research will most likely pn>v-'idc the opportunity to observe and .study an :infrgt]:uentlarge crown fire in that system. Similarly, the .introduction ofinvajiive species is.likely to be detected and studied at manyLTER sites. The LTER Network provides a range of sites in dif-ferent ecosystems that are likely to .sample .large and infrequentevents,

The LTER structure facilitates the use of multiple researchapproaches, Disturbance research at HER sites benefitsfrom a wide variety of approaches, including field observa-tions, manipulative experiments,-.historical reconstruction,modeling, and cross-site studies. This plurality of approaches:is fostered by .focltsiiig-the attention o.f tuany investigatorswith different perspectives:pri a given site, by creatmgloHg-terni databases and making them available online, and byhaving'a network of sites at which similar questions can be

The diversity of approaches is?nieeh/ illustrated by the li:wr-deane research, developed over more than a decade.attlie-'Har-vard pQres.tU'ER site •in.centjra'l'Massachusetts. Harvard For>est researchers :began with retrospective' analysis of effectsoftl'te 1938 hurricane on forests of New England, compilingrecords tram archival sources and-analyzing those data geo-graphically. Further study led- to development of two alter-native models; a meteorological made! that reconstructedhurricane wind velocity and damage on a regional scale, anda topographic exposure '.model that assessed terrain effects ata landscape scale. The models were then tested against ob-served effects and .combined .with extensive historical.records

•to reconstruct hurricane impacts in both New England andPuerto Rico-since .European settlement (Boose et aj. 20GI). Inaddition to observaticai and modeling studies, experimentswere conducted to siniulate hurricane damage by topplingtrees with winches, and the ecological responses were carefullystudied (Foster et at 1997), The experimental hitincartere-vealed. that only a fraction of toppled; trees died immedi-ately, and there was significant continuity of cover by live treesand shrubs t.hraogn the pystdisturbanee period, Goiist'-c|iiei).dy,-despite draniaticaltei'atip.n of forest structure, llierevvas little change of many ecosystempracesses such as nitro^gen cycling, nutrient availability, and soil respiration: (Festeret al. '19-97). Thus, understanding of the disturbance regime:has been infermed by observation, experiment, modeling, andhistorical analysis.

Anicing the many approaches-used to study di*turbance».the ability to conduct long-term landscape-scale esperi-.Kseais-is a liallmarkiOf research at :L*rERs!j;es..S.everai of theforested LTER sites {e,g.,Cowe;eta, Hii¥bard.Broe,ls, and H.J. Andrews) had a history of experimental manipulation(usually cleat-cutting) implemented1 by the US Forest Ser-vice before the ITER pregram began. Tremendous in-sights continue to be. gained through the long-term mon-itoring: of these experiments (e.g., Swank and Grossley1988,Lifens.andBormann 1.995, Elliottetsil 1997), An un-derstanding p:f the characteristics and: dynamics :o:f canopygaps (small disturbances common, in eastern deciduous for-est) lias emerged from experimental studies at Goweeta(e,g.,Clinton and Boting.li993J. Landscape experiments arealso bemg.conducted at Konza Prairie, where replicated- setsof small watersheds have been burned, in the spring at.varying intervals.since 1972 f Knapp et al. 1998), After 25years, .this burning regime has created a landscape ofpatches subjected to different frequencies .of disturbanceby fire. Understanding the response of tallgrass prairiecommunities to different burning: frequencies and themechanisms creating that response would not have beenpossible without the long-term experimental approachand the'-integrated research that is.a general characteristieof the LTER, program.

•Articles

Long-term research provides a focus for modeling disturbancedynamics. Disturbance; models have produced unique in-sights into ecosystem and landscape dynamics atlTEIMtes.J:or example, the hurricane exposure model developed forHarvard'Forest and Luquillo underscored the importanceoflandscape position for predicting disturbance severity (Booseeta!,. 1994). Spatial modeling of fbTestEuttingalternatives in;the: Cascades Mountains, in Oregon identified significanttiriielags in the response of landscape pattern to changes inthe disturbance regime (Waltin et al. 1994). Modelingof thecauses, patterns, and effects of disturbance iSifaeilitated withinthefTER program because detailed long-term data: sets; ex-ist for specific eemystemtors'fpcXis.oti the same site or:gr«up of sites, feeare-h at theBonanza Greefc LCEK site in the boreal forest :of Alasfcv illus-trates hew:iniporUHt;ni,odelirig:contributions can be facili-tated through long-term: research.

Fire is the majordisturbance iaeter in the boreal forest(Wright 1974,Payette 1992)>;aridpostlreSuccession do^nr-nates vegetation dynamics {Zackrissaii 1977, Pa.ye.tte 109>2j-,Succession varies widely across.the region in response to ste-ehastk variability in disturbance and: species recruitment.(Mann and Plug 1999},.Researeliat the Bonanza Greek UBSsite, :begun in 1.98-7, has produced valuable insights aboutdisturbance and.succession^ patternsand rates in the region.Current research addresses how-changes in^climate and dis-turbance regime .might alter the ftinetionitf the Alaskan bo-real ferext.Tor example, fire frequency.influences patterns ofenergy partitioning {Cbflpin el a'l- 2000) and carbon dy-namics (Kiisisclike et al. 1§95).

The relatively simple disturbance regime and •well-studied:successional dynamics-of the Alaskan boreal forest made itideal for computer simulations of climate-vegetatieri^BreintEractmnsat broad scales. A spatially explicit trame^basedmodel (ALFRESGOJ was developed to simulate theresponseof subarctic and Ijoceal ecosystems, to- a changing clim.ateand fire regime pbpp et al, 2000a, 2000b). Modetdevelop-meiit was based an. the kr«3wled.ge of :suece$siona) dynamies .-acquired and synthesized by a cadre of scientists studying thejboreal forest over the p^st 30 to 30 years., CTER:data setsserved both to parameterize :and w test A^PKESGO. Appli-cation of ALFRESCO toihe fbrest-tiuidra ecofone^uggestedthat Jong-term rates and patterns of vegetational changecould be influenced by specific .events leading, to legacies/thatcannot be explained by current climate [Rupp et al. 20()0b),Model simulations also suggested ih&t a warming elimate inAlask;a would caase;a steady increase in tile proportion flf earlysuceess.iona! deciduous forest across the landscape, hi turn*,this increase niighl reduce the predicted decline in: regionalalbedo and thus moderate the positive-feedback to climatewarming (Rupp etal.2QO.Ob, 200.1).

The Bonanza Creek modeling studies have.gerieriated somesurprising results regardingthe response of terrestrial ecosy.s?terns toprediettd mture^chmate. ALFRESCO was used to-sinii-ulate an. instan taneoiis 2-degree Celsius increase in gro.wing-

i temperature fram currently observed climate with

two different precipitation regimes (a. 20% increase-arid 20%decrease from aii-rent precipitation)^ to explore the influ-ence : of eliitiate change on long-term boreal ecosystem dy-.namics. Both: warnmg scenarios predicted a;ti hicrease inthe total number of fires ;c0m pa red turrent clinvate. How-ever,.thedistefbuti0n of firesides waS:Surprfeihg(figufe:4). Asexpected, the vva.rmer.and dria' scenario resulted in fewer small.ftres;(burning 0%-S%:of:to:tala;reai a.ncl an associated shifttowards largerfires (burning > 5% oftdfelarea):. Also as^ex-peeted, the warmer and wetter scenario resulted in fewerforge firesand an associated shiftiowards smaller fires. Flow-e«er> the xiistribution of very large fires :( burning > 25%of to-tal area):i«as unexpected. (:Ggpe4). The warmer and .wetterdiin:ate5cenario.::pi;oduced moi;e:v&y large ttres comparedtotbe.:«anarer and di^'er scenario. Thevvarmer.a.nd dryer sce-nario pradueed frequent- medium-sized fires, which pre^veirfed fiiek from bt31d:ing;up acress-the:teHdscape\dfld I im -ited the number of :lairge firesv In contrast, the warmer- andwetter climate scenario; :led:1tO: fifecjueht small ftres,:which :al-lowe'd the deyelapment of well:" connected, highly flammablelate-successi:onal:stan:ds across the landscape Interestingly,similar results v^ere obtained with a very-different: model forthe landscape of Yellowstone Nationa;! Park, Doming (Gard-ner et al. 1996J. Tlie regional imf)lications-of alternative di-

fimction

With.long-'term researclij, meiriierplay.Bet^enmodelsanddata can be:substantialjy enhanced. Model predietions can betested against dbservalions of the^systeinoyeran extended time,peripd: Modelsialsb Serve as. a research tool, for scientists, al-lowing far the; generation and testing of alternative hy-potheses. In addition, rtlodei parametematian and calibra-tten are iflueh: more feasible when extensive data are availablea rid wh e ii; there is a, critical ma ss of i nves tiga tors and collec-tive knowledge available for the system.

Disturbance-related LTER studies contribute to land andresource management. Long-term disturibancetesearch ismaking sigtlificaht eontrlbutions toimportawt issues irelandand resource management. There is 'substantial interest:amongland managersln using natural disturbance«gimesto guide future mairagernent :pla:ns, wi:th the implicit as-sumption that maintenance of /ecological processes: will beittore likely, under these regimes. For example/.considerablediscussion has:addressed whether the spatial patterns pro-duced by natural di:sturban:ces can serve as a model for thepattern and timingofhmnian disturbances such as forest har-vest (Hunter 1 993v Attiwill 1994 Moiling and MeSe 1M6),Clearly, this ; requires tinders tandihg historical disturbance pat-terns and pi'bcesses. The concepts considered .under therubric of "range of natural variation" are especially Impor-tant in land msnagenieirt. !%o of these concepts are f 1)that-distMrbanefi-;driven spatial and -temporal variability is avital attribute of nearly all ecological systems, and (2) thatpast conditions and processes provide context and guid:-ancc for managing ecological systems today (Laudreset aL

l<mmry2M)3 / Vitl. 53m. / • BioScleiice" 53

MO-

400-

309

200

100

Q Dryssaffiwa.a Wei scenario

>30

Figure 4. Histogram showing ihesire distribution of firesunder two different climate warming scenar io$ using asiiitulation model developed far the Bonanza CreekLTEKsite, Grvwing^season temperature wasinstiMtaneduslyincreased by 2 degrees -Celsius in Iwtn scenarios. The dryscenario (white bars) represents a precipitation regime20% less than currently observed. The vet scenario (graybars} represents: a precipitation regime 20% more thancurrently observed. Results tire far a 1000-ylion period, replicated 100 times.

! 999). Natural variability is defined as spatial and temporalvariation in the ecologicalconditiojis that are relatively un-affected by people, within a period of time and geograplil-•cal area appropriate to an expressed, goal (Landres et al.1999), Improved understanding is needed of how natural: dis-turbance-generated landscape mosaics differ from .human-generated mosaics. The UTER program can"contribute valu-able insights to this discussion, AS illustrated by contributionsfrom the H. J, Andrews LTER site ii.i Oregon.

At theAndrews LTER:site, wildfii"es;duritigthe period A:D1500-1900 strongly shaped the current landscape .distribu-tion of forest age classes, ftegionalsTOthesis of fire history stud-ies in Forest types characteristic of'the Andrews Forest-indi-cates periods of'high and low forest fire extent reflectingcentury-scale climate variability and 'human Influence.:;. Therehas been no stationary reference period against which to in-dex future forest conditions;, foutlaud managers ean.still in-corporate what is knovvn about past disturbances.into theirplanning. For example, uifbrraation on historic frequency,severity, and. spatial pattern of wildfire disturbance Was in-corporated into a new management plan for a 23,9QO forarea of the Willamette National Forest formally designated forinnovative adaptive management (Qssei et al, 1,999).. Thisdisturbad.ce-based management plan was compared with aplan that employed dlspersecl-p'atch dearcutting and a planthtit emphasized reserves .and matrix 'prescriptions for mul-tiple species. The disturbance-based plan produced, moreold -growth habitat and .more natural diversity of structure andcom position at stand, and landscape scales, iind appeared to

besupenpr for sustaining native species and processes. Thedata from LTER sites can be a valuable resource Tor managersof sites vv.ith.iri the.same region.

Challenges and Mure directionsThe CT.ER.4pproach is particularly valuable for understand-ing disturbance dynamics over longtime .periods andplac-ingthose dynamics in a regional context. The LTER Net-work offers unique opportunities for time series observationson a wide array of disturbances and ecosystems. What promis-ing new research directions should, be pursued? Opportuni-ties abound within theTJTER program for synthetic analysesthat compare disturbances and 'ecological responses at differentsites :and: across spatial and temporal, scales. Because sitesdiffer so widely in the types of organisms present and the fre-citieney of disturbance, sailing by organism Itie histories (e.g.,Addicott etal. 1989} and spatial and. teMperalatjributes ofdisturbance:.aiKl succession, (e.g., Tiirner £t al. 1993rralgta \xespecialiy useful for sy«thesis::and;iHtegration. Mariy of th.elarge-scale, long-term experiments at LTER sites provide anexcellent context for understanding evolutionary processes: inresponse to disturbance frequency and 'intensity. In addi-tion, comparisons could :be expanded .to Hon-EJ'ER sitescharacterised, by a .rich history of disturbance-related stud-ies. 3u"or example, HER sites have :made:IrapQrtaot contribu-tions to a synthetic treatment of large, infrequent distur-bances (Fosteret 4 1998, Turner et al. 1998), and- furtherco.lbboraticras and c0;ra:parisans among si^tes should besti;ongly eocooraged. Non^LTER sites may also serve as valu--able locations for independently testing hypotheses generatedbyLTER,

Disturbance-related ••LTjBR.studie&shoaldcapitalijseonthedivexsiliy of ecesystems represented-in the network (table 1}to fu rtherunderstanding of the mechanisms and .tmplicationsof ehanges in natural and anthropogenic disturbances. Biotiemixing has been identified as a majai' challenge in globalecesystetMs research, and :the:CfER:program should providevaluable doeumentatiot'taf species invasions and effects by ask-ing questions such as: Do aiquatfc and terrestrial systems re~:spond similarly to invasions by exotic species? Are invasionsby animal species siinllar;p:r different to, invasioHS by plants?Enormous changes in species composition will, continue iiithe future in response to increasing human effects on theplanet. In addition, climate-induced changes in .disturbanceregiriies' may profoundly affect ecosystem struetu re and func-tion. Ecologists have a responsibility to: anticipate:the conse-queHces of these elia;rigES, and.LTEE sites are in a unique po-sition to study these events effectively.

Disturbance regimes cmcoinpass an extremely wide rangeof temporal and spatial scales. Understanding tile .spatial het-erogeneity in disturbances, and how this, js manifest al differentscales, is-ajso a fruitful direction for Continued work across theLTER Network. Spatial variation in. disturbance effects maylead to important functional patterns across ecosystems, andunderstanding such variation may be important, to drivecomplex system models. The ecosystem, response to. distur-

54 BioSrieivce « linswiryllW 'f Vol. 53-No, I

bance: may influence future disturbance patterns, and dy-namics, and long-term studies could elucidate .such feed-backx/The LTERprogram'sitraditiohaleniphasison under-standing temporal vacation: can be augmented by anincreasing integration of spatial and tempoM variatioil Ad-ditional cross-site studiesiare likely tQ:yieldimpof tant new in-sights and should receive fiign priority. Long-tei'm researchis crucial to gaining :understanding of past^present, and fu-ture disturbance dynainics and eaologieaii-espianses, and theLTER Network is poised to make, continuing contributionsto this goal.

AcknowledgmentsFunding for this research has been provided by the LongTerm Ecological Research Program of the National ScienceFoundation. We thank James R. Miller, latiia L Scho.ennagel,John Hobbie; fnembefs of the LTER Co.drdi.fia:ting.Ci0]nmit-teej and two anonymous reviewers for constructive com-inents on this manuscript.

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Mirhiio AS/ 1991. Conceptual and empirical analyses of biological inva-sion; Sfou-nhu've fisl> iin'asiojimkj n<irth-icn<|)trraie hikes, PhD d.isscr-tiUkw, University ;ef Wisconsin, Madison.

'M'ilc.hunas DG, JUuenrotli WK, 1993, Quanlitative effects, of grazing :onvegetation and soil* over a global range of .environments. EcologicalMonographs. 63: 327~36<1

Milclumas IXi, Uuimwth WK, Burke t€. W8, Ijvcsioc-k grazing: Anima!and plant biodiversity of shortgfass steppe and the relationship to ecosys-tem function, Oikos'.SS: 55*74.

Pavel le S. 1-992. Fsreas,,a conlrolliiig process in liu* No'rtli. AViieriean biXi'aifores!: Pages H5~j<W in Shugan MH> leemans R, finnan -C13, e<to. A Sys-tems Analysis.oJ'i'neGlobal, IJorca! fwesi. Cambridge [United Kingdon):Cambridge University Press.

Hupp TSvSuirfield AM, Chapui FS III. 2(X&uA ftanjc-based Spatially explicitmodel of subarctic vegetation, response to climatic change: C<«flparison•with .a point model, landscape Ecology J&Wlt-ifX),

RuppTS.Cnamn B III, Siflrfield AM, 2000k Response ftFjitijMreticveg«;:i-lion to Jransictif clmwtic change -<m the Seward Peninsula in iiorthwasiAlaska. Global -Change1 Biology 6: 541-435.

-- -, 200 i . MwMiiiglhc ii itiueiice qi" topographic barriers »n t rceline ad-vance ;ii the forest-tundra Bciitaoc in northwsestcto Aiasfei. ClimaticChange 48: 399-4 1 6.

Scau-na PN. 1995. Relative sailes <if iime.and effect iyeness of Aproct'sscs in: a troprai. -nuoiUans rivin forest of Puerto Rto- Riges 103- ] 1 1in Q>st4 JE, -Miller AJ, Potter KW.WHBock KR, «4fc Natural mid An-thropogeriic 'Influences its Huvsal Geomafphologj.'. Washington (DC):Ainerirao Geoj»lwsica! Untoji. Gwfihj'sioi! Ntoiw^raph 89.

Scatena PH, Ursen MC..Wl.-'Pbysieal aspects of 'Hurricane Hugniii Puerto•KtcalUuiropica 23; 3:17-323.

Sateiii) FNJ, Moya.-S, Estrada Cl, CJiiaea ID. 199& TV-first five years in thereargani/aiion ol* al>t>vcgmuiKl feiomass and nuirkni use following;Hiirricuw Hugo in ths BLsfey Experimental Waiewljeds, L-uquillo. Ex-perimental !;rtrcsi, Puerto Rko. Bioiropica 2S: •! 2.-M.40,

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Swank WT, Cmssky !>A J.n «rfs. 1988. Pores!, hydrology and ecology .atCbweetiii New York: Sprinner-Verlag.

Swaiismi F|, Joh'nsoa SI:, 6f(.-y,owS\?. Acker .SA. W$. Hood itisturbance m•<i Ibrestcd mountain taflxcape BioScieHtt 4S; 6B.I-489.

"Rlrnati D. 1984. Plant dorainnoce ajong.au experimental tiutriou estltent,Ucokigy.65:]'W5~l.')S3.

llliivin P, Wedin IX .1-99-1 „ Uyiiamte of nitrogen cafiipctition -betwcsn sue-ciissional grasses. .1-cplojg' 71; 1038-1949,

Turner CL, Blair (M, Schartv. Rl, Neel, -]€, 1997. Soil S and plant responses1ofirc,ta"pogfsj tyah"iisuj>)Mwicn^1832- 1 84S.

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Viin Ckve K, Chap'in B .1 1 1, Dyntess CT, Viercck LA. .1 Wl . Slat* (iictor con-irol«f«bnicnt.fydmgin tfliya tbwsts, Biosciencf'il: I--10,

VareC-ievc K, Vterec'kLA, DymessO". W9.fi. State Factor controj of soils'a.ndIsKSt succession alorit', the fanana River in Jniixuir Alaska, . USA, Arak.aad Alpine Research 28: asS-MO.

Waide BB. 1991. Stuninary- of the response (jfiJtiimal p»|?uliitions: us hw-ri-canto in thefi»sibb(?an. Biotropica 23: 47S-4BO. .

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Jcalii'fns 4; 56*-580i

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Vv'il% MR, &m flo (jH. 1 99 1 . The effect ef H;tirri«:4n« Hugb-OB six 'invcrtc-

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Wilsot! KA. aijl.. In.) pacts of the tuvasive- fusty .etaylish i'On»«&'in northern -W'isemsm lakes, PhD tlissertttion. Universiiy

Wright HE, l974:-'LaH£bitapii.tlcvclopinent-i forest fires, aagemwit. Science 1 Ws: 487- -95.

A paradigm shift in;ccok)g,y. 'Quai'Urly Rwiew-wf 'Biology TYarie JL, yicreck 'A.Vq^v'CJeveK.Adaros'R 1998. f'io«diMg and catsysteni dy-

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of land maiwgement and a recent (iurriauie on forest structtire and«Oin|X!sition ui the l,u(|uillo lixperinieittal Korcst, Puerto Riox forest Eeoi-ogy.and Management 77: 65-76.

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