Riparian Willow Communities on the Arctic Slope of Alaska and Their

Phytocoenologia 32 (2) 145 204 Berlin Stuttgart, June27, 2002

Riparianwillow communities onthe Arctic Slope ofAlaskaand their environmentalrelationships: Aclassification andordination analysis by Udo Schickho ff,Greifswald, Marilyn D. Walker, andDonald A. Walker,Fairbanks/Alaska with 16 figuresand 10 tables

Abstract. We present the first analysis ofriparian vegetation types onthe Arctic Slope of Alaska includingclassification andordination. We classified riparian willowcommunities according tothe Braun-Blanquetapproach, andanalysed environmental relationships of associations tocomplex environmental gradients using Detrended Correspondence Anal- ysis. We also examined synecological differences interms ofcommunity characteristics (e.g. growth form distributions, species richness, soil pHand climatic affinities, phyto- geographic patterns) tobetter understandecological alteration processes andchanging species compositionsalong successional gradients. Data were collected along atransect from the BrooksRange toPrudhoe Bay, primarily inthe watersheds ofSagavanirktok River andKuparuk River .Riparian vegetation inarctic Alaska mainlyconsists ofwillow shrubcommunities whichare functionally important componentsof arctic landscape ecosystems. Acombinationof edaphic conditions(soil pH,soil moisture) andfactors related totopography ,disturbance regime andlandscape evolution (river terrace/stream bank development) controls spatial patterns andfloristic compositionsof riparian plant communities. Classification resulted inthree associations andfour subassociations, each occu- pyingdistinct riparian habitats, and,thus, indicating distinct environmental conditions. TheE pilobio-Salicetumalaxensisass. nov.is atrue pioneer communityalong mountaincreeks (subass. polemonietosumacutiflori)andon gravel bars, floodplains andlower terraces ofrivers (subass. parnassietosumkotzebuei).Thistall willow association indicates sites withfrequent disturbances andcoarser-textured, relatively dry, initial alluvial soils withbasic reaction that have deep active layers andrelatively highsoil temperatures. Theassociation maypersist onriver banksas longas erosion anddeposition ofnew increments ofalluvium occurs, i.e. as longas predominantlyallogenic processes are operative insuccession cycles. It is characterized byrelatively lowspecies richness, considerable percentage ofNorth American endemics, higher amountof more thermo- philousspecies andcomparatively higher cover percentage offorbs. Higher terraces showthe paradoxonof better developed soils anddecreasing productivity ofthe shrublayer. With decreasing river influence andthe transition tofiner-textured, more nutrient-rich, less basic soils, the tall willowcommunity is replaced bythe Anemono-Salicetumrichardsoniiass. nov.(subass. lupinetosumarctici). De- creasing active layer depth,caused bythe insulation ofa thick moss layer andconse- quentlylower soil temperatures, as well as lower rootactivity androoting space, and higher soil moisture reduce the competitiveness of Salix alaxensis -stands andare thuskey factors for this successional replacement withlow willows. TheA nemono-Salicetum richardsoniicharacterizes later stages ofsuccession onriver alluvium withpredomi- nantlyautogenic processes resulting inter alia inan uniquely arctic soil thermal regime.

DOI: 10.1127/0340-269X/2002/0032-0145 0340-269X/02/0032-0145 $15.00 Ó 2002 Gebrüder Borntraeger,D-14129 Berlin ·D-70176 Stuttgart 146 U.Schickhoffet al.

Shrubsand mosses are dominantgrowth forms inthis association, whichis further characterized bycomparatively highest species richness. It also occurs onupland tundra stream banks(subass. salicetosumpulchrae),where environmental conditionsand floristic compositionpoint to a certain affinity tothe Valeriano-Salicetumpulchrae ass. nov. Thelatter association is distributed onolder ,long-deglaciated landsurfaces with paludified, loamy,acid soils withmassive groundice andthick moss layers, resulting in coldsoils, decreased depthof thaw, andincreased soil moisture. It typically occupies the immediate margins ofsmaller tundrastreams andcreeks witha specific streamflow regime (lowdisturbance level), originating inthe gentle topographyof the foothills. Water saturationof the soils lead toreduced decompositionof organic residues andthus to high contents oforganic matter.Percentage ofacidophilousspecies andof more cold-adapted species as well as moss cover andbryophyte species richness are comparatively very high. Thisterminal riparian vegetation typeseems tobe connected tolong-established hydrologic patterns andassociated riparian ecosystem evolution along headwaters inupland tundra, andhas developed inother time scales compared tothe above associations. Dueto synecological andphysiognomic correspondences andfloristic similarities in supraregional andcircumpolar perspective, NorthAlaskan riparian willowassociations can be assigned toexisting higher syntaxonomic units ofthe Braun-Blanquetsystem, established inEurope and Greenland. Strong affinities doexist toalluvial willowassociations ofthe Saliceteapurpureae.We proposeto extend the range ofthis class tothe NorthAmerican Arctic.

Keywords: Alaska, arctic, DCA,riparian, Salix,soil pH,soil moisture, succession, syntax- onomy.

1Introduction Global-scale research onarctic ecosystemshas been intensified consider- ablyin recent years.Research initiatives like the InternationalT undraEx- periment,the USArctic SystemsScience project,or the IGBP-Global Change Terrestrial Ecosystemsprogramme, greatly contributeto our still deficient understandingof arctic environments.The tremendoussurge in research onarctic systemsis largely motivated bypredictions that global warming isexpected tobe mostextreme inhigh latitudes andto result in relatively faster andgreater environmental changes (e.g. Fitzharris 1996; Oechel et al. 1997).In fact, recent climate patternsin the Arctic are generally consistentwith thoseprojected byglobal circulation modelsunder increased atmosphericCO 2 concentrations( Maxwell 1997).It follows, that,due to the highsusceptibility of the temperature-dominatedarctic systems (Billings 1997a),global change will have large effects onsea ice, snow,glaciers, permafrost,and tundra ecosystems, including riparian eco- systemsand adjacent riversand streams ( Oswo o d et al. 1992).T undraeco- systems,mainly characterized bythe qualitative andquantitative compositionof their vegetation types,are likely tochange (e.g. Epstein et al. 2000), beforetheir structureand function, having been relatively stable forthou- sandsof years, can beprecisely assessed. Therefore,there is an immediate need formore detailed vegetation studiesin the vast,remote circumpolar Arctic. Studieson the composition Riparianwillow communities on theArctic Slope of Alaska 147 andclassification ofplant communities as well as ontheir synecology,sym- morphology,synchorologyand syndynamics become increasingly important,but are missingfor many areas; they concentrated ona few relatively small regionsnear humansettlements andinfrastructure ( Walker, M. D. et al. 1994a).This holds true in particular forthe NorthSlope ofAlaska with onlyone permanent road (see below) inan area ofmore than 200,000 km 2, averylow populationdensity ,anda longand severe winter.Whereas the majority ofvegetation studiesin northernAlaska dealt with zonal tundra vegetation types,riparian plant communities have been ratherneglected so far.Inorder to accomplish amoredetailed inventoryof the NorthSlope’ s vegetation, thisstudy focusses on hitherto phytosociologically undescribed riparianplant communities. Since the vegetation ofthe NorthSlope is dominatedby perennial forbs, grasses,sedges, dwarf shrubs,mosses and lichens, taller willow shrublands along river marginsand streamsides form a prominentfeature ofthe tundra landscape.Riparian vegetation mustbe consideredan extremely important componentof arctic landscape ecosystems.Riparian shrublandsare the mostproductive arctic vegetation types( Shaver & Chapin 1991), they providestream bankstability ,andmay reach,together with floodplains,a considerable spatial extent (upto 20 %ofthe total landscape cover;cf. Muller et al. 1998).Moreover ,ripariancorridors play a vital role as reser- voirsof species diversityin arelatively species-poorenvironment ( Walker, M. D. 1995; Go uld & Walker 1997,1999; see also Naiman et al. 1993; Ward 1998 forgeneral considerationson riparianbiodiversity). Addition- ally,riparianshrublands provide organic matter forthe aquatic foodchain, andthey are ofprimaryimportance aswinter forage resource,cover ,nest- ing anddenning habitat forabundant wildlife inthe opentundra, including moose (Alces alces), caribou (Rangifertarandus ), muskox (Ovibosmoscha- tus)andbarren ground grizzly bear ( Ursushorribilis ) (cf. Mo uld 1977; White & Trudell 1980). Riparianvegetation ofthe NorthSlope predominantlyconsists of Salix- shrublands.Almost all riparianhabitats cutbanks,river bars,floodplains andlower terraces alongmajor rivers, upper terraces with furtherdevel- opedalluvial soils,margins of smaller uplandstreams and creeks, and sites along fast-flowing,gravelly creeks inthe mountains are occupied by different Salix-communities.Only on some microsites, e.g. locally along stream channels,along pools of beaded streamsor in flooded areas, minor riparianvegetation typeslike Carex aquatilis - or Carex rotundata -communities occur.This paper concentrates on Salix-communities,for which, like formost of the vegetation in northernAlaska, acomprehensivephytosocio- logical analysisis lacking upto now .There ishardly any detailed informationon the synecological characteristics oronthe floristic-sociological differentiation of Salix-communities.Preliminary observationsof riparian Salix-shrublandsof arctic Alaska were includedin the studiesof Hanso n (1953), Churchill (1955), Bliss & Cantlo n (1957), Spetzman (1959), Ko randa (1960), Cantlo n (1961), Drew & Shanks (1965), Britto n (1967), Hettinger & Janz (1974), Batten (1977), Peterso n & Billings 148 U.Schickhoffet al.

(1978,1980), Ko markova´ & Webber (1980), Walker,D.A.et al. (1982, 1989), Walker,D.A.(1985, 1987), Walker, D. A. & Acevedo (1987) and Walker,M.D. et al. (1989). Salix-shrublandsin western Alaska were describedby Hanso n (1951), Yo ung (1974), Racine (1976), Racine & Anderso n (1979) and Binkley et al. (1997). Mo o re (1982) investigated Salix alaxensis -standsalong the SagavanirktokRiver ,the major river inthe studyarea, with regardto floristics, structure and population biology .At the same river awillow shrubecosystem was incorporatedin ecosystem- level studiesof nutrient cycling, hydrology,plantbiomass, and primary production( Giblin et al. 1991; Shaver et al. 1996).Primary successionof Salix alaxensis -habitatsat Umiat, Colville River,was discussedby Bliss & Peterso n (1992) and Bliss (2000). Riparian vegetation andenvironmental conditionsof the headwaters of an uplandtundra stream (Imnavait Creek) were studiedin detail by Walker, D. A. & M. D. Walker (1996). Walker,M.D. et al. (1994b)pre- sentedthe firstapplication ofthe Braun-Blanquetapproach to the vegeta- tionof the Alaskan arctic foothills.They included in their surveytwo riparian plant communities ( Eriophorumangustifolium -Salix pulchra -community and Salix alaxensis -Salix lanata -communityresp.) without formally describingit according tothe Braun-Blanquetapproach. The onlyother application ofthis approach on the NorthSlope sofar resultedin publica- tions of Ko markova´ & Webber (1980), Ko markova´ (1981,1993) and Ko - markova´ & McKendrick (1988) onthe vegetation ofthe wet coastal plain at Atkasook.These papers,however ,donot contain the descriptionsof associations,which are notyet published.In the entire NorthAmerican Arctic there are veryfew studiesthat have usedthe Braun-Blanquetap- proachincluding unpublished dissertations of Lambert (1968), Gill (1971), Barrett (1972) and Odasz (1983),the studiesof Thannheiser (e.g.1984, 1987) andhis collaborators (e.g. Thannheiser & Willers 1988; Thannheiser & Hellfritz 1989) in the Canadian Arctic, and Co o per’s (1986) classification ofthe arctic-alpine tundravegetation ofthe Arrigetch Creek area inthe southernBrooks Range. The latter includesbrief descriptionsof different riparian Salix-communities.Recently , Go uld (1998) and Go uld & Walker (1999) analyzed the vegetation ofthe HoodRiver corri- dor(Northwest T erritories,Canada), applyingthe Braun-Blanquetap- proach.They described twenty-four communities includingseveral Salix- communities. Areinforcedapplication ofthe Braun-Blanquetapproach in the North American Arctic wouldbe highlydesirable inorder to lay the foundation fora circumpolarclassification ofarctic vegetation (cf. Dierssen 1996 for northernEurope) as well as forthe intendedcircumpolar arctic vegetation map (cf. Walker, D. A. 1995; Walker, D. A. et al. 1995; Walker, D. A. & Lillie 1997).The importance andecological relevance ofthe Braun- Blanquet syntaxain this respect is highlightedby Danie¨ls (1997).Since the international “Workshopon Classification ofArctic Vegetation”in Boul- der/CO,U.S.A.,1992,the completion ofa circumpolar phytosociological/ ecological synthesisof arctic vegetation isa main research focus( Walker, Riparianwillow communities on theArctic Slope of Alaska 149

M.D.et al. 1994a).Moreover, a circumpolar database ofwell-defined vegetation typeswith aconsistentnomenclature wouldbe an integral partof a hierarchic GISfor circumpolar ecosystemmanagement (cf. Walker, D. A. 1997).The useof accurately definedsyntaxa makes the combinationof vegetation studiesand ecosystem studies a loteasier. Itenables abetter extrapolation ofsite measuresof ecosystem fluxes anddynamics to larger areas,which isa uniqueopportunity in the Arctic dueto the pronounced continuityof its vegetation. Inview ofthe presentlychanging arctic envi- ronmentthe potentials ofa commonframework of arctic vegetation and ecosystemstudies with regardto circumpolar ecosystem management shouldbe completely exhausted. Inthis paper a classification andordination analysis of riparian shrub communities onthe NorthSlope ofAlaska ispresented. It is based on 85 releve´salong atransect fromthe BrooksRange toPrudhoe Bay andin- cludes the firstformal Braun-Blanquetdescriptions of riparian associations. The studyaims at analyzing the floristic-sociological differentiation ofex- tremely diverse streamside communities ( Walker,M.D.et al. 1994b)and at revealing their relationshipsto complex environmental gradientsat vari- ousscales. Itis abaseline studyof riparian vegetation ecologythat may serve as afoundationfor future assessments of status and dynamics of arctic environments.

2 Study area The studywas conductedalong a S-N-transectfrom the southernslope of the BrooksRange (EndicottMountains/ Philip Smith Mountains)to the arctic coast inthe vicinity ofPrudhoeBay/ Deadhorse.The transect follows the northernsegment ofDalton Highway,the onlypermanent road in the area (Fig. 1).This gravel access road(“ haul road”), completed in1974 after the discoveryof oil at PrudhoeBay ,parallels the Trans-AlaskaPipeline System(TAPS). Itcrossesthe major ecoregionsof northern Alaska (Brooks Range,Arctic Foothills,Arctic Coastal Plain; Gallant et al. 1995) and makes accessible arelatively undisturbedseries ofecosystems along a latitu- dinal-elevational gradient.It providesthe onlyopportunity for studying a ground-accessibletransect ofarctic andalpine tundrasof northern Alaska (cf. Brown & Berg 1980; Brown & Kreig 1983). The major portionof the transect lies within the drainage systemof SagavanirktokRiver ,the second-largestriver (267 kmlength),after the Col- ville, onthe NorthSlope ofAlaska ( Bo othroyd & Timso n 1983). The drainage systemhas its headwaters inthe northernBrooks Range, including AtigunRiver as the major tributaryalong the transect.Numerous smaller mountainand tundra streams and creeks flow intothe systemon its way tothe Arctic Ocean.Additionally ,the transect traversesheadwaters ofthe KuparukRiver basinin the Arctic Foothillsas well as headwaters ofthe Chandalar andDietrich Riverson the southernslope of the BrooksRange. 150 U.Schickhoffet al.

Fig. 1. Locationof the willowtransect innorthern Alaska. Riparianwillow communities on theArctic Slope of Alaska 151

2.1Physiography The BrooksRange, a northwesternextension of the RockyMountain Cor- dillera, consistsof rugged, east-trending, linear mountainranges, ridges, andhills. In the AtigunPass area (1550 mabove sea level), where the transect crossesthe BrooksRange, glacially sculpturedpeaks attain an elevation ofmore than 2200 m.The physiographyhas been considerably modified byrepeated late Tertiary andQuaternary glacial advances,indicated by numerouscirques at low altitudes,well integrated glacial-valley systems andabraded and streamlined landforms(see Hamilto n 1986 forfurther details onthe major glacial episodes).At present,only small glaciers persist at higherelevations. The mountainsare deeplydissected anddrained by north-and south-flowing rivers in flat-floored, U-shaped valleys. Northof the BrooksRange at Galbraith Lake the mountainsabruptly give way tothe rollinghills andplateaus ofthe Arctic Foothills,which are dissected bynumerous water tracks,beaded andmeandering streams,and partlybraided rivers. This ecoregion can betopographicallyseparated into southernand northern sections (cf. Gallant et al. 1995).In the southern section,elevations varyfrom 350 to1050 mandthe topographyis characterized byirregular buttes,knobs, mesas, east-trending ridges and interven- ing,gently undulatingtundra uplands. Small-scale differences in topogra- phyand landscape age resultfrom different glacial histories.Large areas are covered bydriftsheets of Sagavanirktok River glaciation (mid-Pleistocene), youngerlandscapes were deglaciated ca. 60,000years BP (Itkillik Iphase) andca. 10,000years BP (Itkillik IIphase),respectively (cf. Hamilto n 1986).The northernfoothills are markedby broad, rounded, east-trending ridgesand mesa-like uplands,reaching elevations of180 to350 m. Northof Sagwon,near the confluence ofthe Sagavanirktokand Ivishak Rivers,the foothillsecoregion gradesto the Arctic Coastal Plain, which slopesgently to the Arctic Ocean.This ecoregion ismainly asmooth, poorlydrained and marshy plain. However ,the flat topographyis locally brokenby pingos (ice-cored mounds),low hills,and occasional bluffs boundingriver terraces.The terrain ismarked by permafrost-related features,such as ice-wedge polygons,elongated andoriented thaw lakes,peat ridges,and frost boils. The SagavanirktokRiver becomes abraidedand distributaryriver inthe plain andbuilds a delta intothe Arctic Ocean.

2.2Climate, permafrost,soils, and hydrology The climate in northernAlaska is stronglyinfluenced bythe ocean throughoutthe year.Onthe basisof currently available data three major climatic zones(Arctic Foothills,Arctic Inland,Arctic Coastal) can be differentiated along the transect onthe NorthSlope ofAlaska (Tab.1). The Arctic Coastal zone,which extendsinland about 20 kmfrom the ocean,is characterized bycool summersand relatively warm winters,due to presence ofthe ocean,sea ice, andprevailing northeastwinds. The Arctic Inland zone,which extendsabout 100 kmalong the transect tothe south(from 152 U.Schickhoffet al.

Table 1. Climatic differentiation ofthe NorthSlope ofAlaska (adapted from Zhang et al. 1996). Arctic FoothillsArctic InlandArctic Coast Distance tothe ocean (km) 150 to300 200 to150 < 20 Elevation (m) 300 to 1000 50 to 400 < 50 Air temperature ( °C) Mean diurnal amplitude 10 to 15 8 to 14 4 to 8 Range (extreme low-high) -50 to +30 -65 to +35 -50 to +26 Mean annual -8.6 -12.4 -12.4 Annualamplitude 16.8 21.1 17.5 Degree-day ( °C-day) Freeze 4000 5300 4930 Thaw 800 930 420 Precipitation (mm) Snow 156 126 113 Rain 168 103 85 Annualtotal 324 229 198 Seasonal snowcover Average starting date 27 Sep. 1 Oct. 27 Sep. Range 11 Sep. to15 Oct. 19 Sep to12 Oct. 4Sep. to14 Oct. Average duration(days) 243 236 259 Range (extreme) 226 to 261 198 to260 212 to288 Average maximum thickness (cm) - 43 32 Range (extreme) - 28 to 70 10 to 83 Thaw season Average starting date 28 May 25 May 6 Jun. Range (extreme) 18 Mayto 15 Jun.28 Apr.to6 Jun.26 Mayto 19 Jun. Average length (days) 122 129 106 Range (extreme) 104 to 139 105 to 167 77 to 153 the coastal plain at Deadhorseto the northernfoothills near Sagwon),has considerablyhigher diurnal and annual amplitudes ofair temperature lead- ing toextremely coldwinters andrelatively warm summers. The climate ofthe Arctic Foothillszone (extending fromSagwon to the southinto the BrooksRange) is markedby higherprecipitation andmuch higherwinter temperatures.Increases in temperature andprecipitation mustbe attributedto the higherelevation ofthe foothills,which isabout 900 monaverage inthe southernpart. This is close tothe average height ofthe topof the atmospherictemperature inversionlayer onthe Arctic Slope ofAlaska. Asa resultof the weaker atmospherictemperature inver- sion,mean annual air temperature inthis zone is about 4 °Cwarmer than inthe twozones to the north,although summer temperatures are slightly cooler thanin the Inlandzone (cf. Zhang et al. 1996).T otal annual precipitation increases to640 mm at AtigunPass (1550 m) anddecreases again to 300 450 mm onthe southernslope of the BrooksRange ( Haugen 1982). Asa consequenceof below-freezing mean annual temperaturesand the longperiod of winter cold,the Arctic Slopeis underlainby continuous Riparianwillow communities on theArctic Slope of Alaska 153 permafrost.Active layer thicknessesvary roughly between 30 and70 cm, andincrease fromthe coast tothe Arctic Foothills ( Zhang et al. 1997).The largest thaw depthsoccur along major rivers( Nelso n et al. 1997).Soils of the NorthSlope are thuspermafrost-affected, i.e. their genesis is dominated bycryopedogenicprocesses (freeze-thaw cycles, cryoturbation,ice segrega- tion,frost cracking etc.) resultingfrom cold soil temperatures. Other soil- formingprocesses, such as podzolizationor gleyzation, may be present, butthey are ofsecondary importance. The majority ofthe soilsare poorly drained,have an upperorganic horizonof variable thickness,and have developed onfine-textured materials, silt loams andsilty clay loams.A few moderately well-drained andwell-drained gravelly soilsoccur on ridges andon terraces marginal tothe larger rivers.The soilscan be broadlydifferentiated intocalcareous andacidic soils(cf. Bo ckheim et al. 1998).Apart fromhigher soil pHand concomitantly greater amountsof extractable Ca andMg, and a higherbase saturation,soils of moistnonacidic tundradiffer fromsoils of moistacidic tundrain some other key properties. They have thinnerorganic horizons,a significantly thicker active layer,andgreater cryoturbation.These pergelic nonacidic andacidic tundrasoils, formerly describedas uplandtundra soils and meadow tundrasoils by Tedro w (1977),comprise ca. 70 %ofthe NorthSlope’ s landcover.Like minorsoil types,such as lithosols,regosols, arctic brownsoils, and bog soils, they are nowclassified according tothe recently proposedand adopted Gelisol orderof the USSoil Taxonomy( Bo ckheim et al. 1994). Due tothe presence ofpermafrost as an impermeable layer,the hy- drologiccycle is characterized bya veryshallow subsurfacesystem with a severely limited water storagecapacity .All water leaves the drainage basins either throughrunoff or evapotranspiration.Every spring a major,dramatic ablation runoffevent can be observed( Kane et al. 1989; Everett et al. 1996),that leads toextreme differences between seasonal maximum and minimum streamflow rates.E.g., peaks of 439 and566 m 3 sec 1 during springfloods contrasted with winter flowsof only 0.4 m 3 sec 1 in the SagavanirktokRiver (downstreamof Lupine River) in1971 and1972 (Craig & McCart 1975). Craig & McCart classified stream typeswith different streamflow regimes onthe NorthSlope according tothe originof flow.Theircharacteristics are summarizedin T ab.2. In tundrastreams and creeks,originating in the foothillsand coastal plain,flow peakssharply duringthe snowmelt periodin late May,andfalls offdramatically forthe remainder ofthe summer.Freezeup occursin September.In larger rivers like the SagavanirktokRiver ,originating inthe BrooksRange (mountain streams),flow decreases moregradual after springbreakup before ap- proachingzero in fall. Some flow may continuethrough unfrozen riverbed gravels duringwinter. Furthermore, the hydrographof mountain streams reflects higherintensity of rainfall events inthe BrooksRange throughout the summerseason. As will be discussedbelow ,different streamflow regimes andrelated disturbancesare critical forunderstanding riparian vegetation ecology. 154 U.Schickhoffet al.

Table 2. Characteristics ofmountainstreams (originating inthe BrooksRange) andtundra streams (originating inupland tundra), based ondata from 55 locations in17 Beaufort Sea drainages (adapted from Craig & McCart 1975). Features Mountainstreams Tundrastreams Physical andchemical Surface flow late May tomid-October late Mayto mid-September Groundwaterflow minimal none Summer discharge (m 3/sec) 0.3 100+ 0.1 7+ Temperature ( °C) Summer 10 (4 15) 10 (5 20) Winter 0 1 or frozen frozen Annualvariation 10 17 Color clear/turbid stained pH 8.0 (7.0 8.5) 7.6 (6.4 8.5) Conductivity(umhos/ cm) 176 (78 285) 116 (17 230) Ca (mg/l) 28 (16 37) 9 (3 16)

Benthic invertebrates Standing crop(no./ m 2) 100 1000 Relative diversity low moderate

Fish Most abundantspecies Arctic char grayling (Salvelinus alpinus ) (Thymallus arcticus )

2.3Flora andvegetation The circumpolararctic flora consistsof about 900 vascular species (ca. 0.4%ofall vascular plantson Earth),distributed over a landarea ofca. 13 million km2 oralmost 15%ofthe earth’s land surface ( Billings 1997b). Consideringthis relatively small flora,the NorthSlope ofAlaska is relatively richfloristically . Wiggins & Tho mas (1962) and Hulte´n (1968) listed 450 500 vascular species occurringon the NorthSlope about half ofthe entire arctic flora.The relatively highplant diversityis mainly due tothe dynamic landscape historyof Beringia (the area between Lena and Mackenzie rivers)during the late Tertiary andQuaternary (cf. Ho pkins et al. 1982).During glacial maxima andconcomitant periodsof lower sea levels, land connectionsbetween Asia andAmerica providedan immigra- tionroute for a hugeamount of Asiatic taxa, includingsteppe, forest and tundraspecies. Moreover ,large areas ofBeringia remained unglaciated throughoutthe Pleistocene andserved as refugia forthe regional arctic flora.The Beringian species poolwas furtherextended bytaxa that returned fromareas southof the continental icesheets as well as bynewly evolved taxa ofPleistocene andHolocene age ( Murray 1992,1995). Approximately half ofthe recent NorthSlope flora consistsof Beringian species,the other half hascircumpolar distributions. Riparianwillow communities on theArctic Slope of Alaska 155

According torecent phytogeographiczonations of the Arctic, the study area falls within the SouthernT undraZone of Matveyeva (1998), the SouthernArctic TundraZone and Arctic ShrubT undraZone proposed by Elvebakk et al. (1999),and within the Erect Dwarf Shruband Low Shrub Subzonesproposed by Walker,D.A.(2000) respectively.The vegetation ofthe NorthSlope can be broadlydifferentiated intolowland tundraon the coastal plain andupland tundra in the Arctic Foothillsand mountains ofthe BrooksRange. Lowland tundra is predominantlya wet sedge-moss tundra,upland tundra mainly consistsof tussock tundra ( Walker, M. D. 1995),a widespread vegetation typeon imperfectly drainedlands that is also commonin adjacent uplandregions of northwestern Canada and northeasternRussia ( Bliss & Matveyeva 1992; Chernov & Matveyeva 1997). Walker,M.D.et al. (1994b)published the hithertomost detailed account ofthe tussocktundra vegetation ofthe NorthSlope. They de- scribedthe Sphagno-Eriophoretumvaginatias the zonal vegetation onmesic, acidic slopesthroughout the foothills,dominated by the tussock- formingsedge Eriophorumvaginatum ,low shrubs( Betula nana, Rubus chamaemorus )andmosses ( Sphagnum spp., Hylocomiumsplendens ). Its non-acidic counterpartis the Dryadointegrifoliae-Caricetumbige- lowii,which is widespreadon circumneutral mesic uplandsand hillslopes, primarily inyounger landscapes. Mountain slopes of the BrooksRange are covered by Betula-Salix-shrubtundra, giving way to Dryasoctopetala - dominateddwarf shrubcommunities at higherelevations (1100 1200 m a.s.l.).

2.4Human impact Much ofarctic Alaska still consistsof relatively pristinetundra and riparian ecosystems,only slightly modified byanthropogenic disturbances. As far as ripariansystems are concerned,Alaska can still betermed a“warehouse ofpristine running water systems”( O swo o d 1997).All riversare free- flowing,unregulated rivers; most river corridorsare undisturbedover the whole ofthe river continuum,from headwaters tomouth. Although arctic ecosystemshave been relatively stable forthousands of years, they are very susceptible toanthropogenic interferences (e.g. Billings 1997).This sus- ceptibility ismainly dueto a shortgrowing season, low temperatures,low primaryproductivity ,the presence ofpermafrost,and the extreme sensitiv- ity ofthe vegetative andorganic surface layers toany disruption of their physical integrity andthermal regime ( Reyno lds & Tenhunen 1996). Commonanthropogenic disturbances on the NorthSlope resultfrom energydevelopment, following the discoveryof oil at PrudhoeBay in 1968, andconcentrate along the TransAlaska Pipeline haul roadand in the coastal plain oil fields respectively.Impactsof energydevelopment include disturbance ofrivers and riparian vegetation bygravel miningand crossings, re- moval andmodification ofvegetation byroad and drilling padconstruction andoff-road vehicle trails,snow and ice roadson tundrafor winter trans- portation,dust loads on vegetation fromoil drilling operations,mining 156 U.Schickhoffet al. activities, andtruck traffic, andother impacts suchas oil spills,road salting, surface mining,spoil siting, thermal erosion,and waste disposal( Walker, D.A.et al. 1987; Walker,D.A.1996).During construction of the pipeline, roads,worksites and camps, large areas of Salix-shrublandsin the Atigun andSagavanirktok river valleys were destroyed,mostly through shallow miningof vegetated river barsfor gravel. 23%oftall (>1m) Salix alaxensis - standswere disturbed;subsequent restoration of these habitats was only partially successful( Densmo re et al. 1987). Additionally,large-scale indirect impacts fromsources at lower latitudes, includingclimate change,ozone loss, air pollution,and bioaccumulation ofheavy metals, pesticides andPCBs, adversely affect the NorthSlope’ s environment.Effects ofthese impacts are being increasingly detected in the Arctic (e.g. Braune et al. 1999;see Yo ung & Chapin 1995; Hansell et al. 1998 foreffects onbiodiversity). This illustrates the pervasivenessof global environmental threats,and shows that remote Arctic Alaska can nolonger beconsideredan untaintedwilderness.

3 Methods

3.1Field sampling Inorderto cover the full variety ofriparian shrublandhabitats between the BrooksRange andthe coastal plain,study sites were selected along rivers andstreams of different orders;in total 85 releve´swere completed according tothe Braun-Blanquetapproach ( Braun-Blanq uet 1964),which has become afairly standardizedmethod (cf. Mueller-Do mbo is & Ellen- berg 1974; Dierschke 1994).The southernmostreleve ´ was spaced at Diet- rich Creek (68 °029 N, 149°399 W),justnorth of the arctic treeline onthe southernslope of the BrooksRange, the northernmoststudy site was at the SagavanirktokRiver (braidedsection ofdelta plain) in the vicinity of Deadhorse(70 °119 N, 148°269 W). Phytosociological andenvironmental data collection was conductedby the firstauthor during the periodof 9 July to17 August1997. After a reconnaissance ofthe studyarea locations ofsample plotswere carefully selected alongthe transect inorder to fulfill the requirementsof homo- geneity andminimal area. Sample plotswere ofsquare or rectangular shape. Representative samples ofthe Salix-communitiesrequired minimal areas between 50 m2 (low shrublands)and 100 m 2 (tall shrublands).After estab- lishinga sample plot,height andactual cover ofthe separate vegetation layers (shrub,field, moss,and lichen layer) were measuredor estimated. A detailed inventarisation oftaxa followed,including all vascular,bryophyte, andlichen species.Species cover was estimated according tothe traditional Braun-Blanquetcover-abundance scale (7 classes).A voucherspecimen of each species was collected onthe releve´ sites forfinal identification inthe herbaria ofthe Universityof Alaska at Fairbanks/AK( Salix spp.)and of the Universityof Colorado at Boulder/CO.Nomenclature ofvascular plants follows Argus (1973) for Salix spp. and Hulte´n (1968) formost of the Riparianwillow communities on theArctic Slope of Alaska 157 vasculars.For recent revisionswe consulted Co dy (1996).Mosses, hepatics andlichens are named according to Steere (1978), Steere & Ino ue (1978) and Tho mso n (1979,1984) respectively. Since vegetation isunderstood as aplant community-habitat-system (phytogeocoenosis)sensu Glavac (1996),vegetation samplingwas comple- mented bya detailed characterization ofhabitat conditions.Altitude was measuredwith aThommenaltimeter. Informationon slopeand aspect was negligible inmost riparian habitats, but was notedwhen consideredto be important.Estimates ofa numberof habitat factors,such as frequencyof flooding,flow speed/discharge regime ofrunningwater ,soil moisture,site moisture,snow cover ,%cryoturbation,and animal disturbance,were car- riedout in decimal scales (1 10 or 1 4) inorder to ease the intended transferof data tothe ordinationmatrix. Additionally ,the vertical distance tothe water table was measured,and information on the locality,onthe entire size ofthe sample stand,on microtopography and geomorphic processes,and on the surroundingvegetation typeswas noted.A soilpit was dugon each releve´ site.Soil profileswere describedaccording to Schlich- ting et al. (1995) with regardto depth of active layer,thickness and type ofhorizons, color (Munsell Soil ColorCharts), texture, humus contents, moistureconditions, and root patterns. T obeinaccordance with previous studiesin the region,soil samples (3 ´ 100 cm3 cylindersamples) were collected from10 cm depthin each soilpit. Fresh field samples were oven- dried at 105 °Cfor72 hin camp (ToolikField Station) todetermine percentage weight lossand soil moisture.Laboratory soil analyses (carried outin the Soil,W ater andPlant Testing Laboratory,ColoradoState University, FortCollins/ CO) comprisedsoil pH (saturated paste method),EC, lime estimate, %organic matter,NO 3 N,plant available P,K,Zn,Fe, Mn,Cu, and% gravel, sand,silt, and clay .

3.2Classification and ordination Vegetation was classified according tothe Braun-Blanquetsorted table method,i.e. the releve´swere arrangedin phytosociological tables todif- ferentiate andcharacterize associationsand subassociations ( Mueller- Do mbo is & Ellenberg 1974; Dierschke 1994).On the basisof field im- pressions,the 85 releve´swere arrangedin two separate raw tables.All recordsof tall shrublands( Salix alaxensis -stands)were placed inone table andall recordsof low shrublandsin another.Table workwas carried out bymeans ofa commontable calculation program(Microsoft Excel), where it is easy (i) toobtain bipartite ortripartite divisionsin each table bydrag- ging anddropping rows and columns and (ii) at the same time tofully realize the classification procedurestep by step.An intimate knowledge of the data structurecan be attained thisway .Itis recommendedfor smaller , manageable data sets.Differentiations ofvegetation unitsare basedon diag- nosticspecies (character species,differential species,and constant companions).Determinations ofdifferential species as well as assessmentsof de- grees offidelity ofcharacter species followed the criteria proposedin 158 U.Schickhoffet al.

Dierschke (1994).T abular presentationand nomenclature ofthe described syntaxaare inaccordance with Weber et al. (2000). Inorder to analyze relationshipsbetween variation invegetation and environmental variation,Detrended Correspondence Analysis (DCA) or- dinationswere carried outusing PC-ORD (3.0 for Windows) program (McCune & Meffo rd 1997).DCA was considereda moreappropriate technique thana constrained(direct) ordinationlike Canonical Correspon- dence Analysis(CCA) because the focuswas ongeneration, not on testing ofhypotheses about vegetation-environment relationships, i.e. the problematic dependencyof constrained ordination axes onrecorded environmental variables shouldbe avoided(problematic regardingan unbiasedin- terpretationof all explanatoryvariables ofcomplex-gradients;cf. Økland 1996).Performing DCA, all species andenvironmental data were used. Rare species were downweighted;axes were rescaled basedon program defaults.DCA producesfirst axes showingmajor directionsof variation in the data andrevealing the relationshipof the classification tomajor envi- ronmentalgradients. The latter is consideredvery important in the present study.Classification andordination were perceived as interactive, comple- mentaryprocedures. E.g., preliminary assignments of particular releve´s to subassociationsduring table workcould be later revised andcorrected ac- cordingto positions of samples inthe ordinationspace. DCA ordinations were comparedwith parallel applications ofother ordination techniques available inPC-ORD (e.g. Bray-Curtis, CCA), usingthe same data set. Comparable ornearly congruentconfigurations of ordination diagrams indicated the appropriatenessof DCA forthis general-purpose ecological study. The communitytables provideda valuable data base forfurther analyses at communitylevel, includinginquiries into species richness(alpha diversity),growth form distributions (shrubs, forbs, graminoids, mosses, lichens),geographic range (circumpolar,North American, NorthAmerican- Asian distributions)and physiographic unit (arctic, arctic-alpine, arctic boreal distributions)analyses, and investigations intoaffinities toclimatic zonesand soil pH. Regarding climatic affinity,we assessedthe northern limits ofspecies areas andcorrelated them toclimatic zonesdeveloped by Yo ung (1971),based on the sumof mean monthlytemperatures above 0 °C.Regardingsoil pHaffinity ,we followed the methodof Elvebakk (1982) togroup species intoone of six categories ofpH values (considering mean andrange, within a95%confidence interval) basedon all releve´ sites where aspecies occurred.

4 Results Classification of Salix-shrublandsresulted in three associationsand four subassociations.These communitytypes are notonly marked by their characteristic species combinations,but also by distinct habitat conditions (Tab.3). The floristic differentiation ofthe communitytypes is clearly reflected inthe ordinationdiagram ofall releve´s,even onsubassociationlevel Riparianwillow communities on theArctic Slope of Alaska 159

Table 3. Mean values ofselected environmental variables for the differentiated associations andsubassociations. subass. subass. subass. subass. Selected Epilobio-parnassie- polemonie-Anemono- lupine- salice- Valeriano- environmental Salicetum tosumtosum Salicetum tosumtosum Salicetum variables alaxensis kotzebuiacutiflori richardsonii arctici pulchrae pulchrae Soil pH(paste) 7.2 8.8 6.6 6.6 6.8 5.8 5.0 Soil moisture (vol. %) 24.1 25.4 23.2 47.1 45.1 53.1 61.7 Bulkdensity (g/cm3) 1.4 1.3 1.5 0.9 1.0 0.7 0.5 Gravel (g/100 cm3) 26.0 3.4 41.6 0.6 0.9 0.0 0.6 % Sand 70.4 66.0 73.5 49.0 50.1 45.8 41.2 %Silt 22.2 24.2 20.9 37.5 36.2 41.6 32.7 % Clay 7.3 9.8 5.6 12.9 13.0 12.7 15.0 %Org. matter 3.9 1.9 5.3 13.5 12.9 15.3 24.2 NO3 N (ppm) 2.1 2.0 2.1 3.0 3.0 3.1 3.2 P (ppm) 4.6 5.1 4.2 8.5 9.6 5.3 8.1 Mn (ppm) 98.0 16.0 154.4 136.9 72.8 322.0 514.6 Fe (ppm) 53.2 48.4 56.6 232.0 214.6 282.2 594.8 Flowspeed index 2.1 1.0 2.9 1.6 1.2 2.8 2.3 Vertical distance water table (m) 0.8 1.1 0.6 1.7 2.0 0.9 0.5 Moss layer (cm) 1.8 0.6 2.5 8.4 7.4 11.2 12.3 n 27 11 16 35 26 9 23

(Fig. 2).Actually ,thisordination diagram can beconsidereda graphic rep- resentationof the similarity structureof acombinedphytosociological table ofall releve´s.Each ofthe communitytypes occupies adistinct range within the ordinationspace. Thus, the ordinationresults corroborate the resultsof the classification. Aconsiderablynarrow range is occupied bythe Vale- riano-Salicetumpulchrae,indicating afloristically veryhomogeneous vegetation typewith acomparatively narrowecological amplitude.In contrast,the Anemono-Salicetumrichardsoniiandthe Epilobio-Sali- cetumalaxensisshowa muchmore heterogeneous species composition andoccur over a broaderrange ofenvironmental conditions.As a conse- quence,both associations can be furtherdifferentiated intotwo subassociations. The diagram representsnot only the floristic similarity structure,but also indicates relationshipsof releve´sandcommunities tothe mostimpor- tant environmental gradients.Axis 1correspondsto a complex edaphic gradient primarilyrepresenting soil pH and soil moisture.Releve ´s of moist acidic stream banksare concentrated inthe rightcorner of the diagram, whereas thoseof edaphically drier,nonacidic sites increase inabundance towardsthe left side.V ertical distance tothe water table andfrequency of 160 U.Schickhoffet al. . a e r a y d u t s e h t n i s n o i t a i c o s s a b u s d n a s n o i t a i c o s s a x i l a S f o n o i t a n i d r o A C D . 2 . g i F Riparianwillow communities on theArctic Slope of Alaska 161 floodingshow highest correlation with axis 2,which hasto be interpreted as acomplex gradient ofriver terrace/stream bankevolution or successional gradient with releve´sofyoung, gravelly mountainstreamsides or floodplain sites ofrivers in the upperhalf ofthe diagram andreleve ´sofhigher river terraces with better developed alluvial soilsin the lower half.However ,this interpretationis only valid forthe Epilobio-Salicetumalaxensisand Anemono-Salicetumrichardsoniionriver alluvium. Releve´s of head- water stream bankson oldland surfacesin uplandtundra, mainly belonging tothe Valeriano-Salicetumpulchrae,donot fit intothis successional scheme since theyhave developed in different temporal scales. The influence oflandscape history(deglaciation ages) onriparian vegetation differentiation will be discussedbelow .Releve´ positionsof the Valeriano-Sali- cetumpulchraealong the vertical axis mainly reflects the intermediate positionin terms of height above river/stream water level andassociated floodingfrequency . The ordinationdiagram ofall releve´s(Fig. 2) isplaced togetherwith the correlation coefficients ofenvironmental variables with species axes (Tab.4)

Table 4. Correlation coefficients (Pearson’s r) ofenvironmental variables withspecies axes derived byDCAordination. All Salix samples (N =85) Axis1 Axis 2Axis3 Eigenvalue .043 .026 .143 Gradient length 2.85 2.65 1.99 Soil pH -.871 .152 -.188 Ca -.718 .006 -.147 Mn .668 .035 .026 Soil water .634 .407 -.228 Bulkdensity -.629 .376 .281 Moss layer .596 -.349 .013 Soil organic matter .568 .265 -.067 Flow speed .462 .384 .289 Vertical distance water table -.442 -.627 .003 Frequencyof flooding .436 .491 -.261 Fe .435 -.107 .030 Sand -.409 .320 .060 K .390 .089 .281 Silt .345 -.286 .179 Zn .318 -.003 -.092 Altitude .312 .377 .574 Electric conductivity .306 -.189 .003 Clay .276 -.168 .142 Snow cover .243 .393 .478 NO3-N .233 -.168 -.031 Gravel -.158 .372 .550 P .120 -.201 -.011 Cu .113 -.512 -.067 162 U.Schickhoffet al. . a v o n . s s a s i s n e x a l a m u t e c i l a S - o i b o l i p E f o e l b a t y t i n u m m o C . 5 e l b a T Riparianwillow communities on theArctic Slope of Alaska 163 ) . t n o c ( . 5 e l b a T 164 U.Schickhoffet al. ) . t n o c ( . 5 e l b a T Riparianwillow communities on theArctic Slope of Alaska 165 at the outsetof the resultsto give afirstoverview ofcommunity type differentiation andvegetation-environment relationships as well asto ease understandingof the following discussions.

4.1Epilobio-Salicetum alaxensis Schickhoff et al.2001 ass.nova Nomenclatural typereleve ´:Tab.5, rel. 63 Thistall willow shrubassociation occupies two distinct riparianhabitat typesalong the transect:It occurs on floodplains, gravel barsand lower terraces ofthe SagavanirktokRiver as well as onupland and montane stream banks.Along the SagavanirktokRiver tothe north,this community is well developedup to the HappyV alley area inthe northernfoothills section (Fig. 3);however ,the name-giving taxon,feltleaf willow ( Salix alaxensis),occasionally occursfurther to the north,where the individualsbe- come less vigorousand the shrubstature is reduced. T owardsthe coast,the ecological conditions(climate, depthof active layer) become unfavorable forthe development ofthe Epilobio-Salicetumalaxensis:Itis replaced incomparable habitats bylow willow stands(A nemono-Sali- cetumrichardsonii;see below).The othermain distributionarea ofthis tall willow communitycomprises upland and montane stream banks.It occupies in particular gravelly banksof fast-flowing creeks in the Brooks Range,where it is well developedup to an elevation ofca. 1200 ma.s.l.

Fig. 3. TheE pilobio-Salicetumalaxensis(subass. parnassietosumkotzebuei)is widelydistributed onfloodplains, gravel bars andlower terraces along the Sagavanirktok River; phot.U. Schickho ff,26.07.1997. 166 U.Schickhoffet al. beforeit gives way topioneer herb communities. Occasionally ,it occurs along uplandtundra streams in the southernfoothills, where it is largely restricted togeomorphologically active sites onyounger land surfaces. Withinthe “Alaska Vegetation Classification”( V iereck et al. 1992),this association falls intothe categories “closed tall willow shrub”and “ open tall willow shrub”respectively . The habitats ofthe Epilobio-Salicetumalaxensisare pioneerhabitats.Along a gradient ofriver terrace/stream bankevolution (cf. Fig. 2), thisassociation occupies the least developed sites,e.g. gravelly creek margins,bare gravel barswith initial sandand silt accumulation orlower terraces with well tomoderately drained,coarse-textured soils. T ab.3 shows the comparatively highgravel content,the predominantsand fraction (70 %),the highbulk density ,andthus the relative edaphical drynessin the soilsunder this association. The soilsare generally non-acidic with amean soil pHof 7.2. They are furthercharacterized bylow available nitrogen,a verylow organic matter content,high base saturationand cation exchange capacity.Floodplainand lower terrace soilsshow a verydeep active layer ofabout1 min late July.Thisprovides large rootingspace as well as favorable soiltemperatures for the tall willows, what inturn is reflected ina higherbiomass and a highernet annual productioncompared to the low willow standson upper terraces (see below).It is evident fromthe above habitat descriptionthat the Epilobio-Salicetumalaxensisis atruepioneer community. Thispioneer association is markedby a characteristic species combina- tionincluding character anddifferential species as well as awhole arrayof companionswith highpresence (see Tab.5). There are fourcharacter species, all showingpronounced pioneer character .The amphi-Beringian, thicket-formingfeltleaf willow isable tocolonize bare gravels,accelerates the depositionof sand and silt, and thus facilitates furthersuccession (Bliss & Peterso n 1992).Large amountsof early summerseeds, having nodormancyrequirements, can germinate onnewly exposedsubstrate after the floodwatersfrom spring thaw subside(cf. Gill 1971; Argus 1973; see also Eberso le 1985 forthe capability torapidly colonize disturbedareas). Moreover, Salix alaxensis developsadventitious roots readily sothat growth ofsproutsfrom buried branches often produces dense stands (cf. Viereck 1970; Zasada & Densmo re 1978). Likewise, the circumpolar,arctic-alpine species Epilobium latifolium (syn. Chamaenerion latifolium )is amongthe firsttaxa tocolonize gravel barsand stream banks(cf. Mo o re 1982).E.g., it formsdistinct communities along glacier streamsin SE-Greenland ( LÜnterbusch et al. 1997).It showshigher presence degree andhigher cover along gravelly creeks in the mountains,whereas the amphi-Beringian Astersibiricus has its main distributionalong lowland river bankson sandy-loamysoils. The above describedtwo distinct habitats ofthe association andthe concomitant diverging species combinationresults in adifferentiation oftwo subassociations: Riparianwillow communities on theArctic Slope of Alaska 167

1) Epilobio-Salicetumalaxensis parnassietosumkotzebuei Schickhoff et al.2001 subass.nova Nomenclatural typereleve ´:Tab.5, rel. 62 Thisis the subassociationof floodplains and lower terraces oflowland rivers(T ab.5; rel. no. 59 69).It is on these habitats where the association hasits optimumecological conditions,and where thusmost extensive and productive Salix alaxensis -stands (3 4mtall) can be found.Accordingly , shrubsare the mostprominent growth form, reaching the highestpercentage onthe total sumof plant coveramong all differentiated communities (Fig. 4).Species richnessof vascular plantsand mosses is higher,too (Fig. 5).Habitat conditionsdiffer frommontane stream bankse.g. in higher summertemperatures, a muchdeeper active layer,aslightlyhigher soil moisture,more fine-textured, deeper soils(sandy loam) with muchlower gravel content,and a considerablyhigher soil pH (cf. T ab.3). As evident fromthe DCA ordination(Fig. 6),soil pHis the decisive ecological factor fordifferences infloristic compositionof both subassociations. There isa regular supplyof base cations duringevery springbreakup when the low- landriver banksare floodedwith water flowing downfrom the Brooks Range andits foothills.Due toprevailing limestone bedrockin the headwa- tersthis water ismoderately hardand rich in calcium andmagnesium ions (Craig & McCart 1975; Kling et al. 1992).The sedimentation onlowland river banksof the suspendedinorganic loadof the annualspring flood providesregular mineral nutrientsupply and thus contributes to the productivityof Salix alaxensis -standsof these habitats.

Fig. 4. Weighted growth form spectra of Salix-communities. Percentage ofeach growth form was calculated onthe basis oftransformed cover/abundancevalues ofspecies in releve´s(transformed intoaverage cover percentage according to Ellenberg 1956) in order toreflect actual dominanceconditions. 168 U.Schickhoffet al.

Fig. 5. Species richness of Salix-communities.

The compositionof differential species is afine expressionof the above describedhabitat conditions.Most of the taxa have their main distribution onsandyor gravelly soilsalong stream andriver banks,e.g. Parnassia kotzebuei, Equisetumvariegatum ssp. variegatum, Hedysarumalpinum ssp. americanum, Astragalusnutzotinensis or Elymus alaskanus .Moreover,the vast majority ofdifferential taxa indicates calcareous substrataand basic conditions.Compared to the other Salix associations/subassociations,this subassociationshows the highestamount of basiphiloustaxa (Fig. 7).Along the successional gradient tohigher terraces andupland tundra streamsides the percentage ofspecies defined as basiphilousto circumneutral consider- ablydecreases, whereas weakly acidophilousand acidophilous taxa showa distinct increase. Withinthis subassociation a variant couldbe differentiated (Tab.5; rel. no. 59 65) that occurson higher ground (1.0 2.3m above water table) with slightlydrier and coarser-textured soils, i.e. with ahighercontent of sandfraction andless silt. On these sites, Salix alaxensis isvery vigorous andreaches upto3.5 m inheight. The herblayer showshigher cover and is morespecies-rich. This variant of Castilleja caudata is representedby differential taxa, which are mostlyendemic toNorth America ( Shepherdia canadensis, Zygadenuselegans , Leymus velutinus , Hedysarummackenzii , Carex concinna )orwhose distribution extends a bit furtherto NE-Asia (Castilleja caudata , Anemoneparviflora ).The species compositionmight pointto a lower competitiveness ofcircumpolar taxa underthese specific habitat conditions,and/ orit might be the resultof symphylogenicdevelop- mentsamong taxa ofamoreor less commonevolution in space andtime. Actually,the Salix alaxensis -associationcomparatively showsthe highest amountof North America endemics (Fig. 8).This percentage sharply decreases alongthe successional gradient,whereas the percentage ofcir- cumpolartaxa increases.The latter taxa are obviouslyfavored by environ- Riparianwillow communities on theArctic Slope of Alaska 169

Fig. 6. DCAordinationof Epilobio-Salicetumalaxensis. mental conditionsin upland tundra, similar tovast circumpolar regions. The highercompetitiveness andrichness of endemics inpioneer communities hasserious implications fornature conservation: The destructionof Salix alaxensis -habitats(as happenedin the courseof Dalton Highway buildingon the Arctic Slope) endangersa particularly valuable component ofAlaskan biodiversity.

2) Epilobio-Salicetumalaxensis polemonietosumacutiflori Schickhoff et al.2001 subass.nova Nomenclatural typereleve ´:Tab.5, rel. 75 Thissubassociation (T ab.5; rel. no. 70 85) occursprimarily on gravelly montanestream banksin the BrooksRange upto an elevation ofc.1200 m a.s.l.(Fig. 9).Compared to lowland river banks,stand productivity and vigor of Salix alaxensis is reduced.On the onehand, this must be attributed 170 U.Schickhoffet al.

Fig. 7. pHaffinities for vascular species of Salix-communities. Species were placed into oneof six affinity groups(B =basiphilous, CB=circumneutral tobasiphilous, C= circumneutral, CA=circumneutral toacidophilous, wA = weakly acidophilous,A = acidophilous)by determining mean pHvalue anda range ofpHvalues (based on95 % confidence intervals) ofall occurrences for each species according to Elvebakk (1982).

Fig. 8. Geographic range analysis for vascular species of Salix-communities. tolower summertemperatures and a shortervegetation period.On the other,mineral nutritionand hygric conditions in the shallow active layer are moreunfavorable (cf. T ab.3). Much highergravel contentand higher bulkdensity in the coarser-texturedsoils (loamy sand) along mountain creeks lead toedaphically drierconditions, although vertical distance tothe water table isconsiderably reduced. Mean soil pHdecreases to6.6, i.e. lower base saturation,lower cation exchange capacity,andslight acidifica- tionof the topsoilis indicated. Basiphilousspecies percentage sharply Riparianwillow communities on theArctic Slope of Alaska 171 decreases (cf.Fig. 7).This superficial acidification is enhanced bythe fact, that the annual springflood in the headwaters is mainly restricted tothe stream/creek bedsor flows over still frozenground, and that perpetual allogenic mineral supplyis notgiven. Due tohigher litter fall (highercover ofherb and moss/ lichen layer) andslower decomposition rates inthese higher-elevated habitats,the soilorganic matter contentis higherthan along lowland rivers. Interestingly,the majority ofdifferential taxa ofthis subassociation does notindicate aspecial adaptation tothe moregravelly ,coarser-texturedsoils. E.g., only Poa glauca, Astragalusalpinus ssp. alpinus, Artemisia arctica ssp. arctica, and Cerastium beeringianum primarilyoccur on sandy ,gravelly places. More importantregarding the compositionof differential taxa, that mainly showamphi-Beringian or circumpolar distributions,is their ability tothrive underconditions that are muchless basic.Soil reaction isthe ecologically decisive difference tothe subass.p arnassietosumkotze- buei(cf.Fig. 6).This is clearly demonstratedby the highpresence ofthe name-giving taxon Polemonium acutiflorum , of Valerianacapitata , and by the occurrence of Salix planifolia ssp. pulchra.These taxa are notonly differential species ofthe Anemono-Salicetumrichardsoniisaliceto- sumpulchrae,butalso character species ofthe Valeriano-Salicetum pulchrae,bothoccurring on acidic soils(see below).

Fig. 9. Salix alaxensis -communities are notrestricted tolowland river banks.As true pioneer communities theyoccupy gravelly montanestream banksin the BrooksRange, whichmight showonly periodical streamflow (subass. polemonietosumacutiflori). Stand productivity andvigor of Salix alaxensis is reduced; phot.U. Schickho ff, 18.07.1997. 172 U.Schickhoffet al.

Table 6. Communitytable ofA nemono-Salicetumrichardsoniiass. nova. Riparianwillow communities on theArctic Slope of Alaska 173 Table 6. (cont.)

176 U.Schickhoffet al.

Table 6. (cont.) Additionaltaxa: Cladonia unicalis (rel. 22: +); Cladonia fimbriata (22: +); Calamagrostis lapponica (16: +, 20: 1); Anastrophyllum minutum (20: +); Lepraria neglecta (20: +); Cla- donia subulata (21: +); Cladonia carneola (21: +); Polytrichum strictum (11: 1, 19: +); Cladonia scabriuscula (19: +); Boykinia richardsonii (15: 1, 16: +); Epilobium latifolium (15: +, 16: +); Parnassia kotzebuei (15: +, 16: +); Lagotis glauca ssp. minor (15: +, 16: +); Didymodon asperifolius (15: +, 16: 2); Pedilularis langsdorfii (16: +, 23: +); Cynodontium strumiferum (23: +); Carex krausei (24: +); Amphidiumlapponicum (24: 1); Tortella fragilis (24: +); Tortella tortuosa (5: 1, 24: +); Myurella julacea (24: +); Lecanora epibryon (24: +, 27: +); Solorina saccata (24: +); Salix arbusculoides (29: +,30: 1); Platanthera obtusata (26: +, 30: +); Peltigera leucophlebia (30: +); Shepherdia canadensis (25: +); Androsace chamae- jasme ssp. lehmanniana (25: r); Cetraria tilesii (27, +); Pleurozium schreberi (28, +); Bryum pseudotriquetrum (5: +, 17: +); Entodonconcinnus (3: +, 17: +); Fissidens adianthoides (17: +); Eutrema edwardsii (18: +); Deschampsia brevifolia (3: +, 18: +); Taraxacum phy- matocarpum (18: +); Ceratodon purpureus (18: +,34: +); Castilleja caudata (33: +); Hedy- sarum mackenzii (31: 1, 32: 2); Pohlia cruda (13: +,31: 1); Carex glacialis (32: +); Lomato- gonium rotatum (32: r); Mniumblytii (32: +); Leptobryum pyriforme (32: 1); Salix ovalifolia (34: 2, 35: 2); Dendranthema integrifolium (34: +, 35: +); Oxyria digyna (34, +); Poa alpigena (34: +); Artemisia arctica ssp. arctica (1: +, 34: +); Artemisia glomerata (34: +); Alsinanthe rossii (34: +, 35: +); Salix chamissonis (35: 2); Poa alpina (35: +); Carex podo- carpa (7: +, 8: +); Dodecatheon frigidum (7: +, 8: +); Blepharostoma trichophyllum (7: 1); Hypnumcupressiforme (5: +, 7: 1); Thuidumrecognitum (8: +); Mniumhymenophylloides (3: +, 5: 1); Racomitrium canescens var. ericoides (5: 1, 9: 1);Catascopium nigritum (5: +); Sphagnum squarrosum (9: +); Stereocaulon alpinum (9: +); Aconitum delphinifolium ssp. paradoxum (3: +); Sphagnum teres (2: +, 3: 2); Klaeria starkei (2: +, 3: +); Salix hastata (2: 1); Viola epipsila ssp. repens (2: 1); Timmia megapolitana var. bavarica (2: +); Chondro- phylla prostrata (14: +); Philonotis fontana (14: +); Bryum lisae var. cuspidatum (14: 1); Peltigera didactyla (14: +); Alnuscrispa (1: +); Ledumpalustre ssp. groenlandicum (1: +); Deschampsia caespitosa (1: 1); Thalictrum alpinum (10: +, 11: +); Potentilla hyparctica (10: +); Hierochloe alpina (10: +, 11: +); Hepatic. indet. (10: +); Cetrelias alaskana (10: +); Eriophorum scheuchzeri (11: +). Localities: rel. 1: Dietrich Creek belowviewpoint; 2, 3: OksrukuyikCreek at Slope Mt., left bank;4: LowerHappy V alley Creek, upperterrace, right bank;5: LowerOksrukuyik Creek, near PumpStation 3, left bank;6 8: LowerDan Creek near Dalton Highway, left andright bank;9: LowerOksrukuyik Creek, near PumpStation 3, left bank;10 11: Atigun River above PumpStation 2, right bank;12 14: Atigun River infront ofAtigun Gorge, right bank;15 16: Sagavanirktok River near Slope Mt. Maintenance Station, left bank; 17 18: Sagavanirktok River southof Sagwon, left bank;19 24: Sagavanirktok River near Slope Mt. Maintenance Station, left bank;25 28: Sagavanirktok River near HappyValley LandingStrip, left bank;29 30: Sagavanirktok River southof PumpSta- tion3, left bank;31 32: Sagavanirktok River near FranklinBluffs, left bank;33: Sagava- nirktokRiver southof Franklin Bluffs Camp,left bank;34 35: Sagavanirktok River southof Deadhorse, left bank. Riparianwillow communities on theArctic Slope of Alaska 177

Withinthis subassociation, a variant couldbe differentiated which occurs onedaphically driersites with highergravel andsand content, and which is characterized byan extraordinarilyhigh cover/ abundanceof lichens (Tab.5; rel. no. 79 85).Correspondingly ,lichen species richnessis much highercompared to the subass.p arnassietosum(cf.Fig. 5).Most of the lichens are indicatorsof acidity ,e.g. Stereocaulontomentosum , Cetraria nivalis, Peltigera aphthosa , Thamnoliasubuliformis , and Cetraria cucullata (Wirth 1992).However ,since theyare poikilohydricplants without roots orrhizoids, and their behaviourdiffers from that ofvascular plants,these indicationsonly apply to their immediate environment(often they are at- tached tomoss tissues). This corroborates the above mentionedslight acidification ofthe topsoil.That the mineral soilis still weakly basic,is proved bythe occurrence ofbase indicatorslike Dryasoctopetala or Hypnum bambergeri.Sothe differentiation within thissubassociation must be attributedto soil moisturerather than to soil pHas indicated byaxis 2inFig. 6. Inthe lower section ofT ab.5 companionspecies are listed.Some ofthe mostabundant taxa like Salix glauca , Arctousrubra , Equisetumarvense , or Bistorta vivipara are at the same time mostfrequent companions of the Anemono-Salicetumrichardsonii,indicating floristic andecological affinities between bothassociations.

4.2Anemono-Salicetum richardsonii Schickhoff et al.2001 ass.nova Nomenclatural typereleve ´:Tab.6, rel. 26 Thismore or less openlow willow shrubassociation is widely distributed onbanksof upland tundra streams as well as onupper terraces ofSagavan- irktokRiver, adjoining to Salix alaxensis -standson lower terraces.It occurs overa wide altitudinal range,from mountain stream banksat anelevation of 900 950 mdownto upper river terraces onthe coastal plain.In contrast to Salix alaxensis -communitiesit occupiesbetter developedsites alongthe successional gradient ofriver terrace/stream bankevolution (cf. Fig. 2,axis 2).It follows in the courseof succession, when pioneertall willow shrub communities have preparedthe groundin terms of soil development and/ orwhen sites reach acertain vertical distance tothe water table dueto river erosion.The association correspondsto low willow standsof the categories “closed low willow shrub”and “ openlow willow shrub”respectively by Viereck et al. (1992). The soilsunder the Anemono-Salicetumrichardsoniiare con- siderablyfurther developed compared to feltleaf willow stands(cf. T ab.3): Gravel contentis negligible, andthe soilsare muchmore fine-textured (loam tosandy loam) with aconsiderable increase insilt andclay fractions. Thus,the soilshave muchhigher water capacity andhigher soil moisture, furtherenhanced bya thick,water-storing moss layer .Mossesbecome muchmore prominent in the growthform spectrum (cf. Fig. 4).Organic matter andnitrogen content likewise showa remarkable increase. At the same time, the C/N-ratiowidens, indicating lower decompositionrates. As indicated bya mean soilpH of 6.6, base saturationand cation exchange 178 U.Schickhoffet al. capacity ensurea sufficient nutrientsupply .Summingup, soil conditions improvefrom lower toupper terraces, where acomparatively higher numberof taxa (manyweakly acidophilousspecies; cf. Fig. 7) findssuitable habitat conditions.T otal species richness( D 35.3),vascular species richness (D 25.6),and lichen richness( D 3.4)is higherthan in Salix alaxensis -communities (D 29.8, D 21.2, D 1.9resp.) (cf. Fig. 5).Paradoxically ,the productivity ofthe shrublayer decreases inthe same direction fromtall tolow willow shrubcommunities. This must be attributedto the decrease indepth ofthe active layer.Thicknesses of only 40 50 cm (reducedrooting space) andshorter thaw seasonson upperterraces hamperthe development oftall woodystructures and thus the competitiveness of Salix alaxensis . Inits optimumdevelopment, this association ischaracterized bya 1 1.5m tall shrublayer with acover of70 80%(Fig. 10).T owardsthe coastal plain,where the climatic conditionsbecome moreunfavorable, the average shrubheight decreases to20 30 cm near Deadhorse(see Walker, D.A.1985 fora detailed studyon stature changes along aSagavanirktok River transect).Among the two character species,the amphi-Beringian Salix lanata ssp. richardsonii ispresent in all releve´s,butby far less domi- nantcompared to the name-giving Salix taxa inthe othertwo associations (Tab.7). The character species are moreor less homogeneouslydistributed inboth subassociations (cf. T ab.6), i.e., they occur with comparable cover andabundance on lowland, upland, and mountain streambanks and river

Fig. 10. Thelow willow community A nemono-Salicetumrichardsonii(subass. lupinetosumarctici)is widelydistributed onupperterraces along the Sagavanirktok River; phot.U. Schickho ff,15.07.1997. Riparianwillow communities on theArctic Slope of Alaska 179

Table 7. Dominance values offive highest rankedspecies ineach association as expressed bypercentage onthe sumof the total species cover.Calculation is based oncover/abundance values ofspecies inreleve ´stransformed intoaverage cover percentage according to Ellenberg (1956). Most dominantspecies Percentage ontotal cover ofrespective community Epilobio-Salicetum alaxensis Salix alaxensis 35.0 % Abietinella abietina 4.7 % Aster sibiricus 3.5 % Hypnumbambergeri 2.6 % Salix glauca 2.5 % Anemono-Salicetum richardsonii Salix lanata ssp. richardsonii 14.1 % Tomenthypnumnitens 11.4 % Salix glauca 11.0 % Hylocomium splendens 7.6 % Dryas integrifolia 5.1 % Valeriano-Salicetum pulchrae Salix planifolia ssp. pulchra 30.1 % Hylocomium splendens 7.7 % Aulacomnium palustre 6.9 % Sphagnum teres 4.9 % Sanionia uncinata 4.3 % terraces,indicating that altitude doesnot play a major rolein community characteristics, at least at the associationlevel. The above mentioned edaphic conditionsmuch better characterize the synecologyof thisassociation. Tabularprocessing resulted in two subassociations on upland tundra stream banks(s alicetosumpulchrae)andon upland and lowland river terraces (lupinetosumarctici)respectively:

1) Anemono-Salicetum richardsonii salicetosum pulchrae Schickhoff et al.2001 subass.nova Nomenclatural typereleve ´:Tab.6, rel. 3 Thissubassociation (T ab.6; rel. no. 1 9) is distributedon upland tundra stream banks.It is a transitional vegetation type,already mediating tothe Valeriano-Salicetumpulchraeonmost humid and acid stream banks (cf.Fig. 2).Edaphic parameters are close tothose of the latter association (see Tab.3). Soil pHand vertical distance tothe water table are muchlower , andsoil moisture of the moreloamy soilsis muchhigher than in the second subassociation(l upinetosumarctici).These are crucial factorsdeter- 180 U.Schickhoffet al.

Fig. 11. DCAordinationof Anemono-Salicetumrichardsonii.

mining the differentiation patternin the ordinationdiagram (Fig. 11). Moreover,soil texture,content of organic matter,nitrogen, Mn- and Fe- ions,and thickness of moss layer (Tab.3) differentiate the twosubassoci- ations,and give evidence ofthe transitionalposition to Salix pulchra -communities.This position is likewise reflected incommunity characteristics like growthform percentage (cf.Fig. 4),species richness(cf. Fig. 5) orpH- affinities ofthe species composition(cf. Fig. 7). The name-giving taxonof this subassociation, the amphi-Beringian Salix planifolia ssp. pulchra,as well as otherdifferential species like Polemonium acutiflorum , Valerianacapitata , Anemonerichardsonii , Aconitumdelphini- folium ssp. delphinifolium , Saxifraga nelsoniana ssp. nelsoniana, and Cli- maciumdendroides are at the same time character species ofthe Vale- riano-Salicetumpulchrae(see Tab.7 below).Additionally ,theypoint tosimilar habitat conditionsregarding soil moistureand soil reaction.The majorityof the differential species showamphi-Beringian distribution. Theyindicate moist,rich, peaty-humous soils, and moderately acid toacid conditions.The latter holdstrue not only for vascular plantsrooting in Riparianwillow communities on theArctic Slope of Alaska 181 the mineral soil,but also for mosses covering the soil surface ( Oncophorus wahlenbergii , Aulacomniumpalustre ).

2) Anemono-Salicetum richardsonii lupinetosum arctici Schickhoff et al.2001 subass.nova Nomenclatural typereleve ´:Tab.6, rel. 26 Thissubassociation (T ab.6; rel. no. 10 35) iswidely distributedon upper river terraces inboth lowland andupland. It represents the typical formof the association,occupying better developedsites with advanced soil devel- opmentalong the successional gradient ofriver terrace/stream bankevolution(see above).Sites are characterized bycomparatively highersoil pH, lower soilmoisture, and considerably higher vertical distance tothe water table. The moremoderate habitat conditionsobviously lead tomore favorable living conditionsfor a greater variety ofspecies, except formosses. Total species richnessas well as vascular andlichen species richnessis highercompared to the othersubassociation (cf. Fig. 5). Alongthe transect fromthe BrooksRange toPrudhoe Bay ,asequence ofvariants (cf.T ab.6) representsgradients of decreasing altitude, decreasing soil moisture,increasing vertical distance tothe water table, andincreasing nitrogencontent. The variant of Kobresiamyosuroides (Tab.6; rel. no. 10 14),occurring along uplandrivers, is characterized bydifferential taxa with arctic-alpine andmostly circumpolar distribution,indicating harshclimatic conditions.Moreover, taxa like Kobresiamyosuroides , Dryasoctopetala , and Festucabrachyphylla indicate weakly basic conditionsof less developed montaneriver terrace soils(mean soil pHof sites ofthisvariant is 6.7). The variant of Lupinusarcticus (Tab.6; rel.no. 15 30),occurring along lowland riversin the northernfoothills section, represents the optimum development ofthissubassociation. It occupiesintermediate sites interms ofsoil moisture and soil pH (cf. Fig. 11).It is commonon occasionally floodedbroad upper river terraces with well developed,nutrient-rich soils at analtitudinal range between 260 and550 ma.s.l.Mean soilpH of the sites is6.5, and the sandy-loamysoils are characterized byrelatively high amountsof organic matter ( D 16.8% o.m.).The majority ofthe differential taxa are indicatorsof weakly basic tobasic soils.At the same time they indicate freshsoils or moist soils; pronounced dry-site or wet-site indica- torsare missing. Some differential taxa ( Zygadenuselegans , Leymus velutinus , Carex concinna)are at the same time differential taxa ofthe variant of Castilleja caudata within the Epilobio-Salicetumalaxensisparnassietosum kotzebuei(cf.T ab.5; rel. no. 59 65).This floristic-sociological affinity, also expressedby neighbouring positions of releve ´sin the ordinationdiagram (cf.Fig. 2),reflects similar site conditionsof adjoining vegetation typesalong the gradient fromlower toupper river terraces. The thirdvariant (Tab.6; rel. no. 31 35) is developedon terraces of lowland riversin the coastal plain section below 200 ma.s.l.The sites of thisvariant, often ice-wedge polygons,are characterized bybasic condi- 182 U.Schickhoffet al. . a v o n . s s a e a r h c l u p m u t e c i l a S - o n a i r e l a V f o e l b a t y t i n u m m o C . 8 e l b a T Riparianwillow communities on theArctic Slope of Alaska 183 ) . t n o c ( . 8 e l b a T 184 U.Schickhoffet al. tions(mean soil pH7.8), and they are relatively richin available nitrogen andphosphorus. The favorable nutrientsupply has to be attributedto the runoffpeak in spring,when the banksof the lower SagavanirktokRiver are floodedand provided with minerals andnutrients. Moreover ,airborne nutrientsfrom loess depositions upwind contribute to the remarkable nutrient supply.Basic soilconditions are indicated inparticular by Trisetum spicatum, Carex capillaris , and Salix arctica .Onthe otherhand, harsh climatic conditionsrestrict the productivityof the shrublayer resultingin veryprostrate, creeping growthforms of several Salix species (S. arctica, S. lanata ssp. richardsonii , S. ovalifolia).The shrubsdo not grow higher than 15 cm. The floristic compositionof the mostfrequent companions mirrors the intermediate positionof this association. Some are at the same time most frequentcompanions in the Salix alaxensis -communities(e.g. Arctousrubra , Salix glauca , Equisetumarvense , Bistorta vivipara ),othersshare the same statusin the Salix pulchra -stands(e.g. Betula nana ssp. exilis, Hylocomium splendens, Pedicularis capitata ).Some taxa like Arctagrostislatifolia , Festuca rubra, Tomenthypnumnitens , and Sanioniauncinata are abundantin all three Salix associations.At least the latter are supposeddiagnostic taxa of highersyntaxonomic units.

Fig. 12. TheVa leriano-Salicetumpulchrae,here along the upperKuparuk River ,is commonon paludified, acid banksof upland tundra streams andcreeks inthe Arctic Foothills, especially onolder landsurfaces; phot.U. Schickho ff,25. 07.1997. Riparianwillow communities on theArctic Slope of Alaska 185

4.3V aleriano-Salicetum pulchrae Schickhoff et al.2001 ass.nova Nomenclatural typereleve ´:Tab.8, rel. no. 37

Thisconspicuous low willow communityis restricted inits distributionto mosthumid and acid banksof upland tundra streams and creeks ofthe Arctic Foothillszone (roughly between 400 and800 ma.s.l.).It occupies the immediate marginsof smaller tundrastreams and creeks originatingin the gentle topographyof the foothillsand not in the BrooksRange (Fig. 12).Thus, it istypical fora specific tundrastream typewith aspecific streamflow regime (sharppeakflow in late May),which governsthe envi- ronmentalconditions in terms of disturbance, soil moisture, soil reaction, nutrientfluxes andthe like. The higherelevation ofcommunity habitats is clearly reflected ina slightlyhigher amount of species with arctic-alpine distributioncompared to the twoother associations (Fig. 13) as well asin the lowest percentage ofthermophilous species (Zone4 species),whereas morecold-adapted species (Zone2 species) showthe highestpercentage (Fig. 14). Adjoiningto this association, shrub tundra (mostly dominated by Betula nana)ortussock tundra on paludified soilsprevails with increasing distance fromcreeks. Along somewhat larger streams,where astream terrace is developed andwhere regular disturbancesoccur, this association isreplaced by Salix richardsonii -stands.The latter are restricted toless humidand acid terrace sites with highervertical distance tothe water table, especially on youngerland surfaces. Salix pulchra -communitiesoccupy a distinct,delim- ited space within the ordinationdiagram ofall releve´s(Fig. 2),indicating the narrowestsynecological amplitude ofall Salix-communitiesand a more homogeneousfloristic composition.As aresult,they are muchbetter defined bycharacter species. Salix pulchra -standsare subsumedby Viereck

Fig. 13. Physiographic unitanalysis for vascular species of Salix-communities. 186 U.Schickhoffet al.

Fig. 14. Climatic affinity for vascular species of Salix-communities. Northern limits of plant distributions were assessed withinthe fourclimatic zones developed by Yo ung (1971). Thedifferentiation ofzones is based onthe sumof mean monthlytemperatures above 0 °C(zone 1=0to< 6 °C;zone 2=6<12 °C;zone 3=12 <20 °C; zone 4 = 20 < 35 °C). et al. (1992) under“ closed low willow shrub”and “ openlow willow shrub” respectively withoutseparating it from Salix lanata - and S. glauca-stands. Mostriparian habitats showpioneer-like characteristics, the habitats of thisassociation, however ,cannotbe termed pioneerhabitats. As arule,they are located onolder land surfaces, which are geomorphologicallynot very active. Asthe smaller streamsand creeks drainsmaller watersheds,the water volume is relatively low andso is itserosional impact. Theydo over- flow their banks,but flooding is less severe comparedto mountain streams. Thisis reflected inlower levels ofturbidity and suspended silt loadsof these waters (tundrastreams), which showdistinct differences inchemical characteristics (esp.Ca- andMg-ions, pH, conductivity) compared to mountainstreams originating in the BrooksRange (Tab.9). Moreover ,the sharppeakflow inthe melting periodoccurs at apointin time, whenmost ofthe groundis still frozenand thus protected from erosion. Nevertheless, snowmelt runoffprovides a certain ionic enrichment,which may lead toa certain accumulation inorganic surface soilsduring later melting phases. The ratherstable habitats ofthisassociation showcomparatively mostpro- gressive soildevelopment like leached loamy soilswith verylow soilpH (D 5.0),low bulkdensity ,andrelatively highcontent of silt andclay fractions(cf. T ab.3). On the otherhand, the sites are characterized bya very shallow active layer (30 35 cm),which is mainly dueto perfect insulation byextremely thick mossand mor layers with highwater storagecapacity (Fig. 15).Mosses nearly equal shrubsin cover percentage (cf.Fig. 4),and bryophytespecies richnessis comparatively veryhigh (cf. Fig. 5).W ater saturationof the soilsand lower oxygencontent lead toreduced decompo- Riparianwillow communities on theArctic Slope of Alaska 187 sitionof organic residuesand thus to the development ofthick morlayers andto high percentage oforganic matter ( D 24.2%).Considerablyhigh amountsof Mn- and Fe-ions (cf. T ab.3; Mn-concentrations in plant tissues may reach toxic levels; see Mario n et al. 1989),point to reduction and oxidationprocesses in these stronglyhumous, peaty soils,resulting in re- doximorphicfeatures in the subsoil(iron and manganese concretions,characteristic colorpatterns). Both Mn- and Fe-ions are reducedand possibly complexed with organic acids inthe organic-richsoils ( Everett et al. 1996). Underthese conditions, Salix pulchra -shrubsare remarkablyvigorous. Theyreach highcover/ abundancevalues andaverage heightsof 80 100 cm. As aconsequenceof the hygricand acid, paludified soil conditionsand the decreased rootingspace, taller willows ( S. alaxensis, S. glauca, S. lanata ssp. richardsonii )are nolonger competitive. Salix planifolia ssp. pulchra exclusively dominatesthe stands. Thosecharacter species which showhighest presence degree are at the same time differential species ofthe Anemono-Salicetumrichard- soniisalicetosum(cf.T ab.6), indicating floristic-sociological andsynecological affinity tothat subassociation(cf. Fig. 2;T ab.3). Most of the character taxa are amphi-Beringianin their distributionand indicators for acidophiloussoils and moist sites (Tab.10). T wovariants couldbe prelimi- narilydifferentiated: the typical variant is that of Vacciniumuliginosum , which showsthe characteristic species composition(T ab.8; rel. no. 36 50). The sites ofthisvariant cover the whole distributionalrange ofthe association.Compared to those of the Salix chamissonis -variant (Tab.8; rel. no. 51 58),they are characterized byslightly lower soil moistureand organic matter contents,and slightly higher amounts of nutrients (N, P)andsoil pH.The onlymajor difference insite conditionsbetween the twovariants isthe amountof Mn-ions,which isconsiderably higher in the soilsunder the Salix chamissonis -variant (848.8vs. 336.4 ppm). This may beattributed tohigher water saturation,lower soil pHand a slightlyhigher degree of reductionin the soil.However ,the regional validity ofthe Salix chamissonis-variant hasto be elaborated byfurther studies. It may merely be a

Table 9. Chemical analysis ofwater samples from mountainand tundra streams inSaga- vanirktok andKuparuk River drainages. Ionsare reported in µmol L 1 (data from Kling et al. 1992).

2+ 2 + + + ConductivitypH HCO 3 Ca Mg Na K (µS/cm 1) Mountainstreams Atigun River 166.0 8.0 1105 524 227 144 12 Sagavanirktok River 148.0 8.2 1163 324 189 10 2 Tundrastreams UpperKuparuk River 29.2 7.1 200 95 35 33 8 UpperOksrukuyik Creek 37.9 7.3 298 125 46 28 10 188 U.Schickhoffet al.

Fig. 15. Sites of Salix pulchra -stands are characterized bya very shallow active layer (30 cm inthis example). Thickmoss andmor layers provide perfect insulation ofthe subsoil; phot.U. Schickho ff,10.08.1997.

local variant,since the majority ofits releve´swere recordedalong the upper OksrukuyikCreek. Therefore we refrain fromdescribing it as asubassocia- tion. The differential species ofthe Vaccinium-variant are strongindicators ofmoist, peaty-turfy ,acid soils.This holds true especially for Vaccinium uliginosum, Vacciniumvitis -idaea ssp. minus, Rubuschamaemorus , Ledum palustre ssp. decumbens, Andromedapolifolia , and Empetrumhermaphro- ditum.Intheir majority,the differential taxa showcircumpolar distribution. Onthe otherhand, the differential species groupof the Salix chamissonis - variant containsmore amphi-Beringian elements ( Salix chamissonis , Carex bigelowii, Bistorta plumosa ).Thismight pointto slight differences in the symphylogenicevolution of these phytocoenoses. The mostfrequent companion species (e.g. Betula nana ssp. exilis, Peta- sites frigidus ,abundantmosses) also indicate hygricand acid soilconditions. Bryophytespecies richnessis exceptionally high(cf. Fig. 5).Moreover ,fre- quentmosses like Aulacomniumpalustre , Hylocomiumsplendens , Sanionia uncinata, and Sphagnumteres reach highcover/ abundancevalues andall belongto the mostdominant species inthis association (cf.T ab.7). Riparianwillow communities on theArctic Slope of Alaska 189

Table 10. Attributes ofcharacter species of Salix-communities. Abbreviations for pHaf- finity andnorthern limit are those usedin Figs. 7and14 resp.

Character species Growth pH Physiog-Geographic North- form affinity raphic unit range ern limit

Epilobio-Salicetum alaxensis Salix alaxensis shrub C Arctic NorthAmerica Asia 3 Epilobium latifolium forb CArctic-alpine Circumpolar 2 Astersibiricus forb CBArctic-alpine NorthAmerica Asia 3 Trisetumspicatum graminoid CArctic-alpine Circumpolar 2 Anemono-Salicetum richardsonii Salix lanata ssp. richardsonii shrub C Arctic NorthAmerica Asia 3 Anemone parviflora forb CArctic-alpine NorthAmerica 3 Valeriano-Salicetum pulchrae Salix planifolia ssp. pulchra shrub wAArctic-alpine NorthAmerica Asia 2 Valeriana capitata forb CAArctic-alpine NorthAmerica Asia 2 Polemonium acutiflorum forb CAArctic-alpine NorthAmerica Asia 2 Aconitumdelphinifolium ssp. delphinifolium forb CAArctic-boreal NorthAmerica Asia 3 Anemone richardsonii forb wA Arctic NorthAmerica Asia 3 Saxifraga nelsoniana ssp. nelsoniana forb AArctic-alpine NorthAmerica Asia 2 Climacium dendroides moss CAArctic-boreal Circumpolar 2 Luzula rufescens graminoid AArctic NorthAmerica Asia 3

5Discussion Ourstudy reveals distinct relationshipsof riparian Salix associationsand subassociationswith major landscape-level environmental variables alonga transect fromthe BrooksRange toPrudhoe Bay .Acombinationof edaphic conditions(soil pH,soil moisture) and factors pertaining totopography , disturbanceregime andlandscape evolution(river terrace/ stream bankde- velopment) controlsspatial patternsand floristic compositionsof these riparian vegetation units.Principal environmental gradientsat workin riparian ecosystemsare, thus, included (cf. Malanso n 1993; Hughes 1997). Landscape age, topography,substrateand disturbance effects like annual flooding,erosion and sedimentation are crucial underlyingparameters for the present-daydifferentiation ofthe riparianvegetation mosaic.Environ- mental gradientsaffecting the vegetation inthis study correspond to those well-knownto control plant distributionacross the Arctic (esp.soil moisture,soil pH,landscape age; cf. Webber et al. 1980; Walker, D. A. 1985; Walker,M.D.et al. 1994b; Go uld 1998).Specific riparianreplacement successionscan be derived fromfloristic-sociological traits,synecological characteristics, andspatial patternsof Salix-communities.The Epilobio- Salicetumalaxensisis atruepioneer community along mountaincreeks andon gravel bars,floodplains and lower terraces ofrivers, where it is favoredby frequentdisturbances, coarser-textured soils with adeep active 190 U.Schickhoffet al. layer andrelatively highsoil temperatures. Corresponding to the perma- nenthabitat disturbances,this self-perpetuating pioneer association may persiston river banksas longas erosion and deposition of new increments ofalluvium occurs,i.e. as longas predominantlyallogenic processesare operative in successioncycles. Itis replaced bythe Anemono-Salicetum richardsonii(subass.l upinetosumarctici)onhigher terraces with better developed soils(however ,with ashallower active layer dueto insulationby a thick mosscover andlower soiltemperatures) (Fig. 16).This association characterizes later stages ofsuccession on river alluvium with predominantlyautogenic processesresulting inter alia inan uniquelyarctic soil thermal regime.

Fig. 16. Idealized pattern ofwillowsuccession along the Sagavanirktok riverside illustrat- ing environmental alterations, concomitant directional communityreplacement and changing communitycharacteristics. Riparianwillow communities on theArctic Slope of Alaska 191

Inthe ripariansuccessional sereswithin the gently rollingterrain ofup- landtundra, the Anemono-Salicetumrichardsonii(subass.s alicetosumpulchrae)isreplaced onstreamsides with moreprogressive soil development bythe Valeriano-Salicetumpulchrae.The latter associa- tionis foundon olderland surfaces with paludified,loamy ,acid soilswith massive groundice andthick mosslayers, resulting in cold soils, decreased depthof thaw ,andincreased soil moisture.However ,since the overall Arc- tic Foothillsvegetation patternis nota simple successional sequence dueto the diverse glacial history(cf. Walker,M.D. 1995),riparian vegetation likewise hasto be seenin the light ofa landscape mosaic ofcontrasting deglaciation ages.T erminal riparianvegetation typeslike the Salix pulchra - communities seem tobe connected tolong-established hydrologic patterns andassociated riparianecosystem evolution along headwaters inupland tundra(up to mid-Pleistocene; cf. Hamilto n 1986),and have, thus, devel- opedin other time scales comparedto riparian communities inyounger landscapes.The present-day pattern of riparianplant communities reflects amosaic ofdevelopmental states governedby landscape age heterogeneity. Bothspatial andtemporal environmental heterogeneity influence this pattern. The resultof a combinedcontrol of macroscale andmicroscale environmental gradientson distribution, floristic differentiation andsuccessional developmentsof Salix-communitiesis corroboratedby comparisons with otherecological research onnorthern alluvial habitats. Bliss & Cantlo n (1957) and Bliss & Peterso n (1992) foundthe same patternof plant suc- cessionwith tall Salix alaxensis -standson gravel barsand floodplains and low greenleaf willow communities onhigher terraces alongthe Colville River near Umiat. Asassociated with these floristic andproductivity changes theystressed an increase oforganic matter,adecrease inthe active layer,colder andwetter soils,and decreasing direct river influence. Depend- ing ontexture ofthe alluvium, rate ofmeander migration,stream size, latitude andaltitude, the successional gradient can be steep orgradual (Bliss & Cantlo n 1957,p. 466). Comparable patternshave been reported fromNome and Snake Rivers( Hanso n 1951),from Mackenzie River (Gill 1971),from NE-Alaskan rivers ( Hettinger & Janz 1974),and from Meade River near Atkasook( Peterso n & Billings 1978,1980). Several furtherstudies (e.g. Spetzman 1959; Viereck et al. 1992) attributedthe degeneration oftall willow standswith increasing distance tothe stream channel toaccumulation oforganic debris,rise ofpermafrost table and decreasing soiltemperatures. Feltleaf willow communities obviouslyneed an ameliorated soiltemperature environment.Even within Salix alaxensis - standssoil temperaturesdecrease with distance tothe river.Soil temper- aturesin a younggravel barstand (recorded at 50 cm depthin June) were 5 6 °Chigherthan in an adjacent inlanddecadent standat the Sagavanirk- tok River (Mo o re 1982).The data in Gill (1971) clearly showthat soil temperaturesdecrease inlandas soonas acertain accumulation oforganic material ora mosslayer providesan insulatingblanket andmoisture reten- tion.As aresult,the active layer decreases inthickness along the succes- 192 U.Schickhoffet al. sional continuum.Regarding decreasing river influence, Gill (1971) sup- posedthat low willows ( Salix lanata ssp. richardsonii , S. arbusculoides in hisstudy), succeeding Salix alaxensis inland,cannot withstand alluviation. Theydo not grow on sites which receive annual increments ofalluvium, andshow decreased ability toplace adventitiousroots. Bliss & Peterso n (1992) pointout the decrease innutrients with time as amajor factor favoringhigher competitiveness ofheath shrubsand low deciduousshrubs. However ,infloodplain environments, early successional soilsare generally lower inN andP (e.g. Van Cleve et al. 1996).Our data showan increase innitrogen and phosphorus in later successional soils onhigher terraces: P-concentrations nearly double(cf. T ab.3). Nutrient availability is likely toincrease because oflower mean pHlevels (8.0vs. 6.8in our study). Results of Giblin et al. (1991) alsoshow comparatively favorable nutrientsupply (N, P) for Salix lanata ssp. richardsonii -stands. According toour data, the paradoxonof a decrease inproductivity (standing biomass)on higherterraces with furtherdeveloped soils, expressed by the replacement oftall willow bylow willow shrubcommunities, is more aquestionof decreasing active layer,insulationby athick mosslayer ,lower soil temperatures,lower rootactivity androoting space, and higher soil moisturerather than one of enhanced competition forlimited nutrients. Shaver et al. (1996) showedalong a riverside toposequencethat onlythe combinationof favorable soil fertility,soil thaw andlow soil moisturewar- rantshigh productivity .Even whenthe outercrust of the mosslayer dries inmid-summer, warming ofthe underlyingsoil is prevented due to a low thermal conductivity.Highcontents of silt andclay furtherretard thermal exchange between soilhorizons compared to the coarser,drierdeposits under Salix alaxensis . Salix richardsonii -standsare, thus, more tolerant of lower soiltemperatures and more poorly drained soils. Along thissuccessional gradient,soil pHdecreases whereas species rich- nesssimultaneously increases. This is remarkable,since in mosttundra habitats highersoil pH is associated with highervascular plant species richness (Go ugh et al. 2000).This holds also true along longitudinal river profiles at the landscape scale ( Go uld & Walker 1997).Under extreme environmental conditionslike infloodplain and lower river terrace habitats,the numberof species found,however ,is limited. Onupperterraces with pro- gressive soil development the species poolincreases. Other factors connected with furtherdeveloped soils (higher soil moisture, nitrogen, percentage clay andsilt) as well as biotic factors(vegetation structure,competition)obviously overcompensate the effect ofdecreasing soilpH on species richness.By contrast, riparian willow communities in uplandtundra follow the general patternof higher richness with higherpH, as far asrichness of vasculars andlichens is concerned. Salix pulchra -standson old land surfaces with paludified soilshave lower vascular plant species andlichen richness compared to Salix richardsonii -communities.However ,theyhave the comparatively highestbryophyte species richness.Bryophyte richness seems to be morepositively correlated with soil moisturerather than soil pH(cf. Go uld & Walker 1999). Riparianwillow communities on theArctic Slope of Alaska 193

Regardingthe time frame ofsuccession on river alluvium, 20 30 years are reportedfor early successional stages ( Mo o re 1982).According to Bliss & Cantlo n (1957) and Hettinger & Janz (1974),who counted annual rings,feltleaf willow standsbecome decadent after 45 50 years(see also Walker,L.R.et al. 1986 forinterior Alaska), andlow willow shrub communities are subsequentlybeginning to replace them.However, ages of willow stemsare notvery suitable as indicatorsfor successional time frames,since someof the riparian willows are able tosurvive repeated buri- als byriver alluvium andsend up new sprouts( Gill 1971; Mo o re 1982). Comparedto the Subarctic,where later successional stages are dominated bypoplars, spruces or other trees, willows continueto play the dominant role inarctic ripariansuccession throughout successional seres.Alluvial successionsin the Subarctic are similarly governedby a decrease in depth ofthaw,lower soil temperatures,and deteriorating drainage (e.g. Benning- ho ff 1952; Drury 1956; Viereck 1970; Van Cleve et al. 1996) The dependence ofupland streamside communities onsubstrate,streamflow anddisturbance regime was already intimated by Walker, M. D. et al. (1994b),who assigned Salix alaxensis -S. richardsonii -communitytypes to creek marginswithin rockytill surfaces.The distinctionto Salix pulchra - communities onless disturbed streamsides of older land surfaces with loamier soilsand along hillslope water tracksis in agreement with our observationsin a wider region.Our consideration of Salix pulchra -communities as terminal riparianvegetation typescontrasts with the general con- ception that willows characterize labile, earliest successional stages andbe- come less andless importantas vegetational successioncontinues. Our view iscorroborated e.g. by Bliss & Cantlo n (1957),who found the most poorlydrained, less disturbedsites, most inland along a transect fromthe Colville River,covered with Salix pulchra intergradinginto wet tundra meadows (see also Churchill 1955). Jo hnso n et.al (1966) described Salix pulchra asthe dominantwillow inlater successional stages oninfrequently floodedterraces ofOgotoruk Creek (Cape Thompsonregion). Argus (1973) studieda transect fromthe sandy-gravelfloodplain of acreek tothe upperterrace in the Cape Beaufortarea, where Salix pulchra dominatedthe relatively stable vegetation onthe uppermostterrace with minimal disturbance andsiltation, and poor drainage. Hettinger & Janz (1974) consid- ered the wet sedge-low willow shrub( Salix pulchra )communitythe end pointof riparian successional trendsat lower elevations inthe southern BrooksRange. Withregard to the classification ofriparian plant communities inarctic Alaska, the Braun-Blanquetapproach used in thisstudy proved to be an appropriateanalytical toolto define vegetation typesbased on accurate floristic information,supported by correlations of releve´ groupswith envi- ronmentalvariables via DCA ordination.As aresult,we define associations ona floristic and ecological basis.As already stressedby Ko ma´rko va´ (1993),the Braun-Blanquetapproach is an effective methodto delimit and classify arctic vegetation typesdespite ofits relatively highnumber and allegedly poordefinition. Our results give evidence that aclear delimitation 194 U.Schickhoffet al. anddescription of types for North American arctic vegetation ispossible. Previousphytosociologal/ vegetation ecological studiesalready came up with delimitations ofarctic riparianvegetation types,that partially show remarkable correspondencesto the above definedassociations in terms of floristic structureand synecological characteristics. Go uld (1998) described an Epilobium latifolium -Salicetum alaxensis -communityfrom the HoodRiver ,NorthwestT erritories,Canada, as acommonpioneer com- munityof gravel barsand active floodplains,disturbed by floodingand ice- scouring. Go uld’sdata shownumerous floristic andsynecological correspondences(see also Bliss & Cantlo n 1957, Drew & Shanks 1965, Gill 1971, Hettinger & Janz 1974, Mo o re 1982 forfurther Salix alaxensis - communities in the region).Similar vegetation typesare also foundin mountainousenvironments, exemplified by Co o per‘s(1986) descriptionof the Elymoinnovatis-Salicetumalaxensis,bestdeveloped on broad, gentle alluvial fansof limestone sandsin the central BrooksRange. A specific set ofspecies assemblages (e.g. Salix alaxensis , Astersibiricus , Shepher- dia canadensis , Hedysarumalpinum ssp. americanum, Castilleja caudata , Zygadenuselegans a.o.)can even befoundon floodplainsof interiorAlaska (cf. Farjo n & Bo gaers 1985). Previousdescriptions of low willow shrubcommunities also showmore orless correspondingfindings regarding the above definedassociations. Go uld (1998) described Salix niphoclada -Hedysarummackenzii and Equi- setum arvense -Salix lanata ssp. richardsonii -communitiesfrom protected river banksabove the floodplainof the HoodRiver ,resembling the Ane- mono-Salicetumrichardsoniiofour study .Theyoccur on neutral to basic tobasic/ cation-rich substrates,i.e. in comparable habitats,regardless that mean soilpH is highercompared to this study (7.2 and 7.6 vs. 6.8). Thissuggests that the successionmodel with Salix alaxensis -communities onfloodplainsand lower terraces to Salix lanata ssp. richardsonii -communities onhigher terraces isvalid ina wider regionof the NorthAmerican Arctic. Salix niphoclada ,taxonomically closely related toand presumably hybridizingwith Salix glauca (Argus 1973),forms communities, which are synecologically (cf. Go uld 1998) closely related to Salix lanata ssp. richardsonii -communitiesand which might be vicariant inthis successional seres.Floristically similar Salix richardsonii -communitieshave beenre- portedin northern alluvial habitats e.g.by Bliss & Cantlo n (1957), Gill (1971), Co rns (1974). Salix pulchra -dominatedriparian communities are notrestricted to smaller tundrastreams and creeks. Walker,M.D.et al. (1994b)and Walker, D. A. & M. D. Walker (1996) describeda related Salix pulchra - shrubcommunity ( Eriophorumangustifolium -Salix pulchra -community) along channels ofbetter developedhillslope water tracksin upland tundra with similar floristic composition.Further studies have toshow syntaxo- nomical relationshipsin detail. Similar communitytypes were also described by Lambert (1968) and Hettinger & Janz (1974). The above describedcorrespondences and floristic similarities ofwillow communities ina wider regionunderline supraregional and point to cir- Riparianwillow communities on theArctic Slope of Alaska 195 cumpolarrelationships of arctic communityorganization, that applies in particular toazonal habitats. Ko ma´rkova´ (1981) already mentionedriver- bankand riverbar vegetation inthis respect. Moreover ,Alaskan riparian Salix-associationsshow in many respects affinities toother Arctic riparian Salix-communitiesbeyond North America sothat it seems possibleto as- signthem toexisting highersyntaxonomic units of the Braun-Blanquet system.In terms of physiognomyand synecology ,ourassociations largely correspondto alluvial shrubcommunities describedfrom northern Eurasia (e.g. Mølho lm Hansen 1930; No rdhagen 1943; Secretareva 1979; Klo kk 1981; Ko zhevnikov 1989; Dierssen 1996).Moreover ,as azonal vegetation types,these riparianshrub communities showclose similarities with regardto floristics, even in acircumpolar perspective.Therefore, we proposean integration ofourassociations into the existing Braun-Blanquet classification scheme basedprimarily on phytosociological work in Scandi- navia andGreenland. Inview ofthe existing classification scheme ofwillow shruband forest vegetation onriver alluvium innorthern Europe (class Saliceteapur- pureae;cf.T ab.19 in Dierssen 1996),it is obviousthat the NorthAlaskan riparian willow associationscan be addedto the orderS alicetaliapur- pureae.Withinthis order ,theyshow strongest similarities toboreal-(montane-alpine)-arctic riparian willow communities,which are integrated into the alliance Salicionphylicifoliae.Theassociations of this alliance are characterized floristically bytaxa with moreor less closed circumpolar dis- tributionlike Salix glauca , S. hastata, S. lanata, S. lapponum, and S. phylici- folia.Mainly distributedin alluvial pioneerhabitats ofmiddle andnorthern boreal regions,their range extendsinto the southernArctic andupslope to the lower alpine belt.They typically occupyheadwater streamsidesand creek marginsas well as geomorphologicallyactive river deltas with strong water currentsand water table fluctuations( Dierssen 1996). Onolder land surfaces with gentle topographythese willow communities (composedof the same species) occurin moist colluvial basins,fens andpeaty valley bottomsalong creek margins,often interspersed with tall herbstands. Therefore, geobotanists working in those landscapes mostly assignedthese willow communities tothe classes Betulo-Adenostyletea orS cheuchzerio-Cariceteanigrae,as we also tentatively did,when we tried toassign riparian communities ofToolikLake andImnavait Creek region/Alaska orHood River/ Canada (cf. Walker,M.D. et al. 1994b; Go uld & Walker 1999;see also Danie¨ls 1982 for Salix callicarpaea -communitiesin SE-Greenland). However ,whenconsidering the appearance of riparian willow communities overa wide range ofcircumpolar riverine habitats,the affinities toalluvial associationsof the Salicionphylicifoliae with regardto physiognomy ,ecology,andfloristics are morepronounced. We follow Dierssen (1996) toinclude thoseassociations (e.g. the Filipen- dulo-Salicetumphylicifoliaedescribedby No rdhagen 1943, which is e.g.additionally dominatedby Salix lanata and S. glauca ssp. callicarpaea on Iceland; Dierssen 1996) intothe SalicionphylicifoliaeandS ali- ceteapurpureaerespectively. Salix callicarpaea -communitiesin Green- 196 U.Schickhoffet al. land may also be assignedto the Saliceteapurpureae,as indicated by Danie¨ls et al. (2000).Comparing structural, floristic andecological characteristics,stronger affinities doexist toother alluvial willow associationsof the Saliceteapurpureae(includingthose of montane/ subalpinebelts of the temperate zone)rather than to open woodland, shrub, tall herbor fen communities ofthe classes Betulo-AdenostyleteaorS cheuchzerio- Cariceteanigrae.Wethereforepropose to extend the range ofthe Sali- ceteapurpureaetothe NorthAmerican Arctic. The Braun-Blanquetapproach bears the potential fora commonframe- workto accomodate vegetation unitsof the entire Arctic that oftenshow strongfloristic similarities in similar habitats throughoutthe circumpolar region (Walker,M.D. et al. 1994b),especially at highersyntaxonomic levels. The floristic ties between Scandinavia, the RussianArctic, Greenland andBeringia are strongenough to make anapplication ofhitherto de- scribedvegetation unitsor modifications ofthem toAlaska possible.Fur- therextending the existing vegetation classification ofEurasia and Green- land tothe NorthAmerican Arctic, acomprehensiveand manageable cir- cumpolarsystem could be established that avoidsan inflation ofregional communitytypes not fitting intosupraregional classifications (e.g.,17 Salix alaxensis-communitytypes and a total of888 communitytypes were listed onlyfor Alaska by Viereck et al. 1992).By extending the resultsof regional vegetation studiesto the entire arctic biome ona commonclassification platformwe wouldalso come closer toa circumpolarecological synthesis ofarctic vegetation since floristic similarities orvariations of (vicariant) syntaxarepresent to a great extent synecological affinities ordeviations.

6Conclusions Combiningphytosociological and gradient analyses,i.e. using classification andordination of arctic riparian plant communities as complementary pro- cedures,a wealth ofinformation on floristic-sociological structureand environmental relationshipsof floristically definedvegetation typescan be inferred.T oreveal differentiations inarctic vegetation cover,whose vegeta- tiontypes are oftenhardly distinguishable at firstsight, the above described combinedprocedure is recommended.It combines advantages ofmajor ap- proachesin vegetation science seeking toexplore maximum vegetation ecological relationships,whereas at the same time contributingto a common frameworkof arctic vegetation andecosystem studies. A major implication ofour results is the possibilityto use Salix-communitiesas indicatorsfor riparianhabitat characteristics andlandscape evolution. The above vegetation ecological analyses andformal descriptionsof associationsand subassociations form a furtherstep in syntaxonomicaland synecological research onthe NorthSlope of Alaska. Conductedon the onlyeasily accessible gradient fromthe BrooksRange tothe Arctic Ocean, it wouldbe ofhighscientific interest toextend the scopeof this study to otherregions north of the BrooksRange andto subject the above findings tosupraregional comparisons. W eagree with Matveyeva (1994) andde- Riparianwillow communities on theArctic Slope of Alaska 197 mandto first publish more data onlower unitsof syntaxa(i.e. associations, subassociations,vicariants, variants) beforedefinitely assigningthem toexisting higherunits or tointroduce new higher-level unitswithin the syntax- onomical hierarchyin a circumpolar perspective.Considering the need for arctic ecosystemstudies in view ofrapid environmental changes (e.g.climate warming,pollution from various sources, etc.), the completionof a circumpolar phytosociological/ecological synthesisof arctic vegetation shouldbe atoppriority on the arctic research agenda.

Acknowledgements: Thisresearch was supportedby the Max Kade Foundationgrant KADE-OCG3088 toU. Schickho ff,bythe U.S. National Science Foundationgrant OPP-9400083 toM. D. Walker,andby the U.S.National Science Foundationgrant 9908829 toD. A. Walker.Weare grateful toR. Andrus,Binghamton,NY ,for identifying Sphagnum-mosses, andto W .A. Weber,Boulder,CO,for identifying all other bryophytes.We thankF .J.A. Danie¨ls and J. Oksanen for their valuable comments on aprevious version ofthis paper.

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Addresses ofthe authors: Dr. Udo Schickho ff,Prof., Institute ofBotany and Landscape Ecology,University of Greifswald, Grimmer Str.88, D-17487 Greifswald, Germany; e-mail: [email protected] (corresponding author). Dr.Marilyn D. Walker,Leader,Boreal EcologyCoop Research Unit, Schoolof Agricul- ture andLand Resources Management, University ofAlaska Fairbanks, P.O.Box756780, Fairbanks, AK99775-6780; e-mail: [email protected] Dr.DonaldA. Walker,Prof., Institute ofArctic Biology,University ofAlaska Fairbanks, P.O.Box757000, Fairbanks, AK99775-7000; e-mail: [email protected]