Jonathan H. Titusl, PriscillaJ. Titusl and Boger del Moral, Depart..entof Botany Box 355325 Un verstV of
Washngton.Seatte. Washnqton98195
WetlandDevelopmentin Primaryand SecondarySuccessional
SubstratesFourteenYearsafterthe Eruptionof MountSt. Helens,
Washington,
USA
Abstract
'l
hc eruplion of \{ount St. Helens depo\ited nelv substraresupon which $ctlinds are no$ delebping. Furthcrmorc. rhe blast
damagcd !cgctalion iD wetlands sur|ounding the deposirionarca. This s(udy descfibe\ lvetlands in the blasr zonc norfi of the
crrlcr as documented l.l )e rs afier the 1980 erupiion $'iih 210 100 l11rplots. Five $etland types were rccognized in the lield:
moisl arcas. wcl ureus\!ilh shrllow sl.Lndingr\ rter, seasonalslrcarlrs.pcnnanent streans..rnd ponds or lalcs. The lariables,
elevarion.slopc and tlspccllvere determinedlhot warer springsr\ere di\crjminatcd liom cold warerspdngsbv a nominal !arirble
and substrat. $a! characlcri rcd .rsslableor unslrLble.
Fouf levelsofdisturbancc inrcnsil] were recognized.Spatial\'anablcs.i.e.
rhe position of wetlandson thc hndscape. $erc .rlsoused in anrlysis ofthe data.
Usirg TWI]-SPAN. $e ofganized the legetation into 2,{ wcdand plant clmnunities. Based on canonic,tl corespondence
analysis,$e determinedthat the measuredenlironmcnlal and spatial vrriables were unimporianl in struc(uirg primary succei
sion l vegetation.The lack of a relationshipbc($ccn the wethnd t)pe\ and environmentaltaclors in the prinary \uccessional
$etland communities suggestsihal stochasdccrents. such as seeddispe|sal.have played a lcading role iD eafly primar]' succcs
si()n.Vegetation!r,ithjn sccondarvsuccessionllwetl.rDdsapperrsto be more closely rclatcd lo rhe $etiand tvpes and environrncn
'lhus
tal \'rri bles.
$,e concludc that c.vironmcnhl lactors were nlore important jn structuring lvcllind vegetationin areasrlral
$ere ie\s imprcted b) the eruption lhan in tho\c arcasrhere $etlands \\,erenewly creacd.
lntroduction
The massive1980lateraleruptionof Mounl St.
Helens,Washington.destroyed,altercd and cre
ated many wetland areas.New wctlandsuere
formcd in depressions
within the volcanicsub
\ l r i r l e .T
. h e b l r . l d a n t a g erde g c t n t i , 'inn e x i s t i n g
wetlandsand blanketedvastareaswith air-borne
tephra.This study documenlsthc plant communities of the many wetlands found in the blast
zonc nonh of the crater. These wetlands range
tiom newly fonned wctlands to those that were
only sJightlvalteredby theeruption.Thus.agamut
of recovery pattems occur and arc analogousto
thosc dcscfibed by Grishin et al. (1996.)in
Kamchatka,in that primarysuccession,
secondary successionand simple recovery fronr damagc weredocumented.
Colonizrtionby vegetationin anynewly fomed
wetland requiressuccessfuldispersaland establishmcnt. The chancethat a propagulewill reach
an isolatedsiteis low, and is constraincd
by distanceto a source(del Moral and Wood 1993a),
rPfesentaddfess:l)epa(mcnl of Biologic.ll Sciences.Univefsit! olNevada-Las Vegas.I-.1sVcgas.NV 8915.1'100,t
186
NorthwestScience,Vol. 73, No. 3, 1999
ll L!rl9b\ rh \.nhlN
S.,enrLfi.A\.!
r!r
ALln-eh( r.scr.d
weather(Walton 1990,Titus anddel Moral 1998).
and seeddispersalability (Stdcklin and Baumler
1996).The chancethat the propagulewill lodge
in a favorablemicrositeis determinedby surlace
features(Tsuyuzaki and Titus 1996), the avail
abilityofnuhientsandwater(StocklinandBAumler
1996),and other physicaland biological factors.
Thejoint pr,'h.rhilitythatr proplgule arrrvesin u
suitable site at the right time must, fbr isolated
sites,be extrcmely low (Walton 1990),though it
increasesthrough tine. Conscquently,pdmary
successionin isolated areascan be considercd
stronglystochastic,i.e.,subjectto randon evcnts
(Talling 1951,Margalef1963,del Moral 1993),
iInJlhi: .tol.hir.licilymry. in pan.dct.'rmine
er entual vegetationpatlernsover largescales(Collins
and Glenn 1990. 1991,Collins et al. 1993).In
n e u ,b u r r e nl r e l . o n M o u n tS r .H e l e n . u. c t . i l e s
werc the fir'sttobc colonized(del Moral andWood
1986).The tirst immigrantswere snall-seeded
spccieswith high dispersalability (e.g.,Sal-r spp..
Epilobiunr spp.)or sporeproducin-lspecies(e.g.,
Erlrirclrnr,nosses)(dc1Moral andBIiss l993).
W c p r e d i c lt h a l .i n c d r l ) l r i r n r r ) ' u c c e . s i o n .
environmentalstresscoupled with a limited potential colonization pool preventsa signilicant
corelation betweenspeciescomposltlonand environmental factors (Chessonand Case 1986,
DaveyandRothery1993).Alternatively,sitc-specific conditions could strongly influence early
compositionin aconsistentway (Tsuyuzaki
species
1997).ln recoveringwetlands.i.e., sccondary
the long history of competitivere
successional,
arrangementand opportunity for most available
speciesto colonize implies that environmental
c o n d i t i o nusi l l c o n ' t r l i neo m m u n i l cy o m p',: i t t o n
Thus,
andleadto predictablespeciesassemblages.
we hypothesizethat secondarysuccesslonalwetlands are rnore highly correlatedwith environmental tactors than arc primary successional
wetlands.This study assesscsthe vegetationof
wetlandcommunitiesthathave
early successional
within the blast zone of Mount St.
assernbled
Hclens,the relationshipbetweentheseand morc
devclopedwetland communities,and the intlufactors
spatialandstochastic
enceofcnvironrnental,
on the vegetation.
Methods
StudyArea
The lateraleruptioDof May 18, l9ll0. createda
aroundMount St.Helens
wide rangeof disturbance
(46012'
N. l22ol l'W). Themainforceoftheeruption was directed northwards.hence this study
focuseson wetlandsin the blastzonenorth ol the
by thc
crater.much of which is encompassed
PumicePlajn and the MargaretRange(Figure l ).
The study areaclinlate is marilime with cool,
wet winters and $arm, dry summcrs(del Moral
and Bliss 1993).lnfornation collectedby the
former Spirit Lake RangerStation indicatesthat
annualprecipitationfluctuttcsgreatlyabouta mean
of 237 cm, droughtis conmon dudng July and
August, and meantempelaturesrangefrom a -'1o
C minimum to a 0'C maximum in Januaryrnd a
minimumof 7'C anda maximumof 22'C in August(SpiritLake RangerStation[987 m a.s.l].
PacificNorthwestRiverBasinsCommission1969).
Punicc was depositedover the entjre study
areain depthsranging tiom hundredsof meters
nearthe conek) lensofcentimetersat the edgeof
the blast zone.Where significant depositionsof
the resultingpumicesoils
pumiccaccumulatcd,
are immature and exlremely nutrient poor and
contain very low concentrationsof carbon and
nitrogen(Nuhn 1987,del Moral andBUss1993,
Titus unpubl.data).
A wide variety of wctland comnlunitieswith
varying levels of damageliom the eruption occur within the study area.The crater, 175(f2000
m a.s.l.,has sevcralvegetatedseepsin stableareas. but sffeams in the crater are unstablc and
supportno vegetation.Fumeroleson the lavadome
creatchyddc subsfateswherethesteamcondenses;
areintrequentlyvegetated
theseunstablesubstrates
(onc plot only in this study).The PumicePlain,
covers20 kmr nofthol the
I I 50 to 1300m a.s.1..
cone and was formed by the debds avalanche.
andlatcr
pyroclasticpumicet1ows,tephradeposits
Pumice
mudflows(Foxworthyand Hill 19192).
Moral
et
by
del
was
descibed
legetation
Plain
al. (1995) and Titus et al. (1998).Streamsthat
dissectthe PumicePlain are cither permanentor
seasonal,the latter of which flows only during
snownrelt.The stability of streambanksvarics
wideJydependingupon the stability of upstream
tephradeposits.Many cold water spdngs and
occasionalhot water springsoccur on the Pum
ice Plain. primarily nearthe north shoreof Spirit
Lake. The largestwetlandsalso occur nearSpirit
Lake in this sameirea.
northof the
Spidt Lake, 1300m a.s.l.,located
Pumice Plain, refonr-redon new substratesafter
theeruption.Due to learsof lloodsalongtheToutle
Valley, the lake level was lowered in 1983 via a
constructedtunncl. Wetlandsthen lbrmed on the
newly exposedsurtacesalong the south shoretlf
thc lake;the othershoresof the lakearevery steep.
On steepnorth-t'acingslopeslocatednearthe
Pumice Plain and Spirit Lake, areasthat were
heavily impacted by thc blast were covered by
snow and laced away liom the eruption. These
areasprovided reluge for many spcciesand enabled them to suNive the eruption as rootstock,
parlicularly late-successionalforest understory
s p e c i er\T i r u :('t r l . I q g 8I .A l t e rt h ee r u p l i o ne.r i L
sion quickly removed the pumice layer and exposedrcotstockwasableto resprout.Refugiawetlands include riparian wetlandsin the Margaret
Range,wetlandsalong CedarCreek,Smith Creek
andothercreeksthatflow fiom Windy Ridge,spo
radic wetlandsalong Green River. and wetlands
on the eastsideof the Punice Plajn (Figure l).
.ur
F u r t h e rf r , ' r nl h e c o n e .\ r \ m e\ e g e t a l i t ' n
vived at Meta Lakc (1103 m elevation,llJ km
nofih of the crater), Ryan Lake (1080 m elevation, 26 kn north of the crater). and St. Charles
Lake ( I 262 m elevation,l7 km north of the crater).Thesellkcs havesherelinewetlandvcgetatlon
Mount St. HelensWctlands
1lJ7
Uflil
GhostLake
Margaret Rangc
Norway
Pass
N
t
I i f u f e l . l \ { a po f t h c \ l o u n t S t . H . l c n s b l a s t7 o n e .
188
Titus,Titus.and del Moral
2
3
Kilom6tarr
and wetlandsoccur in low gradientstrgtchesof
associated
streams(Halpelnet al. 1990),but all
treessurounding the lakeswcrc killcd during the
cruption.In contrast,GhostLake ( I 154m eleva
tion. 22 km Dorth of the crater) and Clcarwater
Creek( 13,19m elevation,21 km rorth ofthe cra
ler) havcboth standingdeadtreesandintactpre
eruptionforest on thcir banks *,ith wetlandsthat
u'ere little alteredby the eruption.
Sampllng
We sampledwetlandsdudng the sunmels of 1993
and l99,1byestablishing
220 100-mrplots.Based
on the investigatorskno\\"ledgeof the study area,
wetlands were selectivelychosento adcquately
r e f l c c t h er l n g r co [ $ e tI u r r d\ e ! e l i r l i ' , ne. n \i r o n nrents.anddisturbance
intensitydescribedin the
previoussection.An areawas determinedto be
wetland if the soil was satunted for more than
threeweeksdudng thc growingseason,
roughly
Juneto SeptemberonMount St.Helens(delMoral
andBliss 1993.).
Soil saturationwas cleternrined
by repeatedvisits to suspectedwetland sites.
Wetlandsin the blastzoneon Mount St. Helens
contain large areas of uniform ve8etation and
zonationis not evident.Circular 100-m:pbts were
' c l c ( l e dI l l . . r m fl e h , ' m u ! e n e o uv\e g e t r t i o nc .r
ceptfor linearwetlandslocatedalongcreeks.which
were sampledwith 5 m x 20 m rcctangles.Wetland vascular vegetationdata consistedof percent cover of each spccies.Cover valueswere
estimatedby using a 0.25 m2 PVC frame with
stringsdelineating25 100 cmr squares.thereby
allowingtbr the determination
ofcover vallresto
thenearesl0.01mr tbr speciesvith lessthan l%
coverin a plot. For plantswith lessthan57. cover.
covervalueswereestinatedto the Dearest
0.1%.
Coverwascstimatedto the nearestl7c up to 25%
cover:and. jn 5% incrementsthereaftcrElcvation was determincdtionr contourmaps:slope
u'as convefiedto a 5 state scale(level kl stccp).
and aspectwas convefiedto an exposurescaleof
I = protected.north b 7 = south,exposed.after
Whittaker( 1960).NonenclaturefollowsHitchctrk
and Cronquist( 1976).
Noninal variables."vere
usedto characterize
the fbllowing tive wetland types: $,etlandswith
ephemeralwater: wetlandswith pennanent,shal1o\r'.standingwater:seasonalstreams:permanent
strcams;and pond or lakes.Water depth rvasnot
measuredbecauseit varicsdaily and/orseasonally in mary ofthesewetlands.Sp ng watcrswere
discriminatedby touch lbr two nominal water
temperaturcvadables:hotandcold. Substatewas
characterizedby norninal variablesfbr stableor
unstable (unstablc sites are subjectedto active
substratc
movementby snowmelt in the spring).
Site disturbanccintensitywascharacterized
by
tbur disturbance
levelsas follows: 1 = wetlands
sunoundedby standingdeadtreesand \\,ith survivin-sgroundvegetation;2 = rvetlandssurounded
by f'ullendeadtreesandwith sone sur.rivingground
veggtation:3 = wetlandswhereabovegroundvegetation was eliminated. but sonre belowground
plant organssurvived:and 4 = newly formed
wetlandson primary successionalpost-eruption
substrates.
Thus.plotsin Disturbance
LevelsI to
3 are secondarysuccessional,
whcrcasthosein
DisturbanceLer'el,1areprinary successional
(i.e.,
thePumicePlain).Someplotsin DisturbanceLevel
I appearto be conrposedof climax perennial
u c l l . r n L\ le p e l i r l in i r n da r ep r i m a r i l 5r c ( r l \ L ' r i n !
l i , , m b i o m a s I. o . . w i t h t ' u l l r g e Lh r n p e .i n s p e
ciescomposition.
Data Analysis
Four vegetationanalysistechniqueswcrc conductedto elucidatevegetationpattcmswithin the
blastzone.First,in ordcrto distinguishthe wet
landpltrntcommunities.
TWINSPAN clusteranalysiswasusedbasedon speciescover(Hill 1979).
For eachcommunity.meln speciescover.spccicsrichness,
anddiversity(Shannon-Wiener
index,basee) wascalculated.
DetrendedCorrespondence
Atalysis (DCA)
wasuscdto obtainindirectordinations(Hil1and
Gauch 1980).An indircct oldinationyields an
ordinationdiagramthatshowsthedegrecof sinrilarity in speciescompositionbetweenplots. ot'
the degreeof similarityof speciesdistributions
rvithin plots.As speciesdistributionsdiverge.thc
distanccsbetveenpointson the speciesordination diagran increase.First, thrcc DCAs were
conducted:thc cntire data set. primary successionalsubstrateplots.and secondarysuccessional
subsffateplots werc analyzed.Speciescoverpercentagcswere log-transtbrmedto improvc plot
spreadin ordination space,and infrcquent speciesvere downweighted.
Downwcightingdecreases
the inUuenceof a specieson the outcomeof the
ordination.FinalJy,a fourth DCA was conducted
in orderto assess
the rclationshipbetweenthe four
disturbancelevels.To do this.eachplot wasrcpre
sentedby its disturbancelerel in the DCA.
Mount St. HelensWetlands 1139
Spearmanrank correlation tests$,ereused to
makecomparisons
betweenthefjve wetlandtypes
(.Zar 1981) and to dctcrminc how inportant the
u e t l u n dt l p r ' r r e i n s l r u e t u r i nrgh r r e B e l r t r ornn
the primary and secondarysuccessionalareas.
Speaman'slest determinesthe degreeoI correlationbctwccntwo wctlandtypes.i.e.,ifthere is
a significantresultthe two wetlandtypeshave
similarvegetation.
To cempletethis analysis.the
. t r \ e r , , 1e i r . hs p e i i e si n l h e n l l l t . l h i r l. r ' m n r i \ e
eachwetlancltype were averagedand theseaveragcvalucswcrc rankcdandassigncda scorebased
on thcir rankcdposition.The outcomewas correctedtbr ties in the ranks,and then testedbv
Spearmans test.
Relationshipsbetweenthe enviror')mental
va ablesand the plot and speciesdatawere explored
by CanonicalCorespordenceAnalysis (CCA, ter
Braak 1990).CANODRAW and CANOPOST
graphics were used. r'hich produce ordination
biplot diagramsthat showtherelationshipbetween
plots or speciesand environmentalvariables
(Smilaucr1993).On thcbiplots.Axis I rcprcscnts
the dircctionof thc grcatcstarlount of variation
aDdAxis ll represeDts
the secondgreatestamount
of variation in the data set.CCA determinesthe
perceltage of variability in speciescomposition
ofthe plots that can be explainedby the environ
nental variables.
The entire data set was analyzedby CCA to
detemine overall pattems,and subsequentanalyses
wereconductedon plotsin bothprimaryandsecondary successional
environments.In order to
assessthe contributionof each environmental
variable in deternriningvegetationstructure.
interset correlations and t-values for each vaLriable are examined.Ilterset correlatiors arc the
cor:relationof each environmentalvariable with
theCCA ues. The t r rluesoI erch elr ironmen
tal variable are compared\\"ith the Studentt-distribution;a variablemay contributesigniticantly
to vegetationstructure if it greatly exceedsthe
appropriatecritical value (i.e., t>3). Thus. envi
ronmental variablesthat contdbute significantly
to obseNed pattems were inferred (Jongmanet
al. 1987.ter Braak1990).Notethatvariablesthat
have high correlationswith the CCA axesdo not
necessarilycontdbute signilicantly to obsen'ed
patterns.In addition.in CCA the relationship
bet\\"eenthe speciesdata and the envilonmental
datacan be testedto determineifthe relationship
is stlongerthanrelationshipsgeneratedrandomly
by usingthe Monte Carlo tesl (ter Braak 1990).
190
Titus,Titus.anddel Moral
This was testedtbr the llrst axesand all the axes
combined(crJledthe trace).In all of the CCAs.
speciescover percentageswere log-translbrmed
and inlrequentspccicsdownwcighted.
This was
doneto inprove speciesspreadon thc ordination
diagrams.Finally.a CCA was conductedusing
wetlandtypeasthe only environmental
variable.
This allowed us to determinethe importanceof
the relationship betu'eenwetland types and the
\i'etlirndplantcommunities.
Geographicposition has beenfound to play a
role in determiningwhich speciescolomze a site
(Primack and Miao 1992: del Moral and Bliss
1993).Borcardet al. (1992)outlinedhow CCA
can separateenvironmentalvariationtiom variation in the specicsdatacauscdby spatialt'actors.
Spatial positions of the plots were assigncdby
mapping the location of each plot on a grid of
squareswith 25 east to west rows, (X), and l6
north to south columns. (Y.).Thereby,each plot
had a X and Y location and each squarercprcsented0.6,1km2. [n order to assessthe importance ol complex spatial patterns.the matrix of
two dimensionalcoordinates,
X andY, vas expandedinto a cubic trend surfaccrcgressionby
adding locationsrepresentedby X'], XY. Y:, Xr,
X']! XY']andYr. Thesenine spatialvariableswcrc
analyzedusing "forward selectionof cxplanatory
variables",a multivariateextensionof stepwise
regression.
Speciesdata $ere analyzedby the
followingtraits:(a) constrained
by the environ
ment after spatial factors are removed. i.e.,
nonspatialenvironmental
variation,which is the
poflion of the speciesvariation that can be explainedby the environmcntal
variablesindependcnt of any spatialstructure,(b) constrained
by
spatiallactors atter removing environmentalef:
tects,i.e.. spatialpattemsin the speciesdatathat
are not sharedby the environmentaldata,(c) constrainedby spatiallystructuredenvironrnental
variables.i.e..spatialstructuringin the species
datathat is sharedby the environmentaldata,and
(d) unexplainedvuiation, which includesvaria
tion due to unmeasuredenvironnental variables
and stochastic
tuctors(Borcardet al. 1992).
Results
P antCommun
t es
Twenty-lbur wetland plant communities were
distinguishedby TWINSPAN analysis(TablesI
and 2: speciescomposition of each plant com
munity availablefrom the fust authorupon request).
T A B L E L N u m b e fo f | l o t s . i u c c e s s i o n as]t a t u s ( 1= p r i m v . 2 = s e c o n d a r y )m. c a n l o t a l p e r c e n ct o v e ro f a l l s p c c i c ! .s p e c i e s
richress and dilefsiry (Sh.rnnon-Wienerindcx. basee) of 2.1\\iedrnd communiiic\ lion the bla\t zone of Morlnl Sr
Helen\.
Plant Cornnlunidcs
Sample
Sizc
l.,A.qu.rticvegetatur
2. EquiseILnIut it1!ile\\er1^nd
3. Cdrer lrl.,r-rllzr $eI nreado$'
4. Cdrexdqudtilis Carct 1.,r/l.r,l,i' $et meados
5. C.rrc..r..rr(r.?rr wet neado$
6. Dislurhed hcrbaccousstrcumbunks
6a. Priman' successionaldisl. herb. streanbrnks
6b. Secondar! \uccc\sional disl. herb. stre.rIl1banks
7. Reb!! ?.,.rdbili.l thickcts
8 . . l r u r ! l i , l r d i . rt h i c k c l s
9. Secondarysucc. .Jd/i-r\inhc sis/Equisetunarv s?
10. Prim.ry succ. .\,/d !ltuh(nti\/fquisetwt an'? seA
I l. Prinary succ. Sdrlf rlttl,"nsis/Equi*tum u^.?nsc B
ll. .!'.alirr-inrr,rrir primat succes\ionalwetlands
| 3..\al ir sitttu nsislCam rr.rrrrll $ etlands
l.l. Primar) succcssiondEqrl,r.rfi d/-f.,nie wetland!
15.Jtr/].rr ,lrlornr.' wetl.rnds
l6. rr.?rr drri.r/dtr.r $ethnds
17. ,r.!
! ./frl,rr0,,./ii \aetlands
1lJ..rrk?! r?, ri! wcrands
19../,n.,!.,!i/,1nl! \rerlands
20. bTrd /drdb1,auetlands
20a. Z ldrilb/ld primrry successronal\{cllands
2Ub.7: l.rlttln secondar,vsLrcccssionirl
wcllands
21. S.,?r/.r .,r.,rnrdtrr ! uellands
)). Et)ilobi un \\ ul :oni i \rerlands
)3. Epilobi ktrLdn eu!1fu/i,,r wetlands
21. CdLdDktRra
stis canadtrr;.i u etlands
In a fcw cases,communitiesoccured in both primary andsecondarysuccessional
substates.however,in most casesspeciescompositiondillered
si-qnificantlybetweenprirnarl,andsecondarysuccesslonalareas.Most widespreadspecies.with
fhc cxccption of Sa/r,rsrtc/rcnslsand EtltLisetunt
arferJe. wefe confined to cither primary or sec
ondary successionalsubstrates.For example,
J r l , ?/ /, . rm c : r d o \ \r.) n l ) ( \ ( r t r r ( d i n p r i m a r l . u c cessionalareas.while Care-rmeadowswere only
in secondary
successional
areas.
In the DCA analyses,
aquaticvegetationplots
(composedof submergedor f'loating-leavedyeg
etation) werc rcmoved tiom the analysesdue to
high dissimilaritywith theothcrcommunities.The
DCA of the entiredatasetcompriscd203 plots
and 20,1spccies.The tirst t$'o axes of the DCA
ordination explain 9.3 and 8.3'lc of the yadance
ll
l8
l9
6
l1
2
t5
I
5
2l
.l
t1
2-l
5
IE
1
.l
3
I
6
t2
l0
2
I
9
Status
7t
Co\'er
l.l
2
2
2
1
1.2
)
2
2
2
I
I
I
I
I
88
1
91
21
28
11
120
3l
63
12
39
5l
11
,18
75
I
I
I
l.l
I
2
I
I
I
I
6)
69
91
87
lll
Species
Richness
6.8
l5.8
19.9
13.1
18.2
19.9
20.6
26.0
22.5
l5.l
t3.li
1.1.3
19.6
11.1
10.0
10.3
1.3
r3.0
10.5
10.8
9.0
r0.0
65
t0
6
12.0
F.0
16.8
Spccies
Diverlilv
0.60
0.68
1.51
r.l1
1.IJ9
1.91
1.36
1.98
2.11
2.01
1.76
0.78
Lt8
1 t. 6
t.71
0.83
0.94
t.0t
t.04
0.61
1.33
1.1'l
l.l1
0.i12
l . l' 1
1.15
L56
r.96
in the spcciesdata, respectively.Thus, the axes
explain onJy a small percentageof the variation
in the spcciesdata.Axis I in thisordinationis,1.7
half changcslong, which is indicative of a data
setwith a widc rangeof speciescomposition.In
the primarysuccessional
dtrtaset( I l6 plotsIonc
outlicr plot removedland 104 species),
the first
two axcs accountfbr 14.,1and 9.97r of the vari
ance, rcspectively (Figure 2). In the secondary
successional
dataset(86 plotsand 177species).
theseaxesaccountfbr I l 7 and 9.0% ofthe vari
ance.Iespectively(Figure 3).
Disturbancc
levelsseparated
noderatclywell
in ordinationspace(Figurc4). Primary successionplots(Disturbance
Level 4) andheavilyin
pactedplots (Lcvel 3) were separatedlionr Ler'els land2alongAxis[.Lcve]sI and3 aredistinct
from Levels2 and,1alongAxis II.
Mount St. HelensWetlands 191
TABLI
2. Dcscriptionsof2,l wedand communiriesfroln the blasl /one of Nfounr St He]ens.
P l a n tC o m m u n i t !
l. Aquxlic!cgctation
Occur\ in deep $aler in all fie
ponds and lakcs cxanrrned.
). Equisenunll tiatil( \'ter-
Onl)- occurs in deep rlater at Rlltn
Lakc.
3. Curer inletntta \\er
1. C{e\ uquatilis Carex
l.rril &/fl/i! r'el meadow
Occurs in lo\l disturbrncc $ctland
.reas br" Cho!! l-ake and abng
Clea|wrter Creek.
Most conrnlon al0ng lhc shoreof
Mela L-ake.
Connon Spccicsl
Notes
Por.,rag.ton lbliosus
Potomogctm naldns
Ra,tL,n:ultls dquatiI i s
Floating lealed rnd sub'
nerged \cgctation. fhe alga
C u r e rh r u n n e s c e n s
\'pha luti|otul
Equisehm db-cn\?
Lquisetto an'(nt(
Ettltis(tun flwietile octrns itl
deep lvater.Nlost ofthe 0thcr
spccicsoccur on the pond
.trr:f rll.l.,?rir is oi|en co
dotL\inanr. EIi)phntum
dr.qrrrdirliu, occur\ occa
sionall!.
EquisetunLdb-ens( i5 com
Ranun(LlussctLeratus
.!.r1Lrli/.l.nrir
5. ( n/er.dr.r..r.i
wcr
Occu|\ along $c lhore of St.
CharlesLake.
and
Sd/,.rrir.h(rri-f is often co-
Equisetan ancns(
Potumos&,t hliosus
Sptltgdni n emersw
T)"pheknifiniu
6. Distufbed hefbrceous
O c c u r si n b i g h e re l e v a t i o nh e a v i l )
dislu$cd secondarysuccessional
.lutlcus rcrtcnsi'nus
sites.such as along crccks in
Punice Plain refugia and the
\hlerid|a sitLhen\i\
MargafetRange: also in primary
succcssionalsitesalong crecks on
r h e P u m i c cP l a l n .
7 R , , f u . \ 1 , ! . / , r / , / i rI h i ( k ( r \
Occufs only on the banks of Ryan
Lake.
8. Alrr.i ri?rdrr? thrckets
Occurs in moist. heavilr_dislurbcd
secono v successlonarareas.
9. SecondarySucccssional
SaI i r \it chefish/ Equi sr nm
192
habitats.Slopesare steep.
espccially ln secondafy
Di!ef\e \econdarYsucccs
A I h)"I i an li I i r t'cD u,1
u
Epilobium o gusIiJolint1 sional typei cover i\ high due
to densemultiple !elctation
lalers. This vegelationis
common rs fbrest understor,"in lvcl sjtes throughoutthe
Pacilic Noihlvcn (Kunze
1 9 9 , 1C. h f i s t ) & T i l u s 1 9 9 7 ) .
Ath|rian liLixf(nlinu
Epih)biu t 't txsonii
F o u n di r h c a \ i l y l m p a c t e ds r t e s
An(tpltal i s murllaritut ld
a l o n gc f e e k si r H a m o n y B a s l n
and the M,ugaret Range.and alorg
q)ih)biutI utltsonii
thc Creen River and Bean Crcek:
Juncu\ m'!ften\it]nus
rlso lbund in lcss disiurbed sites
by Gho\r and Meta Lakcs a.d
Coldsalcr Creek.
Titus.Titus,and del Moral
These areasare olicn species
ich u,ith more than 20 species
pef pl01.Reco!eing habitals
0i this t_vpeare mLrchmofe
diversethrn rcccntl) jnilialed
Underston is diverse wilh
unrally lnorc |han l0 specle\
in a plot. ,1r1,lJr nrx.rrdthick
ets are conmon on prlrnar)'.
successionrlsubstrates0r &e
Pumicc Plain, but only in
Thesc arcasare olien species
fich with more than 30 species
ln a plot,
TABLE 2 Continued.
Planl Communitvl
10. Primaq Successional
Suli; \it(hnsi slEquke nL
drr.r.\r wellandi ( lypc A)
C n m r n u nS p e r i e s l
Occur\ rcr6s the PunriccPlait in
se|eral habltats.usurllr- on moist
Epibbitm \,a^onii
.luncut np tn'itrt s
Othef speciesaI bw covel
underslorydilcrs liom the
seconda|vsuccessionalSaltr
I it(hqs islEq isetun a rt.nsr
Iype.
I L P r i m a r yS u c c c s s i d l
Suli\ tiklrcnsis / Eqlis.tun
dn,.,]re $ethnds (Type ts)
Comnlon in the norlhcrn part of the Anqhuli\ narydtitacea
Pumice Plain: occurs most comEikhirn
tratsonii
only al01g pernanenr streams.
Hlpochacis rddicakt
C o n t a i n su p l a n di n v a s i ! e
12..taiB !ikl,"rrir doninatedprnnarv successional
wetlands
Common acros\ rhc Pumice Plain.
usuallv on the b.rnksof permarcnl
stfeamsialso occurs in the cratef.
Lquketunt anens( | Dal
prcserl of oDry pfesenlrn face
1 3 . S d l it ' i k h ( n r i s / C a r e r
Occurs in the northem part 01 the
Pumlcc Plain 0n ephemeralmoist
And?huIiv urgaritdcea
Epi I obi LnLang us! i fit t w1t
Etil&tun watsonii
Il\pochdetis nkli.d
AnaphdLisnarearitat ea
Understory dlffcrs liom othef
.tulir !ir(i"nrir 1)pes$ith a
grcatcr cover of upland inva
Putstctno cdrh1e11ii
Sxtiltusulin ustned
1L Equircxtt rLntnse
rcdands
Occurs only in thc northern plllt of
thc PurniccPlain und is the mosr
Epilobiutit udtsottii
conmoD well.rnd rype on the Plain. II)"po xerh tulk:dtu
Large areas0f wct silt sedi,
ments arc doninated bv
monoculturesof a?,i.l(rr,?
dr-r.,/i.1?r
rvith inlicquenl
occLrrcncesof other species,
This \egerarionrvpe occurs
mon liequenth in moist and
standingwater habitars.
15.J&fl.&r-&,rldrrrr wetl.rnds Lo\ divcrsir] Jur.rr rrd.rrl/rr
wctlands cover exten\ive ephern
Equisetu,t dtr?n:e
eral moist afeasin the northcm pal1 Epilobialt wx!,tii
of the Pumicc Plain.
O t i e n l o c a l c do n m o i s ts i l t
16. Junc^ ut !i(uluhr\ \"tel
lands
Habil.rt is sinlilrr to "/!,(x.1
J u | t u s b u f u n i u \ a n dE E . t i s a
/rm drrsn.l? are olten co-
11.Ju|cus drwltnn ldii
Habilal is simillr to./rrcrr
18. .rrr.ri
H a b i t a ti s s i m i l a ft o J r ' . ! r
Equiselumarr.n*
O c c u l s0 n t h e n o | t h e n d o f t h e
Pumice Plair in cphcncr.rl lnoist
habjrr|s und in standing$arer and
LL,ibhiun \ats0iii
EEtisctutn drtens?
larrii wetlands
19. ./r/l.rftn jit)1ta re!
20. 1Vhu luttalie \)crlanrls
Epilobiun ]ratJ,nii
Equi\(nunanense
Jen.r! ,l/lo'rtur is co-domi
Occurs in sranding!lat!'r on pri
Epil iun \rdtenii
mar] successional\ubstralesin dre Lquiutum u^,(nse
Punice Plain, and in secondar,v
successionalaleasalong the shore\
0 f s e ! e r a lo f t h e t o n d s .
Lqltisetum arr?n\t is altcn cadominanr and ilr.llr ,,y'),i.r.1
i s u s u a l l yc o m m o n .
Not strongly associaledwith
the oiher Jr,1.r.,rspecies.
This rype occurs acros\ the
di\turbancc spcctnrm.Second
ilr) successidralsites arc
similar lo prin:rv \ucce\
sional \iies in richncss.but are
Mount St. HelensWctlands 193
TABLE l. Continued.
P l a n rC o n r n r u n i l y
2 1 . 5 1 1 4 " ! su n t r i t x n u !
!!cllands
OccuISirhequently in standing
$'ater acfossthe Punice Phins.
1). Lpik$i un \\1 tsot t i i
Occurs along both permancnland
efhemerrl streamson the Punice
Plain.
13.?ikiiun
o guslil ir"l
.drd./.rlllr wetl.rnds
Connnon Species'
Notes
Eleo(hutit rtluntil
Associaled$ith hot water
spri'1gs.
The preponderanceot
Lpi I obi un u at sonii ^\]rl \h.
sifeanbllnk habitat seprrate
rlli\ fto|n Equi:e!un att'ens?
and rr.&r brlrriirr rclland\.
Occurs along both permanenland
cphcncrirl sllcams on lhe Pu ice
Plain.
Andp httli s margarikk ed
EpiIobiunt utltsonii
Occurs in dricr habiratsand
contains more upland in\'.r\i\.'e
spccicsthan ]tpilornfi
Occurs alolig pcrmancn! slrcams
l.)r the Pumice Plain.
Anqhdlis mdrgdrikkeu
Epi|obiunt unK rtiloliun
Epilobiun $atsonii
Diverse pf imary'.succession.Ll
c o m m l r n l t ytl h i s i v p e r s
c0rlposed |)1both upl.rrd and
LVegetationrlpes not discusscdalc as lbllows: moss donrinatedwetlandr occur patchil,vacfossthe Pumice Plain\l.ll.r.rdrrrrr
prtche\ oecur in seepagearcar I l\poc hueris radkrt r dominatcdrvetlandsoccur or the Pumice Plain and in the c mtefl patche\ of
Petdsitesligidus occnt tn somo oi lhc sccondarylucccssionalareaslalgal mats are common along both hot and cold spring run\
rSpccic\ composilioll ofeach plant communit) a\'ailablefrom ihe first aulhor upoDrequest.
o .,ta'
3. 0
10
10
--
2424n"12
2s zq 24i2'-.^
2a 2a.^2a
12:i2
12tz21ti'
12 1i-12
'tz
lz 12
12
1l
1't '',
..
11
l0
1 4 't4
M
14
14
14
19
19
19
19 19
1[ 11
14 14 $
14
11
2'l
14 15 ,t5
14
la
. r- 1 5
tl
1 4 1 4 , , 1 1 61 6
15
2222-- i;- 20
2 2 ' zz z m f f
2f;'oozz zo2o
"o "o
io
"o
1t
l7
s
't1 ,r1r119
11 1,1
t.
4.0
FiSure 2. Detrcrdcd CoficspondcnccAnalr_\isof primary succelsional plots. Nutnberscorrespondto communiliesin Tables
I and2.
194
Titus.Titus,and del Moral
3 3
3
3
-3
t
-o-n66e66 e
6
7
a
g
4 4
6
ara, ,
9
s 9
9909 I
Figure 3. Dcrcrded CorrespondenccAnrlysis of \econdary \uccessi(nrulplots. Numbersconespond to communities
in lhblc\ I ard L
Correlationsbetween Vegetation of the
Wet and Types
S p c u r m . trni r n Lc o r e l r t r o n .l m , r n gp r i m r r S: u e cessionalwetlandtypesshowmodest,but signif-rcantcorrclations(Table3). Vcgetationin wetlands
u i t h p e r m a n c ns th : r l I r u. l u n d i n gu l c r i . n , ' l\ i " nilicantly corrlrlatedto vegetationalong ephem
eral streams.which contains many upland spec i r \ I n \ e r u n d r r y s t t c c c . . i l n r rel n r i r o n m e n r r
wctland types ale not corre]ated,$'ith the exception of ephcmeralstreams.which are prinarily
found in heavily impactcdsites.If the heavily
i mpactedsecondary
successional
sites(Disturbance
Level 3) areremovedfrom thc analysis.therelationshipsdo not change:horvever.the epheneral
strcamgroupthenhasan insufficientsamplesize.
canbe explainedby theenvironmental
variables.
For the entire data sct, both axis I and the trace
are significant as analyzedby the Monte Carlo
test(P=0.0l);thatis,therelationship
betweenthc
specicsand envifonmcntaldata is strongerthan
randomly generatedrelationships.
^ ^^l
u a " o ' r c a r /u1 ^o. . "^ ^e^ ^s^ ^o^ ^o^ ^' t o e r c e A
Ta t y s , s
Wetlandcomrnulities did not segregatein the
CCA biplot. indicating thar they are not charac
terizedby a consistentcombinrtionof environmentrlr rriublc..The In,'.1inllonanl(nvironltren
tal variablcon axisI, basedon intersetconclations
fTirhl(+) andl-\ r lue.. $ asd i.rurhunc(le\ e| (l 19.76).Axis II hasils strengesr
relationships
$'irh
elevation(t = 10.53)and the nominal variable
"ephemeral
streant'(t = 10.08.1.
Also on axis ll.
the Dominalvariable"wet stream".althoughnot
highly conelatedwith axis II. was significant(r
= 8.78).
Entire Doto s'et. Aquatic plots wcrc distinctive
andwcreremovedfrom thc analysis.Of the !ariabilityin specicscompositionofthc plots.24.761,
Prirnar,tSuccassitnalPlols. The CCA of plots
on prin-rarysuccessionalsubstratesis shown in
I i g u r e5 . O l t h r r r r r i r h i l i r li n s p c e i erso m p o : i -
/--^^^i^^.
M o u n tS l . H e l e n sW e t h n d s l 9 - 5
^3
r3_
ri.t 3.
3
3
4
4
",':',^ff!ii:i""
. ,,lilf3';.
1a'l
.1111',,
1
1 t"
1
,|
2
4
4
4
4
4 , 44
4
4
1
4
,:,,,ii:ir'^''i
i' ,
12
2 1
2a
'2
2'
2
2 2
2
;'
10011'oo
oXo
44' ! 44
444
4 2
a
44
2
5.0
I igure.l. Dclrended ConcspondenceAn.rl)sis of all plors. Numbers e thc DisturbanceLc\cl\. l=lo\\' rnpacl: 2=nrodefate
imprct: 3= hcavilt inp|clcd lecond ) succcssion:'l=prinrry succession
T A t s L E L S p c a f l n a nr . r n kc o n c l a t i o n sb e t \ \ c c n! l e t l a n dt l p e s .T h e c o v e fo f e a c hs p e c i e si n t h e P ] o l \ c o m p r i s i n ga s c l l a n d t ! p e
$ c r e a v e r a g e dr . a n k e dc.o r r e c t c di o r t i e si n t h er a n k s .a n dt h e nt e s l c dh ! S p e m a n ' sl c n . A s i g n i l i c a n l r c s u (l ii n b o l d )
indicatesrhat (he nro \ietland llpe\ have vcgctationthal is significantly correlalcd.1npfiDarl succc\sionalwetland
q pe!. pond $r\ not t.stcd due to small lalrlple \ize. and in \econdary succcssionalwetlaDdtlpcs, standmgratcr wa\
nol testeddue lo snall s.rnple sr/c.
\!ttl.rnd Typc
Prinrarv Successi0nrl
m o i s t ! s . e p h e m e r a\l r c a m
r=0.366p<0.005
r=0.316p<0,02
n n ) i s t! s . p c r a n e n t\ t r e l m
r=0.597p<{1001
s ( a n d i n g$ , a t e r\ s . c p h e r n e f asl r c a m
F0.17,1p>{).1
r=0.555p<0,001
r(ri\t vs. nrndiig \\atef
\tlndrng $alef vs. Pcl a.enl \Ilelllll
s u c a ) n\ s . p e f m a n c nsl t r e a n l
cphemeriU
ephcmcral stfe.rn \s. poncl
permrnent sfc.1m vs. pond
196
Titus,Titus,anddel Moral
r=0.550p<0.001
Secondaf) Succc\sional
p<0.001
r=-0,,119
r=0.170p>0.I
r=0.0,16p>0.5
r= 0.038p>0.5
r=-0.258p<0.01
r=0.167p>0.05
24
21
14
24
.Hot
12
23
24
21
t5
1520 t6
14
srABtL 24 24
14
14
14
6 z o 1 6 22
2020
20 '14
20
.WETSTRE
.DRYSTRE
11
12 t.
rot"u." tr3r4a
19t9
1114
t"l!',o
rt
Figurc5. Canonjcal CorfespondenceA nal) sis plot en\ ircnm.nl biplor of pri ary succelsionalplots.NuInbe|s corfespondto
comlllunities in T bles l rird:. Abbrc!ialion\ ol cn\ ironmental\,'ariablesare defined in Trble.l. Seetexi fbf interyret'
ing the dirgruln.
rion of the plots, 19.0%can be explainedby the
environmentalvariilblcs.Nole that disturbance
levelis not includedin thisanalysissinceall plots
Both axis I andthe trace
areprimary successional.
aresigniticant(P=0.01.Montc Carlotest).In the
n r J i n a t i r rhni p l , ' tJ r r g l r r n rn., . r m i neanl r i r o n m e n
tal variablcs are representedby points and continuous environnental variablesare rcprcscnted
by vectors.The directionof a vcctorrepresents
thedirectionof maximumchangeofthe environ
mentalvariableacrossthe diagrrm andthe length
of a vector is propofiional to the magnitudeol
varichangein thatdirection.The enr,ironmental
ablcsthat have long arrows ale more closely correlated to the vegetatiouthan those with short
arrowsand are usuallymore jmportantin intluencingcommunityva ation.The vectorshould
alsobe considered
asextcndingbackthroughthe
origin. A point that representsa plot can be related to each vector by dlarving a perpenclicular
from the line of the vector up to the point represcnting thc pbt. Plots with thef perpendicular
projectionsnear to or beyond the tip of the vector will be stronglycorrelatedwith and influcnccd
variable.thosenearthecenter
by thatenvironmental
will be less affected.AIso, the position of a vector with respectto an axis indicatcshow closely
conelatedtheaxisis with thatenvironmentalfhctor
(Kent andCokcr 1992).
Basedon t-vtrlucslbr Axis [. the variablesel
cvation.pemranentstreamandhot watercontribute
the most to vegetationstructure.Elevation,slope
and ll.\pecthavc thc highestcorelations with axis
1(Table4). Althoughslopeandaspectcontribute
significantly 1()the pattem, they are less impor
tant than someof the wetlandtypcs and watcr
Axis ll is not stronglycorrelatedwitl]
temperaturc.
anyenvironmeDtal
vadable,nonethelesselevation
andsubstrate
stabilitycontributethe mostto this
(i.e..
andcxposurcdecreases
axis.Slopesincreasc
Mount St. Helens\4'etlands 191
TABLE .1. CCA irterset correlatioDsoi er!irunncnt.rl variable\ with the caronicrl uxes.All uDal)ses$ere conducted $'ith
a q u r r i c\ c g c t a t i o nr c o v e d . S c o r e s a r e t i m e s l 0 0 0 . V a r i a b l e s t h a t c o n t r j b u r e s t r o n g l y t o r h e o . d i n a t i o n ( > 1 . 0 ) a r c l n
bold. Varirble\ rhar hr\'e high cofrelrt(ms with CCA rxes do not necess il) contfibute \ignific.Lnrlt to obsened
prlrerns. Thus. large i crscr cofcldtio] lalucs iuc nol nccc\saril] \igrilicanl.
All Plots
Ax I
Ax2
Vaiablc
106
Il0
;103
Nloi!rSite-N{OIST
-109
-122
$'et Site wET
281
184
164
-194
320
Ele\rtx)n'ELEV
S l o p eS l - O P E
,A.specr,A.SPICT
Ephcmcnl Slrcanr DRYSTRFPernranentStreain wt I Sl ltE
Pond-POND
Hol Watcr t{OT
Cold $'atef'COLD
S r a b l eS u b s t r a t eS l r \ B l L
Unst.rbleSubstr.rte-U\STAB
DisrurbanccInrcnsil! DIST
22.2
222
-19,1
291
865
-216
-266
266
198
198
-120
Most of thc communitiesdo not segregate
in
the CCA biplot. irdicating one or more of the
lbllouing: plots cornprisingeachplantcilrnmunity do not bclong to a similar set of environmental variables:r'egetationis respondingto unm e l ' u r . . l r r r i r r b l c . J: I L I / , \ r\.c g e t r r i o np r l l e r n :
a r e5 t o c h r . t i ci . e . .t h ep i l l t e r n i.n l h e \ e ! e l i r l i , , n
.r|c duc ltJ rlndonr flctors nther thrD cn\ iron
mentalvariables.Weak specieselvironment pattems werc lurther ref'lectedby the common spe. i e . - i n t h r t m r \ . 1r \ f l h c c , ' r n m o n. p c c i u .\ \ e r c
locatednearthe ceDterofthe biplot (not shown).
$'hichindicatesthatthey inhdbita varietyol environnents. Weak tendencies,such as the aftinitr-ol Equisetuntarterse firr moist areasand,Sa1n
sitchensisfor seasonalstreams.were apparent.
Most Jrrcrrs speciescluster aroundthe wet substratevariablg.
P1ots.Thc CCA of
Secondary SLtct.'essionaL
plotson secondarysuccessional
includsubstrates,
ing non-aquaticpond vegetation(i.e..emergent
vegetationneara pond shore).is shownin Figu r ( t r .O l I h ( f l i ' l r l r r r h r l i t l r n r p e e i ei' o m p o r i tion,31.71/acan be explained by ervironmental
variables.Both Axis I and trace are signilicant
(P=0.01,MonteCarlo test).
Titus,Titus,and del Moral
Sccondar]
SuccL'ssional
Ar I
Ax2
6\2
507
-60r
-2119
-389
786
t59
,163
-218
t11
.136
more northemaspects)with increll.sin-q
elevation.
Wetandmoist sitesgenerallyoccurat lowerelevations,whereasstrcamsoccur at higherelevations.
198
Primar!
Succersional
Axl
Axz
-378
178
'7',7
1l
369
150
r3n
- 13,1
6
5t-)
1i5
338
ll8
t9l
-393
107
171
106
-399
609
609
-720
8,1
112
494
9',7
152
25
-503
Axis I is correlated
with elevation(t = I 3. l9).
(r=7.79),
ephemeralsfeams(t= I1.50),distui'bance
and permanentstreams( = 9.15):substrate
stability is not signiticant(Table4). Axis II is cone
latcdwith ponds,disturbance
and moistareas.It
is cleal from Figure 6 that, with incrcasing elevation. slope increases,whereasexposureand
substratcstabilitydecreases;
ephemertlstreams
are the most common wetland type at higher elcvations.Thus,thc "disturbedstream' \'egetation
type is tbund in a portion of the diagramthat il
lustrtrteshigherelevation,unstablesubstrate,
steep
slopes,and ephemeralcharacter.
Plotsare strongly clusteredin severalptfis of
the CCA biplot (Figurc6). Mtrny communities
retain their coherencein the CCA biplot. indicating that the plots comprisingeach community
h c l o n Et , ' a . i r n i l a r . e to l m e r s u r e eL nl ri r , r n m e n
tal variables.C4re,rmeadowseccupythe lett pafi
of the biplot and vary along axis II. Carex rneadows are usuaily associated
with ponds,except
Cttrc\'lrtcrntl)to murd,ru..u hich llen ur'iur in
wet areasnot associatedwith ponds. Thcrc is a
cleardistinctionbetu,eenheavilyimpactedstrean
side sites and Cai'e-rnreadows,becauseCare,t
mcadowsoccurin cnvironmentsthatsuff'eredrelatively little disturbancefrom the eruption.Utllikethc prinrarysuccessional
CCA. Sa/ir sr'lcltenstr
tnd Equisetuntarvensed<tnot occur in such a
g
:3.1
Morsr.'J
)
a
4l
9' s9 9
8
sa s
WETSTRE
STABLE
r g C
e,
;5_5
20'
41X"4
':444
2O
'44
Figure 6. CanonicalCorespondcncc Anrlysis plot envifonmentbiplot of secondarysucccssionalplors. NumberscolTe\pondto
conlmunirics in Tablc\ | and L A bbfel i.rtionsof envifonncnt.rl laiablcs are defined in Table L Seete{t lbr interprel
ing thc diagrirn.
rvide anay of enlironments and are not in the
centerof the bipl()1.
WetlandTtpesin CCA. CCAs were conducted
datauson primaryand secondarysuccessional
ing only rvetlandtype as the environmentalvad
able.Resultswclc lcsledbytheMorte Carlotest.
cornIn bothprimary and secondarysuccesslonal
munitics,wellandtype $as significant(primary
successional
tirstaxisP=0.01,tr:iccP=0.01;sec
ondary successionalfirst axis P=0.01, trace
P=0.01).This indicatesthat the relationshipbetween wetlandtype and speciesdistributionis
dillerentliom a randonldistdbution.andthatthere
relationship
bctweenwetlandtype
is a significant
and vcgctationpatterns.
SptttialAru ysis.Forthethreedatascts(complete.primary successional
and sccondarysuc
cessional),lirrward selcctionindicatedthat most
spttial variableswere signilicant.Althou-qhthc
amountof variabilitycxplainedby spatialvari
ableswas not large.it wascomparableto the vai
ability explainedby rncasuredenvironmentalfac
tors alone(Table5). Howcver,multicollineadty
"Infla
between the spatial variableswas high.
tion valucs">20 in theCCA outputindicatehigh
variablcs.
multicollinearitybetweenenvironnrental
If only those spatialvariablcswhich have low
areused,explainedspatialvari
multicollinearity
ability is reducedby about one-half. If inflation
valuesare reducedbclow 20, the following spa
tial variablesare rctained:Y. Xr and XYr for the
completc dala set; Xr, XzY and Yr fbr the primary succession
dataset:andY. X' and XY'firr
successional
dataset.lfonly X and
thesecondary
Y are uscd. cxplained spatial vadability is considerablyreduccd.
spalial.and
In thesealalysesgnvironmental,
spatially-structured
environnrcntalsourcesof
Mounl St. HelensWetlaDds 199
TABLE 5. \iriuce prrtition ing of wel l.Lndlegetaiion from
\ { o u n ! S r . l l c l c n s .T h c ! r l i . r t i | ) nd u e t o e n ! i r o n nrentrl factofs alier remoring thc cJlccl oJ thc
geogr.rphicaL
matrix. the !rriation due to the geo
s.aphical rna(ri1 alicr rcrlo!ing the eftect of the
en!ironmenral \'ariables.spatiall)"strucLurcden
\ ironmenlal \'ariation. and thc rcmaining unex
phined varirtion are partjtio cd. Spatial\&iablcs
used$ere detenninedbf forr\ ard sclcclion.Spt
ti.rl \.rrirble\ \iefe: .rll plds (X .Y. X:. XY Yr.
X . X r Y X Y r ) . p r i i r a r y s u c c e s s i N a(l X . Y X r .
Y : . X r . X r Y Y i ) . s e c o n d a r vs u c c c s s i o n a( X
l .Y
X]. XYY]. X., X]Y XY:.Y]).
Sourccol \'arirlion
tl \'rriance pallilqsl
A11
Primary
Secondary
Plots Succession.rl
Successio'ral
l l 7
l-1.,1
\palirll] snuciured
enlifonnent.rl
I L0
ul1cxplained/stochr\tic 6.1.0
ll.2
Il . l
10.5
6t.2
|li.l
r6.l
,lli.7
variationaccounllbr 35-507. ofvegetationstructure. Measuredenvironmeltal factors are nrore
lmpofiantin thc sccondarysuccessional
vegeta
tion thar in the otheranalyses.Unexplaincdvtrriation is high in all threeanalyses,althoughit is
lessthan50% in thc secondarysuccessional
analy
sis.Unexplained
variationincludesvariationdue
1(]unmeasurcd
cnvir()nmental
tictors andstochastic
lactors.suchas seeddispersal.
Discussion
Communities
Wetlandcomnunitiesin thc lcss impactedareas
ol this stud),rlso occur at similar elevations
throughoutthe rcgion whereLhcrcis suittble substratc(Franklinand Dyrness1988,Kunze 199.1.
Titus et al. 1996.Christvand Titus 1997).Like
wise. vegetationsimilar to thc primary succassionrl rletlandcomrnunities
on thePunice Plain
is found in highly disturbedwet areastiom sea
levelup to theelerationof theItmice Plainthroughout theregion(Tilusel al. 1996.J.Titus.pers.obs.).
For cxample.$'etlandsdonirated b1-Epik tiunt
rlalsoiiii and.t-ri1irsll.l?dr.rl.roccrr in wet. logged
sttes.Juntusbufoniusand.1terlls iuc foundalong
l , r g g i r r ng r i r L j .r ,n. d E 4 u i . 't,t t u , . l n ,r , ' , o c e u r ri n
wet aleasaroundrord construction.
200
Titus,Titus,and del Moral
The most speciesrich and diversecomnunr
ties found in this study are the secondarysuccessional '4lnrs, Salix/Equiseturn,and Rubus
spectubilis thickcts. This is due to the dch assemblageof understoryherbaceous
species.These
communilic.rre l. '\ rleJ in di\turbancc
pronc:ilc..
suchas streambanks,and both early and late successionalspeciesoccur in thc understory.Primary
successional
Salir communitiesare lessdiverse
h e . i r u \ et h e ] c o n t l i n f e u r r u n t l e r s t , ' rs)p e c i c s .
i.e.,thelalersuccessionaJ
species
arenot prescnt.
T h e. S a / i r l t ,l t e t r ' i ' / C , r n .nrt c r t c n . (l lr r n n r u n i t )
rs more diverse than the othef primary successional Szrli-rcommunitiesbecauseof the occurr e n r eu l h o l h u p l r n da n d u c t h n t l p i , ' n q q .1p q
ciesin the understory:
this is due to thc fact that
Sctlissitchensis/Carexnertea.ril occurs in drier
substratcsthan the other Sall.rcomnunitics.
Many pioneerspeciesmost conmonly found
in uplandscolonizedboth wct anddry areasacross
thc PumicePlain.Thesespecieshavc wide ecological anplitudesand can inhabitmany diflcr(Grime
ent kindsofsitesifthe siteis unoccupied
et al. 1988.dcl Moralet al. l995.Titusetal.1998).
These speciesare representedin high densities
in the seedrain (del Moral and Bliss 1993.dcl
Moral andWood 1993a).For example,wet areas
in hi-qhimpactsitesottencontainspecicssuchas
Anapha lisnnrgar i taceu. Ep il obitrnt ungustiJol i unt
antlH1-pocltueris
ru.li..rta. howevet it is unlikely
that thesewill persistil PumicePlain wetlands.
These speciesare less common in wetlands in
the low impact areas.The preponderanceof E4uisetwnan,ense
clominated
wctlandson thehlmice
Plain mav be due to the aggressiveclonal spread
of this speciescombinedwith substrate
instability. r'hich makeE. anense sitesdifficult 1brother
speciesto colonize.No infomration regardingthe
density of E. .Jr-ler?se
sporesin the seed rain is
availablc,probablybecause
thesporcsarctoo small
to be documentedin most seedtraps.
A major differenccbetweenprinary and secondary successionalareas is the predoninance
ol ./a,'rr:us
dominatedwetlandson the Purlice Plain
and the predominanccol Care-rdominatedwetlandsin fessdisturbedareas(Table1 tnd2). Curex
speciesforn wet meadows in reiatively undis
turbedarcasthrou-qhout
theCascadeRangc..rtrr?d/.r
species,
with the exccptionofJ. ensilolius.generallyvegetatewetlandsin disturbedsitesthroughout the region.It is possiblethat the Cdl.e-rspecjes
havenot invadedPumicePlain wetlandsdue to
dispersallimitations,but upland Cnre,rspecics.
e.g.. Cqrcr tnertettsii ar.d Curcx linnophila. are
widespreadacrossthe Punicc Plain (Titus et al.
1998.).
Thus.it is morc likely thatthe charactcristicsofPurniccPlainwetlands,suchashigh sub
state instability,arcnot amenableto wetlandC'.rr.r-r
species.
D i s t u r b a n c eL e v e
The intensity of disturbancefrom the eruption
stronglyinflucnccdspeciesconpositionwithin
o u r . l u d ) i r r u : rD. t t r i n gt h e e r u p l i , , n\-c B L l a l i o n
wasconrplctclydestroyedin prirnarysuccessional
sitesand hcavily impactedin DisturbanceLevel
sites.Sincc the entp
3 secondarysuccessional
siteshave detion, some prinary successional
velopedvegetation
thatrcscmblesthatofheavily
sitesand the
impactedsecondarysuccessional
boundarybclweenthesetwo disturbancelevels
(levels3 and,1)overlapsir the DCA ordination
(Figure,1).This overlapis causedby prinary suc
cessional Salr:; tlickets that resentbleheavily
impactedsecondaq,successionalsir1i-rthickets.
mostprimarysuccessional
sitesdiller
Ne\€rtheless,
fiom heavily impactedsecondarysuccessional
sites;this is plimarily due to later successional
and forcst speciesthat sproutedfrom rootstock
thatsurvivedthe eruption(del Moral et al. 1995.
in substntefea
Tituset al. 1998)anddiffer-ences
turcs. Many DisturbanceLevcl 3 secondarysuccessionalsites are ephcmcral streamson steep
slopes,u,hereasthc PumicePlain is relatively flat
andoftcrsa widel varietyof wetlandtypes.Dis
turbanceLe\el2 plotsareoftenfloristicallysinilar
to DisturbanceLevel 1 plots, particulafly those
dominatedby Crrrc-rspecies.
Wetland Types
W e t l r r ntd1 p c .u e r e f o u n Jt o b e r n r r r ei r n p o r l r r ) l
in structulingthe vegetationin sccondarysucplots
plotsthanin primarysuccessional
cessional
(Table3). This may be a resullof wide ecologispecal amplitudesofnany primary successional
ciesthatcanoccupyanymoistsite.For example.
severalspecies(e-g.,Agrostis?ruretd, Equi.\etLtnr
allense. and .lunctrsbufonius) ore prcvalenl in
plots acrossall wetlandtypcs,and their inclusionpleventwetlandtypcsandcommunitiesfiun
scgregatingclearly in the DCA ordination (Figprocceds.it is likely that
ure 2.).As succession
thesewidespreadspecicswill becone rcstricted
to certainenvironmentsor disappearSpeciessuch
as.luttcLlsbufotius ma1-become less commor
u'hensubstmtesstabilizeandotherspeciesinvade.
sitesdiffer
WetlaDdqpes in secondaysuccessional
fionr each other in speciescomposition and the
plant communitieshave t'ew speciesin comrnon.
Speciesin thesecommunities.when comparedto
wetlands.
thosethatdominateprimarysuccessional
appearto haverelativelynarrowdistributions,i.e.,
t h e . es n e c i e . , ' c e iunr f u $ e rc o m m u n i l i e \ .
Success
ion
Not all of the sites in this study are necessadly
successional,rather some may be nerely recovering liom biomassremoval.For example,Carer
Thcsesedges
meadowsare stablecommunities.
apparentlysurvivcd the eluption urder ice and
sproutedthrough accumulatedash. ln contrast.
highly disturbcd sites with Sa/lr silc&e,slr tncl
ALtlussittLratuthickets will probably change 1n
l h e c o r n i n gd e c , . l e .r . : h | u b c , - rerr i n c r c l s e . .
may rcsemblethc
Eventually,thesccomn.runities
high shrubcor,ersitesobsen'edin krw impaclareas.
sites
wetlandson pdmarysuccessional
Herbaceous
will changein ways that areditficult to detemine
at present. It is likely that Tlphe lqtitoliq wll,l
will even
dominatesomesitesandSa/ir.iltr:/rensis
tuallydominateothers(Kcddy l989,Tuetal. 1998).
Clonal speciessuch ds Equisetum ortense and
T.r'plrtlatifttlidrnay tetaiDsite dominanccindefi
Tsuyuzakil989. Prachrnd
nitely (Keddy 19119,
Pysek199,1).
Unstablesitesmay continueto be
dominatedby Equisetrundrve se a\dlot Juncu.r
biry'rnirsfor many years if substratesdo not sta
bilize. Thus at present.thereis little evidencethat
primllry successionalwetland sitesare undergoing dircctionalsuccession
towardsrelativclyundisturbedwetlandcommuniticsand little leason
to believcthat they soonwill. Most of the Pumice Plain u'ill eventuallybecomeforest that will
resemblc the pre eruption Punricc Plain
(Kruckeberg1987).Becauseof the geologically
unstablenatureof the Punice Plain. hydroJogic
changesmay causesome substratesto bcconle
t l r i e l t h r o u g ht i m e . W e t l r r n JI.h i r lr e r n a i nm i l )
become Tluja plitaLr dominated tirrest, r'hich
occurs elsewherein thc region (Franklin and
Dyrncss 1988,Kunze 1994,Titus ct al. 1996,
ChristyandTitus 1997).lfthe shoreofSpirit Lake
stabilizes.the vcgctationmay eventuallyresemble
pondshoreregetationlbund in low impactsites.
although there is no evidenccof this at present.
Mount St. HelensWetlands 20l
Contribution of Spatia and Env ronmenta
Variables
In uplandarcason Mount S1.Helens.geographic
positionplaysaD impo(ant role in dctermining
which specicscoionizesa sitc (del Moral and Bliss
1993).Spatialanalysesenablea partitionto be
made betweenenvironmental.
spatial.spatially
stl'Lrclurcd
environmental,andundetennincdcomponents(Borcardet aJ. 1992).ln the conplete
andsecondarysuccessional
analyses,disturbance
intcnsity is highly correlatedwith both the X ard
Y lactors.This is becausedisturbance
decrcases
with increasingdistancefron thc volcanoin all
dircctions.Likewisc in theset$,o analyses.
spa
tial valiablesare highly correlated.which indicatesthat sinrilar pattemsare explainedby boft
X and Y variables.Howcver, lbrward sclection
indicatedthat.evenwith high conelations.a large
nunber of spatial variablcs u'ere significant in
structur'lDgvegctationpattems.A reasonfor this
is the patchydistributionof wetlandcomplexes
acrossthelandscape.
Eacholthesewetlandcompleres$ r. :rrnpledh1 'er crulplors.r..ru.ing
groupr
ofvegctativelvsimilarplotsto behighlyclumped
in spatiallyscallcredwetlandlocations.This clunpiDg ofvcgetativelysimilar plots may causea large
numberof spatialvariablesto exhibit significance.
Thc amountof variationin the vegetrtionattrib
uted to spatialvariation is most likely due to this
landscapeeffcct. Thus, although a large numbcr
of spatialvariablesare signiflcant. that does not
m e r n l h c l l h . ) . r r ea l l i m p o n l n t i r r \ t r u ( l u r i n g
the vcgetation.In any case,the amount ol vanation due to thc spatialand environnrentalvariables
r r . . i m i l r ur n d b , ' l h r e r e l r t\ c l y u n i m p , n l i n
stl'rlcturingthe vegetatim (Tlble 5). Unexplained
vadation(vrrittion due to unmeasured
enlironnrental variablcs and/or stochastictactors) rvas
fbund to explain 61.2% of thc variatior in thc primary successional
pk)ts.as opposed|() ,18.7%lbr
secondalysuccessional
plots (Table5).
F n \ i r o n r n c n l J! ill r i r h l c st h l r r e i n r p o r t l nitn
maturewctlands,suchas soil chemistry,arc unlikel),to be importantin eally succcssional
land
. r ' i r p e sr .v h e r c: l l i l : c r e c \ l r r m ( . 1 )n u t d e n pt o , ' r
(Tu et al. 199t1.Titus unpubl.dara).Thus. ir is
likelv thatstochastic
factors\r,ereinitially impod t
in thc colonizationof primarysuccessional
rvet,
lands.i.e..a wetlandis colonizedby a particular
specicsbased upon thc random chance of a
propaguleof thal parricularspecieslandingin the
wetland.Al presgnt.nany of thesewetlandsare
202
Titus.Titus,anddel Moral
denselyvcgetated,much more so than most up
land sites (del Moral et al. 1995) and thcrefbre
othcr lactors. such as competition. are also impofianl. However. in high-disturbanceprinary
successional
wetlands,suchas unstablestream
banksandarcasofliequentsedimentdeposition,
stochasticfactors.suchassecddispe$al.may still
h e i m p o fJl n l f , ' re . t l b l i s h r n c h
ne
t c a u . oe p e n . i t c s
remainplentiful.In theseareaschancewould favoI the locally prevalentspecies.
Stochasticfactors are unlikely to be in1portant in structuringlow impact sites,becausevegetation has occupiedthesesitesfor long periocls
of timc and vegetatilccoveris high. Thus.it is
p r o b r b l cI h i t lu n m e t : u r e Jp r e c r u p t i , , e n \i r o n m e n t arl u r i r r h l e r t. l h r r I h i r nc h l n c e .r r e i m p o r
tant in the low impactsitesand are responsible
tbl mostofthe Lrncxplained
variation.
In uplandprimarysuccessional
studies,it has
becnpositedthat initial speciesasseurblages
are
a.Il?oc and may be novel. Invading specieshave
broadlyoverlappingtolerancesandtherefbremany
species
couldpotentiallyoccupya microsite(dcl
Moral and Wood 1993b,del Moral el al. 1995).
Establishmenthas been attributed to stochastic
facbrs operating through a lottery model (Sale
1977,Lavoreland Lebreton1992).Unlike upl a n r lp r i n a r l \ u c c e \ \ i o n a.li r c r . u h e r c . p c e i e .
gcnerallycannotcxcludeotherpioneerslrom their
vicinity(delMonl ct rl. 1995).in wetlands
plants
canexcludeotherpioncers.This is duc to the npid
r r , . h i e r e m e,n, lt h i g h c o \ c r i n t h c u c t c n r i r o n
ment via prolific seed production and clonal
growth.Thus.it is unlikely that a carouselmechanisnr (van der Maarel and Sykcs 1993).where
dift'erent speciessuccessivelyoccupy the same
microsite.will occur.
ln the u'etlandsstudiedhere. most of the primary successional
communiliesarenot novel,bul
are similar to wetlandspecicsassemblagcs
fbund
in heavily disturbedsitesthroughoutthe region.
If the unexplainedvariation in communitieswas
lar€elydetemrinedby stochastic
factors.would
it be possiblethat similar assemblages
would
devclopin disturbcdsitesthroughouttheregion?
On thePumicePlain,initial speciescomposition
mry have becn stochasticliom the pool of wet
land speciesthathavepropagulcsin the seedrain.
i.e.. 1bulder effects are impoftant. That is, seed
andcloneproductionby locallyestablishcd
spe
cics would quickly outweighany lolg distance
(Moody
dispersalbyspcciesnot locallyestablished
andMack l988. del Moral andBliss 1993.Stttcklin
estabThus,whcrea species
andBiurnler 1996.).
becoores
dominantis due
lishesandsubsequently
of dispersaland not due
to the chanccprocesses
te environmentalvariables.Becauseof the limitedpool ofwetland speciesin the sccdrain, communities that cmergevia the sbchastic dispersal
proccssaresimjlar to wctlandvegetrtionthrough
out the region. The pafiicular locatiot of an E4risetun atense, Salixsilcfunsisor Jwtats bufonius
u e t l u n t li ' \ l o . h a . t i ch e r . r u : cI h c s es p e e i ecsl n
colonizea wide varietyof sites.and bccauseof
the relatively snrtrll availablespecicspool simjThus.
occurelsewhere.
lar wetlandassemblages
thesewetlandconmunitiesare fbund through
out the region wherevera disturbanceoccurs.
H ,' u e re r .f u t u r e , . h r n B ci n. \ e ! e l i r l i ( ' n . ( \ t n p o . i t i o n r r i l l h c d r i \ e n h y l i r c l , ' r :s u c hr s e n r i r , , n mentally nediated competition.
Conclusion
Wc concludethat cnvironmentalf'actorswere not
of great importanca in thc speciesassemblyof
wetlandcommunitieson the PumicePlain.Sto-
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chastictactorsare importanton the PumicePlain
and are linked with a small speciespool of po
which yield commonand rectentiald()minants.
ognizableconrmunities\\"ith little linkage to etvironmentalfactors.Thus. similar sites yield
different communitics.Vegetationdevclopment
in the luture will be strongly determinedby initial tbunder effects.In 10wimpact areas.unmeasurcd environmental variables appear to be of
greaterimportaDce.
Acknowledgments
Thanksto S. Tsuyuzaki.J. Bishopand S. Moore
in the lield. J. Leps assistedvith
lbr assistance
data analysisand the facilities of the Faculty of
Biology, Universityof South Bohcmia,Ceske
Budejovice,CzechRepublicwere invaluablc.J.
Leps. P Smilauer.J. Christy. E. Rykiel Jr., and
an anonyllous reviewergreatly improvcd the
manuscript.Many specieswere idcntified by M.
Arnot,A. Yen.S. Gage,S. MooreandJ. Christy.
A grant from the W$hington Native Plant Soci
ety madethis researchpossible.NSF GrantsBSR89-06544and DEB-9406987to R. del Moral
helped supportthis research.
del N{orul.R. and L. C. Bliss. 1993.Mcchanjsmsof pfimrry
s u c c c s s i o ni :n s i g h t sr e s u l m g t i o m t h e e r u p l i o no f
Mounl St. Hclens.Advancesin Ecological Research
2:l:l-66.
del Mc'ral, R., J. H. Titus. ,rnd A. M. Cook. 1995.Earl] pri
nary successionon Moun! St. Helens.Washinglon.
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