Role of Ecosystem Development and Climate Change in Bog Formation... Author(s): D. R. Foster and H. E. Wright, Jr.

Role of Ecosystem Development and Climate Change in Bog Formation in Central Sweden
Author(s): D. R. Foster and H. E. Wright, Jr.
Source: Ecology, Vol. 71, No. 2 (Apr., 1990), pp. 450-463
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1940300 .
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Ecology, 71(2), 1990, pp. 450-463
? 1990 by the Ecological Societyof America
ROLE OF ECOSYSTEM DEVELOPMENT AND CLIMATE
CHANGE IN BOG FORMATION IN CENTRAL SWEDEN'
D. R.
FOSTER
Harvard Forest,Harvard University,
Petersham,Massachusetts01366 USA
H. E.
WRIGHT, JR.
Limnological Research Center,218 PillsburyHall,
University
ofMinnesota,Minneapolis,Minnesota 55455 USA
Abstract. Bog development,in termsof the rates of horizontaland verticalaccumulation of peat and the timingof landformdevelopmentof open-waterpools, was examined
on two concentricraised bogs in central Sweden. The resultsare compared with three
models (allogenic,autogenic,and neutralmodels) of bog developmentin orderto evaluate
the relativecontributionof environmentalfactorsvs. ecosystemcontrolof developmental
processes. Both mires began to grow on discreteupland sites -6000 BP and then spread
concentricallyand accumulated peat verticallyat approximatelycontinuous rates to the
present.Radiocarbon dates frompool sedimentsare progressively
youngerfromthecenter
to the margin of the mire, suggestingthat pool formationis triggeredautogenicallyby
changesin hydrology.The resultsconformto hydrologicalmodels ofmireformationbased
on groundwatermound equationsand suggestthatautogenicprocessesexerta major control
over bog expansion,landformdevelopment,and the formationof conspicuous featuresin
the stratigraphic
record.
Key words: autogenic;bog, climate; mire;paleoecology;Sweden.
peat stratigraphy
was formulatedbeforethe developThe paleoecological reconstructionof past environ- ment of the theoreticalunderstandingnecessary to
mentshas reliedhistoricallyon macrofossil,microfos- evaluate it thoroughly(Birksand Birks 1980, Godwin
sil, and lithologicalinformationderivedfrompeat and 1981). The consequence has been a continual reevallake sediments.Mires were the focus of many early uation of many basic assumptionsand a concomitant
studiesin partbecause of therelativeease of obtaining increasein the understandingof bog dynamicsand its
withenvironment(Casparie
stratigraphic
samples fromopen exposuresin the peat complexinterrelationship
cuttingsthatoccur throughoutcentraland northwest- 1969, Damman 1979, Clymo 1984).
A particularcase is theuse ofrecurrencesurfacesfor
ernEurope (Godwin 1981). Mires continueto be used
stratigraphic
correlation,dating, and environmental
forvegetationaland climatic reconstructionsbecause
The idea was introducedbyBlytt(1876),
theyofferparticularadvantages over lake sites, nota- reconstruction.
bly: (1) raised bogs, which receive theirmoistureand and subsequentlyexpanded by Granlund (1932); renutrientsprimarilyfromatmosphericinputs,may be currencesurfacesare layers of fresh,well-preserved
more sensitiveto environmentalfluctuations
thansites peat overlyinga discretelayer of humifiedpeat and
in the surroundingupland (Barber 1981, Aaby 1986, were interpretedas synchronoussurfacescaused by a
Moore 1986), (2) the abundance of macrofossilsand change fromdryconditions(humifiedpeat) to moist,
theirgenerallygood stateof preservationin peat pro- cool conditions(well-preservedpeat). Five or more of
vide directinformationon the vegetationat the sam- these layers were identifiedon classic Swedish sites
plinglocation (Tolonen 1971, Janssens1983, Dupont (Granlund 1932), and these in turnwere used forcor1986), and (3) peatlands collect local microfossilrec- relation across a bog (Granlund 1932), a landscape
ords that document small-scale vegetation patterns (Lundqvist 1958), and broad regions(Godwin 1946,
across the mire and adjacent areas (van Geel 1978, Berglundet al. 1984). Aspectsoftherecurrence-surface
conceptwere criticizedratherearly,on the basis of an
Jacobson and Bradshaw 1981, Tolonen et al. 1985).
As a resultof thesedistinctivecharacteristics,
mire- intuitive understandingof bog growth and climate
based studies have been and continue to be of major change(Friihand Schrdter1904, von Post 1925, Conimportancein the fieldof paleoecology(von Post and way 1948). Only with the availabilityof radiocarbon
Sernander1910, Granlund 1932, Nilsson 1983, Aaby dating and improved appreciation of bog hydrology
1986). However,much oftherationaleforinterpreting has it been shown that the classic recurrencesurfaces
were often asynchronous (G. Lundqvist 1963, J.
Lundqvist 1969), that the fundamentalmodel of bog
1 Manuscriptreceived9 January1989; revised24 May 1989;
accepted 26 May 1989.
developmentand environmentalresponse underlying
INTRODUCTION
April 1990
BOG FORMATION IN SWEDEN
451
thisinterpretation
was incomplete(Ingram 1983, Cly'4 A-fil
mo 1984), and that interpretation
of the stratigraphic
~
~
~ ~ , ~~
record required consideration of complex interrelationshipsand inherentdelays in systemresponse(Casparie 1969, Frenzel 1983, Tolonen et al. 1985).
In the presentstudy we examine two other stratigraphiccharacteristics
ofnorthernpeatlandsthathave
been widelyused forreconstructing
regionalenvironmental conditions.These are: (1) the use of basal radiocarbon dates as indications of peatland inception
and inferredclimate change, and (2) the use of pool
sedimentsor otherindicatorsof surfacepattern(hummock/hollow,hummock/pool)developmentas proxies
forchangesin regionalmoisturebalance. Our objective
is to determinethe relative role that allogenic environmentalfactorsand autogenic developmental processes play in bog developmentand in controllingthe
formationof surfacelandforms.Resolution of these
1.Aralpooraho
Hmamsnsown
h
issues should help futurepaleoenvironmentalreconFIG.~~~~~~~
structionswhile addressingfundamentalquestions of
ecosystemdevelopmentand function.
We firstexplore relevantmodels of mire developmentand patternformationas controlledby allogenic
and autogenic processes and outline expected stratigraphicpatternsbased on each model. The fieldwork
and resultsare thendescribed,followedby a discussion
o 300 m
"
of theirsignificancein termsof the proposed models
and theirimplicationforthe understandingof ecosystemdevelopment.The fieldsitesin thisstudyare Hammarmossen(Figs. 1 and 2) and Nittenmossen(Fig. 3),
FIG. 1. Aerialphotograph
ofHammarmossen
the
showing
raised bogs in centralSweden thatcontain well-devel- well-developed
thebog
whichgivewaytowards
poolsystems,
oped hummock, hollow, and pool microtopography margin
andupland
hummock-hollow
to a concentric
pattern
forest
ofPinussylvestris.
(Granlund 1932, Sjbrs 1948, Foster et al. 1988).
ACM
Raisedbogecosystems
Raised bogs are wetland ecosystemscharacterized
by a convex cross section and a broad distributionin
cool temperateareas especiallyin the NorthernHemisphere(Gore 1983). As a consequence of theirdomed
shape bogs are ombrotrophic,i.e., theirsurfaceis isolated fromgroundwater,and nutrientsand moisture
are derivedfromatmosphericsources. In the resulting
acid and nutrient-poorenvironmentdecomposition
rates are slow and organic materialaccumulates to a
peat depth generallyrangingfrom 1 to 10 m, making
raised bog systemsthe singlegreatestsink fororganic
material among wetland types, worldwide (Ingram
1983). The surfaceof raised bogs is covered predominantlyby Sphagnum, sedges, and ericaceous plants
thatforma microtopographicpatternof firmelevated
hummocksor ridgesand low,moisthollows.Especially
in northernand maritimeregionsthemicrotopography
is accentuatedinto robustand broad ridgesand deep
hollows and open-waterpools. Such mires are widespread and formprominentparts of the landscape in
Canada, Scandinavia, European Russia, and central
Siberia (Ivanov 1981, Sj6rs 1983, Foster and Glaser
1986).
Bog and bog-pool development:environmental
interpretations
and empiricalmodels
Three distinctmodels of raised bog developmenton
upland sitesmaybe outlined(Fig. 4): (A) peat initiation
across a broad area followed by gradual vertical increase in heightbut no lateralexpansion,(B) peat initiation at numerous loci followed by expansion and
fusionoftheseparatemiresintoa singlemirecomplex,
and (C) peat initiationat a centrallocus followed by
gradual increasein heightand horizontalextent.Each
model predictsverydifferent
resultsfora seriesofbasal
peat dates. For example, onlyformodel (A) would the
basal date in several cores be always the same. The
other models would give a series of differentdates.
Thus these models should be easily differentiated
throughthestudyofa seriesofbasal radiocarbondates.
At least threedistinctmodels forpool formationcan
be distinguished(Fig. 5), including:(A) allogenic,climatic model, (B) autogenic,ecosystemmodel, and (C)
neutralmodel which statesthatpools have always existed on the mire. For each model contrastingpredictions emergeconcerningpools and their(i) age distribution,(ii) spatial distribution,and (iii) morphometry.
D. R. FOSTER AND H. E. WRIGHT,JR.
452
A
Ecology,Vol. 71, No. 2
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~~~~'~410
Fio.2. ap f Hmmamosen howng:(A) he istibuionandsiz oftheopenwatr pols(dak),(B)theloctio
andag o baalraioarbn ats nd ntrplaed sohrne fo bg xpnsin cf Fste e a. 988, nd(Cth0lcaio
and gesfpols sudie inth current
study.
IN:
545o
>X8:
;O7O
showingthe
ofNittenmossen,
FIG. 3. Aerialphotograph
bythefusionofadjacentpoolsand
largecentralpoolformed
ridges(dark)andhollows
theconcentric
ofhummock
patterns
(light).The locationofbasalpeatsamplesis indicatedbythe
datesareshown
basalradiocarbon
opendotsandcorresponding
to theright.
In thefirst
model(allogenicmodel;Fig.5A) poolsform
acrossa boginresponseto broad-scale
simultaneously
climaticchange.Pools therefore
shouldbe similarin
age and depth.In modeltwo (autogenicmodel;Fig.
5B) pool formation
is triggered
by autonomousprocessesas portionsof themirepass through
a critical
whichmightbe controlled,
stageofdevelopment,
e.g.,
bybogmorphology
sensuAario 1932),
(Grossformen
peat depth,or hydrology.
Pool formation
would be
timedependent
andcentripetal,
andpoolagesanddepth
mightbe correlated
positively
withbasal peatage and
depth.In thethirdmodel(neutral
model;Fig.5C) pools
formsimultaneously
withpeatinitiation.
Pool ageand
depthwouldbe equivalentto thatoftheadjacentpeat.
The classicinterpretation
ofpoolsas hydroclimatic
features
thatrespondcloselyto regionalclimaticvariation(allogenicmodel)is bestsummarized
inthestudies byBarber(1981) on BoltonFellMoss. In thisview
the relativearea of hummockvs. pool on the mire
surfaceis directly
controlled
surbyclimate(moisture
plus).Duringdryperiodshummocks
buildandexpand.
A changeto moisterconditions
causespool formation
and expansionand a generaldeclinein theheightand
size ofhummocks
(cf.also Godwin1954,Ruuhijirvi
1960,Walker1961,Aartolahti
1967,Sakaguchi1979).
Pool sedimentsare interpreted
as contemporaneous
productsofregionalclimatechangeand therefore
are
tobe traceableacrossbroadareasas temporal
thought
markers.
stratigraphic
In theautogenicmodelit is envisionedthattheregionalclimatemaybe suitableforthedevelopment
of
April 1990
BOG FORMATION
Present
Present
Present
t
IN SWEDEN
453
SITE DESCRIPTION
Hammarmossen and Nittenmossenare located in
thesouthernportionoftheBergslagenregionofcentral
Sweden. Across this landscape large raised bogs are
characterizedby the extensive development of a surfacepatternofbroad hummocksand pools (Sjdrs 1948;
Fig. 1). Hammarmossen is well describedwithina regional contextforits gross morphologyand stratigraphy (Granlund 1932) and the flora,water chemistry,
and vegetationof both mires have been discussed by
Sjdrs (1948). Hammarmossen is situated on a broad
outwashplain of fine-to-medium
sand thatslopes very
gentlyto thesouth.Nittenmossenhas formedon a level
sand plain that is bordered by shallow lakes on two
sides. The surfacepatternof hummocksand pools on
these mires formsa concentricpatternthat parallels
the surfacetopographyof the bog dome.
METHODS
Field work at Hammarmossen duringthe summers
of 1986-1987 consisted of surveyingthe bog topographyand pool elevations, morphometricanalysis of
selectedpools, and coringof pool sedimentsand peats.
The sand plain east of the bog was firstsurveyedwith
nessbutnothorizontal
extentthrough
time,in (B) separate a transit-leveland staff,and thenthe bog surfacelevel
peatlandsfusethrough
lateralexpansionand verticalaccu- was surveyedfromthe southend to thecenter.At each
mulation. and in (C) a single locus for peatland initiation
survey point across the southernhalf of the bog the
increases in depth and extentthroughtime.
pool patternson mires,but that the specifictimingof
theirformationis dependenton thebog passingthrough
some critical threshold.The thresholdmightbe, for
example,a minimumpeat depth,surfaceslope,or water
table depth.As portionsofa mirepass the(presumably
hydrological)threshold,pools will form(Fig. 5B).
The thirdmodel (neutral model; Fig. 5C) is based
on the interpretationof Swedish researchersthat in
some situationspools date fromthe initiationof mire
formation(Osvald 1923, Granlund 1932, Lundqvist
195 1). In this model pools startdirectlyon or very
close to the mineral soil as wet depressions and are
permanentfeatureson themire.Pools therefore
extend
from the peat surface down to the mineral soil (cf.
Granlund 1932).
The models outlined above cover the basic interpretationsof landformdevelopmenton mire systems,
withtheexceptionofpurelyphysicalmechanismssuch
as mass movement or freeze-thawaction (cf. Drury
1956, Pearsall 1956, Schenk 1966, Lindsay et al. 1985).
Additional complexitiescould be added, e.g., changes
in pool size and number in response to a fluctuating
climate. However, the evidence from most sites in
northernEurope and NorthAmerica is thatonce large
pools are initiatedtheypersistthroughtime,and thereforethissimple templateis used (Boatman 1983, Kuznetsov 1986, Foster and Fritz 1987).
We turnnow to therelevantevidence in two Swedish
raised bogs.
-TIME-
/1;7-=
FIG. 5. Schematic representationsof threemodels of bog
landformdevelopmentthroughtime showingthe formation
ofpools in plan view and cross section.In theallogenicmodel
(A) pools are initiated synchronouslyacross the bog in response to externalenvironmentalchange, in the autogenic
model (B) pools develop progressivelyacross the mire from
the centeras controlledby characteristicsof the mire (e.g.,
peat depth) and in the neutralmodel (C) pools are initiated
at the time of bog formation.
454
D. R. FOSTER AND H. E. WRIGHT, JR.
depth of the underlyingpeat was determinedwith a
thinmetal rod. At several pointsthe peat stratigraphy
was examined witha Russian peat corer.
Water
Fifteenpools werestudiedmorphometrically.
depthwas measuredat 1-5-m intervalsalong the long
axis and acrossthemaximumwidthofeach pool. Cores
5 cm in diameterwere obtained fromthe deepest part
of eightpools by sampling froma small rubberraft.
The uppermostorganic ooze (gyttja)was collected in
a clear plastic tube provided witha piston so thatthe
water gyttjacontact was preserved; the lower sediments were collected with a square-rod piston corer
(Wright1967). The loose gyttjawas extrudedin 2-cm
incrementsintoplasticbags. The remainderofthecore
was wrappedin plasticand aluminumfoilfortransport
to the laboratory.
The cores were sampled and analyzed at the Departmentof QuaternaryGeology,Universityof Lund.
Each core was splitin halflengthwise,and one-halfwas
cut into incrementsof 2-5 cm, which were washed
throughtwo sieves (100- and 250-,ummesh) to obtain
coarse and finefractionsformacrofossilanalysis.Samples weretakenforradiocarbondatingofthebasal peat
at 15 locations.The basal 3-8 cm ofpeat was uniformly
made up of verydark and well-decomposedsiltymaterial that frequentlycontained charcoal. The dated
sample was takenjust above thislayer.For each of the
eightpool cores the contactbetweengyttjaand underlyingpeat was firstidentified.The uppermost5 cm of
peat and the adjacent lowermost5 cm of gyttjawere
sampled for radiocarbon dating. Samples were submittedto the radiocarbon laboratoryat the Department of QuaternaryGeology at Lund Universityfor
dating(Hakansson 1986, 1987).
At Nittenmossencores were taken every50 m on a
transectfromnearthecenterofthemireto themargin.
was described in the fieldand
The peat stratigraphy
six basal samples were collected for dating at Lund.
The Russian corerwas used forexaminingthe stratigraphyat many places across the mire.
Ecology,Vol. 71, No. 2
1 and 8). Pools range from <2 m in lengthto >100
m. The largestpools, which are subcircularin outline,
are positionedon gentlyslopingareas towardsthecenter of the mire. More nearlylinear pools are aligned
along contoursbetweenthe crestof the mire and the
margin.The marginalslopes containelongatedhollows
filledwith water and support
that are intermittently
carpets of Sphagnum cuspidatumand S. papillosum.
Coalescence of adjacent pools and hollows is recognizable fromthe scalloped and complex outlines(Fig.
1) and fromthe appearance in cross section of double
basins separatedby an interveningridge(e.g., pools 4
and 5 in Fig. 8). Shape is thereforegoverned in part
by slope position and in part by such dynamic processes as coalescence and expansion.
Three main types of pool morphometrywere observed in cross section: steep sides and flat bottom
(pools 6 and 7; Fig. 8), an undulatingbottom(pools 1,
2, 3, and 8), and two basins separatedby a sharpridge
of peat (pools 4 and 5). Most pools have very welldefinedand solid banks. The seasonal maximumwater
depth rangesfrom40 to 240 cm and is generallycorrelatedwithpool age (Table 2, Fig. 9B). The waterin
all of the pools is colorless. Macrophyticpool vegetation is generallylimited to Sphagnum cuspidatum
growingalong the margins or floatingin the water,
whereas diatoms and other algae comprise a major
component of the gyttjasediment (S. Fritz,personal
communication).
Pool stratigraphiesand age
The sediment beneath pools consistentlyincludes
gyttja,rangingin thicknessfrom35 to 110 cm, a sharp
transitionto Sphagnum peat, 25-85 cm thick,and a
basal layer5-8 cm thickofwell-humified
siltypeat and
mineralsoil. The gyttjacontains variable amounts of
detritusof Sphagnum cuspidatum,sedge leaves, and
pine needles and is distributedin a thicklayer across
the bottomof the pools. Rough estimatesof the rates
of peat accumulation below pools and sedimentation
of gyttjain pools can be derivedfromthe radiocarbon
RESULTS
dates (Table 2), assuming constancyof rates and no
Bog development
compaction or subsequent decomposition (however,
As documentedbythebasal radiocarbondatesHam- cf. Clymo 1984). Peat accumulation ranges from -9
marmossenand Nittenmossendeveloped throughcon- to 110 cm/1000 yr,with most cores rangingfrom33
tinuous horizontaland verticalaccumulation of peat to 90 cm/1000 yr. Gyttja deposition has a narrower
(Figs. 2 and 3; Table 1). Both miresformed -6000 BP range of 20-60 cm/1000 yr,with most pools ranging
withpeat initiationnear the centerof the mire and in from30 to 50 cm/1000 yr.
The pools sampled are situated on portions of the
the regionof what is now the deepestpeat. Horizontal
expansion occurred concentrically.At Nittenmossen mire that range from2000 to 6000 yr old, as interthereis an indicationthatthe rateof horizontalexten- polated fromthe isochronesdrawn from 17 basal rasion decreased with time (Fig. 6A). At Nittenmossen diocarbon dates (Fig. 2; Foster et al. 1988). Based on
thebasal age: depthcurveis approximatelylinear(Fig. the age of peat immediatelybelow pool sedimentthe
6B) whereasat Hammarmossenit is concave (Fig. 7A). pools range from 1940 to 4200 yr in age. Two pools
occurin each oftheperiods from2300 to 2400 BP and
Pool characteristics
4000 to 4200 BP, and threepools date to the interval
The bog pools at Hammarmossen exhibit consid- 2940-3060 BP. One pool has a date of formationof
(Figs. 1940 BP.
erablevariationin size, shape,and morphometry
BOG FORMATION IN SWEDEN
April 1990
455
1. Radiocarbon dates fromHammarmossen and Nittenmossenbogs, Sweden. Paired dates fromthe upperpeats and
lower gyttjasedimentsbeneath pools at Hammarmossen have been given the same core number. Errorsquoted include
standarddeviations of count rates.
TABLE
Core number
Laboratorynumber
Sample depth
(cm below water
or peat surface)
Radiocarbon date
(yrBP ? 1 SD)
Material
1
2
3
4
5
6
7
8
9
10
11
12a*
12 b
13 a
13b
14a
14 b
15 a
15 b
16
17
18 a
18 b
19 a
19 b
20 a
20 b
21 a
21 b
22a
22 b
23
Lu-2526
Lu-2527
Lu-2528
Lu-2529
Lu-2530
Lu-2531
Lu-2532
Lu-2533
Lu-2534
Lu-2535
Lu-2536
Lu-2537
Lu-2538
Lu-2539
Lu-2540
Lu-2541
Lu-2542
Lu-2543
Lu-2544
Lu-2545
Lu-2822
Lu-2823
Lu-2824
Lu-2825
Lu-2826
Lu-2827
Lu-2828
Lu-2829
Lu-2830
Lu-2831
Lu-2832
Lu-2833
Hammarmossen
25-30
105-110
215-220
300-305
310-320
365-375
365-375
175-185
335-345
235-245
130-140
90-100
100-110
210-220
220-225
305-310
310-315
290-300
300-310
310-315
365-375
302-307
307-312
198-203
203-208
270-275
275-280
250-255
255-260
219-224
224-229
95-100
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Basal peat
Gyttja
Pool peat
Gyttja
Pool peat
Gyttja
Pool peat
Gyttja
Pool peat
Basal peat
Basal organic
Peat
Peat
Gyttja
Pool peat
Gyttja
Pool peat
Gyttja
Pool peat
Gyttja
Pool peat
Basal peat
20 ?
920 ?
2320 ?
3140 ?
3950 ?
5190 ?
5240 ?
2130 ?
5820 ?
2390 ?
1450 ?
1300 ?
1940?
2040 ?
2390 ?
3770 ?
4200?
2580 ?
4010 ?
4130 ?
6540 ?
2710 ?
2640 ?
1470 ?
2940 ?
1780 ?
3060 ?
2230 ?
3030 ?
1860 ?
2300 ?
940 ?
45
45
50
50
60
60
60
50
60
50
45
45
50
50
50
60
60
50
60
60
70
50
50
50
50
50
50
50
50
50
50
50
A
B
C
D
E
F
Lu-2852
Lu-2851
Lu-2853
Lu-2854
Lu-2855
Lu-2856
Nittenmossen
328-335
340-347
293-300
237-244
118-125
23-30
Basal
Basal
Basal
Basal
Basal
Basal
4910
6070
5450
4290
2290
520
?
?
?
?
?
?
60
70
60
60
50
50
peat
peat
peat
peat
peat
peat
*a and b are two samples fromthe same core.
Pool age is closely relatedto both basal depth (Fig.
7A), and basal age (Fig. 9A), and more loosely to pool
depth(Fig. 9B). Pool ages fallon a concave line against
basal peat depth that closely parallels the curve for
basal age and depth (Fig. 7A, B). The basal age below
the oldest pool cored is -4800 BP witha pool age of
4010 BP. The youngestpool cored, formedin 1940
BP, is underlainby basal peat - 2000 yrold. The mean
intervalbetween peat initiationand pool formation
was 550 yr,witha maximum of - 1100 yr.The pools
apparentlyformin a relativelyshort time following
peat initiationand thenpersistformany thousandsof
years.
The onset of gyttjasedimentationshows a much
looser relationshipto basal depth and age (Fig. 9C).
This is partlya consequence of the variable lengthof
thetemporalhiatusthatseparatespool formation(based
on the age of the uppermostpeat) and the onset of
gyttjadeposition (based on the age of the lowermost
gyttja;Fig. 10). In fivepools therewas a hiatus >600
yr(640-1470 yr)and in threepools the difference
was
350-440 yr.The durationofthehiatusdoes notappear
to be relatedto any obvious characteristic
ofpool morphometry,slope position,age, or sedimentcharacteristics.
DISCUSSION
Mires are dynamicecosystemsthatchange progressivelyby verticaland horizontalaccumulationof peat
and subsequentvariationin hydrology,
and
chemistry,
vegetationcomposition. Thus, althoughsystemcharacteristicsand processes may vary within a narrow
rangeover the period of an ecological or hydrological
fieldstudytheymay changesignificantly
over thetime
456
D. R. FOSTER AND H. E. WRIGHT, JR.
70001
A
60001
m00
5000.
4000-
Li
:
3000-
C,)
2000-
m 1000
0
0
E
40
80
120
160
200
240
DISTANCE Cm)
0
C.
B
B
235m
I
oQ
100-
ILI
Ld
*
*150
m
m
300-
*100 m
0om
0
2000
- E -
U - G - W=
0
where P = precipitation, E = evapotranspiration, G
200-
CLi
<
cally supportthe single locus model of bog developmentdiscussed above (Model C, Fig. 4).
The interpreted
historyofdevelopmentforthesebogs
also conformsclosely to that derived fromrecenthydrodynamicmodels of bog formation(Clymo 1978,
1984, Ingram 1978). The models in turn describe
changes in bog shape and hydrologythat may have
direct application towards understandinglandform
changeson the mires.
The simplest models assume (1) initiationfroma
centrallocus, (2) developmentacross a flatnearlyimpermeable substrate,(3) peat groundwaterdischarge
around the perimeter,and (4) constantmositureparameters(Clymo 1978, 1984). Under theseconditions
U, peat groundwaterdischarge,is definedby the hydrologicalbudgetas
P
m
~~~~200
Ecology,Vol. 71, No. 2
4000
* 50 m
6000
BASAL AGE (yr BP)
FIG. 6. GraphsfromNittenmossen
showing(A) therelationship
between
theageofbasalsamplesanddistance
along
thetransect
fromthecenterto themargin
ofthebog(seeFig.
3), and (B) therelationship
betweendepthofbasal samples
and radiocarbon
age.
E
Bog development:empiricalresults
and theoreticalmodels
A
500
o
100-
I
I-
150-
w
200-
L
scale ofpaleoecologicalreconstructions
(Ivanov 198 1).
The significant
questionconsideredin thecurrentstudy
is: To what extentare the major changes recordedin
a signalof autogenicdevelopmentof
peat stratigraphy
the mire systemand to what extentare theya result
of allogenic, primarilyclimatic factors?The conclusions are of interestnot only fortheirbearingon paleoenvironmentalreconstructionbut for their relevance to understandingecosystemdevelopment and
integration.In orderto understandthebroadercontext
forlandformdevelopment,it is thereforeessential to
explorethe formationand dynamicsof entirebog ecosystems.
=
groundwaterleakage throughthe substratum(considered to be insignificant
relativeto U), and W = change
in storage.Ingram (1982, 1983) has shown that the
watertable of a raised bog is maintainedby impeded
drainage and dynamic equilibrium between recharge
and supply and may be described by equations for
250<
*
300-
W~~~~~~~~~~~~
Al,
350
400
*-
0
2000
4000
0
6000
AGE (yr BP)
150-
B
E
I-
m
D0
A
200A
250_
*
A A
A
Hammarmossen and Nittenmossen were formed
6000 BP. Basal radiocarbon dates at the two sites < 300w
suggesta rathersteadyrate of horizontalspread from
CL
~~~~~~~~~AA
a singleinitiallocus,withno markedalterationthrough
350'
time (Figs. 2 and 3). The relationshipbetweenage and
2000
3000
4d00
depth forbasal samples at the two sites comprises a
POOL AGE (yr BP)
concave or a linear function(Figs. 6 and 7). The abFIG. 7. GraphsfromHammarmossen
showing
(A) theresence of abrupt changes along these curves suggests lationship
betweenthedepthofbasal samplesand theirage
continuous bog development. The resultsunequivo- and (B) therelationship
betweenbasal depthand pool age.
457
BOG FORMATION IN SWEDEN
April 1990
1300 yr BP
1940
18
3060
1860
2300
2
-
6
2040
2390
2230
~~~~~~~~~~~~~~~~3030
8
4010
294[0!.
7
40
3770
41
4
I00 ~cm
MN
FIG. 8. Cross-sectionaldiagrams of the pools investigatedin this study(numberedat lower lefteach diagram) showing
depth of water (black), thicknessof gyttjasediment(stippled),and depth of peat (white). Radiocarbon dates are given for
samples (e) taken at the base of the gyttjasedimentand the top of the peat.
As demonstratedby Clymo (1984), if U* and k are
constant,then fora bog of given radius LX the height
will accumulate to HX witha hemi-ellipticalwatertaIf thereis no physical obstructionthe mire will
ble.
U*/k= H2/(LI - 12),
spread horizontally,and new values of L and H will
where U* = the value of U in long droughts(R. S. be reached. Althoughthere is no direct hydrological
Clymo,personal communication),H = heightof bog constrainton furtherincrease in bog size, decay prosurface,L = bog radius, I = distance from the bog cesses will eventuallylimit height to a steady state
margin,and k = hydraulicconductivity.From this (Hmax).This will occur at the point whereproduction,
equation the maximum waterheightat the bog center in termsoforganicmatterreachingthecatotelm(sensu
should be
Ingram1982) ofthemire,is equalled bythecumulative
slow decay throughoutthe catotelm (Damman 1979,
H = (U,*L2/k)?'2
mounds(cf.Marino 1974). Shape and size
groundwater
of groundwatermounds may be described as hemiellipticalin cross section,followingthe equation
TABLE
2.
Characteristicsof pools on Hammarmossen Bog, Sweden. Pools are arrangedin orderof decreasingbasal age.
Pool no.
Maximum
length
(m)
Maximum
width
(m)
Maximum
water
depth
(cm)
Transition
gyttja/
peat
(cm)
2
1
6
5
7
3
8
9
37
62
35
50
45
12
63
9
17
38
13
16
25
9
14
5
-200
-197
-240
-130
-190
-150
-165
-40
-310
-295
-275
-203
-260
-225
-224
-100
Basal
depth
(cm)
Basal
age
(est. yr
BP)
Pool
age
(14C yr
BP)
Gyttja
age
(14C yr
BP)
-325
-330
-285
-280
-277
-245
-235
-190
4800
4700
4200
3800
3400
3000
2400
2000
4200
4010
3060
2940
3030
2390
2300
1940
3770
2580
1780
1470
2230
2040
1860
1300
Gyttja
Basal peat
accum.
accum.
rate
rate
(cm/1000 (cm/1000
yr)
yr)
25.00
50.72
8.77
44.91
45.95
32.79
110.00
73.43
29.18
37.98
19.66
49.66
31.39
36.76
31.72
46.15
Ecology,Vol. 71, No. 2
D. R. FOSTER AND H. E. WRIGHT, JR.
458
5000
0-
A
B
E
m*.
4000-
100Lii
(9
0
0
0
nj3000-
0
0
0
0~
*
2000-
3000
2000
4000
0
200*
2000
5000
4000
80
180C
_ 200200t
E
3000
POOL AGE (yr BP)
BASAL AGE (yr BP)
E
100
D
*
0~~~~~~~~~~~~~~~~~~~~~~~~~~~
90
300
Ct)
320i~~~~~~~~~~~~
340
*
0
3800
3000
2000
1200
3001200
2000
3000
3800
GYTTJA AGE (C-14 yr BP)
GYTTJA AGE (yr BP)
FIG. 9. Graphs showingthe relationshipsbetween(A) basal age and pool age, (B) pool age and pool depth,(C) gyttjaage
and basal depth,and (D) gyttjaage and gyttjadepth at Hammermossen.
thehorizontalextentofthemire
Clymo 1984). At Hmax
(Lmax) will be controlled by hydrology.
In theabsence ofexternalenvironmentalchangethis
model describes the consequences of peat accumulationin termsofchangingmorphologyand age-relations
on a growingmire.Startingfroma centrallocus a raised
bog may then spread concentricallyacross a flatplain
as it increases in height.As the mire grows,its gross
POOL
2
F
0.
1
I
i
NUMBER
5
4
6
I
I
I
3
i
7
8
I
I
duration of peat accumulation (black), the hiatus.be2000the
oL 4000-
...
...
5 0005000
tweenpool formation
and gyttjasedimentdeposition(dark
stippled),
andtheduration
ofsediment
deposition
(lightstippled).
morphologyshould change in conformancewith the
hemi-ellipticalgroundwaterequation withthemargins
becomingsteeperand the centerbecomingflatterand
broader(Ingram1982). As a consequenceofslow decay
throughoutthe catotelm the surfaceproduced at any
given time should graduallysink down throughtime
(Fig. 11). Curves of age vs. depth should thereforebe
concave forboth a singlecore (Aaby and Tauber 1975,
Clymo 1984) and fora seriesofbasal dates takenfrom
the centerof a concentricmireto the margin(Fig. 12).
At Hammarmossen the concave curvilinearityof the
basal dates vs. depth followsthis pattern.At Nittenmossen this plot is more nearlylinear.
The developmentalpatterndescribed by hydrodynamic modellingand conforminggenerallyto the results from Hammarmossen and Nittenmossen (sequence of basal dates; curve of basal age vs. depth)
suggesta long continuityof generalprocesses and hydrologyon bogs (Clymo 1984). The hydrology,in particularthe depth to watertable and seepage gradient,
should be largelycontrolledby thisgrossmorphology,
especiallythe topographicgradient(Ivanov 1981, Kurimo 1984). Steep marginalslopes would be expected
to have steeperseepage gradientsand greaterdepth to
the watertable than the increasinglybroad and more
level bog expanse (Rycroft1971, Damman and Donham 1981, Bragg 1982). Likewise, the distributionof
vegetationand microtopographic
featureswould be ex-
April 1990
BOG FORMATION IN SWEDEN
459
Time
Age
-6E4
0
E
2
0~
200
100
0
100
200
DISTANCE FROM CENTER(m)
FIG. 1 1.
Idealized curves of developmentthroughtime fora 10 000-yr-oldbog, showing,on the right-handside, the bog
surfaceat 2000-yr intervalsafterbog formationand, on the left-handside, the currentlevel of formersurfacesat 2000-yr
intervals.The dashed line connectsthe heightof each marginallocation after2000 yrof peat accumulation and shows that
peat accumulation rate increases fromthe centertowards the marginand then decreases rightbeforethe margin.Modified
fromClymo (1984).
pectedto have a regulardistributionalpatternthatfollows these radial changes in depth to the water table
(Ingram 1983). Thus, althoughany given area of the
with time and with its relmire changes significantly
ative position on the mire,the entiremire maintains
similar dynamic processes throughtime. This understandingoflocal and broad-scalechangecan be applied
to the interpretation
of landformdynamics.
Patternand timingofpool formation
At Hammarmossenradiocarbondates fromthe top
of the peat beneaththe gyttjaof eightpools show that
theselandformsdeveloped from4200 to 1900 BP. The
dates form a concave curve against peat depth and
closelyparallelthebasal age: depthcurve(Fig. 7). Pool
formationhas been active duringa largeportionof the
developmentalhistoryofthebog and is correlatedwith
the depth and age of the underlyingpeat.
These resultsappear to be inconsistentwiththe allogenic and neutralmodels of bog-pool development
butare consistentwiththeautogenicmodel and suggest
that a major factorcontrollingpool formationmay
have been thedevelopmentalprocessesofthebog massif, namely horizontal extension and vertical accumulation of peat. Pools have developed consistently
fromthe bog centerto the marginin accordance with
decliningage and depth of peat in this direction.This
is in agreementwith fieldobservationsof the current
process of pool formation.The marginal30-40 m of
the mire,which formsa steep descent to the forested
outwash plain, has a ratherhomogeneous vegetation
over
of Sphagnum,dwarfshrubs,and Pinus sylvestris
a peat surfacewithlittlemicrotopography
(Fig. 1). Just
upslopefromthissteepmargin,wherethesurfacelevels
slightly,the trees clump into apparently random
patches.The small openingsbetweenclumpsare slight
depressions,some ofwhichcontainstandingwaterduring wet periods. Slightlyfartherupslope these wet
patches become elongatedinto sublinearhollows parallel to thecontours.Growthofvascularplantsis sparse
because of the wetness,and a loose and deep cover of
Sphagnumcuspidatumdevelops. Withmorepersistent
floodingthe Sphagnum cover apparentlydegenerates
(cf. Boatman 1972), and the peat beneath begins to
decompose, resultingin open water.As the surroundinghummocksrisethroughpeat accumulationthewater
in the pools becomes deeper and gyttjasedimentation
betweenthe raseals the pool bottom. The difference
diocarbon dates forthe top of the peat and the base of
the overlyinggyttjais a measureof the hiatus between
pool formationand gyttjasedimentationplus the extent that peat may be removed by decompositional
and erosionalprocesses(Seppili and Koutaniemi 1985,
Foster and Fritz 1987).
The basal dates and fieldobservationsmaybe related
back to the hydrodynamicmodels thatsuggestthatthe
AGE (yr BP)
0
200E
I
o
400-
5000
10000
150 m.
Tansect*I'>
Trasc
of basal 1
120
date
~
Central profile
90 mr
60m
aL
x
om (=center)
600Catotelm decay = 0
FIG. 12. Idealized curve fromthe hydrodynamicmodel
of bog development showing the concave relationshipbetweendepth and age fora peat core taken fromthe centerof
a bog. Age-depthcurves ( ) forotherlocations at a distance fromthe center are labelled. The dotted line (
)
connectingthese curves shows the relationshipbetween age
and depth for a transectof basal samples taken from the
margininto the centerof a bog. The dashed line --- -) is the
linear relationshipexpected if accumulation is constantand
no decay or compactionoccursin thecatotelm.Modifiedfrom
Clymo (1984).
460
D. R. FOSTER AND H. E. WRIGHT, JR.
Ecology, Vol. 71, No. 2
TIME
/
i
-
Z
'-.~~~~~~~~~~A
ofthehistoricaldevelopmentof Hammarmossenshowingplan view and cross-sectional
FIG. 13. Schematicreconstruction
view. The mire was initiatedfroma centrallocus and expanded outward and upward. Hummock-hollowpatternsoccupy
steep marginalslopes and pools develop on deeper, more level centralareas. Pool age and depth decreases fromthe center
to the margin.
developmentof the bog is accompanied by changesin
bog structureand hydrology.At any given time the
vegetationand landformpatternconformsto thegross
morphologyand watertable relationsofthebog. Steep
well-drainedmarginal slopes with a relativelygreat
depthto the watertable supportlinear,hummockand
hollowstructures.Hollows become deeperand broader
progressively
towardsthecenterofthemire,thuspools
occuron gentlerslopes witha higherwatertable. However, as the bog actively spreads, marginal locations
accumulate peat more rapidly than the central mire
expanse (Clymo 1984). Therefore,forany specificlocation in the mire thisleads to a change in slope from
relativelysteep to progressivelymore level as peat accumulates and the marginmigratesoutward (Fig. 11;
Ingram 1983). Throughtime the small linear hollows
initiatednear the bog margindevelop into pools that
deepen and broaden as the mire continuesto expand
outwardand upward (Fig. 13).
The resultsfromthis study suggestthat landform
development,at least on an unconfinedbog capable of
expansion as well as verticalgrowth,may be primarily
controlledby the autogenic changes occurringduring
bog development. In comparison with the significant
hydrologicalchanges (depth to water table, thickness
of the acrotelm,changes in seepage gradients)driven
by changingbog shape, the effectsof such extensive
factorsas precipitation/evaporation
ratiosappear to be
relativelyslight.
ofhumification,
peat structure,
peat composition)may
also arise in response to autogenicallycontrolledhydrological conditions. Allogenic processes should
thereforebe interpretedas an overlayon this internal
signal and can be understood only afterthe internal
signal has been accounted for. For example, in the
currentstudyof Hammarmossen the dates withinthe
pool age: basal-depthcurve may be divided into four
groups(4000-4200, 2940-3060, 2300-2390, and 1940
BP). It is possible that these clusters may represent
some responseto fluctuationsin moistureavailability.
This response,however,is superimposedon the major
signal of autogenicbog development.
In anotherregardit is clear thatthe examinationof
a verylimitednumberofbasal peats,pools, or hollows
may provide a misleadinginterpretationof landform
developmenton a mire. Basal dates frommires have
oftenbeen used as indicatorsof major environmental
change: e.g., change to climaticconditionssuitable for
peat accumulation (cf. Moore 1986) or upland modificationby humans (Moore 1973). In the case of Hammarmossenor otherconcentrically
expandingbogs,the
majorityof basal dates provide no environmentalinformation;they only mark the contemporaneousextent of ecosystemdevelopment. However, the oldest
dates fromthe centerof the mires do mark the onset
of peatland initiation.At Hammarmossenand Nittenmossen the centraldates of 6000 BP coincide with a
knownperiod of lower lake levels in southernSweden
(Digerfeldt1988). Additional studies will be required
Implicationsfor stratigraphicstudyand
to determinewhetherthe historyof moisture availenvironmentalreconstruction
abilityin centralSweden was different
fromthatfarther
The lessons fromthis study concerningthe strati- south,or whetherpeatland initiationwas unrelatedto
graphicreconstruction
of past environmentsare man- changingwater balance and reduced rates of decomifold.Major changesin landscape appearanceand land- position at these sites. Similarly,the timingof pool
formdynamicsmay have occurredon theseraisedbogs initiation provides little direct evidence of climate
primarilyas a resultof autogenic processes. Presum- change or landformstability.Many studies have disably, many otherchanges in stratigraphy
(e.g., degree cussed the relative longevityand age of pools, with
April 1990
BOG FORMATION IN SWEDEN
considerabledisagreement(Ratcliffeand Walker 1958,
Moore 1977, Sakaguchi et al. 1982, Smart 1982). The
Hammarmossenresultssupportthose studiessuggesting thatpools of all ages and landformsin all stagesof
developmentcan occur on mires(Nichols 1918, Kuznetsov 1986).
Poolformation and dynamics
This studycontributesto answeringthe question of
landform dynamics, in addition to addressing the
broaderimplicationsofthedrivingforcesbehindthese
dynamics. During this researchit has been assumed
that pool developmentis the resultof biological processes under hydrologicalcontrol. This conformsto
the general interpretationof many researchers(Sj6rs
1948, Ratcliffeand Walker 1958, Moore 1977, Smart
1982) and is supported by detailed studies (Ivanov
1981, Kuznetsov 1986, Foster and Fritz 1987). The
currentstudyadds the observationthatthe hydrological and geomorphologicalcontextin whichpools form
changesthroughouttheirdevelopment.
Pools are initiatedas hollowson relativelysteepslopes
over shallow peat. As the mire develops the hollows
graduallydevelop into pools (Sernander 1908, Boatman 1983). This evidentlyoccurs as a result of two
processes. Presumablythe rate of peat accumulation
in the hollows is less than that in the adjacent hummock, and thereforethe hollow surfacedecreases in
heightabove the water table (Kashimura and Tachibana 1982, Boatman 1983, Kuznetsov 1986). This is
supportedbystudiesofproduction/decomposition
rates
in hummocks,lawns, and hollows (Clymo 1970, Clymo and Reddaway 1974, Pakarinen 1978, T. Moore,
in press),by stratigraphic
studies showingthe gradual
progressionfromhollow to pool (Moore 1977, Smart
1982, Foster and Glaser 1986, Foster and Fritz 1987)
and by long-termobservations(Mets 1982, Boatman
1983). In addition,and reinforcing
thistrend,the mire
aroundthehollowpresumablybecomes morelevel and
thegeneralwatertablerisesthroughtime(Ingram1982).
As the surfacelevel of the hollow decreases relativeto
thewatertable,theplantsgraduallybecome weakened
and are ultimatelyreplaced by open-waterconditions.
The microtopographical
developmentof small-scale
featureson the mire must thereforebe viewed within
the geomorphologicalperspectiveof the broad-scale
peat mass. This conformsto Aario's (1932) distinction
between Kleinformenand Grossformen.As the mire
becomes flatterand pools develop fromhollows, the
narrowhollowsmay also expand laterallyand coalesce
withadjacent areas to formbroaderpools. The broadbased pools on the central expanse of the mire are
derivedfrominitialhollows thatwere elongatedalong
a much steeperslope.
ACKNOWLEDGMENTS
We gratefullyacknowledge the assistance of M. Thelaus
withthe fieldwork,
theintroductionto the Swedish landscape
461
by H. and G. Sjdrs,the hospitalityand supportof Professors
N. Malmer and B. Berglundat the Universityof Lund, the
helpfulcommentsof B. Aaby, R. S. Clymo,H. A. P. Ingram,
N. Malmer, G. Motzkin, and M. Santelmannon the manuscript,and the technicalassistance of B. Flye. Research supportwas providedby the National Science Foundation (BSR
8806386), the Swedish National Research Council, and the
Swedish Royal Society. ContributionNumber 399 fromthe
Limnological Research Center,Universityof Minnesota.
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