The Growth of Sphagnum: Experiments on, and Simulation of, Some Effects of Light Flux
and Water-Table Depth
Author(s): P. M. Hayward and R. S. Clymo
Reviewed work(s):
Source: Journal of Ecology, Vol. 71, No. 3 (Nov., 1983), pp. 845-863
Published by: British Ecological Society
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JournalofEcology(1983), 71, 845-863
THE GROWTH OF SPHA GNUM: EXPERIMENTS ON, AND
SIMULATION OF, SOME EFFECTS OF LIGHT FLUX AND
WATER-TABLE DEPTH
P. M. HAYWARD*
AND
R. S. CLYMO
College,LondonNW3 7ST
ofBotany,Westfield
Department
SUMMARY
ofbothshadingand ofwater-table
weremade to determine
theeffects
(1) Experiments
depthon thegrowthofthreespeciesof Sphagnum.The species(and theirusual habitats)
were: S. capillifolium
(hummocks);S. papillosum(lawns); and S. recurvum(pools and
flushedlawns).
(2) Water-tabledepthhad littleeffecton growthmeasuredas increaseof drymatter;
associated withplant size.
shadingreducedgrowthand therewere specificdifferences
depthand shading.
interactions
betweenwater-table
Therewereno significant
(3) For growth measured as growth in length,there were highly significant
in responseto shade and, to a lesser
interactions,
individualspeciesbehavingdifferently
depth.
extent,in responseto water-table
(4) In Sphagnum lawns in two naturalhabitatstherewas a negativecorrelation
In experimentalconditions
betweendepth of the water-tableand surface-roughness.
increasedbothas thewater-table
was raisedand as shadeincreased.
surface-roughness
(5) A computersimulationof growthof Sphagnumin a lawn was able to reproduce
theobservedvariationsin surfaceroughness.In mixedlawnsoftwospecies,theone in its
theother.
'natural'habitatout-grew
INTRODUCTION
Most attemptsto measurethegrowthof Sphagnumhave been concernedwithobtaining
values of productivity
(e.g. Leisman 1953; Overbeck& Happach 1956; Pearsall &
Gorham 1956; Bellamy & Rieley 1967; Clymo & Reddaway 1974; Ilomets 1974;
havebeenmadeto measuregrowthin relationto
Pakarinen1978). Relativelyfewattempts
the environment.
Some of these(Chapman 1965; Clymo & Reddaway 1971; Boatman
Sphagnumfromone positionto anotherin the field.
1977) have entailedtransplanting
Otherstudies,by Green (1968) and Clymo (1970, 1973) have includedlaboratoryor
butonlyin a
in whichtheenvironment
was morepreciselycontrolled,
gardenexperiments
limitedway.
The aim oftheworkpresented
herewas to obtainmoreprecisedetailsaboutthegrowth
a quantitative
modelof
in
relation
of Sphagnum
to itsenvironment.
Fromthisinformation,
of
was
thegrowth a Sphagnumcarpet
developed.
The earlierwork indicatedthat two environmental
variables-lightfluxand water
an
on
fluxis important,
butso is the
influence
Total
light
overriding
growth.
supply-have
variationof incidentfluxon a smallscale, betweenone individualSphagnumplantand
anotherin a singlelawn.These effects
maybe due to 'external'shadingbylarger,vascular
byneighbouring
Sphagnumplants.
plantsor to 'self-shading'
Watersupplyto thecapitulum(theapical tuftof expandingbranches)has been shown
to depend,in a complexway,on thestructure
oftheSphagnumplant(whichdiffers
among
* Presentaddress:DepartmentofBotany,University
ofLeicester,LeicesterLE 1 7RH.
$02.00 (? 1983 BritishEcologicalSociety
0022-0477/83/1100-0845
845
846
Effectsoflightand wateron Sphagnumgrowth
species)and on water-table
depth(Hayward& Clymo 1982). In general,however,water
and,fora givenwatercontentin
supplyis inversely
relatedto thedepthofthewater-table
willbe at a greaterdepthbelowhummockspeciesthanitwill
thecapitula,thewater-table
and easily
be below lawn species.Thus, water-table
depthcan be used as a convenient,
watersupply.
measurableway,ofexpressing
habitats,relatively
whichis, comparedwithmostterrestrial
Bogs have a water-table
of speciesappearsto be relatedto thedepthof the
close to the surface.The distribution
water-table
(e.g. Ratcliffe& Walker 1958; Clymo & Hayward 1982). This impliesthat
willaffectplantsof different
speciesin different
changesin the depthof the water-table
is not a plane surfacebeneaththe
ways, complicatedby the factthatthe water-table
(Popp 1962; Goode 1970; Boatman,
hummocks,
lawnsand pools ofthemicrotopography
is closeto thesurface,smallchangesinits
Goode & Hulme 1981). Becausethewater-table
on bog plantsthantheywouldifthewater-table
depthhave a proportionally
greatereffect
wereat a greaterdepth.
in theseexperiments
and
depthweretherefore
controlled
Bothlightfluxand water-table
were includedin the model of Sphagnumgrowth.The model had severalquantitative
consequences.Of these,the one whichcan most easily be measuredin the fieldis the
surfaceroughnessofa Sphagnumcarpet.Thereis someindication(Clymo 1973) thatthis
of
Here we reportmeasurements
is negativelycorrelatedwithdepthof the water-table.
surfaceroughness
bothinthefieldand inexperiments.
THE SPECIES USED IN EXPERIMENTS
wereused in
S. papillosumand S. recurvum,
Plants of threespecies,S. capillifolium*,
bog microhabitats.
experiments.
These species are typicallyassociated withdifferent
Sphagnumcapillifoliumis naturallya species of drierareas such as hummocks,S.
betweenbog pool and hummock,at or
papillosumformslawns at a heightintermediate
and S. recurvum
is foundinpools and wetflushedlawns.
just abovethewater-table,
METHODS
and water-table
Growthin relationto lightflux
depth
An experiment
was made withplantsof the above threespeciescollectedfromMoor
House N.N.R., Cumbria,fromtheareas aroundBurntHill and Bog End (NationalGrid
referencesNY 753329 and NY 765329 respectively)
growingin pots in a gardenin
offivedepthsto
foreach species,offactorialcombinations
London.It includedtreatments,
the water-table(0, 3, 6, 10 and 14 cm below the surface)and fivedegreesof shade
light).We choseto use shade,with
(absorbanceof0, 0.54, 0.80, 0.91 and 0.96 ofincident
in irradiance,ratherthan constantartificial
the consequentfluctuations
lightpartlyfor
convenience,and partlybecause we wantedto be able to relatethe resultsto field
behaviour.
Plantswerecutinitially
to allowall cutendsto be at least1
to (known)length,sufficient
cm underwaterat thestartoftheexperiment
(Table 2). The lowest1 cm ofeach plantwas
couldbe
stainedwitha cationicdye (crystalviolet)so thatanyloss duringtheexperiment
detected.Therewere no losses. All plantsof the same size and taxonwerewell mixed
* Nomenclature
of Sphagnumfollowsthatof M. 0. Hill (1978); thatof vascularplantsfollowsClapham,
Tutin& Warburg(1981).
P. M. HAYWARD AND R. S. CLYMO
847
The fortyplantsweretiedintoloose
beforebeingallocated,fortyplantsto each treatment.
bundles,so thattheycould be recoveredeasily,surrounded
by othercut but unmarked
plants(to reduceedge effects)
and packed at naturaldensityintopots madefromsquare
section2-litreplasticbottlesby cuttingoffthetop partleavinga 10-cmsquare,20-cmtall
containerwithstraightsides. The shorterplantswere placed on an underlying
bed of
choppedSphagnumto bringthecapitulaoftheplantsin all treatments
to thesamelevelin
thepot.The apparatusshownat thetop of Fig. 1 was used to pack theplantsin thepots.
The leftcontainerwas a slidingfitinsidetherightone. The plantswerepackedas shown
and theleftcontainerplus plantsslidintotherightcontainer.The leftcontainerwas then
withdrawn
withthestringwhilethepistonheldtheplantsin positionintherightcontainer.
The filledpots wereclose packedon a flatlevelsurfaceand weresurrounded
by a 'guard
ring'of otherpots filledwithSphagnumon whichno measurements
were made. The
and position
lastedfor77 daysfrom17 June.Duringthistimetheorientation
experiment
ofthepotswas changedrandomly
each weekaccordingto a Latinsquarearrangement.
a tubeinserted
The waterlevelin each potwas maintained
intoitsbase (Fig. 1).
through
The tubewas attachedto a devicewhichkeptthewaterlevelconstanteitherby allowing
excessrainto runaway or by addingdistilledwater.Withthisarrangement
therewas no
obvious accumulationof evaporitesat the surfaceof the plants.As the plantsgrew,the
at a constantdepthbelowthesurfaceofthe
potswereloweredto maintainthewater-table
Sphagnumlawnin each pot.
The potswereindividually
shadedby coveringthemwith0, 1, 2, 3 or 4 layersofblack
polyestergauze. The absorbanceof lightby thismaterialwas measuredon a recording
String
Ofherplants
40 plants
Homogenized
cut to length
dead plants
Sphagnum
surface Water-table
4l-Air
FIG. 1. Methodused to maintaina constantwaterlevel in pots duringthegrowthexperiment.
Onlytwoof thefivelevelsare shown.Waterwas cycledby raisingiton a streamof airbubblesin
withdistilledwateras required.The
a verticalglass tube.The 25-litreMariottebottlewas refilled
upperdiagramshowstheapparatusused to pack plantsin containers.
848
Effectsoflightand wateron Sphagnumgrowth
spectrophotometer.
Between wavelengthsof 350 nm and 1000 nm it was nearly
independent
of wavelength
withan averagevalue,between400 nmand 700 nm,of0.337.
The absorbanceof multiplelayerswas close to additive.The gauze did affecttherateof
evaporationof water;theratewithgauze was about 20% belowthatwithoutgauze. But
therateat whichwatermovesup Sphagnumis high(Clymo& Hayward1982) so itseems
thatthewatersupplyto thecapitulawas muchaffected.
unlikely
The size of the experiment
(seventyfivepots,20 000 plantsof which3000 werecut
accuratelyto size) precludedreplication,
so second orderinteractions
were used as an
estimateoferror.If anything,
theyshouldtendto over-estimate
error.However,in orderto
obtainmoreinformation
aboutthetypesof errorinvolved,a further
fifteen
pots wereset
up. All thesecontainedS. capillifolium,
all had a water-table
3 cm belowthesurfaceand
all had one layerofgauze shading.Threeharvestsweremade; after5, 36 and 45 weeks.
At harvest,twotypesof measurement
of growthweremade: lengthand mass.Increase
in lengthwas measureddirectly
witha ruler.Increasein drymass was measuredusingthe
'capitulumcorrection
method'ofClymo(1970). The newgrowthwas cutofftogether
with
an extra1 cm to includeall theoriginalcapitulum.Fromthedrymassofthismaterialwas
subtractedan estimateof themass of theoriginalcapitulumbased on a linearregression
betweencapitulummass and themass of a unitlengthof stemimmediately
belowit.This
methodallows forchangesin the size of the capitulumduringthe experiment
and for
materialcarriedupwardsbyinternode
within
theoriginalcapitulum.
extension
The resultsof growthin mass wereexpressedin twoways: as growthperplantand as
growthpermass of unitlengthoffullyelongatedstem(thesectionfrom1-4 cm belowthe
apex whenthe experiment
began).The latterwas used as a meansof comparingplants,
thoseof different
sized apices; stemmass is correlated
particularly
species,withdifferent
withcapitulummass and hencewithapex size (Clymo 1973).Thisprocedurealso reduced
variabilitybetween individuals.During the experimentsa few plants forked.The
proportion
doingso was about 0*01, and thetwo axes of such plantsweretreatedas a
singleindividual.
Fittingsurfacesto results
surfacesdescribedbypolynomial
Whererequired,multi-dimensional
equations(detailed
in the relevantplaces) werefittedto experimental
resultsby minimizing
a badnessof fit
was the sum of squareddeviationsfromthe measuredvalues.
function,
g. The function
Arbitrary
values of thepolynominal
coefficients
are chosenand thevalue ofg calculated.
New valuesof thecoefficients
are thenchosenin such a way as to reducethevalue ofg.
The processcontinuesuntilno further
reduction.
of g can be obtained.A moredetailed
is givenby Clymo (1978). The SIMPLEX option(Nelder& Mead 1965) ofthe
description
CERN programMINUITS (libraryD506, D516) was used to locate minimaand the
MIGRAD option(usingDavidon's 1968 method),
whichis extremely
fastneara minimum,
to refine
theestimate.
Surfaceroughnessin thefield
Surfaceroughnesswas estimatedby the pointframe('shoe-box')technique,as the
varianceof height(Harper,Williams& Sagar 1965; Boorman& Woodell 1966) after
removalof largerscale (5-10 cm) topographic
surfaces(Clymo 1973). A setofthirty-two
10 cm long pointers(culms of Deschampsia cespitosa) were supported
light-weight
and parallelin holesintwohorizontal
vertically
piecesofstainlesssteelgauze.The pointers
in a 4 x 8 rectangular
werearranged8 mmapartin each direction
of
grid.Measurements
P. M.
HAYWARD AND
R. S. CLYMO
849
surfaceheightweremadeby photographing
thetopsofthepointersagainsta graphpaper
background.Pointersand cameraweremountedon an aluminium
frameincorporating
a
mirrorat 450 angle which allowed the camera to be pointeddownwardsto record
horizontal
viewsofthepointers.
The resultsgivenare ofresidualvarianceinheightafterfitting
a surface,with25 degrees
offreedom,
ofthegeneralformz = a + bx + dX2+ ey2 + fxyto theheightsofthepointers
(x andy are thetwoaxes oftherectangular
grid;z is theheightabove an arbitrary
zero; a
A surfaceofthisnaturewas fitted
to f are parameters).
fortworeasons:theframeholding
thepointersdidnothaveto be horizontal,
and topographic
featureson a scale ofabout 10
cm wereignoredwhilstkeepingthe small scale (1 cm) roughness(Clymo 1973). Each
framethusgave a singlevalueofroughness.
Field measurements
weremade at randompointswithinan area chosenbecause itwas
occupiedby a carpetof the requiredspeciesof Sphagnumat about the requiredheight
above the water-table.
Garden measurements
weremade on the centreof experimental
cores.
Surfaceroughnessinexperiments
Cores, 30 cm in diameter,of S. capillifolium,
S. papillosumand S. recurvumwere
collectedwitha stainlesssteelcutterfromCoom RiggMoss N.N.R., Northumberland
and
BrishieBog on theSilverFlowe N.N.R., Galloway(NationalGridreferences
NY 690796
and NX 447833, respectively).
Care was takento disturbthecoresas littleas possible,and
surfacedisturbance
was negligible.
The cores weremaintainedoutdoors,as describedfor
thegrowthexperiment,
withfactorialcombinations
oftwodepthsofwater-table
(3 cm and
15 cm belowthesurface)and twodegreesof shade (absorbanceof0 and 0.80, from0 and
2 layersof gauze respectively).
Two cores (one in thecase ofS. recurvum)
wereallocated
randomlyto each treatment,
one corefromeach site.No subsequentdifferences
in surface
roughnessattributable
to originwereobserved.Vascularplantsgrowingabove thelevelof
the Sphagnumcapitula (mainlyCalluna vulgaris,Eriophorumangustifolium
and E.
vaginatum)werecut offperiodically
and measurements
of surfaceroughnessmade,with
theapparatusalreadydescribed,
at intervals
fora year.
RESULTS
Growthin massand lengthin responsetoshade and water-table
The growthof plantsin theextrafifteen
Sphagnumrubellumpotsis shownin Table 1.
in measurements
of growthin bothlengthand mass was relatedto theamount
Variability
of growth.For thesepots and all theothersin theexperiments
(ninetyin all) therewas an
capillifolium
plantsin pots (see text).The experiment
began
in September.
Harvestsweremadein October(after5 weeks),May (after36 weeks)
and July(after45 weeks).Values are themeanforfivepotsexceptthoseforgrowth
in lengthwhichare themeanforall individualplantsin fivepots (about 200 in all).
Valuesinbracketsare standarderrorsofthemean.
TABLE 1. Growthof S.
Growthinlength(cm)
Growthin drymass (mg)
Etiolation(cm mg-')
5
Harvestafter(weeks)
36
45
1.30 [0-04]
1.62 [0-38]
0.97 [0-25]
3 48 [0.06]
4-10 [0-50]
0.91 [0-15]
6.53 [0-21]
8.17 [1.47]
0.97 [0-15]
Effectsoflightand wateron Sphagnumgrowth
850
(r = 0.81) betweenstandarddeviationand meangrowth
linearrelationship
approximately
appliedto all the resultsbefore
was therefore
in lengthof plants.A log transformation
ofreducingthecoefficient
effect
incidental
had the
analysis.Thistransformation
statistical
ofvariationfroma meanof35% to 17%.
of theplants,
resultfromtheextrapots was thatthemorphology
A secondimportant
did not
mass),
in
growth
by
divided
in
length
as
growth
(quantified
as
etiolation
measured
not
was
experiment
main
the
of
scale
time
the
that
implies
This
changebetweenharvests.
of
different
experiments
with
obtained
been
have
would
results
relative
same
critical;the
length.
depthhad
is shownin Table 2. Water-table
Dry matterincreasein themainexperiment
with
(associated
differences
specific
were
there
littleeffect,shadingreducedgrowthand
of
results
the
with
as theyagreein general
are surprising
plantsize). None of theseeffects
interactions.
significant
very
(Clymo1973).Therewereno
otherlessdetailedexperiments
however(Table 2); individual
interactions
Growthin lengthshowedhighlysignificant
a
lesserextent,in responseto
to
and,
in responseto shade
speciesbehaveddifferently
an
'optimum'shade and forS.
depth(Fig. 2). For all threespeciestherewas
water-table
2. Mean values of growthof three species of Sphagnum in the pot
carriedout in a gardenin London,on the effectsof shade and waterexperiment
For
fromlogarithms.
table depthon growth.Values have been back transformed
indicatelength(cm) to which
see text.Values in parentheses
detailsof treatments,
Italicvaluesin bracketsareF values
plantswerecut at thestartof theexperiment.
on growthin length
froman analysisof variance.Mean valuesforthemaineffects
meanvalues
The interaction
are significant.
are not givenbecause theinteractions
are shownin Fig. 2.
TABLE
Degreesof
freedom
Species
S. capillifolium
S. papillosum
S. recurvum
2
[85.2***]
3.0
8.5
6.2
[40.1***]
6.2
7.3
13.1
[39.2***]
9.2
7.4
5.8
4.0
2.9
[32.7***]
14.4
12.0
8. 1
65
4.7
Shade Layers
0
1
2
3
4
Absorbance
0.00
0 54
0.80
0.91
0.96
4
depth
Water-table
(cm)
0
Plantlength
(cm)
(5)
4
3
6
10
14
[1-1]
4.8
(5)
8
8
16
Residualmeansquare
30
[0-7]
Length
(cm plant-')
[52.7***]
[26.2***]
[18.8***]
7.6
5.4
8-7
[1-6]
[1-11
[1-5]
[1-9]
[2.9*1
[1-91
[13.2***]
[7.2***]
[2.3*]
0 018
0.011
5-4
5.9
5.6
(1 1)
(11)
(15)
Interactions:
Species x shade
Species x water-table
Shade x water-table
*0.01 <P<0.05.
**0.001 < P < 0.01.
*** P 0.001.
Growthin
Mass
Mass
(mgplant-1) (mg(mgcm-'stem)-')
0.016
8-4
9.1
8-4
851
S. capiii1folium
(I)
>
l~~~~~~~~~~~~~~5
q
(a)
5
(b)
'''''.'.''-"tA" .R:...R
Ng,'''.M
....~..
..........
,
N
,
>,
..N ........
~~~~~~~~~~~~.............
se5
(1)Spapiiiosum
00
0:1
X:::
:::: E.X;o::::-...................:::::
::::::: w
>n..
.,,...
*X:-....
...........
(d) .
(
1
.
I0~~~~~~~~0
S. recrvSum
(iii)
4
(c)
0
.....,.,..
/
>~~~~~~~~~~~~~~~~~~~~~~~~
SO
d0 A
r0
2
(III) S. recurvum
5c
0
0
(f\
(e-5)
::'e(Q 3X
?er
o
y
\1
.
4 141 \c>\tX((\
FIG. 2. Growthin lengthof Sphagnumin relationto shade and water-table:
(a), (c), (e) growth
(bars represent95% C.L.); (b), (d), (f0surfacesfittedto the resultsin (a), (c) and (e),
by a leastsquaresfitto thecubicequation
respectively,
L =a + bW + cS + dWS + eU'2 + fS2 + gWV3+ hS3 + iW2S + jWS2
whereL = growthin length(cm), W = depthof water-table(cm), S = absorbanceof gauze
shade, a to j are parameterswhose value definesthe shape of the surface.The values are
estimatedbytheminimization
procedure.
Effectsoflightand wateron Sphagnumgrowth
852
reducedtherateofgrowthin lengthat
papillosumand S. recurvum
a fallinthewater-table
on rateof
In unshadedconditions,
water-table
depthhad a largeeffect
theshadeoptimum.
buthad relatively
growthin lengthof S. recurvum
and to a lesserextentofS. capillifolium
withthoseofClymo(1973)
littleeffect
on S. papillosum.Theseresultsare againconsistent
and also those of Sonesson et al. (1980) who measuredthe elongationof S. riparium
in different
degreesofshade.
(whichis in sectionCuspidataand relatedto S. recurvum)
ways in responseto the
It is clear thatgrowthin mass and lengthvaryin different
treatments.
Stemlengthbetweenbranchfasciclesincreasesand branchlengthdecreases
(i.e. theplantsbecamemorestraggling,
or etiolated)witheitheror bothgreatershadeand
are showninTable 3.
higherwaterlevel.The effects
ofshadeand waterdepthon etiolation
inmass)forthethree
inlength/growth
3. Meanvaluesofetiolation
(growth
carried
outina gardeninLondon,
usedina potexperiment,
speciesofSphagnum
ofshadeandwater-table
Unitsofmeasurement
are
on theeffects
depthongrowth.
from
cm plant-'(mg (mg cm-'stem)-')-'.Valueshavebeenback transformed
are all highly
of species,shadeand water-table
The maineffects
logarithms.
significant
(P < 0.001).
TABLE
Layers
0
1
2
3
4
Shade
Absorbance
S. capillifolium
S. papillosum
S. recurvum
(0-00)
(0-54)
(0-80)
(0.91)
(0.96)
0.16
0.31
0.53
0.66
0.91
0.11
0.31
0.48
0.63
0.57
0.20
0.41
0.51
0.54
0 29
Water-table
depth
(cm)
0
3
6
10
14
0.55
0.56
043
0.37
0.32
0 49
045
0-41
0 27
0 24
0.65
0.51
0.37
0.28
0.20
measurements
Surfaceroughness
Naturalconditions
Measurementsof surfaceroughnesswere made on lawns of S. papillosumin field
conditionson Brishie Bog on the Silver Flowe N.N.R. and at a site in a pooland-hummock
complexbetweenBog Hill and SpurHill at Moor House N.N.R. (National
NY 771328). The resultsfrombothsites(therewas no significant
Gridreference
difference
betweenthesites)are shownin Fig. 3. A negativecorrelation
betweenvarianceand depth
ofwater-table
was notlinear.A plotoflog varianceagainst
was foundbuttherelationship
water-table
linebutno functional
is implied.
relationship
depthwas neara straight
Experiments
thatinwhichthecoreswerecollected(44
By themiddleofthegrowingseasonfollowing
weeks fromthe introduction
of thewaterlevel and shade treatments)
values of surface
effects
attributable
to species,
roughnesswereas shownin Table 4. Thereweresignificant
shade and water-table
interaction
betweenspecies
depthand therewas also a significant
and shade. These trendscontinuedthroughout
the growingseason; both shadingand
P. M. HAYWARD AND R. S. CLYMO
853
S2O
E
E
10
o8
>6
-00
-
4
LO
L
10 _
Lo
2
I
0
I
10
I
I
20
I
30
fi
Depthof water-table(cm)
FIG. 3. Surface roughness of natural lawns of Sphagnumpapillosumin relation to depth of
water-table. The line shows the regression:
log10 (y) = 1.33-
00297
x; r2 = 0.90,
where y is the residual surface variance (mm2) and x the water-table depth (cm). (a),
House; (0), Silver Flowe.
Moor
4. Surfaceroughness,assessedby residualvariance(mm2)oflawnsofthree
speciesof Sphagnumgrownin controlledconditionsfor44 weeks.Values are the
mean of duplicates,but for single measurementsfor S. recurvum.Values in
parenthesesare lightabsorbanceby shadingmaterial.Italic valuesin bracketsare
F values froman analysisof variance.The F-value of the interaction
species x
shadewas [12.3*].
TABLE
Shade
[1 72***]
Shaded (0-80)
Unshaded (0-0)
Water-table
depth (cm)
[35**]
3
15
3
15
S. capillifolium
17
13
9
2
Species [39**]
S. papillosum
S. recurvum
33
27
12
7
26
23
19
10
*0.01
<P- 005.
** 0.001 < P < 0.01.
*** P < 0.001.
raisingthewater-table
increasedthesurfaceroughnessof all speciesalthoughwater-table
intherangeofconditions
investigated
herehad thesmallereffect.
A MODEL OF THE GROWTH OF A SPHAGNUM CARPET
The variousinfluences
on Sphagnumgrowthare generallyconsideredin isolation.If their
is
overalleffect to be studiedtheymustbe appliedto the systemtogether
and allowedto
act together.
Two sortsofeffect
maybe imagined:
(i) wheretheoveralleffectis thatof theapplicationof morethanone factorin a simple
additiveway; or
(ii) wherethe factorsmay interactin such a way as to producea resultwhichis not
fromthemaineffects
alone.
predictable
854
Effectsoflightand wateron Sphagnumgrowth
In themodelof a Sphagnumcarpet,whichis now described,measuredinteractions
were
included,but all othereffects
weresimpleadditiveones.The modelallowsan examination
of the consequencesof puttingthe pieces togetherin a situationwherea sufficiently
wouldbe too largeto be practicable.It also providessome
detailedfactorialexperiment
calculatedeffects
whichcan be checkedindependently-values
forsurfaceroughnessare
themostobvious.
The modelis shownpictorially
to theleftof Fig. 4 whereSphagnumplantsare shown
growingverticallyin a pure lawn which is flat and horizontalwith a well defined
water-table
belowit.The capitulaarenotalwaysall at thesamelevel,however,and so the
questionmustbe considered:What happensto an individualifitscapitulumis notat the
generalsurfacebut is eitherabove or below it? To the rightof Fig. 4 are shownthe
radiantfluxand thewatersupplyto whichthecapitulumof such an individualmightbe
subject.Bothof theprofiles
have twodistinct
parts:one above,theotherbelowthemean
surface.
Incident radiation
Capitulum
/
Surface
Wateravailability
Watertable
Increasing
oflightfluxand water
FIG. 4. A conceptualmodelofa Sphagnumlawnwith,to theright,
profiles
availabilityto thecapitulumof an individualwhichis notgrowinglevelwiththerestof thelawn
and whose capitulumis eitherabove or belowthemean surface.The partofthemoistureprofile
shownby thebrokenlineis speculativeand has notbeendemonstrated
by experiment.
For shortdistancesabove the surface,incidentradiantfluxis constantat a value
determined
by the generalclimateand 'external'shadingfromvascularplants(Calluna
withdepthor, moreprecisely,
vulgaris,etc.). Below the surface,fluxfallsexponentially
withcumulativedrymatter,i.e. Beer's Law can be applied.The slope of the attenuation
ofshadingand water-table
linedependson thespeciesinvolvedand on theprevioushistory
shaded
conditions
&
and
a
(Clymo
Hayward 1982);
high water-tableboth reduce
i.e.
is
related
to
absorbance
'etiolation'
absorbance,
inversely
(see above).
The moistureprofilebetweenthesurfaceand thewater-table
dependson severalfactors
of
the
Both
water-table.
includingspeciesand thepreviousposition
hydraulic
conductivity
no simplemathematical
and densityof thepeat also affectsupply.Thereis consequently
descriptionof water supply analogous to Beer's Law for light,althoughin specific
conditionsthemoistureprofilecan be described(Hayward& Clymo 1982). Fortunately,
P. M. HAYWARD AND R. S. CLYMO
855
thisproblemcan be avoidedby usingtheresultsfromthegrowthexperiment
described
notto watersupply.Polynomial
above as growthwas relatedto depthof thewater-table,
equationsfittedto thoseresultshave thusbeen used in themodelto describegrowthin
termsof both lengthand drymatterby substituting
values forshade (calculatedfrom
belowa particular
Beer'sLaw) and thedepthofthewater-table
capitulum.
The effect
of theheightthatan individualplantis exposedabove thegeneralsurfaceon
watersupplyto, or on growthrateof,theplant,has notbeeninvestigated
experimentally
theamountof
and is indicatedby a brokenlinein Fig. 4. In thecomputermodeltherefore,
growthcalculatedfromthe heightof the capitulumabove the water-tablehas been
it by a value whichwas variedto simulatedifferent
modifiedby multiplying
exposure
The exposureeffect,
froma simpleexponential
effects.
function:
E, was determined
E = exp (k *DL)
wherek is a constantand DL is theheightof an individualabove theaveragelevelof the
six surrounding
plants (Table 5). (Capitals denote names of variables used in the
computerprogram.)Withk = 0, therateof growthwas unaffected.
By makingk positive
or negative,
therateofgrowthcouldbe increasedwithexposureor decreased.
The componentsof themodeland theirinter-relationships
are shownin Fig. 5. Most of
the model,that partwithinthe box, concernsindividualSphagnumplantsin the lawn
separately.Five parametervalues are requiredbesidesthosedescribing
growthrate and
the attenuation
of light.These fiveparametersare listedin Table 5 together
withtypical
values. In thefollowing
resultsthe numberof plants,n, was set at 25 throughout.
These
plantswerearrangedin a 5 x 5 gridwithalternaterowsoffsetby halfa columnso that
each plant was surroundedby six equidistantothers.The whole arraywas wrapped
i.e. theplantsat theleftedge are consideredas
aroundtoroidally
to minimize
edgeeffects;
adjacentto thoseat therightedge,and thoseat thetop as adjacentto thoseat thebottom.
TABLE 5. Parameters
required
inthemodelofSphagnum
growth
showninFig.5.
Symbol
Description
Chosen beforeeach run
Numberofplants,n,inthelawn
C(1)
k in theequation:E = exp (k.DL) for
C(2), C(3)
effect
ofexposureon elongationand
productivity
respectively*
Externalabsorptionof lightby
C(4)
Calluna vulgarisetc.
C(5)
Depthofwater-table
belowmean
surfacelevel
Builtintothemodel
Parametersforgrowthfunction
GL(1.. 10)
determining
elongation
(foundfromgrowthexperiment)
Parametersforgrowthfunction
GM(1.. 10)
determining
productivity
(foundfromgrowthexperiment)
Parametersforfunction
AC(1 .4)
extinction
coefficient
determining
(fromHayward 1980)
Typicalvalue(s)
Units
25
-1.39 (growthhalved
whenDL = 0.5 cm)
cm-l
0 and 0-3 (testedup to 1.0,
i.e. 10% transmission)
0-14
cm
See Table 6
cm plant-'
See Table 6
mgplant-1
See Table 6
* DL is thedistanceof an individual
above themeanheightofthesix surrounding
plants(SRND). In Fig. 5,
DL = X(j) - SRND. The effect
operatesonlywhenDL is positive,i.e. theindividualis emergent.
856
Effectsoflightand wateron Sphagnumgrowth
LEVNGTe
MTTE
average
A(6 o n
e
A()
J
e
prodAuc7tvityJ_
box for each of j =I fo n plant
Ge~~rjpeat
,f--
i
Xt
etiolation
/elongation
_THGROWTHe
A
~~--~
lengthabove
datum
X(j
TL
m-
TL
surfacerioughness
A''
k~~~~~J~~~~~~~
FUNCIONproductivity
~R(j
-kNCTIONI
exposure
effect
(EE)
-
length
(DL)
Rmean lengthof 6
surroudingplant
ng
varianceverage
/
/
~~~~water-tal
heightabo
((EXT)A I
--
/tum
X/
(SRND)
~~A(2
-
fl
water-tabledepth
belowindividual
(W)
dtum
in)
exposure
effect
(EE)
\I
shadingon
plant (S)
A(j + 1)
matter
a
above
/n
/
extinction
coefficient
bB(i
C4
vrg
se If-shad
A(9)
3
average mnter
(
I
matter
oof
A5)
FIG. 5. Flow diagramrepresenting
a model of Sphagnumgrowth.Flows of 'matter'and of
'length'are representedby solid arrows and causal relationships('flowsof information')
by
broken lines. The conventionsare those of Forrester(1961): 'X' representsa measurable
physicalquantity,'R' a rate,'A' some otherauxiliaryvariableand 'C' a constant.Lettersin
parenthesesare othernames used in the model.Everything
withinthelargerectangular
box is
repeatedforeveryplantin a carpet.(A flowof 'length'indicatesthatthe particularvalue of
lengthgrowthof the plantsin a giventimeinterval,determined
by thevariablesand constants
shown,is handedon unchangedduringsubsequenttimeintervals.)
It is theoretically
possiblefortheseedge constraints
to cause wave-likepropagationof
effects,
buttherewas no signofthishappening
inthesesimulations,
For each plantin thelawn,growthin bothlengthand mass was calculatedas indicated
by the solid arrowsin Fig. 5. The rateof growthwas determined
by a 'growthfunction'
and dependedon the shade ('external'and 'self')and depthto thewater-table
as already
described.The growthfunctions
werethepolynomialequationsdescribing
fittedsurfaces
such as thosein Fig. 2. The parameters
oftheseequationsare givenin Table 6 forthetwo
speciesused inthemodel.Mixedlawnsofdifferent
speciescouldbe modelledby specifying
theappropriate
parameters.
The water-table
was keptat a specified
depthbelowthemean
surface,i.e. bothroseat thesamerateas theplantsgrew.
The amountof shadingon the individual(S) had two components:the amountof
externalshade, C(4), and the amountof 'self-shade'.The latterwas zero unless the
capitulumwas belowthemeanlevelof its six nearestneighbours
whenDL was negative
of self-shade
werecalculatedfromBeer'sLaw. If theplantswererigid
(Fig. 5). The effects
as impliedby themodel,and thesun wereoverheadat all times,thiswouldbe
structures,
incorrect.
However,formostof thetimethelightfallsobliquely,so thepathlengthis at
least proportionalto DL (path lengthequals DL dividedby the sine of the angle of
incidence).Also, theplantsare not rigidstructures
and tendto crowdovereach otherso
thatevenifoverhead,thesunmustoftenpenetrate
a canopyofneighbouring
plantsbefore
reaching a shorterindividual.Thus Beer's Law, in practice, seems a reasonable
forestimatedself-shade.Depth in the carpetwas used in thecalculation
approximation
P. M. HAYWARDAND R. S. CLYMO
857
see text) of
TABLE 6. Values for parameters(estimatedfrommeasurements,
equationsused in the modelof Sphagnumgrowth.The two speciesin the model
and (ii) S. papillosum
were:(i) S. capillifolium
Z*
Length,
(cmplant-')
(i)
(ii)
Parameter
1-5
0.79
a
0 57
0.097
b(W)
10.6
5-1
c(S)
-0.77
0.057
d(WS)
-0-014
-0 082
e(W2)
-7.2
-3.8
f(S2)
0.00026
0.0030
g(W3)
0.30
1.2
h(S3)
0.018
0-018
i(W2S)
0.36
-0-28
j(WS2)
Mass,Z*
(mgplant-')
(i)
(ii)
1.7
10.9
0.72
1.3
5.5
3.5
-3.6
-0-60
0.027
-0-028
5.2
-9.0
-0-00030
-0-0069
3.9
-4-4
0-031
0.13
1.22
-0-046
(i)
0.89
0.085
-0 73
-0.093
EXTt
(ii)
0.71
0.022
-0-58
-0-027
* 'Growth
ofthegrowth
fitted
to theresults
werepolynomial
experiment.
Theywere
functions'
equations,
of the form:Z = a + bW + cS + dWS + eW2 + fS2 + gW3 + hS3 + iW2S + jWS2 whereW= depthof
whosevaluedefines
anda to j areparameters
the
ofshading
plantmaterial,
water-table
(cm),S = absorbance
Thevaluesareestimated
Theequations
werevalidover
procedure.
bytheminimization
shapeofthesurface.
0-14 forWand0-1.3 forS.
theranges
coefficient'
(from
equations
Hayward1980)wereoftheform:EXT = a + bW + cS + dWS.
t 'Extinction
EXT was
therange3-15 forW and 0-0.8 forS. Outsidetheselimits
The equationis validonlywithin
constant.
assumed
toremain
the
ratherthan cumulativedrymatter(Hayward 1980) in orderto avoid complicating
depth,
coefficient
(EXT) was calculatedfromthemeanwater-table
model.The extinction
oftheseequationsare
C(5), and theexternalshade,C(4) (Hayward1980).The parameters
giveninTable 6.
(DL positive)
Whenthelengthof an individualwas greaterthanthatof itsneighbours
the rate of growthwas modifiedto take account of increasedexposureas previously
explained.
in
important
of surfaceroughnessdescribedabove are particularly
The measurements
of the
the contextof the model because the value of surfaceroughnessis independent
assumptionsmade in the model and can be calculatedfromit. The model calculates
surfaceroughnessas thevarianceof thelengthsof all theplantsin themodelcarpet.This
measureis similarto thatused in thefieldbutnotidentical;thelengthsmeasuredin real
plants.
thoseto theapicesofdiscreteindividual
lawnswerenotnecessarily
The completemodel was writtenin FORTRANand run as a subroutineof a general
fora
by one of us (R.S.C.). It controlsinput,managesiterations
programSYSFLOwritten
series of time steps and produces printedand graphicaloutput.Forrester's(1961)
conventionswere followed.At each iteration,all new values of variables(lengthsand
weights)werecalculatedbeforeany rates.The orderof individualplantsand theorderin
whichthenewvalueofvariableswas calculatedwerethusofno importance.
Resultsfrom the computermodel
The model describedin the previoussectiondoes not allow forindividualvariation
It couldeasily
betweenplants;all behavein thesameway in a particularsetofconditions.
The
be made stochastic,butthereis too littleknownat presentto make thisworthwhile.
by choosingthe
'forced'at thestartto simulateindividualdifferences
modelwas therefore
at random)followeda normaldistribution
initialvalues so thatthe lengths(distributed
witha meanof 0 cm (datumlevel)and a standarddeviationof0 5 cm.The onlyexception
858
Effectsoflightand wateron Sphagnumgrowth
(2 S.D.) and theeffectof
was thattwo plantswereset at -1 cm and + 1 cm respectively
recordedindetailforthesetwoplants.
shadingand exposurewas subsequently
and S. papillosum
The modelwas firstrunwithpurelawnsof each of S. capillifolium
oftheconstantsC(2) to C(5). The modelwas testedwithdepths
and variouscombinations
of between0 cm and 14 cm withabsorbance('external'shade)up to 10
to thewater-table
(10% transmission;equivalentto a path lengththroughCalluna vulgarisof 6-9 cm
(Grace & Woolhouse1972)).
The most noticeablefeatureof theserunswas thatthe model Sphagnumpopulation
of parameters
tendedtowardsone of two statesdependingon theparticularcombination
used. The populationeitherwentinto a steadystatewiththe averagerates of growth
constantand surfaceroughnessstabilized,or it becameunstablewithsomeplantsfalling
rateawayfromthemeansurface.
behindand othersgrowingat an increasing
irrevocably
Two typicalresults,showingstableand unstablebehaviour,are shownin Fig. 6. There
weretwo sets of conditionsunderwhichthe modelbecame unstable.The firstof these
occurredin cases withno externalshade, C(4) = 0, and withcombinationsof high
was increasedwhen
The instability
and onlya smallnegativeexposureeffect.
water-table
the exposureeffectwas made zero or positive(i.e. withincreasedgrowthabove the
weremainlycaused by longplantsgrowingaway from
surface).These unstableconditions
thesurface.This consequenceis expected,butsuchplantsare neverobservedinnature,so
itis probablethatk is inrealitynegative.
The modelalso becameunstablewhentherewas someexternalshade and thedepthto
was great.Changingthevalueofk (positiveor negative)intheequationfor
thewater-table
was largelydue to short
In thiscase theinstability
exposureeffectmade littledifference.
plantsbeing'leftbehind'.The modelwas notdesignedto respondto sucheventsand plants
whichbehavedin thisway (and whichwouldhave diedin reality)simplylefta 'hole' inthe
model lawn. In naturethis hole would be filledby sidewaysmovementor growthof
ofapicesto producetwoplantswherebeforetherewas one
or by thesplitting
neighbours,
(Clymo& Hayward1982).
Some of the model resultsof surfaceroughnessare shownin Fig. 7. The resultof
changingthe exposureeffectwith no externalshade is shown in Fig. 7 (a) for S.
As expected,the resultsshow a decreasein surfaceroughnesswithmore
capillifolium.
withexceptionof a small(thegraph
negativevaluesof k and also withlowerwater-tables,
incombination
withthemost
is plottedon a log scale) increasewiththelowestwater-tables
ofthissize couldbe detectedinthefield
thatan effect
negativevaluesofk. It is improbable
orin experiments.
downto 6 cm and ofshading
on surfaceroughnessofdepthofthewater-table
The effect
and S. papillosumis shownin
withabsorbanceof from0 to 1I0 forbothS. capiltifolium
Fig. 7(b) and (c). Sphagnumcapillifoliumbehaved much as it did in realityin the
lawnsdescribedabove (see Table 4). Water-table
depthhad a similareffect
experimental
on the surfaceroughnessof S. papillosumalthoughit was in generalsmallerwithlower
generallyincreasingroughness,exceptwherethe
Shade had a largereffect,
water-tables.
These resultsdo not
absorbanceof lightwas 0.3 whenit was apparentlyat a minimum.
of shadeon S. papillosumwas muchgreater
conflict
withthoseofTable 4 wheretheeffect
thanwas thatofwaterlevel.
species.For theseteststhemodel
The modelwas nextrunwithmixedstandsofdifferent
a flatcarpetin whichwereplacedtwoplants
was arrangedwithone ofthespeciesforming
of theotherspecies.In variousteststheseplantswerearrangedto be eitherlevelwiththe
generalsurfaceor 1 cm above or below the surfaceas in previousruns.Figure8 shows
P. M.
HAYWARD AND
(a)
R. S.
859
CLYMO
A(O0)
8
,
-\\\/
/
6
6.
4
,, 2
A(3)
A(8).
,
A
A2
/
A(17)
/
s2
_,
...........
..(6)
)
A(
.
cn
cv
0
(b)-
10
o
t
8
/
- -
/
A
AO)-
.h--
. ...
A(17)
A()
~~~ V
82
A
--
A(7)~~~~~~~~~~~~~~A8
> 6
<)
100G
200
300
400
500
600
Time (days)
FIG. 6. Resultsof computersimulationforthe growthof Sphagnumcapillifolium
run for600
timestepsof one day: (a) withno externalshade and thewater-table
3 cm belowthesurface;(b)
withabsorptionby externalshade of 0-3 and water-tableat 12 cm. Variablenames are those
used in Fig. 5 and theirvalues have been scaled on a commonaxis by multiplying
by theitalic
values shownbelow in brackets:A(2), averagelength(cm) [0-40]; A(3), average mass (mg)
[0-25]; A(4), variancein length(surfaceroughness)(cm2) [20]; A(5), variancein mass (mg2)
[1.0]; A(6), averagerate of elongation(cm day-') [40]; A(7), averagerateof production(mg
day-') [401; A(8), averageetiolation(cm mg-1)[101. Growthin lengthof plantsinitially1 cm
above and below the mean surfaceis shownby shadingaroundA(2). Exposureeffectwithk =
-1 39, A(10) [10], and shade, A(17) [2 51, are shownforthesetwo plantsrespectively.
After
600 days,thevalue ofA(5) in (b) was 35 mg2.
someof theresultsobtainedas themeangrowthin lengthofthesecondspeciesin relation
to thatofthemainspecies.Bothof thesegraphsshowsimilareffects.
In bothcases,when
therewas no externalshade, S. papillosum grew fasterthan S. capillifoliumdid when the
water-table
was nearthe surface.Withthewater-table
below2 cm,therateof growthin
lengthofthetwospecieswas aboutthesame,althoughS. capillifoliumgrowingin a lawn
of S. papillosumgrewslightly
fasterwhenthewater-table
was betweenabout2 cm and 8
cm depth.In all thesecases the modelreacheda steadystate.Plantsinitiallyabove or
Effectsoflightand wateron Sphagnumgrowth
860
a)
0
(a) S.capo///fo/wum
0
o
4
~910480,
E~
k
I _
C
a)
0
U)
_J~
\
/
~I
I
I
,-,1002
C,r
C
I
I
Water-table depth(cm)
0>-
(b ) S. capi///ifo/niJm
u
I
39
* ~~~-1
06
1010
0~~0
.3
( c) Sp,apoi//osum
\ 20 7
0
000
0
15
0?
V
witha
FIG. 7. (a) Surface roughness(log scale) of model lawns of Sphagnumcapilitfolium
varietyof water-tabledepthsand no externalshade. Each set of pointswas recordedwitha
different
exposureeffect.Values at therightare k in theequationofexposureeffect(see Table 4).
indicatesconditionswherethe model was unstable(plottedpoints are then the
(--O--),
minimumvalues attained);*showsthe surfaceroughnessof thelawn at thestartof a runwhen
lengthswerenormallydistributed
randomnumberswithS.D. =0.5. (b) Surfaceroughnessof
combinationsof
model lawn of Sphagnumcapilifolium(withsame unitsas (a)) withdifferent
water-tabledepthand externalshade. The exposureeffectwas set to -1.39. (c) As (b), but
Sphagnumpapillosum.
below the surfacetendedto stabilizeat the same level as those startingat the general
surfacelevel.
producedgreatergrowthinmassexceptat thelowest
Sphagnumpapillosumconsistently
water-table
depthsexamined,wheregrowthwas similarto thatof S. capillifolium.
This
may simplybe a consequenceofthevaluesof meanstemweightused to converttheunits
pointis probablythatthe
of growthin mass to mg (mg cm stem-1)-1.The important
difference
is greaterthenearerthesurfacethewater-table
is.
in growthin lengthwere
Withtheexternalabsorbanceoflightset at 0.3, thedifferences
downto 5
greater;S. papillosumnowgrewfasterthanS. capilifoliumwiththewater-table
S. papillosumplantsfromstabilizing
veryfarabove
cm. A largeexposureeffect
prevented
butinthereversesituation,
S.
thesurfaceof a S. capillifolium
lawnwitha highwater-table
capilifoliumplantsweresome distancebelowthesurfaceat theend of therun.Withthe
behindat 200 days,so the
water-table
at 0 cm,theseplantswerestillslowlyfallingfurther
model community
was probablyunstablein theseconditions.It was certainlyunstable
when S. papillosum was grown with a low water-tablein a S. capillifoliumlawn,
P. M. HAYWARD AND R. S. CLYMO
10
E
' C
0i5s
?
*
(a) S.cap/l/ifo/lum
amongstSopap/llosum
M
*-A--- @
0
+
~~0----------ff-
o a moel
FIG.; 8. Reslts
,:,
-25
15 o0)
h20
~
L
0n
L
0
L
~
S
J
03
-
i
Shade
-
0.0
-M~~~~~~~
0
fr0-
L
5
p
o
20
dy
0
Shade
0ge03
\
i
(
w
capillifoliumgrowing0 a0
25
pillifolium.
In (b), the S. papillosumplantsinitiallyDepth
~~~0
a'
*
M1
ofphagnmgroth:theffetofntroduingasecon
-5~~
0
(b) S.pap/llosum amongst Scap/l/ifolium
"Shade 0
11/
4D
861
5
10
(cm)
Depthof water-table
-a-n
Depth of
---
--------
5
of
water-table (cm)
j s
FIG. 8. Resultsof a modelof Sphagnum
a secondspeciesintoa
growth:theeffectofintroducing
lawn. Graphs show difference
in growthbetweenthesecond speciesand therestof
Sphagnum
patcual
thelawnwhenall theplantsstartedat thesame level.The symbols+ and - showthedifference
in growthof plantsinitiallyat 1 cm above or belowthesurface,at water-table
depthsof 0, 3, 8
and 14 cm.
whe In each
th
plnt
wer
intal
in growth
beo in length
th
sufae
Intbltin mass (M,
case ----) by is dense
and
graph,difference
(L,
shownfora periodof 200 days withabsorbanceby externalshade of 0.0 (g) and of 0.3 (0).
The value of k in the equationforexposureeffect(see Table 4) was
-1i39
throughout.
(a) S.
capillifoliumgrowingin a lawn of S. papillosum;
(b) S. papillosum
growingin a lawn of S.
In (b), the S. papillosum
capillifolium.
plantsinitiallyat -1 cm, at water-table
depthsof 8 and
14 cm,bothfinished
at -2e6 cm (shownby arrows).
whenthe plantswereinitially
belowthesurface.Instability
caused by dense
particularly
has been seen before,whenlawns of single
shade in associationwitha low water-table
specieswereconsidered.
DISCUSSION
The existenceof a relationshipbetweenwater-tabledepth and surfaceroughnessis
and it can be simulatedtoo. Thereare at least two mechanismswhichcould
confirmed,
accountforthisobservation.The firstconcernstheeffectof watersupplyon emergent
is near the surface,therateof growthin lengthof a plantjust
plants.If the water-table
above thesurfacewillbe high,despitetheexposureeffect.
But as it growsaway fromthe
willcause therateto fallso thatin a steadystatetheplantis
surface,theexposureeffect
growingat thesamerateas therestofthelawnbutjust above it (Fig. 2). Ifthewater-table
inlengthofan emergent
is lowered,therateofgrowth
reducedandthe
plantis immediately
rest of the lawn catches up. Thus surfaceroughnessis inverselyrelatedto depthof
water-table.
The secondmechanism
concernstheeffect
of shadingon a plantwhichfallsbehind,and
the
reduction
shade.The rate
reinforce
of
surface
wherethereis no external
may
roughness
of growthin lengthis greatest
whentheradiantfluxincident
on a lawnis highbuta plantis
a fewmillimetres
shadedbyitsneighbours
above.Such a plantwouldthentendto return
to
the surface,thoughwitha reducedapex size, because growthin mass is reducedby
is also apparentfromtheresultsin Fig. 2. Whenthereis external
shading.Anothereffect
in
increase
shadefromneighbouring
shading,any
Sphagnumplantswilltendto reducethe
862
Effectsoflightand wateron Sphagnumgrowth
Thiseffect
individual
and itwillfallbehindirrevocably.
rateofgrowthin lengthofa shorter
was observedin themodel.
pointsarise fromthe modelrunswithmixedlawns.First,thetwo
Severalinteresting
the otherin theirown 'natural'habitat.Even if
species each appear to outperform
theywillgrowbetterthanthoseof theother
individualsof one speciesare in a minority,
or near the surface,forS.
is eitherlow, forS. capillifolium,
species if the water-table
the
papillosum.Whethersuch individualscould competeto the extentof eliminating
predominantspecies cannot be determinedwithoutstudyingtheir branchingand
abilities.
reproductive
at whichthegrowthof thetwo speciesis equal
Secondly,thedepthof thewater-table
to 5 cmdeepwith
seemsto be about2 cmbelowthesurface,withno externalshade,falling
is usually
an absorbanceof lightby externalshade of 0.3. AlthoughS. capillifolium
consideredto be a hummockspecies, it can grow successfullyin places wherethe
& Walker1958; Clymo& Hayward1982).
is nearlyat thesurface(Ratcliffe
water-table
wherethe
thisresultdoes implythatS. papillosumgrowingon a hummock,
Nevertheless,
Thismay,ofcourse,
maybe 20-30 cm belowthesurface,is at a disadvantage.
water-table
to
be compensatedby otherfactorssuch as its ability,comparedwithS. capillifolium,
multiply.
betweenthe two species are greaterwhen theyare shaded
Thirdly,the differences
whichare oftenshadedmorethanopen lawns,wouldseem
externally.
Again,hummocks,
to putS. papillosumat a disadvantage.
ACKNOWLEDGMENTS
the
and forpreparing
We thankP. Ratnesarfortechnicalhelpwiththegrowthexperiments
figures;two anonymousrefereesand Dr B. Moss forhelpfulcomments;and theNatural
heldbyP.M.H.
ResearchCouncilfora studentship
Environment
REFERENCES
statistics
ofa 'miniature
bog'.Oikos,1,33-40.
Bellamy,
D. J.& Rieley,
J.(1967).Someecological
ina bogpoolontheSilver
Flowe
ofSphagnum
cuspidatum
Boatman,
D. J.(1977).Observations
onthegrowth
ofEcology,
65,119-126.
National
NatureReserve.
Journal
development
on Long
Boatman,
D. J.,Goode,D. A. & Hulme,P. D. (1981).TheSilverFlowe.III. Pattern
ofEcology,
69,897-918.
LochB andCraigeazle
mires.
Journal
micro-topography.
formeasuring
Boorman,
L. A. & Woodell,
S. R. J.(1966).Thetopograph-aninstrument
Ecology,
47,869-870.
ofthe
III. Somewaterrelations
Chapman,
S. B. (1965).TheecologyofCoomRiggMoss,Northumberland.
bogsystem.
Journal
ofEcology,
53,371-384.
Flora of theBritishIsles. Cambridge
E. F. (1981).Excursion
Clapham,A. R., Tutin,T. G. & Warburg,
University
Press,London.
ofmeasurement.
58, 13-49.
R. S. (1970).Thegrowth
ofSphagnum:
methods
Journal
ofEcology,
Clymo,
ofEcology,
61,849-869.
R. S. (1973).Thegrowth
ofSpagnum:
someeffects
ofenvironment.
Journal
Clymo,
MoorsandMontaneGrasslands
(Ed. by0. W. Heal
British
Clymo,R. S. (1978).A modelofpeatboggrowth.
& D. F. Perkins
withW. M. Brown),
pp.182-223.Springer,
Berlin.
Ecology(Ed. byA. J.E.
Bryophyte
Clymo,R. S. & Hayward,
P. M. (1982).The ecologyof Sphagnum.
Smith),
pp.229-289.Chapman& Hall,London.
ofSphagnum
andpeataccumulation.
E. J.F. (1971).Productivity
(bog-moss)
Clymo,R. S. & Reddaway,
Pennines.
Moor
as: AspectsoftheEcologyoftheNorthern
Hydrobiologia,
12, 181-192.(Reproduced
Council,
London).
HouseOccasionalPaperno.3,pp. 1-15,NatureConservancy
Wils.onPennine
blanket
bog.
rateofSphagnum
rubellum
Clymo,R. S. & Reddaway,
E. J.F. (1974).Growth
Journal
ofEcology,
62,191-196.
forminimization.
Computer
Journal,
10,406-410.
Davidon,W. C. (1968).Variance
algorithm
MIT Press,Cambridge,
Mass.
Forrester,
J.W.(1961).Industrial
dynamics.
P. M.
HAYWARD AND
R. S.
CLYMO
863
ofHull.
Goode, D. W. (1970). Ecological studieson theSilverFlowe NatureReserve.Ph.d.thesis,University
studyofthegrowthand productivity
Grace,J.& Woolhouse,H. M. (1972). A physiologicaland mathematical
in Calluna. Journalof
II. Lightinterception
and photosynthesis
of a Calluna-Sphagnumcommunity.
AppliedEcology,10,63-76.
of Sphagnumimbricatum
the spatial and temporaldistribution
Green, B. H. (1968). Factors influencing
Hornsch.ex Russ. in theBritishIsles.JournalofEcology,56,47-58.
ofsoil
Harper,J.L., Williams,J.T. & Sagar, G. R. (1965). The behaviourof seedsin soil.I. The heterogeneity
the establishment
of plants fromseed. Journalof Ecology, 54,
surfacesand its role in determining
151-166.
on the growthof Sphagnum.Ph.D. thesis,Universityof
Hayward, P. M. (1980). Effectsof environment
London.
Hayward,P. M. & Clymo,R. S. (1982). Profilesof watercontentand pore-sizein Sphagnumand peat, and
theirrelationto peat-bogecology.ProceedingsoftheRoyal SocietyofLondon,Series B, 215, 299-325.
Hill, M. 0. (1978). 1. Sphagnopsida.The Moss Flora of Britainand Ireland (Ed. by A. J. E. Smith),pp.
Press,London.
30-78. CambridgeUniversity
Ilomets,M. (1974). Some resultsof measuringthe growthof Sphagnum.Estonian Wetlandsand theirLife
(Ed. by E. Kumari),pp. 191-203. AcademyofScienceoftheEstonianS.S.R., Valgus,Tallinn.
Leisman,G. A. (1953). The rateof organicmatteraccumulationon thesedge matzones of bogs in theItasca
StatePark regionofMinnesota.Ecology,34, 8 1-101.
Nelder,J. A. & Mead, R. (1965). A simplexmethodfor functionminimization.ComputerJournal,7,
308-3 13.
Overbeck, F. & Happach, H. (1956). Uber das Wachstum und den Wasserhaushalt eininger
Hochmoorsphagnen.
Flora, Jena,144,355-402.
ecologyofthreeSphagnumspeciesin southFinnishraisedbogs.
Pakarinen,P. (1978). Productionand nutrient
AnnalesBotaniciFennici,15, 15-26.
Pearsall,W. H. & Gorham,E. (1956). Productionecology.I. Standingcropsof naturalvegetation.Oikos, 7,
193-201.
Popp, E. (1962). SemiaquatileLebensraume(Biilten)in Hoch- und Niedermooren.I. Die Standortsfaktoren.
47, 431-464.
undHydrographie,
Internationale
Revue dergesamtenHydrobiologie
Ratcliffe,
D. A. & Walker,D. (1958). The SilverFlowe,Galloway,Scotland.JournalofEcology,46, 407-445.
T-A. (1980). Growthof SphagnumripariumAngstr.in
Sonesson,M. Persson,S., Basilier,K. & Stenstrom,
factorsin the Stordalenmire.Ecology of a SubarcticMire (Ed. by M.
relationto some environmental
Sonesson),pp. 191-207. SwedishNaturalScienceResearchCouncil,Stockholm.
(Received1 December1982)