VOLUME
35, Nos. 1 AND 2
DECEMBER
PATTERN AND PROCESS
1947
IN THE PLANT COMMUNITY*
BY ALEX. S. WATT, BotanySchoot,University
of Cambridge
(WithelevenFiguresin theText)
CONTENTS
THE PLANT COMMUNITY AS A woRKING
THE EVIDENCE FROM SEVEN COMMUNITIES.
The regenerationcomplex
.
.
Dwarf Callunetum
..
Eroded Rhacomitrietum
..
Bracken
...
GrasslandA (Breckland)
..
Grass-heathon acid sands (Breckland) .
Beechwood.
.
.
SUPPLEMENTARY
SOME IMPLICATIONS
.
.
.
.
.12
.
.
..
..
.
.
.
.
.
.
.
2
.
2
3
5
6
8
11
.
20
.15
.16
.19
.
.
Animalsand micro-organisms .
.
The nature of the plant community .
REFERENCES
1
.
.
EVIDENCE
COMsPARISON AND SYNTHESIS
PAGE
MECHANISM.
.
.19
.22
THE PLANT COMMUNITY AS A WORKING MECHANISM
The plant communitymay be describedfromtwo points of view, for diagnosis and
classification,and as a workingmechanism. My primaryConGern
is with the second of
these. But inasmuchas the two aspects are not mutuallyexclusive,a contributionto our
understandingof how a communityis put together,and how it works, may contain
somethingof value in descriptionfordiagnosis.
It is now half a centurysince the study of ecology was injected with the dynamic
concept,yet in the vast output of literaturestimulatedby it thereis no record of an
attemptto apply dynamicprinciplesto the elucidationofthe plant communityitselfand
to formulatelaws accordingto which it maintainsand regeneratesitself. Pavillard's
assessmentof the dynamic behaviour of species comes very near it, but is essentially
concernedwiththe 'influence(director indirect)of the species on the natural evolution
of plant communities'(Braun-Blanquet & Pavillard, 1930). As thingsare, the current
descriptionsof plant communitiesprovideinformation
of some,but not critical,value to
an understandingof them; how the individualsand the species are put together,what
determinestheirrelative proportionsand their spatial and temporalrelationsto each
other,are forthe most part unknown. It is true that certainrecentstatisticalworkis
out towardsthat end, but the applicationofstatisticaltechnique,the formulastretching
tionoflaws and theirexpressionin mathematicalterms,willbe facilitatedifan acceptable
qualitative statementof the nature of the relations between the componentsof the
communityis firstpresented. Such a statementis now made based on the studyof seven
communitiesin greateror less detail, fordata of the kind requiredare seldomrecorded.
The ultimateparts of the communityare the individualplants,but a descriptionof it
in termsof the charactersof these units and theirspatial relationsto each otheris impracticableat the individuallevel. It is, however,feasiblein termsof the aggregatesof
* Presidentialaddress to the BritishEcological Society on 11 January1947.
J. Ecol. 35
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2
Patternand processin theplant community
individualsand of species whichformdifferent
kinds of patches; these patches forma
mosaicand togetherconstitutethe community.Recognitionofthe patch is fundamental
to an uinderstanding
of structureas analysed here.
In the subsequentanalysis evidence is adduced to show that the patches (or phases,
as I amncallingthem)are dynamicallyrelatedto each other. Out ofthisarisesthat orderly
change whichaccounts forthe persistenceof the patternin the plant community.But
thereare also departuresfromthis inherenttendencyto orderlinesscaused by fortuitous
obstacles to the normaltime sequence. At any given time, therefore,structureis the.
resultantof causes whichmake fororderand those that tend to upset it. Both sets of
causes mustbe appreciated.
In describingthe seven communitiesI proposein the firstexamplesto emphasizethose
featureswhichmake fororderliness,in the later to contentmyselfwithlittlemorethan
passingreference
to theseand to dwellspecifically
upon departuresfromit; in all examples
to bringout special points forthe illustrationof whichparticularcommunitiesare well
suited or forwhichdata happen to be available.
For the presentthe fieldof inquiryis limitedto the plant community,divorcedfrom
its contextin the sere; all reference
to relicsfromits antecedentsand to invadersfromthe
next state is omitted. I am assumingessentialuniformity
in the fundamentalfactorsof
the habitat and essentialstabilityof the communityover a reasonableperiodof time.
THE EVIDENCE FROM SEVEN COMMUNITIES
The regeneration
complex
Regarded by some as an aggregateof communities,by others as one community,the
regenerationcomplexfittingly
servesas an introductionbecause its studyemphasizesthe
underlyinguiniformity
of the natureof vegetationalprocesses. It consistsof a mosaic of
patchesforming
an intergrading
seriesthemembersofwhichare readilyenoughassignable
to a fewtypes or phases. The samplesof these phases are repeated again and again over
the area; each is surroundedby samplesof other,but not always the same phases.
As the workof Osvald (1923) and of Godwin& Conway(1939) shows,thesephases are
dynamicallyrelatedto each other. For TregaronBog the sequence is brieflyas follows.
The open waterof the pool is invaded by Sphagnumcuspidatumwhichin turnis invaded
and thenreplacedfirstby S. pulchrum,thenby S. papillosum,by whosepeculiargrowth
a hummockis formed.This hummockis firstcrownedby Calluna vulgaris,Erica tetralix,
Eriophorumvaginatumand Scirpus caespitosus,later by Calluna vulgariswith Cladonia
.silvaticaforminga subsidiarylayer. The proofof the time sequence lies in the vertical
sequence of plant rem1ains
in the peat itself.
Each of the,phases of the regenerationcomplex was at one time regardedas a community,and the wholeas ait aggregateofcommunitiesdynamicallyrelatedto each other.
Tansley (1939), forexample,stillcalls themseral. The resemblanceto a sereis close; each
phase fromthe open waterto the hummockwithCalluna dependson its antecedentsand
is the forerunner
of the next,the sequence dependingon plant reaction.
The resenmblance,
however,is partial only,fora cycle of change is completedby the
replacemlentof the hummockby the pool, a topographicalchange broughtabout by
differential
rates of rise in the different
patches. That is, the immediatecause does not
resideexclusivelyin the patch itselfbut in the spatial relationbetweenpatchesand their
relativechangesin level. Thus the Calluna ofthehummock(lies or is killedand is followed
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3
ALEX. S. WATT
in theplacevacatedby it by otherspecieswhichhave had no director indirecthand
in its death. Thus in the full cycle we may distinguishan upgradeseriesand a
downgrade.
anddevelopsunder
onitsneighbours
mosaicis dependent
Each patchinthisspace-time
partlyimposedbythem.The samplesofa phasewillin generaldevelopunder
conditions
ofphaseswillvary.
thesame,forthejuxtaposition
butnotnecessarily
similarconditions
of
a phaseandtheir
patches
of
the
of
development
the
rate
affect
to
be
expected
Thismay
phasesthere
duration.But onthedurationofthefullcycleofchangeand itscomponent
is gainedthatin the upgradeseries
is inadequateinformation,
althoughthe impression
papillosum
slow,thenfastintheSphagnum
perannumis at first
thenetrateofproduction
phase,thenslowagainin thefinalphases.
of diverse
complexas a community
This briefsummarypresentsthe regeneration
thereis changein timeat a givenplace,
pattern.Although
a space-time
phasesforming
is
thesame;thethingthatpersistsunchanged
remainsessentially
thewholecommunity
phases.
of
sequence
in
the
theprocessand its manifestation
DwarfCallunetum
complexis a specialcase. It is oneofmy
It maywellbe arguedthattheregeneration
are paralleledin a widerangeof comobjectsto showthatthesedynamicphenomena
all ofwhichcan hardlybe set asideas specialcases.
munities,
complexthereoccurpartialsequencesin spacewhich
in theregeneration
Occasionally
betweenthe space and
withthe timesequence. Completecorrespondence
correspond
underhighlyspecializedconin certaincommunities
timesequenceis found,however,
are foundin the Arctic(Walton,1922),wherethe prevailing
ditions.Such conditions
and theirunidirectional
oftheplantsin thecommunity
thealinement
windsdetermine
of highly
vegetativespread. Similarphenomenaare shownby the dwarfCallunetum
exposedplaceson theslopesoftheCairngorms.
The Callunetum
consistsof stripsof Callunaseparatedby stripsof bare wind-swept
intotheshelter
soil;all theCallunaplantsliesidebysideandinline,theirapicesspreading
other
In
placesthe
createdby the plantitselfand theirold partsdyingaway behind.
is double,consisting
stripof vegetation
mainlyof Callunaand Arctostaphylos
uva-ursi;
so far
bothspecieshave thesamehabit,but theydiffer
conditions
undertheprevailing
foundto leewardof Calluna
is generally
in theirspecificmake-upthat Arctostaphylos
growth
(Fig. 1). The spatialrelationbetweenthesetwospeciesand theirunidirectional
overtheerodedsoil,Callunagrowsoverthe
growsforward
suggestthatas Arctostaphylos
shootsthoughnotkillingthe
theleaf-bearing
suppressing
olderpartsofArctostaphylos,
of
relation
is foundinthedead
the
dynamic
oldstems.Thisis,indeed,thecase,andproof
remainsofArctostaphylos
belowtheCalluna.
on therefinedepending
(ormore-phase
is a three-phase
Thecommunity
justdescribed
mentof the analysis)systemwiththe phasesarrangedin linearseries. In the dwarf
each
ofmoresheltered
spreadis replacedbycentrifugal;
Callunetum
placesunidirectional
The
Calltnaplantis freeto spreaduntilcheckedby otherplantsin its neighbourhood.
is a background
ofCallunawithscattered
patches
staticpatternofthematurecommunity
Cladoniasilvaticaand baresoilin it. But thetimesequenceremains
of Arctostaphylos,
to three
thesame,withfourphasesin thefullcycle,whichmaybe shortened
essentially
or evento twophases.
1-2
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4
Patternand processin theplant community
Brieflysummarizedfromdetails obtained by Dr G. Metcalfe (to whom I am also
indebtedforFig. 1) duringthe CambridgeBotanical Expeditionto the Cairngormsand
kindlyplaced by him at my disposal, the historyof the relations between the phases
is as follows,the kind of evidencebeing the same (except for the positionalrelation)as
that obtained fromthe linear series. The young vigorousshootsof Calluna are usually
dense enoughto exclude lichens; the older,fewerand less vigorousshoots are unable to
01(1brokeni
stemis
Positionof
Calluna roots _
Barearea
o rctost(aphylos
Calltna
4
Fig. 1. Diagrammaticrepresentationin plan and elevationto show the spatial relationbetweenCalluna and
in the 'double strips' separatedby wind-sweptbare soil in the DwarfCallimetumof highly
Arctostaphylos
exposed places at approx. 2500 ft. on the northernslopes of the Cairngorms.The double strip moves
forwardin time; Arctostaphylos
invades bare soil, and Calluna moves on and suppressesthe adjacent
Arctostaphylos.
do so and Cladonia silvaticais oftenabundant on the old parts of the plant. On its death
Cladonia becomesdominant,anchoredon the dead stems. (The counterpartofthisphase
in thelinearseriesis the stripofdead Calluna stemsbehindthe live; owingto the violence
of the wind, Cladonia, althoughpresent,is unable to dominate.) In time the Cladonia
mat disintegrates(in muchthe same way as it does in Breckland(Watt, 1937)) and bare
soil is exposed, often with some remains of Calluna stems on it. If Arctostaphylos
happens to be near such a gap (and a much ramifiedsystemof non-leafbearingstems
occursunderthe Calluna mat) invasionis followedby completeoccupation. In timeby
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5
ALEX. S. WATr
vegetative spread fromthe marginCalluna replaces the Arctostaphylos.
The relations
betweenthe phases are indicated diagrammaticallyin
Fig. 2, which also shows the short-circuiting
in the
Calluna
absence of Aretostaphylos.
Clearly Calluna is the dominantplant; the locally
dominantArctostaphylos
and Cladonia are allowed to
occupy the ground which Calluna must temporarily Arctostaphylos
Cladonia
vacate, Arctostaphylos
as a phase in the upgradeseries
and Cladonia in the downgrade. Althoughno data
are available it may be pointed out that from the
annual ringsof the woodystemsof Calluna and ArctoBare soil
staphylossome estimate of the duration of the phases Fig.2. Diagramillustrating
thedynamic
is made possible.
relations
betweenthechiefspecies
in Dwarf Callunetumin less
ErodedRhacomitrietum
exposed places than those men-
tionedinFig.1.Thearrows
indicate
At somewhat higher altitudes on the Cairngorms
the directionof change.
an eroded communitydominated by Rhacomitrium
lanuginosumshows phenomenasimilarto those of the Calluna strips. For here,too, the
directionof the prevailingwind, by determining
the directionof spread of the plants,
imposes on the patches of vegetationa space sequence whichis also a time sequence.
The followingveryshortaccountis based on detailskindlysuppliedby Dr N. A. Burges,
to whomI am also indebtedforFig. 3.
J.
10
20
30
40
Rh.
50
cm.
Rh.Q. Rh.d.
6
7
80
90
100
Fig. 3. Profile
downwind
oracrossa patchofvegetation
in erodedRhacomitrietum
at approx.3500ft.on the
northern
The spatialsequenceis also a timesequence.Li., Lii. andP. = bryophytes,
slopesofCairngorm.
E. =Empetrum
hermaphroditum,
C. = Cladoniarangiferina,
V.= Vaccinium
J.= Juncustrifidus,
myrtillus,
Rh.=Rhacomitrium
lanuginosum,
Rh.e.=R. lanuginosum,
erodedface,Rh.d.=R. lanuginosum,
dead.
The communityconsistsof a networkof patchesof bare soil and of vegetation;each of
the lattershowsfromthe lee side to the exposed and erodedwesternside (Fig. 3) a series
ofphases characterizedrespectivelyby bryophytes,
Empetrumhermaphroditum,
a mixture
of Vaccinium myrtillus,V. uliginosumand Rhacomitriumlanuginosum,and finally
Rhacomitrium
itself. Proof cf the temporalsequence is foundin the verticallayeringof
peaty remainsfoundunderthe last phase. The Rhacomitrium
phase is erodedby the wind,
the exposed accumulatedhumus dispersedand mineralsoil once again laid bare. Since
the stemsof Empetrumstretchfromend to end of the patch the numberof ringsenables
an estimateto be made ofthe durationofthe wholeand ofits phases. The rate ofadvance
is approximately1 m. in 50 years.
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Patternand processin theplant community
6
Bracken
to Calluna in the dwarfCallunetumof the less exposed
The relationof Arctostaphylos
places is deducedfrominternalevidenceand supportedby evidenceprovidedby the study
of theirspatial and temporalrelationsin the specialized double strips.The Pteridietum
(Fig. 4) whichhas been studied(Watt, 1947) providesa closeparallel,forthe area in which
Hinterland
Margin
'at ofro,,lj
Soil stirfacae
Depthiof' origini orfronds
()
(b)
of'rhioe
Plani:
arranicgen1ii
Phases
Gras
m-
PioneerBuiltlidig
Mature
Degencrate
Fig. 4. Diagram to illustratethe spatial and temporalrelationsbetweenthe phases in the marginalbelt and
in the hinterlandof a brackencommunityon LakenheathWarren(Breckland). In (a) the changein height
and in continuityof cover of the frondsare indicated: also the change in depth of originof the fronds.
In (b) the relativesize and directionof growthof the main axes of the brackenplants are shown. In the
marginalbelt the axes are parallel, in the hinterlandthey forma network. In (c) the phases in the
marginare in linearseries: in the hinterlandthey are irregularlyarranged.
it occursconsistsof a hinterland(comparablewiththe continuousCallunetum)in which
the frondsare patchilydistributedand the axes of the brackenplants forma loose network; marginalto the hinterlandis a belt of brackeninvadinggrass-heathin whichthe
individualplants lie side by side, with theirmain axes parallel to each otherand their
apices in line (cf. the stripof Calluna).
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ALEX. S. WATT
7
Sucha marginal
beltwithunidirectional
spreadcan be dividedintoa seriesofzonesor
phasesbylinesrunning
parallelwiththeinvadingfront.
Thesephasescanbe distinguished
bydata fromthefrondand rhizome.
Theinherent
circumstances
makeit clearthat,from
back to front,
the space sequenceofphasesrepresents
a sequencein time. Amongthe
overdispersed
frondsofthehinterland
therearepatcheswithoutfronds(thegrass-heath
phase)and patcheswithfrondswhichvaryamongthemselves
in featuresoffrondand
rhizomein muchthe same way as the sequencein the marginalbelt; theseare distinguishedas thepioneer,building,
matureand degenerate
phases.Thesephases,together
withthegrass-heath
phase,forma cycleofchangein timeat one place. Comparison
of
the two setsof data (Watt,1945,1947)-fromthe zonesofthe marginalbeltana the
patchesofthehinterland-shows
closeagreement
between
therelations
between
thevalues
within
eachset,an agreement
allthemorestriking
whenallowanceis madeforthedifferent
circumstances.
Forinthemarginal
beltthephasesareinlinearseriesandbrackeninvades
grass-heath
freefrombracken;in the hinterland
the grass-heath
phase is vacatedby
brackenbutseldomcompletely
beforereinvasion
takesplace.Thereis thussomeoverlap
betweenthebeginning
and endphasesofthecyclicseries.
The evidenceforcyclicchangederivedfromthespecializedmarginal
beltis supported
by theinternal
evidencefromthehinterland
itself.Forin thegrass-heath
phasewithout
fronds
thereare abundantremainsofdecayingrhizome,
and underthelitterin thelater
phaseswithbrackenare the recognizable
remainsof Festucaand Agrostis
ofthe grasAheathphase. Phaseswith,and a phasewithout,
alternatein time.
fronds
are the same. In the
The processesinvolvedin the marginalbeltand thehinterland
marginalbeltthenaturaltendency
is checkedby
ofthebrackento spreadcentrifugally
intothegrass-heath
lateralcompetition;
spreadis thusdirected
byecologicalopportunity
whichis freefrombracken.Once establishedthe sequenceof phasesdependson the
as affected
development
ofthe brackenitselfand aevelopment
by its ownreaction.In
is similarly
the hinterland,
directed
wherethe axes forma loosenetwork,
rejuvenation
to areasvacatedby thedeathofthe
by ecologicalopportunity
and essentially
restricted
ofestablished
invasionand
bracken.Thephaseswhichfollowarea biologicalconsequence
are limitedto thearea ofthatinvasion.
ofthecycle,thereis changein the
Runningparallelwiththechangein thevegetation
factorsofthehabitatand in totalhabitatpotential.The variablefactorsofmicroclimate
theamountoflitter,thestateofthe
and soil,e.g. shelter,
lightintensity,
temperature,
ofthephasessuperposed
on the
humusand its distribution
in the soil,are differentiae
original
foundation
ofa uniform
habitat.Theyarecloselylinkedwiththephasesbecause
ofonephasebecomepartcause
theeffects
theyare theeffects
oftheplantsthemselves;
causedby thevariable
ofthenext.Thusthespatialvariationin thehabitatis primarily
cover. Further,
vegetational
takinga broadview,wemaynotethatintheupgradeseries
thereis an accumulation
of plantmaterialand an increasein habitatpotential;in the
downgrade
bothare dissipated.
The significance
ofthedifferential
actionofthephaseson microclimate
maybe illusor pioneer
tratedby the effect
offrostduringthe winter1939-40. In the grass-heath
phaserhizomeapiceswerekilledat a maximumdepthof20 cm.; in the maturephase,
protected
bya blanketoflitter,themaximum
depthoflethaldamagewas 6 cm. Action
of thiskindretardsdevelopment
the
and prolongsthe durationof a phase,upsetting
smoothcourseofthetimesequence.
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Patternand processin theplant community
8
GrasslandA (Breckland)
The vegetationof GrasslandA (Watt, 1940) is obviouslypatchy. In its hollows and
hummocksthe habitat presentsa strikingresemblanceto the regenerationcomplex,and
and soil
here, too, vegetationalvariation is linked with variation in microtopography
habitat.
of the communityupon dynamiclinesfourphases are recognized:
In a reinvestigation
is
but littledifficulty
the hollow,building,mature,degenerate.Thereare intermediates,
four
of
these
one
or
other
to
community
typical
of
the
parts
all
experiencedin assigning
phases.
0cm.
5-
ED
M
D.~K
80
~~~~H.
.
<
DB
~~~~~~~~~D
NMD?
H
B
D
H.
Fig. 5. The relativesize and spatial relationsofthe phases in a plot of 160 x 10 cm. in GrasslandA. The relation
is seen in the profiletaken along the upper edge of the plot.
betweenthe phases and the microtopography
Mature
Early
Degenerate
Late
Hollow
Building
-CM.cm
33
2-2
0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
the
Fig. 6. Diagrammaticrepresentationof the phases showingchange in floraand habitat and ixwdicating
in the soil.
'fossil' shoot bases and detachedrootsof Fedtua ovinau
is shownin Fig. 5; the patchesare irregularin size and shape
Theirspatial distribution
and theirjuxtapositionvaries. These fourphases forma time sequence,the evidencefor
(Fig. 6). The whole 'life' ofthe comwhichis emphasizedin the followingreconstruction
munitycentresroundthe reactionsand lifehistoryofFestucaovina,its growthand reproduction; it has everyrightto be called the dominantplant even althoughit occupiesless
than one-halfof the total area (approximately45?%). The seedlingbecomes established
among the stoneswhichfloorthe hollowphase; as the plant growsand spreads the level
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9
soilinsidethetussockrises.Thissoilisfreefromstonesanditsaccumulation
ofthemineral
particles
veryprobablyalsoto wind-borne
ofantsandearthworms,
is dueto theactivities
in the splashofheavyrain.The youngvigorousfescuewith
and particleswater-borne
the
longstalksconstitutes
on relatively
longleavesand manyinflorescences
relatively
phase; in it theshootsoftheplantsare stillattachedto theparentstock. By
building
increasesin height,attaininga maximumof
ofsoilthehummock
accumulation
further
leavesand fewer
fescue,withshorter
vigorous
muchless
about4 cm.,and thencarrying
tussockis replacedby
withshorterstalks.The originalmany-branched
inflorescences
ofone or twoshootswith
and consisting
smallplantseach lyinghorizontally
numerous
roots;theyhave arisenfromthelargerplantby theseparationof
descending
vertically
thedeathand decayoftheparentstock,theevidenceforthisbeingthe
through
branches
which
ofthebasesoflateralbranches
ofthehummock
thesoilprofile
throughout
presence
persist.
thus
and
to decay
are resistant
lichens
In thespacesbetweentheindividualplantsin thematurephase,thefruticose
Theyspreadand ultimately
becomeestablished.
and C. rangiformis
Cladoniaalcicornis
forma mat belowwhichthe remainsof fescueare found.This is the earlydegenerate
rootsarefoundin thesoil. In thelate degenerate
phase. 'Fossil' shootsand unattached
lichensPsoradecipiensand Biatorinacoeruleoby the crustaceous
phase,characterized
rootsare again found. In placestheseroots
unattached
and
shoots
'fossil'
nigricans,
to the erosion(alreadysuggestedby
witness
bear
and
projectabove the soil surface
the pittedsurface)whichgraduallywearsdownthe hummock.Some vestigesof the
thesedisappearand theerosion
surviveintothehollowphase,buteventually
hummock
pavementofflintand chalkstonesis onceagainexposed.
can be assignedto one or otherof
Everypartofthesurfaceofthetypicalcommunity
ofthewholeandsummarizes
thesefourphases.A singlesetofthemis fullyrepresentative
ofthe comThe understanding
at workand theirmanifestations.
in itselftheprocesses
thephases
of
relations
of
the
elucidation
the
on
is
based
mechanism
a
working
as
munity
core.
its
very
area),
(minimum
its
representative
minimum
to eachother.Theyare
whole
a
as
mayvary
whichthephasesmaketo thecommunity
Nowthecontributions
and
onesourceofvariability,
amongthemselves
and alsofromyearto year.To eliminate
ofgreatimportance,
at thesametimeto takeaccountofit as an ecologicalphenomenon
uponareasofequal sizeintheseveral
ofthecommunity
I proposeto basethedescription
ofthe
phases(andto call thewholetheunitpattern)and to makea separateassessment
phases.
the
by
occupied
areas
relative
inFig.7;
graphically
A arepresented
ofGrassland
Someofthedatafortheunitpattern
The
shown.
are
clearly
of
the
phenomena
continuity
as wellas theunifying
thediversity
holophytic
related.The
offescueand 'baresoiland stones'areinversely
coverpercentage
excludedfromthebuildingand maturephasesand increaseto
are virtually
bryophytes
fromfescueis least. Forthelichens,onthe
inthehollow,wherecompetition
a maximum
can
liesin thephasewherethefungalpartners
appropriately
otherhand,themaximum
phases.
previous
during
accumulated
remains
the
utilize organic
of seedlings
on the distribution
ofthe phasicmicroenvironments
The selectiveeffect
directly
varies
of
seedlings
number
1.
The
total
in
Table
shown
fateis
andtheirsubsequent
seedof
the
failure
of
the
adults
suggests
withthe'bare soiland stones'.Thedistribution
the
to
and
shows
restriction
phases
in thematureand degenerate
lingsto reachmaturity
to the hollowphasebecausethe greatbulkof
buildingand hollowphaseand virtually
ALEX. S. WATT
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IPatternand processin theplant community
10
100
;~~~~~~~~~~~~~~~~~~~
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o~~~~~~~~~~~~~~~~~~~~~~
A
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10004,b4,
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ol
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.'
/
Bu'ilding
Mature
.
..
Hollow
Degenerate
Phase
F*ig.7. Graphical presentation of some dlata for Grassland A. Note that most of the data for the
building phase are from late stages.
Table 1. GlrasslandA
Species
Arenaria serpyllifolia
Avenapratensis
(,alamintha acinos
(,irsiumnlanceolatumn
Crepis virenq
E'rigeron acre
Festuca ovina
Hieraciurn pilosella
Koeleria gracilis
Senecio jacobaea
Total
Arenaria 8erJpyllifolia
(Val,mintha acino,9
Cerawturn semidecandrubm
C,repis virens
0rigeronacre
B
Galiuq annlicum
HieraciumPpilosella
Saxifraga tridactylites
Total
Phases
Hollow
Mature
Degenerate
Building
Total no. in 40 plots, each 5 x 5 cm.
Seedlings
1
0
O
O
2
0
O
O
6
8
3
3
0
1
0
O
21
710
24
68
31
17
25
66
10
0
2
7
6
5
9
0
1
0
O
1
0
O
O
172
67
32
63
Adults
O
O
1
I
3
11
2
I
19
.
O
0
0
0
0
0
0
0
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0
0
0
0
1
0
0
0
1
1
3
8
0
3
7
4
0
26
ALEJX. S. WATT
11
the adults recorde(dfromthe buildingphase are in the immediatesurroundof the fescue
tussockand not in it. This restrictionto the hollowphase in all probabilityholds forthe
survivingfescueseedlingsas well. Thus at any given time the initiationof the cycle of
change is restrictedin space, and at any given place to the hollow phase in the time
sequence.
methodsthe relativeareas of the phases hollow,building,
Estimated by two different
mature,degenerateare,respectively,25-7,15-1,16-3and 42-9. As faras I can judge from
the annual chartingoftwo plots since 1936,but withoutactuallynotingthe spatial limits
of the phases, Grassland A is remarkablystable both in its unit patternand the areal
extentof its phases.
a
- -Marginalbelt
cn
i h
ring (Fi.)fmdimo(a)f
th community.Atypical
ofthe phassfromtheanitrpattern
(b)
of
in plan (a) and profile(b) of an Agrodisringset in a background
Fig. 8. Diagrammatic
representation
aladonia 8ilva6tica.In (b) the darkerline indicatesthe changein height(and/ornumber)ofAgrostisshoots,
the thinnerlinlethe changein thicknessof the Cladonia mat.
on acid sands(Breckland)
Grass-heath
toprovide
has beensufficiently
onacidsandsin Breckland
Thegrass-heath
investigated
of the plant communityand also to
supportingevidenceforthe dynamicinterpretation
demonstratethe usefulnessin descriptionof separatingthe estimateof the relativeareas
ofthephasesfromtheunitpattern.
consistsof rings(bothsolidand hollow)of Agrostis
the community
Fundamentally
of
tussocksof Festucaovinaset in a background
and
occasional
tenuits
-andA. canina,
to
other
its
relations
because
is
omitted
to Festuca
reference
silvatica6.
ClIadonia
(Further
are notfullyelucidated.)Each ringis derivedby vegetative
speciesin the community
in itselfthe spatialand temporalchangeswithin
spreadfromone plantand epitomizes
zone of
A typicalring(Fig. 8) of mediumsize consistsof a peripheral
the community.
shootsarisingfromlarge plants with
numerous,vigorous,vegetativeand flowering
rhizomesradiatingoutwards,an innerzone (maturephase) withfewerand shorter
shootsat all, on smallerplantswhose
shoots,or no flowering
vegetativeaindflowering
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12
Patternand processin theplant community
rhizomes form a loose open network(cf. Bracken, p. 6). In the centre of the ring
Agrostisas a live plant is absent, but its abundant dead remains are found below
the mat of dominantCladonia silvaticawhichis from3 to 4 cm. thick;to thisthickness
it has gradually risen froma thicknessof 1 cm. onlyin the middle of the peripheral
zone.
Fuller investigationof the dynamicbehaviourof the ringsjustifiesthe recognitionof
phases withinthe marginalbelt and in a fullcycle additionalphases oTdegenerationand
rebuildingfollowingdisruptionof the lichen mat (Watt, 1938). But to keep the issue
simpleno morethan the threephases need be furtherconsideredsincetheymay complete
a shortcycle; samplesfromunit areas in each constitutethe unit pattern.
The mutualrelationsbetweenAgrostisand Cladonia have not been investigatedto the
degreenecessaryto decide whetherthe death ofAgrostisis due to age, or in some measure
to the influenceof Cladonia or to both acting together. Similar problemsarise at this
stage of the cycle of changein all the communitiesexamined; the problemsare universal
because they concerndeath and its causes. Their solution,althoughimportant,is not
necessaryto establishthe fact of replacement.
The extensionof the Cladonia phase at the expense of the maturephase (innerzone
withAgrostis)is not,however,merelya questionof time; it is primarilydue to drought,
betweenthe Agrostisplant of the peripheralzone (which survives)
which differentiates
and that of the maturephase (whichdies). As the resultof a severedroughtthe mature
phase passes wholesaleand abruptlyto the Cladonia phase, and the extentof the change
will depend on the area of the phase capable of being affectedby it. An area occupied
by a largenumberQfsmallpatchesofAgrostiswillbe relativelylittleaffectedby drought,
whileone whichis almostentirelyin the maturephase, throughthe spread and fusionof
rings,will be almostwhollyaffectedby it (Fig. 9). This explains the violentfluctuations
met within the populationsof Agrostisin this community;in certainareas fluctuations
have been so widethat estimatesof abundanceforone yearare of no value as a diagnostic
character.They do, however,reflectthe vagariesin meteorologicalfactors. On the other
hand, data fromthe unit patternwould be much less affected.
Beechwood
cause may
The effecton the structureof a communityof droughtor other efficient
persistlong afterthe cause has ceased to operate. In fact,at any given time,theremay
be no correspondencebetween structureand the currentmeteorologicalfactors.The
to communitieswithlong-liveddominants. But first
pointis best illustratedby reference
a briefnote on phasic changein beechwoods.
The patchinessin some all-aged beechwoodson the Chilterns,reputedlymanaged on
in termsofthetemporalsequenceofphases as revealed
a selectionsystem,is interpretable
of
life
of
the
even-agedbeechwoodsof the same ecologicaltype
the
history
pure
study
by
on the South Downs (Fig. 10) (Watt, 1925). To the three phases recognized-Bare,
Oxalis, Rubus-there should be added a fourth,the gap phase, to whichregenerationis
confinedbecause it is excludedfromotherphases. At any givenplace thereis a cycle of
change consistingof an upgrade seriesof phases in whichthereis a continualchange in
ecological structure,associated with changingage, rate of growthand densityof the
dominanttrees and correlatedwith changes in the fieldlayer, and a downgradeof the
dying,dead and rottingstemsand the vegetationof the gap.
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ALEX. S. WATT
18
For the time-productivity
curve for the upgrade series we have the foresters'yield
tables for managed woods. Althoughthe data take no account of the yield fromthe
subsidiaryvegetation,we may assume thisto be small in relationto the yieldof the trees
and withoutappreciableeffecton the course of the curve. This is of the familiargrowth
type,risingslowlyat first,thenfast,then slowlyagain.
The phasic phenomenaof woodland are on a scale to be immediatelyobvious. Again
because of the scale, but also because we have the data, woodlands affordan excellent
illustrationofthe cyclicrelationbetweentreespeciesand ofcauses havinglong-continued
:
o~~~~~~~~~~-
(a)
(b)
at different
Fig. 9. Diagram totshow the effectof droughtoinAgrostisin two areas of Festuco-Agrostidetum
stages of development. In (a) thereare many small (young) patches of Agrostis;in (b) the patches,have
grownand fusedsQ,that the bulk of the area is in the maturephase. In (c) and (d) droughthas killedthe
Agrostisin the maturephase only of (a) and (b).
in structure.Both pointshave been dealt withrecently
effects
by Jones(1945); brief
is all thatis necessary
reference
here.
Viewedagainsttheidealofsustainedyield,aimedat mostsimplyamongforesters
by
allocatingequal areasto eachage classor groupofage classes,theprimevalwoodby all
accountsmoreoftenthannotshowsan unequaldistribution
oftheareasoccupiedbyits
overperiodsoftimein, say,,thenumberof
age classes. Asidefromminorfluctuations
deathsamongoldtrees,thereareexceptional
ofrareorsporadicoccurrence,
such
factors
as storms,
which
create
a
of
dimensions.
fire,drought,
epidemics,
gap phase exceptional
If thewholegap phaseis regenerated
aboutthesametime-and variou'scircumstances,
both inherent
and external,likeperiodicity
in seed yearsasndsuitablemeteorological
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14
Patternand processin theplant community
factorsforseedlingsurvival may help in that direction-then thereis initiatedan age
class of abnormialarea. This will persistlike a tidal wave movingalong the age classes
until at least the death of the trees; it may even influencethe structureof the next
generation.In otherwords,the relativeareas underthe age classes (as a superrefinement
of phasic subdivision)need bear no relation to currentmeteorologicalfactorsbut be
explicablein termsof some past event whichhappened,it may be, 200 or 300 years ago.
ft.
100
50-
?..?
10
Bare
(a)
~
Oxalis
-
1j~?
Rubus
ft.
100
50
Gap
Oxalis
Bare
Rubus
(b)
Fig. 10. D)iagramto illustrate(a) the phasic change duringthe life historyof an even-agedpure beechwood
and1(b) the distributionin space of the phases whenthe old wood is leftto itself.
of'explaining'
A seriesofsporadicexceptionaleventswillobviouslyincreasethedifficulties
currentrelativeareas, as also will the selectivedestructionof phases theirjuxtaposition
in space. Fortuitousfluctuationsof climateand othercauses may thus bringabout major
departuresfromthe normalor ideal wood.
The second point refersto the normal inclusionin the cycle of change of a phase
kinds of beechwoods,forexample, by
d(ominatedby anotherspecies of tree,in different
ash, oak or birch. Such a phase (e.g. birch) may occupy small or large areas, but irrespective of the area it occupies, if the relationis cyclic and not seral (that is, one of
alternatingequilibriumin whichsuccessfulregenerationofthe beech is inhibitedby itself
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ALEX. S. WATT
15
butpromoted
bytheconditions
brought
aboutbytheotherspeciesoftree),thensuch
a phaseis partandparcelofthesystem.
To recognize
thebirchwood
andbeechwood
as
separate
entities
maybeconvenient
inclassification,
butsuccessful
practical
management
ofsuchwoodsdepends
ontheirrecognition
as int-egral
partsofa coordinated
whp.le.
SUPPLEMENTARY EVIDENCE
Doubtless
becauseofitsuniversality,
itsfamiliarity
anditsapparent
lackofsignificance
thefactoftheimpermanence
oftheindividual
orgroup
ofindividuals
hasseldom
evoked
comment.
Butcher
(1933)has,however;
calledattention
tothechanging
pattern
ofriver
vegetation
overa short
period
andsuggests
progressive
impoverishment
ofthesoilofthe
riverbedas thecauseofthedisappearance
ofSiumerectum
froma givenplacein it.
Tutin(1938)has attributed
themarked
reduction
in areaoccupied
by Zostera
marina
primarily
todeficiency
inlightintensity;
thispredisposed
theplantsto disease,
which
is
a symptom
ofweakness
rather
thana causeofitsvirtual
disappearance.
Without
contesting
theaccuracy
oftheimmediate
diagnosis,
I wouldsuggest
thatbothphenomena
maybe morefully
andsatisfactorily
explained
against
thebackground
ofcyclicchange.
Phenomena
similar
to thosedescribed
havebeenseenbutnotinvestigated
in communities
dominated
respectively
by Polytrichum
(in Breckland),
Ammophila
piliferum
arenaria
and Carexrigida-Rhacomitrium
lanuginosum
ofmountain
tops. Poole(1937),
from
a studyofmixedtemperate
rainforest
inNewZealand,
concludes
thatDacrydium
ontheonehand,andQuintinia
and
Weinmannia
racernosa
onthe
cupressinum
acutifolia
other,
alternate
intimeat oneplace..
thefewspecies
Thephenomena
arenotconfined
totemperate
regions.
Inthehigharctic
are represented
by scattered
individuals,
and communities
are distinguished
by the
factors
quantitative
relations
oftheirchief
components
and/or
byhabitat
(Griggs,
1934).
ofsoiland
Wheretherearefewindividuals
competition
is negligible
orabsent;factors
from
climate
aretheprimary
selective
agents.Griggs
saysthere
is notonlychange
place
to place,butchangein timeat a givenplace(herefers
to unpublished
datafrom
perinplaceandin
manent
quadrats
as thebasisforthislatterstatement).
Thisdistribution
time,is,according
to Griggs,
erratic,
butnodetailsaregiven.
fromopeninitialcomAt thefirst
remove
in thelowarctica two-stage
succession
oftheorganic
munities
to theclosedassociation
ofheathandbogshowstheemergence
thedistribution
of
environment
intoimportance.
Competition
nowis a factor
limiting
species.
inwhich
Attheother
ofplantcommunities
extreme
istropical
forest,
themostcomplex
thatsucha
thebioticenvironment
playsan outstanding
part.It is notto be expected
Yetpresumptive
complex
community
willbeamenable
toa simple
dynamic
interpretation.
ofthe
evidence
viewsonthestructure
is givenbyAubreville
(1938)whohasexpressed
He finds
ofcommunities.
forest
akintothoseadvocated
closely
herefora selection
that
or most
although
therearein tropical
forests
verymanyspeciesoftreethedominant
abundant
speciesarerelatively
fewinnumber
(abouta score)theirrelative
proportions
from
varying
placetoplace,somedominating
inoneplace,someinanother.
Butalsoat a givenplacethereis a changeinthedominant
specieswithtime,for,in
areoften
thesubsidiary
of
the
the
trees
dominants
layers young
(the
future)
specifically
doesnot
different
fromthedominants
oftheuppermost
layer.Aubreville,
however,
a regular
at a givenplace;infact,as faras canbe judgedfrom
mention
cycleofchange
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16
Patternand processin theplant community
his account the assemblage of species in a newlyformedgap depends on the fortuitous
occurrenceof a numberoffactors,includingdistance ofparents,mobilityand abundance
of seed, frequencyof seed production,dispersal agents and their habits, and various
factorslike animals eating the seed, dur4tionof life of the seed, light intensity,root
competitionand the kind of undergrowth,affectingthe survival of the seed and the
establishmentof the seedling. Althoughthe earlierstages of gap colonizationmay show
diversityin floristiccompositionthe later ones, accordingto Dr P. W. Richards (oral
communication),are much moreuniform.
If we assume withAubrevillethat the generalclimaticand soil conditionsare uniform
over a largearea thenthe differentiating
biologicalcharactersofthe manyspeciesassume
primaryimportance,and variation in floristiccompositionfromplace to place is the
resultantof the impact of these biologicalcharactersand environmental
factors,largely,
but not exclusively,residingin the plant communityitself;that is, the communityitself
largelydeterminesthe distribution,densityand gregariousnessof its componentparts.
COMPARISON AND SYNTHESIS
The analysis of the seven communities,documentedby observationsover a period of
time,hy comparativestudiesand by appeal to the registrationof the time change either
in the plants themselvesor in their subfossilremains,shows that there is an orderly
sequence of phases in time at a given place and that the spatial relationbetweenthe
phases can be interpretedin termsoftheirtemporalrelations. I now proposeto correlate
the observedphenomenaand to offera simplegeneralizationand explanation.
The cycle of change in all the communitiesis divisibleinto two parts,an upgradeand
a downgrade.The upgrade seriesis essentiallya process or set of processesresultingin
a buildingup of plant materialand of-habitat potentialand the downgradea dispersion
of these mainlyby fungi,bacteria,insects,etc., but also by inorganicagents. At each
stage, of course,there is both a buildingup and a breakingdown, but in the upgrade
seriesthe balance is positive,in the downgradenegative.
When bracken was being investigatedthe fieldphenomenaevoked the spontaneous
application of the names for the phases, pioneer,building,mature,degenerate.They
undoubtedlyconveyed impressionswhichlater study hardened,forthey are obviously
applicable to several of the communities.The quantitativegraphicalexpressionof the
change for the upgrade series for,woodland and the qualitative descriptionfor other
communitieslead me to suggestthat a curveof the growthor logistictype expressesthe
generalcourse of change in total productionin the upgrade series of each of the communities(Fig. 11 and appendedcorrelationtable). This is doubtlessan oversimplification
ofthe wholeconcretesituation. But thereare data enoughto show that we are dealing
with somethingfundamental,with in fact, some biological law which operates in the
plant communitv.
For the downgradeseries a curve of similarformis tentativelysuggested.There are
manypartialassessmentsofthe courseofthe changes,but noneto myknowledgedealing
withthe whole seriesin a comprehensiveway.
The basis of these regularphenomenaof change may be sought in the behaviourof
populationsand in the sequence of a plant successionon the restrictedarea of a patch.
The communityas we have seenconsistsofpatches,each oflimitedarea,and differentiated
by floristiccomposition,age of dominantspeciesand by habitat.
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17
ALEX. S. WATT
Patchinesscan be readilyenoughdetectedin all the communitiesinvestigatedand in
the othersreferredto. In forest,the communityabout whose structuraltypes more is
known than about any otherkind of community,patchinessis widespread.There is a
formof 'irregularforest'that would appear to be an exception,althoughit is not known
how farit is local or temporary,and untilit has been examinedby the use of criteriain
addition to the distributionof crown classes, judgement must be suspended. A comparable structuremay be foundin othercommunitiesand at the momentno claim can
be made that patchiness,as understoodhere,is universal. It certainlyis widespread.
Downgradeseries
Upgradeseries
Fig. 11. The graphsillustratein a generalway the assumed courseof change in the total productivityof the
sequence of phases in the upgrade and downgradeportionsof the cycle of change. The appended table
communities.
correlatesthe phase sequences of the different
Plant
community
Pool
Regeneration
complex
Bare soil
Dwarf
Callunetum
Bare soil
Eroded
Rhacomitrietum
Pteridietum
GrasslandA
Acid grassheath
Beechwood
(sere 2)
Hummockwith:
Calluna
Erica
Cladonia Degenerate
Scirpu8
C(adonia
Calluna
Phase
Sphagnum Sphagnum Sphagnum
cu8pidatum pulchrum papiloum
Calluna
Arctostaphylos
Bryophytes
Empetrum
Pioneer
Grassheath
Hollow
(iadonia Pioneer
Gap (late)
Bare
stage
RhacomitriumVaccinium
Building
Rhacomitrium
Eroding face
Mature
Degenerate
Building
Building
Mature
Mature
Degenerate
Ciadonia
Rubu8
stage
Gap (early)
OxaWZ
stage
The widespreadoccurrenceof patchinessis of interestin itself,but the persistenceof
the patch in timeundercontinuingnormalconditionsis fundamental.Why,forexample,
startingwith a set of patches, do we not get a generalblendingof the whole with the
vegetationof everysquare foota replica of everyother?
in the fundamentalsoil and
May I firstremindyou that we are assuminguniformity
we get, as we have
this
foundation
climaticfactorsof the community.Over and above
ofthe soil and clinate whichare closelylinkedwiththe phases; these
seen, modifications
are in the firstinstancereactionsof the phases and in the second,
floatingdifferentiae
causes. We have, in fact,habitat variationin space as in time. (Also it should be noted
that the habitat factorsinherentin a phase modifyand are themselvesmodifiedby the
habitats of surroundingphases.) Seeds and propagulesfallingon a plant community
environment(varyinghabitat and competitivepowerin the
fall on a diversified
therefore
2
J. Ecol. 35
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18
Patternand processin theplant community
is restricted
to thephasewheretheplantssurvive;formany
phases),and establishment
to it,formed
speciesthisphaseis thegap phase,orphasecorresponding
by thedeathof
oftheupgradeseries.Forplantsspreading
inthemature
thedominant
phaseorend-phase
effective
invasionislimitedtothegrass-heath
thesameholdsgood;inbracken,
byrhizome
intothepioneerphase.
it maybegininthematurephaseandcontinue
phase,although
inspace,itis alsorestricted
ofmanyspeciesthusrestricted
Butnotonlyisestablishment
colonizaintime,to theperiodduringwhichthegapphaseis receptive.Checksto further
thussettinga timelimitto the period
tionmaybe madeby the invadersthemselves,
takesplace. In thisway,thereis a tendency
withinwhichestablishment
towardsthe
this
Besides
or
restricted
of
even-aged
near-aged
populations..
period
during
production
there
are
factors
take
other
whichcolonization,
may
place,
many
whichtendto produce
someofwhichhavealreadybeenalludedto andwillnotbe further
even-aged
populations,
discussed.
theplantsestablished
inthegaporcorresponding
Mainlybecauseofcompetition,
phase
are unableto invadeand suppresssurrounding
phases.Thusprevented
fromspreading
oflimitedarea withcontinuous
thepatchbecomesa microcosm
laterally,
but restricted
ofthatareabehaveslikea population
foodsupply.Hencethepopulation
offliesorhuman
ofspace and food. It is unableto spread,but it may
beingsundersimilarrestrictions
develop.
in theupgradeseriesmaytakeoneoftwoforms,
'rhis(levelopment
eitheras an evenor
as
a
seriesofdominants
ofa dominant
as ina plantsuccession.
age(Ipopulation
species
the successivephasesare distinguished
In theformer
the
by the speciesaccompanying
itself.The justification
doziinantbutprimarily
by theage and densityofthedominant
or shootsas a basis forsubdivision
of age in individuals
of the recognition
lies in the
form(height,
needsoftheindividual
withage,itschanging
changing
spread,rootdistriandanchoring
areaofabsorptive
bution,
surface,
etc.)anditschanging
competitive
pow'er.
is obviousin a plant,whosegametophyte
The needforsuchrecognition
and sporophyte
orinan insectwithcomplete
areseparateandhavedifferent
requirements,
metamorphosis,
andhabitsofthelarvadiffer
so widelyfromthoseoftheadult;but
in whichthestructure
it holdsalsoforhigher
plantsbecausetheirrequirements
changeduringtheirlifehistory.
The secondtypeof changeis foundin communities
wherethe successivephases(or
thatis, thephenomena
someofthem)have distinctdominants,
are strictly
comparable
comeinwhentheenvironment
withthoseina sere,wherenewdominants
becomessuitable
forthem.
Now in boththese,theeven-agedpopulationand theplantsuccession,
the courseof
tenmporal
changeintotalproduction
perunitareais represented
bya curveofthegrowth
& Jenny,
type(PearL1926; Braun-Blanquet
1926).The reasonforthegeneralequation
ofcorresponding
oforderly
phasesnowbecomesobvious.In both,thephenomena
change
havetheirbasisin theuniversal
ofpopulation
rhythmic
phenomena
development
and of
oftheplantcommunity
I)lantsuccession.It wouldthusappearthatcertainphenomena
withinthescopeofmathematical
can be brought
inquiry.Thisopensup an entirely
new
fieldin the studyof naturalcommunities
withthopossibility
of puttingthe practical
ofcommunities,
as theforester
has donetheforest,
management
upona soundbasisof
naturallaw.
The description
of the plantcommunity
is based on the unitpattern,whichis the
ofthecontinuous
present
expression
processofdevelopment
and declinein a population
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19
ALEX. S. WATT
of the plant
broadlyinterpreted.The qualitativeand quantitativeassessment,therefore,
communitymay be expressedin termsof the temporalchangesin such a population.
Now it has been shown mathematicallyand confirmedexperimentallythat certain
ecological requirementslive togetherand maintainstabilityonly
species with different
in a definiteproportion(Gause, 1935). In like mannerwe may assume (althoughindeis necessary)that the plant communityin a constantenvironment
pendentconfirmation
will show a steady state or a definiteproportionbetweenthe constituentphases. This
steady state we may call the phasic equilibrium,that is, the communityis in harmony
with itselfand with its environment.Departuresfromthis phasic equilibriumeitherin
space or in time could then be measuredand correlatedwiththe changedfactorsof the
environment.
this procedureare obvious. There are doubtlesssome comThe difficulties
confronting
munities(e.g. GrasslandA) of whichit would be possibleto say withsome assurancethat
they are in phasic equilibrium. But there are otherslike grass-heathon acid soil, and
woodland,where,on account of the lag in responseor repeatedand fortuitouschecksto
normalorderliness,or the long delay betweenthe incidenceof the disturbingfactorand
returnto a possible normal,anythinglike phasic equilibriummay rarely be achieved
even althoughthe tendencyis always in that direction. In such circumstanceswe may
copy the foresterwhose normalforestmay be definedin termsof areas as one in which
equal areas are occupied by a regulargradationof age classes. If we thinkin termsof
naturalgroupings(phases) of unequal duration,then the areas would be proportionalto
the durationof the phases, the total equalling the durationof the cycle of change. The
standardof comiparison
or normalwould then be based on the unitpatternand duration
of the phases. With such a standard,departuresfromit could then be assessed and comparisons made, but attemptsto correlatesuch deviations with changes in the causal
environmentalfactorswould be confrontedwithgreatif not insurmountabledifficulties.
But once again thereis broughtinto prominencethe need fordata on the durationof life
of the plant in a varietyof habitats and as affectedby competition.
Relative areas of the phases is onlyone aspect of the generalproblemof the textureof
the patternor of the way in whichthe communityis put together.The othertwo aspects
are the size of the individual sample of the phase and the arrangementof the phases.
Both these are importantbecause the conditionsof establishmentand growthof plants
vary withthe size of the sample of the phase and withits surroundings.
SOME IMPLICATIONS
Animalsand micro-organisms
I have thus faralluded almostsolelyto the higherplants and describedthe communities
in termsof them,but it should be clear fromthe insistenceon processthat organisms
otherthan higherplants play an importantand sometimesa fundamentalrole,and that
in the systemof whichthe plant communityis a part theyoughtto be investigated.On
readingBioecology(Clements& Shelford,1939) I confessto a feelingof disappointment,
because the data given to supportthe concept of t-hebiome fall shortof demonstrating
that intimacyand integrationbetweenplants and animals whichwe are led to expect.
Perhaps this is due to the lack of data for the communitiesdescribed.The degree of
intimacyvaries,but all organismswhetherloosely attached or occupyingkey positions
in the systemshould be consideredpart of it.
2-2
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20
Patternand processin theplant community
Tragardh(1923) has given a good illustrationof the interactionsbetweenplants and
insects in pinewoodsin northernSweden wherethe nun moth is a normal inhabitant,
whichneverbecomesepidemicas it does in pure pine or sprucewoods in centralEurope.
This he attributesto the presencein the open pinewoodsof Calluna, Cynoglossum,
etc.,
which are the food plants of species of caterpillarswhichprovide a constantsupply of
hosts forthe polyphagousparasitic species of Piimplawhich also attack the nun moth
and so keep it in check. In the dense coniferforestof centralEurope fromwhichCalluna
is excluded the populationof Pimpla risesand fallswiththat of the nun mothbut with
a distinctlag; it also dies out withthe extermination
of its foodsupply.The reappearance
of the nun mothis followedby rapid multiplication,and much damage is done before
Pimpla arrivesand increasesin numbersto be an effectivecheck to it. In the Swedish
forest,on the otherhand, a continuityof the populationof the parasite is assured here
because of the presence of alternativehosts feedingon Calluna, etc. Here we have
betweencomponentsof the tree layer and the fieldlayer and the insects
interrelations
peculiarto each forminga complexweb whichmakes forstability.This stabilitymay be
assured only so long as the food supply of the alternativehosts is presentin sufficient
amount. Furtherexcellentexamples of the intimacyand complexityof the relations
betweeninsectsand plants are given by Beeson (1941).
weremade to the varietyof relaThis addresswould be too long if adequate reference
tionshipbetweenorganismsassociative,co-operative,antagonistic,symbiotic,etc.-and
to the changein the natureofthe foodsupplyduringthe periodcoveringthe death ofthe
plants in the maturephase and the completedisintegrationof the accumulatedorganic
But that the changeis regularis shownby the workof Blackman & Stage (1924)
mnatter.
on the sequence ofinsectson living,dying,dead and decayingwalnuts,of Graham(1925)
on the earlierstages in the rottingof a log,of Saveley (1939) forinsectsand otherinvertebrates on dead pine and oak logs, and of Mohr (1943) on the changes in the species
of cow droppings.
populationduringageingand disintegration
Comparable sequences are known among fungi. Parasites,facultativeparasitesand
saprophytessucceed each otheron dyingand dead stems,and Garrett(1944) writesof
the successionof micro-organisms
on dead and decomposingroots. Generallyon organic
the
first
and are succeededlater by the cellulosefungi.
remins
sugar fungidevelop
The part these organismsplay is essentialto the maintenanceof the community;no
assessmentof the communallifecan be completewithoutadequate appreciationof what
is takingplace.
The suggestionmay be made that the lifehistoriesand foodhabits of micro-organisms
in general,and insectsand fungi,in particularthose withcomplicatedlifehistories,may
be more profitablystudied against the backgroundof phasic change and that the
parallelismbetweenthe change of food and habitat requirementsduringthe lifehistory
of an organism(e.g. some insectswithsoil larvae) and the spatial changein the condition
and floristiccomposition,or in age of dominant,of juxtaposed phases may be more
-thanfancifulspeculationabout originsand survival.
The natureoftheplant community
Tansley (1935) has coined the termecosystemforthe basic units in nature-units in
whichplants,animals and habitat factorsof soil and climateinteractin one system.The
term is primarilyused in referenceto the largergeographicalunits. The words steppe,
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ALEX. S. WATT
21
prairie,moor,bog apply to such systemsand not to the plants alone. But the use of the
termecosystemis elastic; it may be appliedto thesmallerunitslikethe'plant community'.
The firststep in the shatteringof this unifiedsystemis the separationof the living
plants and animals (the biome)fromthe non-livinghabitat,the componentsof the latter
being regardedas factorsaffectingthe former.The next step, which Tansley justifies
partly on the groundof practical convenience,is to associate the animals with climate
and soil as factors affectingplants. Further subdivisionrefersto the plants alone.
Accordingto Lippmaa (1933) the plant communityis composed of synusiae or strata
whichhe regardsas the fundamentalunits of vegetation.The finalstep is Gleason's; to
juxtapositionofplantindividuals'(1936b).
himtheplantassociationis 'merelya fortuitous
of the componentspecies of the
cites
the
randomicity
In supportof his view Gleason
association.This is by no means universal(Clapham, 1936; Blackman, 1935). The restricpatterntends
tion of establishmentin space and in timeto certainphases in a diversified
towardsthe groupingof age classes (that is, thereis overdisperseddistributionin time),
even althoughthe speciesas a wholemaybe at random.Further,twospeciescharacteristic
of a phase may be randomlydistributedbut show a high degreeof associationbetween
them.
This assumedrandomicityalso enables Gleasonto cut the communityintosmallpieces,
each of whichhe regardsas representativeof the whole. Such a proceedingignoresthe
significanceof the diversitymanifestedin pattern; this patternsets a lowerlimitto the
representativeminimum.
It is, however,importantto realize that Gleason uses the wordindividualin the sense
ofan ecosystemofwhichtheplantis thecentre,thatis, in whichthe plant and its environmentformone interactingsystem.The individualplant (in the,ordinarysense) is a part
must
productof its environmentand itselfmodifiesthe environment.Position therefore
be important.
Gleason himselfcogentlyargues that by splittingan association into its synusiae,
Lippmaa is separating synusiae which are 'united by relativelystrong genetic and
dynamicbonds' (1936a). The same argumentis valid if it can be shown that in lateral
spacing as well as in verticalthereis dependenceof one part on another,as is held to be
shownby the evidenceadduced here. His skeletonpictureof a two-layeredcommunity
consistingof a treeand a herbbelow it is divorcedfromits spatial mouldingcontextand
set in a momentin time. In time,both will changeand theirsuccessorsbe influencedby
their collective surroundings.In short Gleason has minimizedthe significanceof the
relationsbetween the componentsof the communityin horizontalspace and in time.
These relationsconstitutea primarybond in the maintenanceof the integrityof the
plant community;theygive to it the unityof a co-ordinatedsystem.
This co-ordinatedunit may be studied as a plant community,the central object of
studyofplant sociology;roundthisunitknowledgeis accumulatedforuse and classification. The functionalrelations of the communitymay be systematizedin laws and
hypotheses.
To drawa dividinglinebetweenthe approachto the studyofnaturalunitsthroughthe
ifnot impossible. But clearlyit
plant communityand throughthe ecosystemis difficult,
is one thingto studythe plant communityand assess the effectoffactorswhichobviously
of all the componentsof
and directlyinfluenceit, and anotherto studythe interrelations
the ecosystemwithan equal equipmentin all branchesof knowledgeconcerned.With a
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22
Patternand processin theplant community
it be climate,soil,animalsor plantswhichare elevatedinto
limitedobjective,whether
to the subordinate
the centralprejudicedposition,muchof interestand importance
even
studiesand perhapsevento thecentralstudyitselfis setaside.To havetheultimate
fragments
intotheoriginalunityis ofgreat
ifidealisticobjectiveoffusingtheshattered
practicalbecauseso manyproblemsin natureare
scientific
and practicalimportance;
it
oftheecosystemn
ratherthanofsoil,animalsorplants,and scientific
problemns
because,
is ourprimary
businessto understand.
What1 wantto say is whatT. S. Eliot said of Shakespeare'swork:we mustknow
all ofit in orderto knowany ofit.
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