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FunctionalEcology19937, 236-241

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Seed size and shape predict ersistencensoilK. THOMPSON, S. R. BAND and J. G. HODGSONUnit of ComparativePlant Ecology (NERC), Department fAnimal and Plant Sciences, The University,SheffieldJO TN, UK

Summary1. An understandingf seed persistence n the soil is important o vegetationmanagement nd weed control,butexperimentalollectionof seed bankdata istedious and expensive.We reporta rapid, simplemethod forpredicting eedpersistencenthe oil. The method s tested na range fBritish,mostly erbaceous,species.2. Diaspore (seed or fruit)weight s plotted againstvariance of the three ineardimensions f thediaspore.All diasporeswithin n area of thegraphdefined yamaximumweight nd variance are persistentnthe soil. The criticalweight s thesame forfruitsnd seeds, but the critical arianceofdiasporedimensions ppearsslightly igher or ruits. hegreatmajorityfdiaspores utside his egion re shortlived, nd therelativelyew mbiguous asescanbe resolved yreference ohabitat.3. The generalityfthesuspectedunderlying echanismuggests hat hemethodcan also be appliedto floras utside north-westurope.Key-word: eed bankFunctional cology 1993) 7, 236-241

IntroductionInformedegetation anagementequiresccurateandreliablenformationn the characteristicsfmany lant pecies.One of themostmportantfthese relates o seed persistencen the soil. Anunderstandingfpersistenteedbankssthekey omany spectsof practicaland managementoragriculturer conservation,nd to the effectiveconservationfmany are pecies. heworld'smostperniciousrableweeds llpossess persistentiableseed bank (Holm et al. 1977). Furthermore,n theincreasingly isturbed nvironmentreated bymodern and use, soil seed banks re ofgrowingimportancen the managementnd restorationfvegetationKeddy& Reznicek 982; JeffersonUsher 987;Bakker 989;Hodgson Grime 990),and t s ometimesossibleopredicthe esponsefvegetationo various ypes fmanagementromknowledgef he ompositionf he ccompanyingseed bank Marks Mohler 985;VanDerValk&Verhoeven988).Managementhichermitsnter-mittentegenerationrom persistenteed bank scrucial o thesurvival fmany are or decliningspecies Keddy Reznicek982;Rowell,WaltersHarvey 982).Apersistenteedbank ftenonfersdegree fresiliencentheface fmodernntensivelanduse; pecieswhich re urrentlyncreasingheirabundancen Britain re twice s likely o have apersistenteedbankas thosewhich redeclining(Hodgson Grime 990).Yet even n he ompara-tivelywell-studieduropean flora,data on the

persistencef eeds n the oilareavailablefor nlyminorityf pecies nd are often funcertainuality.Outside Europe even less informations available,but to offset hese deficiencies xperimentallys aformidable ask. Collection of originalseed bankdata, eitherby germinatingr extracting he seedscontained in soil samples, is prohibitivelyabourintensiveand expensive, the results of a singleinvestigationre frequentlynconclusive, nd exca-vation of soil frombeneath rare species is clearlyundesirable.The observationhas frequently een made thatthere s a connection etweenpossessionofa persis-tent eed bankandseedsize and shape Thompson&Grime 1979; Thompson 1987; Leck 1989). Forreasons which are almostcertainly onnected withtheprobabilityfburial,persistenteeds tendto besmall and compact, while short-lived eeds arenormally arger and either flattened r elongate.However,a previous ttempt opredict eed persis-tence required nformationn germination hysi-ology as well as size and shape (Grime 1989). Thispaper reports rapid methodof predicting ersis-tence n the oil frommeasurementsf seed size andshape alone.Materials ndmethodsPlantdiaspores re usually eferred o as seeds,eventhough manyof themare in fact fruits.Here weanalysedata on seeds and ndehiscent,ingle-seeded

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237 fruits eparately. Size is quantified s the air-dryPredictingeed weight f 100 diaspores. n order o givequantitativepersistencen soil expression o therelationship etweendiaspore ize,shape and persistence, quantity s requiredwhichexpresses the extent to whichshape differs romsphericity.The quantity we have chosen is thevariance of diaspore ength,width nd depth, afterfirst ransformingll values so that ength s unity.This variance has a minimumvalue of zero inperfectly phericaldiaspores, a maximum alue ofabout 0 3 in needle- or disc-shapeddiaspores, andvaries ery ittle etween ndividual eeds of he amespecies. Five replicatediasporeswere measuredforeach species. In most cases measurementswerecarried ut on the dispersule, ather han he germi-nule. Thus'most rass aryopsesweremeasuredwiththe persistentemmas nd awns withwhich hey renormally ispersed, lthough he nutofCarex laccawas measuredwithout ts utricle.Medicago fruitsrather than seeds were measured for the samereason. The achenes of Compositae,however,weremeasuredwithout hepappus. nthefew xamples fspecies with fleshyfruits Arum maculatum andHedera helix),measurements ere made on isolatedseeds.Seed persistencen the soil is frequentlylassifiedwithrespect o both ongevity nd size of the seedbank relative o annual seed productionThompson& Grime1979),butfor hepurposes fthis aperwehave adopted a simpler chemeproposedby Bakker(1989), and described n Thompson 1992). In thisclassificationhree lasses of seed persistencen thesoil aredescribed: ransient,ersistingnthe oil forless than1 year; short-termersistent, ersisting ormore than 1 but less than5 years; and long-termpersistent,ersistingor t least5 years. n practice,theboundary etween the first wo classes is oftenrather lurred, nd in thispaper we attemptmerelytodistinguishhe hird,ongpersistent,lassfrom hefirst wo. Seed persistencedata employed n thispaperwereabstracted rom n unpublished and asyetunfinished)atabaseofthe oilseed banksofthenorth-west uropean flora in preparation by K.Thompson ndJ.P.Bakker.Thisdatabase presentlycontains data abstracted from c. 200 publishedsources, nd nferseedpersistence othfrom irectestimation f seed longevity nd fromthe depthdistribution f seeds in the soil (Thompson 1992).The classificationf pecies ntopersistence ategor-ies in Table 1 musttherefore e regardedas pro-visional, lthoughwe are confident hatvery ew, fany, specieswill eventually urnout to have beenmisclassified.Measurementswere made for 97 species (44speciesof eedsand 53 speciesoffruits, able 1). Allspecies are British atives r naturalized liens, andalmost ll are herbaceous.The specieswere hosen norder to provide approximately qual numbersoffruits nd seeds and of transient nd persistent

100 (a)x x10 X X

01 S-' >(t* , x01 et

0-01 -0*001 - { IE 0 0-05 01 0.15 02 025 0.3 035Variarncef eeddimensionscmF3 100-? (b) x

10 - x X x-1- - - - - - - - - - ->-

1 - ** | " X X% X0*1 -* 0

0'01 I I 'I I I0 0.05 01 015 02 025 03 0.35Variancef ruitimensionsFig.1. Relationshipetween iaspore eightndvarianceofdiaspore imensions,n a) 44 pecies f eeds nd b)53species f fruits. ata from able 1. (X) Specieswithdiaspores hich ersistn the oilfor t east years; x)specieswith iaspores hichersistn he oil or 5 years;(#) pecieswhose eedbank ypeannot edeterminedtpresentwing o insufficientr contradictoryata.Thedashed ine ncloseshe egion ithin hichll diasporesexaminedre ong ived.

diaspores,togetherwith a wide rangeof families,seed sizes and shapes. An additional onstraint asthatseeds had to be available from he collectionsheld at UCPE.ResultsDiasporeweight, ariance ndpersistencereshownin Table 1,whileweightsplotted gainst ariance nFig.1. It is apparent that persistentdiaspores,whether eeds or fruits, ccupymore or less thesame, unique region of the size/shape space.Compactdiasporeswhichweigh essthan mg re allpersistentnthe oil.For moreflattenedrelongateddiaspores,persistence ppearsto be determined yshape,andall diaspores bove a critical ariance reshort ived. This critical ariancewas identified yeyefrom ig. 1,and sverymuch working ypothe-sisat this tage.The exact ocation f he inedividingtransientrom ersistent iaspores n the owerpartof Fig.1(a) is particularly ncertain.Neverthelessthere s some evidencethat the critical ariance sslightlyigher or ruitshan or eeds; thepersistentfruits ith moderately ighvariance whichwouldplace them n the transient egion of Fig.1(a)] aremostlygrasses in the genera Poa, Agrostisand



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Table 1. Diasporeweight, ariance fthree iasporedimensionstransformedo that ongest imension sunity) ndpersistencen thand 53speciesoffruits. omenclature ollows lapham,Tutin& Warburg1981). Persistencelasses: p) specieswith iasporeswhichyears; t) specieswithdiasporeswhich ersistn the oil for 5 years; u) specieswhose seed banktype annotbe determinedtprescontradictoryataSeeds Fruits

WeightSpecies Family Variance (mg) Persistence Species Family VArummaculatum Araceae 0*020 4.540 t Achilleamillefolium Compositae 0Hederahelix Araliaceae 0-084 20-430 t Carlinavulgaris Compositae 0Impatiens landulifera Balsaminaceae 0-063 7-320 t Centaurea cabiosa Compositae 0.Campanularotundifolia Campanulaceae 0-119 0-067 p Matricariamatricarioides Co'mpositae 0.Arenaria erphyllhfolia Caryophyllaceae 0-041 0-061 p Erigeron anadensis Compositae 0.Cerastiumontanum Caryophyllaceae 0-038 0-120 p Gnaphalium liginosum Compositae 0.Silene dioica Caryophyllaceae 0-026 0.646 p Hieracium ilosella Compositae 0.Silenenutans Caryophyllaceae 0-048 0*298 p Leontodonhispidus Compositae 0.Silenevulgaris Caryophyllaceae 0*066 1.199 p Seneciovulgaris Compositae 0.Spergula rvenis Caryophyllaceae0-022 0-449 p Coronopus didymus Cruciferae 0Stellariamedia Caryophyllaceae0-076 0-327 p Carexflacca Cyperaceae 0.Chenopodiumlbum Chenopodiaceae 0-062 0*560 p Agrostis apillaris Gramineae 0Helianthemum ummularium Cistaceae 0*051 1-280 p Anthoxanthumdoratum Gramineae 0Convolvulusrvensis Convolvulaceae 0-039 10-420 p Arrhenatherumlatius Gramineae 0Alliariapetiolata Cruciferae 04132 3-451 u Avenulapratensis Gramineae 0Arabidopsis haliana Cruciferae 0-069 0.020 p Brachypodium innatum Gramineae 0Capsellabursa-pastoris Cruciferae 0-094 0-123 p Brachypodiumylvaticum Gramineae 0Cardaminehirsuta Cruciferae 04132 0.101 p Bromuserectus Gramineae 0Erophilaverna Cruciferae 04127 0-032 p Bromus terilis Gramineae 0Thlaspi rvense Cruciferae 0-092 1.163 p Dactylisglomerata Gramineae 0Droserarotundifolia Droseraceae 0-248 0-018 u Deschampsia lexuosa Gramineae 0Ledumpalustre Ericaceae 0-236 0-020 t Desmazeriarigida Gramineae 0Juncus ffusus Juncaceae 0*089 0-010 p Festucaovina Gramineae 0Origanum ulgare Labiatae 0.100 0-098 p Festucarubra Gramineae 0Anthyllisulneraria Leguminosae 0-117 3-529 t Holcus lanatus Gramineae 0Cytisuscoparius Leguminosae 0-089 6-734 p Koeleriamacrantha Gramineae 0



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Table 1. (Continued)Seeds Fruits

WeightSpecies Family Variance (mg) Persistence Species Family VaLathyrus ratensis Leguminosae 0-036 13-688 u Loliumperenne Gramineae 0-Lotuscorniculatus Leguminosae 0-040 1V278 p Poa annua Gramineae 0-Ononisrepens Leguminosae 0-027 5-427 u Poa trivialis Gramineae 0-Trifoliumubium Leguminosae 0-053 0-227 p Ballotanigra Labiatae 0-Viciacracca Leguminosae 0-005 23-661 t Galeopsis tetrahit Labiatae 0-Vicia hirsuta Leguminosae 0.017 24190 p Stachysylvatica Labiatae 0-Alliumursinum Lifiaceae 0.061 4-270 t Teucriumcorodonia Labiatae 0-Hyacinthoides on-scripta Lifiaceae 0.032 64170 t Thymus raecox Labiatae 0.Nartheciumssifragum Lifiaceae 0-302 0.060 u Medicago upulina Leguminosae 0-Chamerionngustifolium Onagraceae 0.171 0-041 t Melilotus ltissima Leguminosae 0-Epilobiumhirsutum Onagraceae 04118 04138 p Polygonum viculare Polygonaceae 0-Oxalis acetosella Oxafidaceae 04148 1F0lO t Rumex cetosella Polygonaceae 0.Plantago anceolata Plantaginaceae 04150 1-891 p Ranunculus ep ns Ranunculaceae 0-Pyrolaminor Pyrolaceae 0-216 0.001 t Dryasoctopetala Rosaceae 0.Digitalis urpurea Scrophulariaceae0-072 0-072 p Potentilla recta Rosaceae 0-Euphrasia officinalis Scrophulariaceae0-149 0*130 t Filipendulaulmaria Rosaceae 0-Linariapurpurea Scrophulariaceae0-101 0-133 u Galiumaparine Rubiaceae 0-Viola arvensis Violaceae 0-087 0-400 p Galium axatile Rubiaceae 0-Anthriscusylvestris Umbelliferae 0-ChaerophyllumemulentumUmbelliferae 0-

Coniummaculatum Umbelliferae 0.Daucus carota Umbelliferae 0-Foeniculumvulgare Umbelliferae 0-Heracleum phondylium Umbelliferae 0.Myrrhisdorata Umbelliferae 0.Torilisaponica Umbelliferae 04Urtica ioca Urticaceae 0-



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240 Holcus. While the regionof the graphwithinwhichK. Thompson all diaspores re persistents relatively ell defined,et al. it s nottrue hat ll diasporesoutside his egion reshort ived.Large, moreor ess spherical iaspores nparticularmay be either persistent r not. Herehabitat eems o be thebestguide.Specieswith arge,round, hort-lived iaspores seeds orfruits) re allcommonest n woodland, hedgerowsor tall herbcommunitiese.g. Vicia cracca,Hyacinthoides on-scripta,Galium aparine),whilethosewith ersistentdiaspores re plants fdisturbed rable Convolvulusarvensis,Galeopsis tetrahit) r fire-prone abitats(Cytisus coparius) Grime,Hodgson& Hunt1988).A strongphylogenetic ffects also apparent nTable 1. Families n which hediaspores re normallysmall and compact (e.g. Caryophyllaceae,Cruciferae) re generally ersistent, hilefamilies nwhich he diaspores re largeand either lattened relongated e.g. Umbelliferae) re mostlyhortived.These trends re also evidentwithin amilies; mall,compact grass and compositefruits re persistent,whiletheir arger elatives re not.

DiscussionDIFFERENCES BETWEEN SEEDS AND FRUITSSome interesting ifferences etween seeds andfruits merge from ig.1. Seeds, whichuntil hed-ding are enclosed within he protectivewallsof theovary, ncompass rather arrower angeofshapesthan fruits.However, there are some importantexceptions. he four pecieson theright fFig. 1(a)are all small-seeded lants f unproductive,nfertilehabitats (Pyrola minor, Nartheciumossifragum,Ledumpalustre,Dorsera rotundifolia). he signifi-cance, ifany,of elongatedseeds in these typesofhabitats s unknown.The data llustratehat he ikelihood f diasporepersistingn the soil is determined o a largeextentnotby ts developmental rigin i.e. whether t s aseed or fruit), utby tssize and shape. Within hesample tudiedmore eeds thanfruitsormed ersis-tentseed banks simply ecause seeds, on average,are morecompactthan fruits. ize seems to differrather little between the two groups (Fig. 1),althoughheupper nd ower ize imits reoccupiedbya fruit nd a seed respectively.

PREDICTING PERSISTENCEOn the basis of the relatively mall sample of theBritish,mostlyherbaceous, floraexamined here itseems that eed size and shape canpredict iasporepersistence n the soil. Additionalhabitatdata seem

capable of resolving those few cases where anyambiguity emains.The mechanism nderlyingherelationshipn Fig. 1 s almost ertainlyase ofburial,linked to the relativefreedom rompredation on-ferred by burial (Thompson 1987). Most likelymechanismsof burial (penetrating racks in soil,being washed into soil by rain, ingestion byearthworms) ill operate more effectivelyn small,compact diaspores. Exceptions include self-burial,e.g. in Avena and Erodium Stamp 1984), and antdispersal.Avenafatua s one conspicuous xampleofa large, elongated diasporewith a persistent eedbank which fails to conform o the relationshipnFig. 1(b). Many genera with an elaiosome (e.g.Luzula) have relativelymall,round eeds consistentwith ig. 1,while ome with arge,rounded eeds arealso persistente.g. Ulex). We are not awareofanyspecieswhich ombines nt dispersalwith transientseedbank,butmorework s needed to determineheexact relationshipbetween ant disperal and seedpersistence.However,outsidethesesmalland per-haps atypicalgroups,the workof Peart (1984) hasdemonstrated hat mall diasporesnotonlybecomeburiedmore easily,but that hey lso requireburialfor uccessful eedling stablishment.Of course seed persistencen thesoil snot whollydetermined y seed size and shape. Germinationrequirements, ormancymechanisms nd resistanceto pathogens also contribute o persistence.Forexample, phenolic defence compounds are impli-cated in prolonged eed persistence Kremer 1986;C. Moss, G.A.F. Hendry,K. Thompson & P.C.Thorpe, unpublished observations). Neverthelessseeds do not persistfor long periods on the soilsurface, nd burial s clearly n essentialpreludetopersistence.Artificial urialnearly lwaysoveresti-mates thepotentialforpersistencen the soil. ForinstancePons (1989) commented n the results faseed burial experiment:The results ndicate thatMoliniacaerulea eedscansurvive or ongperiods nthe soil, but the author knows of no recordsofpersistenteed banksofthis pecies'. Under normalcircumstances iasporesof some species frequentlybecomeburied,while thers o so only arely. hosediasporeswhichoftenbecome buried consequentlyexperiencedifferentelectionpressures rom hosewhich o not, nd thusprobabilityfburialhascometo be associated with suite ofother haracteristicsconcerning germination,dormancy and defencechemistry.Measurementsof seed shape and size,which dmittedlyffectnly hefirst ssential tep nseed persistence, an perhaps thereforeprove asurprisinglyood guide to subsequentseed persis-tence. tremains o be determined hethermechan-isms wherebyseeds become incorporated nto apersistenteed bank re similarworld-wide, ut f o,we anticipatethat this method of predicting eedpersistence an be easily xtended o other loras ndclimates.



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241 Acknowledgementsperistencg no This workwas supportedby theNaturalEnviron-mentResearchCouncil.

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Cutting. luwer,Dordrecht.Clapham A.R., Tutin T.G. & Warburg, E.F. (1981)Excursion loraof theBritishsles,3rd edn. CambridgeUniversity ress, Cambridge.Grime,J.P. (1989) Seed banks n ecologicalperspective.Ecology of Soil Seed Banks (eds. M. A. Leck, V.T.Parker& R.L. Simpson),pp. xv-xxii.AcademicPress,London.Grime,J.P.,Hodgson, J.G. & Hunt, R. (1988) Compara-tivePlant Ecology.UnwinHyman,London.Hodgson,J.G. & Grime,J.P. (1990) The roleof dispersalmechanisms, egenerativetrategies nd seed banks nthevegetation ynamicsftheBritishandscape. SpeciesDispersal nAgriculturalabitats eds. R.G.H. Bunce&D.C. Howard), pp. 65-81. Belhaven,London.Holm, L.G., Plucknett, .L., Pancho,J.V. & Herberger,J.P. 1977) TheWorld'sWorstWeeds.Hawaii UniversityPress,Honolulu.Jefferson, .G. & Usher,M.B. (1987) The seed bankinsoilsof disusedchalk quarries n theYorkshireWolds,England: implications or conservationmanagement.Biological Conservation2, 287-302.Keddy, P.A. & Reznicek, A.A. (1982) The role of seedbanks n the persistencef Ontario'scoastal plainflora.AmericanJournal fBotany 9, 13-22.

Kremer, R.J. (1986) Antimicrobial ctivity f velvetleaf(Abutilon heophrasti)eeds. Weed Science34, i17-622.Leck, M.A. (1989) Wetland seed banks.Ecology of SoilSeed Banks (eds. M.A. Leck, V.T. Parker & R.L.Simpson),pp. 283-305.Academic Press, London.Marks, P.L. & Mohler, C.L. (1985) Succession afterelimination f buried eeds from recently lowed field.Bulletin f the Torrey otanical Club 112, 376-382.Peart,M.H. (1984) The effects fmorphology, rientationand position of grass diaspores on seedling survival.Journal f Ecology 72,437-453.Pons, T.L. (1989) Dormancy, erminationndmortalityfseeds in heathland nd nland and dunes.Acta BotanicaNeerlandica 8, 327-335.Rowell, T.A., Walters, .M. & Harvey,H.S. (1982) Therediscovery f thefenviolet,Violapersicifolia chreber,atWickenFen. Watsonia 4, 183-184.Stamp, N.E. (1984) Self-burialbehaviour of Erodiumcicutariumeeds.Journal f Ecology 72, 611-620.Thompson,K. (1987) Seeds and seed banks. NewPhytolo-gist106 suppl.), 23-34.Thompson,K. (1992)The functionalcology f eedbanks.Seeds: the cologyofRegenerationn PlantCommunities(ed. M. Fenner), pp. 231-258. CAB International,Wallingford,K.Thompson,K. & Grime,J.P. (1979) Seasonal variation ntheseed banksof herbaceous pecies n tencontrastinghabitats.Journal fEcology 67, 893-921.Van Der Valk,A.G. & Verhoeven,J.T.A. (1988)Potentialrole of seed banks and understoreyvegetation inrestoringuaking ens rom loating orests. egetatio 6,3-13.Received11 March1992; revised 2 June1992; acceptedJuly 992


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