Journal of Ecological correlates of seed persistence in soil in theEcology 0887\75\ 052Ð058 north!west European ~ora

KEN THOMPSON\� JAN P[ BAKKER\$ RENEłE M[ BEKKER$ andJOHN G[ HODGSON��Unit of Comparative Plant Ecology\ Department of Animal and Plant Sciences\ University of Shef_eld\Shef_eld S09 1TN\ UK^ and $Laboratory of Plant Ecology\ University of Groningen\ PO Box 03\ 8649 AAHaren\ the Netherlands

Summary

0 Using the data in a recently published seed bank database for north!west Europe\we describe how a species| seed bank behaviour can be characterized by a single{longevity index|\ and investigate how representative the information in the databaseis of the north!west European ~ora[ We also test the hypotheses that seeds of short!lived species are more persistent than those of long!lived species\ and long!lived seedsare characteristic of species living in disturbed habitats[1 The data are not representative of the north!west European ~ora as a whole^ theyare a fair re~ection of a research e}ort that has been largely directed towards grasslandand arable weeds[ Data for rare species\ non!agricultural habitats in general andwetlands\ rocky habitats and native woodland in particular\ are scarce or absent[2 Annuals and biennials almost always have more persistent seeds than related per!ennials\ and this di}erence is most striking when\ as in Ana`allis arvensis and Aphanesarvensis\ the short!lived species have moved well away from the {core| habitat of thefamily[ Confamilial monocarps and polycarps do not di}er consistently in seed mass[3 Gradients of habitat disturbance are accompanied by predictable changes in seedpersistence\ which are themselves often "but not always# accompanied by parallelshifts in seed size[ These results suggest that increasing habitat disturbance "i[e[increasing density!independent mortality# always selects for increased seed persist!ence\ con_rming both theory and previous analyses[ However\ increased seed per!sistence is not always associated with reduced seed size[ This is because persistencedepends not only on seed size\ but on other traits\ many of them physiological[ Inmany habitats the probability of seed burial is strongly linked to seed size and shape\but in arable habitats cultural practices have broken this link[

Keywords] conservation\ disturbance\ life history\ seed bank\ seed size

Journal of Ecolo`y "0887# 75\ 052Ð058

et al[ 0882^ Hendry et al[ 0883^ Khan et al[ 0886#[Introduction

The recent publication of a comprehensive seed bankdatabase for north!west Europe "Thompson et al[The accumulating data on soil seed banks have

recently begun to yield an increasing understanding 0886# allows us for the _rst time to explore someof the factors responsible for interspeci_c variation current theories with a large data set[in seed persistence in the soil[ The hypotheses that Seed size is largely\ though by no means entirely\ apersistent seeds are small\ compact and well!defended\ family trait "Hodgson + Mackey 0875^ Peat + Fitterboth physically and chemically\ have all received some 0883#[ Since seed size and persistence are negativelysupport\ but only in relatively small data sets "Thomp! correlated "Leck 0878^ Thompson et al[ 0882#\ weson + Grime 0868\ 0872^ Thompson 0876^ Thompson might expect both to be similarly constrained by phy!

logeny[ Therefore in this paper we _rst consider brie~ythe phylogenetic pattern of seed persistence\ althoughCorrespondence] Ken Thompson\ Unit of Comparativea fuller treatment of this subject will be publishedÞ 0887 British Plant Ecology\ Department of Animal and Plant Sciences\

University of She.eld\ She.eld S09 1TN\ UK[Ecological Society elsewhere[ Secondly\ we explore the relationship

053 between seed persistence and life history\ and how far are representative of the north!west European ~ora[Nomenclature follows Stace "0880#[Ecological this relationship is simply a consequence of variation

in seed size[ Many authors have shown that seed sizecorrelates of seedpersistence in soil and adult longevity are positively correlated "Salis!

A LONGEVITY INDEXbury 0831^ Baker 0861^ Silvertown 0870^ Rockwood0874^ Foster + Janson 0874^ Hodgson + Mackey The simplest option would be to calculate the arith!0875^ Mazer 0878#\ although some recent analyses metic mean of all type 0\1 and 2 records for a species[have cast doubt on the generality of this relationship One objection to this procedure is that the persistence"Rees 0882\ 0885^ Hodkinson et al[\ in press#[ Leish! scale "0Ð2# is not linear[ Nor is it obvious which trans!man et al[ "0884# found that an apparent correlation formation would be appropriate\ since the longevitybetween seed size and life history arose from inde! of type 2 has no upper limit[ Another di.culty is that\pendent correlations of both variables with plant as explained in Thompson et al[ "0886#\ type 2 recordsheight and growth form[ are the best but by no means the only evidence of

Thirdly\ we test the prediction that plants of dis! long!term persistence[ For at least two large classesturbed habitats should have more persistent seeds of seed persistence data\ the conservative approachthan plants of more stable habitats[ Bet!hedging mod! adopted by Thompson et al[ "0886# underestimatesels predict that in variable habitats\ plants in which longevity[ Data derived from seasonal sampling pro!only a fraction of one year|s seed output germinates grammes "e[g[ Thompson + Grime 0868# can allocateimmediately will be favoured by natural selection species only to types 0 or 1\ even though in reality"Cohen 0855^ Bulmer 0873^ Venable + Brown 0877^ many may be type 2[ Also\ if the history of the veg!Rees 0882#[ The success of this strategy is improved etation is unknown\ as it frequently is\ species en!if seeds are able to make judgements about the quality countered in the seed bank but not in the vegetationof the environment\ and there is abundant evidence are allocated to type 1[ With better data\ many ofthat seeds have evolved to make use of a wide range of these records would undoubtedly be correctly allo!information\ including temperature\ light and various cated to type 2[ Thus\ for purposes of calculating achemical stimuli\ in order to do this "Gorski et al[ longevity index\ we have adopted the simplest possible0866^ Thompson + Grime 0872^ Pons 0878^ Keeley view of the data\ which is to recognize only two classes+ Fotheringham 0887^ Thanos + Rundel 0884#[ We of record^ transient "type 0# and persistent "types 1also explore the possibility that greater seed per! and 2#[ Therefore our longevity index is de_ned as]sistence in unstable habitats is wholly or largely aconsequence of the well!documented pattern of vari! S "type 1 ¦ type 2#

S "type 0 ¦ type 1 ¦ type 2#ation in seed size between habitats "Salisbury 0831#[We begin\ however\ by showing how the per!

which can have any value from 9 "no persistent re!sistence data for individual species\ which are oftencords# to 0 "all records persistent#[ In the subsequentvery numerous\ and also at times extremely variable\analyses reported here\ we use this index as a measurecan be condensed into a single {longevity index|[ Weof the persistence of individual species[ However\then go on to determine how representative the datanone of our conclusions would be greatly altered ifin Thompson et al[ "0886# are of the north!west Euro!we had used mean persistence score[pean ~ora as a whole\ at least partly in order to pro!

vide guidance for those who would like to _ll in thegaps in the existing database[

REPRESENTATIVENESS OF THE DATA

We have already "Thompson et al[ 0886# pointed outMethods

that plants from some families and habitats appearmore or less frequently than one would expect in theThompson et al[ "0886# allocated each published re!

cord for every species to one of three longevity classes^ database[ This can be illustrated partly by comparingthe number of records for a species in the databasetype 0 "transient\ persistence ³0 year#\ type 1 "short!

term persistent\ persistence ×0 year but ³4 years# and its abundance in the British Isles "Fig[ 0#[ Clearly\almost half of the interspeci_c variation in numberand type 2 "lon`!term persistent\ persistence ×3 years#

records[ There is also a fourth category that records of records is accounted for by species abundance^common plants have more records than rare ones[when seeds are present but cannot be assigned to one

of the three seed bank types\ but this type is imma! Note _rst\ however\ that the relationship is not linear^as plants become less common\ information aboutterial to the present discussion[ For comparative pur!

poses\ especially when dealing with large numbers of their seed persistence becomes logarithmically morescarce[ Secondly\ there is a good deal of scatter aboutspecies\ something much simpler is required\ i[e[ some

form of longevity index that summarizes the infor! the relationship^ some plants are much more\ andÞ 0887 British

mation available for a particular species[ In inter! some much less\ frequent in the database than oneEcological Society\preting such longevity indices\ we also need to con! would expect from their abundance[ One way ofJournal of Ecology\

75\ 052Ð058 sider how far the data in Thompson et al[ "0886# exploring this variation is to calculate the ratio of

054 scale can be found in Grime et al[ "0877#[ Figure 1 isa plot of the ratio of hectads occupied in Britain byK[ Thompson et al[the species to number of seed bank records of thespecies against the a.nities for two contrasted habi!tats[ Clearly\ species that are relatively frequent inwetland habitats tend to be underrepresented\ whilespecies frequent in arable habitats\ not surprisingly\tend to be overrepresented[ There is also some evi!dence that specialist species of woodland and of open\rocky habitats are underrepresented\ but interpret!ation of these patterns is hampered by small samplesize[ To some extent\ this is surprising\ since studiesof wetland and woodland seed banks were not uncom!mon\ at least relative to the abundance of these habi!

Fig[ 0 Relationship between the number of hectadstats in the landscape "Thompson et al[ 0886#[ These"09 × 09 km squares# in the British Isles in which a species

occurs and the number of records for that species in the seed results are\ however\ consistent with Hodgson "0875#\bank database of Thompson et al[ "0886#[ who showed that wetlands contained a dispropor!

tionate number of rare species\ while arable habitatshad many common but very few rare species[ Manystudies of woodland seed banks have looked only atplantations\ and the study of seed banks of rockyhectads "09 km × 09 km squares# occupied in Britain

to the number of seed bank records for the species[ habitats is technically di.cult[The lessons for those who would like to add to ourMost of the species with low values of this ratio "i[e[

overrepresented in the database# are artefacts\ in the stock of knowledge on seed persistence\ rather thanjust duplicating what already exists\ are "i# pay par!sense that they are rare in Britain but more common

elsewhere in north!west Europe[ A few are species that ticular attention to wetlands\ native woodlands\ rockyhabitats and non!agricultural vegetation in general\are relatively uncommon but nevertheless of major

economic importance\ with Alopecurus myosuroides and "ii# ensure that all species\ both in the establishedvegetation and the seed bank\ are fully and correctlyprobably being the best example[ Those species with

high values "i[e[ underrepresented in the database# are identi_ed[more interesting[ Six of the top 09 are wetland herbs\while three are species of Salix[ Seeds in this genus

PHYLOGENY\ LIFE HISTORY AND HABITATare very transient "recalcitrant in some species# andtherefore unlikely to be found in the persistent seed The relationship between seed persistence and phy!

logeny was examined by conducting a nested analysisbank\ but large numbers of transient records mightbe expected[ A more likely explanation is that this of variance "NESTED function in MINITAB 00# onsupposedly taxonomically di.cult genus is recorded"either in the seed bank or vegetation# simply as Salixsp[\ since such inexact data were omitted from thedatabase[ A similar problem a}ects some other{di.cult| genera\ notably Carex[

Comparison with quantitative habitat data forspecies found in central England "Hodgson et al[ 0884#suggests that most generalist species "those occurringin a variety of distinct habitats# are well!representedin the database[ However\ for specialist species "thoselargely or entirely con_ned to a single habitat type#\the position is more complex[ Species recorded in asurvey of over 09 999 quadrats in central England"Hodgson et al[ 0884^ J[ G[ Hodgson\ unpublisheddata# can be allocated quantitative a.nities for skel!etal\ arable\ pasture\ spoil\ wasteland\ woodland andwetland habitats[ A.nities are expressed on a _ve!point scale\ where 4 is very common and characteristic

Fig[ 1 Ratio of the number of hectads "09 × 09 km squares#of the particular habitat ") frequency ×3 times thatin the British Isles in which a species occurs and the numberin the survey as a whole# and 0 is largely or wholly

Þ 0887 British of records for that species in the seed bank database ofabsent from the habitat ") frequency ³9[14 timesEcological Society\ Thompson et al[ "0886#\ plotted against a.nity for two con!that in the survey as a whole#[ Further details of theJournal of Ecology\ trasted habitat types[ See text for more details of habitat

a.nity scores[ Bars are standard errors[75\ 052Ð058 survey methods\ habitat de_nitions and measurement

055 the longevity indices[ We explored the relationshipbetween persistence and life history within families\Ecological

correlates of seed by comparing mean longevity index of the mono!carpic and polycarpic members for all families thatpersistence in soilcontain both life histories[ The data for particularfamilies summarize information for very widely vary!ing numbers of species\ from two "Cannabaceae\Lythraceae\ Urticaceae# to × 099 "Gramineae\ Com!positae#\ although even those families with few speciessometimes represent large quantities of data[ The twospecies of Urticaceae\ for example\ together have 024records in the database[ We also compared mean seedmasses of confamilial monocarps and polycarps[

In order to investigate the relationship betweenseed persistence and habitat\ we employed the _ve!point quantitative index of habitat preferencedescribed above[ First we compared mean seed lon!gevities of habitat specialists\ i[e[ species with at leastone habitat a.nity score of 4[ Secondly\ we examinedthe way in which mean seed longevity varies alonggradients of a.nities for several habitats\ and com!pared this with variation in mean seed mass along thesame gradients[

Fig[ 2 "a# Mean seed longevity indices and "b# log seedResultsweights for confamilial monocarpic and polycarpic species[Seed longevity data from the seed bank database of Thomp!

SEED PERSISTENCE AND PHYLOGENY son et al[ "0886#[ Bars are standard errors[

Seed persistence is much more a species attribute thanis seed mass\ with just over half the total variancefound at this level "Table 0#[ Nevertheless\ seed per!sistence is clearly not independent of phylogeny\ and data are clearly consistent with the hypothesis that

shorter life histories are correlated with increased seedthus we expect the relationship between size and per!sistence to be strongly in~uenced by phylogeny[ We persistence[ It may be signi_cant that two of the fami!

lies which lie above the 0]0 line in Fig[ 2a are hard!use the method of phylogenetically independent con!trasts to analyse this relationship\ and others involv! seeded "Malvaceae and Geraniaceae#\ although the

much larger quantity of data for the "also hardseeded#ing seed size\ in a separate paper "Hodkinson et al[\in press#[ Leguminosae are consistent with the general trend[ A

striking feature of Fig[ 2a is a group of families inwhich seed persistence in the monocarpic members is

SEED PERSISTENCE AND LIFE HISTORYvery much greater than in the polycarpic members[These tend to be families in which the great majorityFigure 2a plots mean monocarpic longevity index

against mean polycarpic longevity index for 28 fami! of species are slow!growing perennials of relativelyundisturbed habitats\ but a very few atypical specieslies[ The mean longevity index of polycarps is greater

in eight\ while in 18 families the opposite is true "sign are weeds "e[g[ Primulaceae\ Violaceae\ Rosaceae#[For the 29 families that contain monocarpic andtest\ Z � 2[18\ P � 9[990#[ Moreover\ the mean

di}erence in longevity index between life histories is polycarpic members and for which we possess seedweight data\ there is little evidence of any link betweensigni_cantly greater in the latter group than in the

former "9[21 vs[ 9[04\ t � −1[53\ P � 9[901#[ These seed weight and life history within families "Fig[ 2b#[

Table 0 Percentage variance in seed mass and longevity index at each taxo!nomic level————————————————————————————————Source Subclass Order Family Genus Species********************************

Þ 0887 British Seed weight� 4[9 9[9 30[8 27[0 04[9Ecological Society\ Longevity index 9[9 19[3 09[0 04[5 42[8Journal of Ecology\ ————————————————————————————————

�From Peat + Fitter "0883#75\ 052Ð058

056 Fifteen families lie above and 04 below the 0]0 seeds of short!lived species are more persistent thanthose of long!lived species[K[ Thompson et al[ relationship between monocarpic and polycarpic seed

weights[ This _nding is consistent with other evidence The answer to the _rst question turns out to be afairly unequivocal {no|[ The seed bank data are a fairthat seed weight is essentially a family characteristic

"Peat + Fitter 0883# that is largely independent of life re~ection of a research e}ort that has been largelydirected towards grassland and arable weeds[ Datahistory within families "Hodgson + Mackey 0875#[for rare species\ non!agricultural habitats in generaland wetlands\ rocky habitats and native woodland inSEED PERSISTENCE AND HABITAT

particular\ are scarce or absent[ The increasingFor habitat specialists "Fig[ 3#\ it is clear that seed per!demand for reliable autecological information fromsistence varies enormously between habitats[ The pat!those concerned with management and conservationtern in Fig[ 3 largely con_rms that found by Hodgsonof our native ~ora is unlikely to be met by new exper!+ Grime "0889# and is essentially a gradient of habitatimental data\ if these follow the same pattern[ Thestability[ Stable\ relatively undisturbed plant com!sort of approach proposed by Thompson et al[ "0882#munities such as woodland and pasture have generallyand supported by recent research "R[ M[ Bekker etlow seed persistence\ while highly disturbed habitatsal[\ unpublished data#\ i[e[ to use seed size and shapesuch as arable _elds and spoil have high persistence[as predictors of longevity\ seems the most practicalIt might be expected that varying seed persistenceway to make progress[in di}erent habitats would be accompanied by pre!

Answers to the _nal two questions are closelydictable changes in seed size[ Examination of gradi!related[ The results presented here are consistent withents of habitat a.nity suggests that this is often\ butthe predictions of theory "Cohen 0855^ Bulmer 0873^not always\ the case "Fig[ 4#[ Seed weight and per!Venable + Brown 0877^ Rees 0882#^ plants of stablesistence vary inversely along gradients of preferencehabitats generally have seeds with low persistence infor woodland and spoil^ as plants become more con!the soil "Figs 3 and 4#[ Not surprisingly\ this trend is_ned to each habitat\ their seeds become both largermost marked in woodland[ Here other processes mayand less persistent "in the former# or smaller and morebe at work\ since the dense plant canopy createspersistent "in the latter#[ However\ mean seed weightintense shade that selects strongly for large seed sizeand persistence do not exhibit a similar correspon!"Salisbury 0831#[ Large seeds are likely to be short!dence along gradients of a.nity for wetland or\ mostlived\ _rst because large seeds su}er more intensestrikingly\ arable habitats[ Increasing frequency inpredation "Thompson 0876#\ and secondly becausearable habitats is correlated with increasing seed per!large seeds themselves reduce the perceived variabilitysistence\ as expected\ but this is not accompanied byof the environment "Venable + Brown 0877#[any signi_cant change in seed size[

The lack of correspondence between changing seedsize and persistence along a gradient of a.nity forDiscussionarable habitats casts an interesting new light on the

We set out to examine whether the data in Thompson phenomenon of seed persistence[ Thompson et al[et al[ "0886# are representative of the north!west Euro! "0882# presented evidence that in the British ~orapean ~ora\ and to test the hypotheses that long!lived small\ compact seeds were generally more persistentseeds are characteristic of disturbed habitats and that than large\ ~attened or elongate seeds[ More recent

work "R[ M[ Bekker et al[\ unpublished data# con_rmsthis relationship for several semi!natural habitats in_ve north European countries[ The suspected under!lying cause of this relationship is that seeds of theformer type would be more easily buried by rain\animals or gravity "Peart 0873#[ We suggest that inarable habitats\ cultural practices have broken thislink between ease of burial and seed morphology[Burial by ploughing is clearly independent of seed sizeand shape[ Therefore\ we expect that species suited tothe arable habitat\ by virtue of life history\ herbicideresistance\ phenology or other traits\ experienceintense selection for physiological traits "chie~y inhi!bition of germination by darkness# that are consistentwith seed persistence in the soil[ Avena fatua\ whichnow has many physiological attributes associatedwith seed persistence\ including stimulation of ger!

Þ 0887 Britishmination by light "Hilton + Bitterli 0872# and nitrateEcological Society\ Fig[ 3 Mean seed longevity indices of habitat specialists fromions "Hilton 0873#\ is proof that a large\ elongate seedJournal of Ecology\ seven major habitat types[ Data from the seed bank database

of Thompson et al[ "0886#[ Bars are standard errors[75\ 052Ð058 is no barrier to the evolution of seed persistence in

057

Ecologicalcorrelates of seedpersistence in soil

Fig[ 4 Gradients of mean seed longevity index and mean log seed weight in relation to increasing a.nity "on a 0Ð4 scale# forfour contrasted habitat types[ Data from the seed bank database of Thompson et al[ "0886#[ See text for more details of habitata.nity scores[ Bars are standard errors[

the arable habitat[ We suspect that Anisantha sterilis\ size and shape\ likelihood of burial\ germination physi!ology and chemical and physical defences against pred!a member of a genus that is otherwise uniformly tran!

sient\ is currently undergoing a similar intense selec! ators and pathogens[ The conclusion that seed per!sistence has more evolutionary ~exibility than seedtion for seed persistence in the UK "Peters\ in press#[

Both theory and previous analyses suggest that mass is also supported by the _nding "Table 0# thatmost variation in seed persistence occurs at a loweradult longevity should be negatively correlated with

seed persistence\ even if environmental variability is taxonomic rank than variation in seed weight[In the above analyses we have failed to takeconstant "Rees 0882#[ Since monocarpic species also

predominate in disturbed habitats\ it is not surprising account of the possible confounding e}ects of seeddispersal[ Seed size may be related to dispersal mode\that annuals and biennials almost always have more

persistent seeds than related perennials "Fig[ 2a#[ This although the relationship is not a simple one[ In many~oras\ di}erent modes of dispersal are associated withdi}erence is most striking when\ as in Ana`allis arvensis

and Aphanes arvensis\ the short!lived species have very di}erent means and ranges of seed mass "Leish!man + Westoby 0883^ Leishman et al[ 0884#[ How!departed most from the {core| habitat of the family[

However\ at least among the species investigated here\ ever\ in the She.eld ~ora\ unassisted and wind!dis!persed species\ which together make up ×79) of thelife history within families is not consistently associated

with seed size "Fig[ 2b#[ These results suggest that ~ora\ di}er hardly at all in seed mass "Leishman et al[0884#[ Published analyses of the relationship betweenincreasing habitat disturbance "i[e[ increasing density!

independent mortality# always selects for increased seed dispersal and persistence have su}ered from avery crude classi_cation of species as either {e}ectivelyseed persistence\ as theory predicts\ but that increased

seed persistence is not inevitably associated with dispersed| or not "Rees 0882\ 0885#[ Not only doesthis capture little of the real diversity of dispersalreduced seed size[ Although seed mass and persistence

are correlated "Thompson + Grime 0868^ Thompson capacity\ it causes statistical di.culties by leading tovery few contrasts in phylogenetically independentet al[ 0882#\ the relationship is not a close one "Hod!

kinson et al[\ in press#\ and the majority of variance analyses "Rees 0885#[ A new analysis\ in which windÞ 0887 British

in seed persistence is not accounted for by seed size[ dispersal capacity is characterized as seed terminalEcological Society\Persistence depends not only on seed size\ but on a velocity\ does not reveal any signi_cant correlationJournal of Ecology\

75\ 052Ð058 constellation of variables\ including "inter alia# seed between seed dispersal capacity and persistence in the

Keeley\ J[E[ + Fotheringham\ C[J[ "0887# Mechanism of058 soil "D[ J[ Hodkinson et al[\ unpublished data#[ Wesmoke!induced seed germination in a post_re chaparralK[ Thompson et al[ therefore suspect that explicit consideration of dis!annual[ Journal of Ecolo`y\ 75\ 16Ð25[

persal would change rather little the relationships Khan\ M[\ Cavers\ P[B[\ Kane\ M[ + Thompson\ K[ "0886#between habitat preference and seed size and per! Role of the pigmented seed coat of proso millet "Panicum

miliaceum L[# in imbibition\ germination and seed per!sistence in Figs 3 and 4[sistence[ Seed Science Research\ 6\ 10Ð14[

Leck\ M[A[ "0878# Wetland seed banks[ Ecolo`y of Soil SeedAcknowledgementsBanks "eds M[ A[ Leck\ V[ T[ Parker + R[ L[ Simpson#\

This work was supported by the Natural Environ! pp[ 172Ð294[ Academic Press\ San Diego[Leishman\ M[R[ + Westoby\ M[ "0883# Hypotheses on seedment Research Council and by the Netherlands

size] tests using the semiarid ~ora of Western New SouthOrganization for Scienti_c Research[ We are gratefulWales\ Australia[ American Naturalist\ 032\ 789Ð895[to Henry Ford for assistance with the Ecological

Leishman\ M[R[\ Westoby\ M[ + Jurado\ E[ "0884# Cor!Flora Database[ An earlier draft was improved by the relates of seed size variation] a comparison among _vecomments of Jonathan Silvertown\ Lindsay Haddon temperate ~oras[ Journal of Ecolo`y\ 72\ 406Ð429[

Mazer\ S[J[ "0878# Ecological\ taxonomic\ and life historyand three anonymous referees[correlates of seed mass among Indiana Dune angio!sperms[ Ecolo`ical Mono`raphs\ 48\ 042Ð064[References

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Thanos\ C[A[ + Rundel\ P[W[ "0884# Fire!followers in chap!Phytolo`ist\ 85\ 20Ð23[arral] nitrogenous compounds trigger seed germination[Hilton\ J[R[ + Bitterli\ C[J[ "0872# The in~uence of light onJournal of Ecolo`y\ 72\ 196Ð105[the germination of Avena fatua L[ "wild oat# seed and

Thompson\ K[ "0876# Seeds and seed banks[ In] Frontiers ofits ecological signi_cance[ New Phytolo`ist\ 84\ 214Ð222[Comparative Plant Ecolo`y "eds I[ H[ Rorison\ J[ P[ Grime\Hodgson\ J[G[ "0875# Commonness and rarity in plants withR[ Hunt\ G[ A[ F[ Hendry + D[ H[ Lewis#\ New Phytol!special reference to the She.eld ~ora[ I[ The identity\o`ist\ 095 "Suppl[#\ 12Ð23[ Academic Press\ London[distribution and habitat characteristics of the common

Thompson\ K[ + Grime\ J[P[ "0868# Seasonal variation inand rare species[ Biolo`ical Conservation\ 25\ 088Ð141[the seed banks of herbaceous species in ten contrastingHodgson\ J[G[ + Grime\ J[P[ "0889# The role of dispersalhabitats[ Journal of Ecolo`y\ 56\ 782Ð810[mechanisms\ regenerative strategies and seed banks in

Thompson\ K[ + Grime\ J[P[ "0872# A comparative study ofthe vegetation dynamics of the British landscape[ Speciesgermination responses to diurnally!~uctuating tem!Dispersal in A`ricultural Habitats "eds R[ G[ H[ Bunceperatures[ Journal of Applied Ecolo`y\ 19\ 030Ð045[+ D[ C[ Howard#\ pp[ 54Ð70[ Belhaven Press\ London[

Thompson\ K[\ Bakker\ J[P[ + Bekker\ R[M[ "0886# The SoilHodgson\ J[G[ + Mackey\ J[M[L[ "0875# The ecologicalSeed Banks of North West Europe] Methodolo`y\ Densityspecialization of dicotyledonous families within a localand Lon`evity[ Cambridge University Press\ Cambridge[~ora] some factors constraining optimization of seed

Thompson\ K[\ Band\ S[R[ + Hodgson\ J[G[ "0882# Seedsize and their possible evolutionary signi_cance[ Newsize and shape predict persistence in soil[ FunctionalPhytolo`ist\ 093\ 368Ð404[Ecolo`y\ 6\ 125Ð130[Hodgson\ J[G[\ Grime\ J[P[\ Hunt\ R[ + Thompson\ K[

Venable\ D[L[ + Brown\ J[S[ "0877# The selective inter!"0884# The Electronic Comparative Plant Ecolo`y[ Chap!actions of dispersal\ dormancy\ and seed size as adap!man and Hall\ London[tations for reducing risk in variable environments[ Amer!Hodkinson\ D[J[\ Thompson\ K[\ Askew\ A[P[\ Hodgson\ican Naturalist\ 020\ 259Ð273[Þ 0887 British J[G[\ Bakker\ J[P[ + Bekker\ R[M[ "in press# Ecological

Ecological Society\ correlates of seed size in the British ~ora[ Functional Received 17 February 0886Journal of Ecology\ Ecolo`y[ revision accepted 10 Au`ust 088675\ 052Ð058