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Spread of the Invasive Alien Vine Vincetoxicumrossicum Tradeoffs Between Seed Dispersability andSeed QualityAuthor(s) NAOMI CAPPUCCINO ROBIN MACKAY CANDICE EISNERSource The American Midland Naturalist 148(2)263-270 2002Published By University of Notre DameDOI httpdxdoiorg1016740003-0031(2002)148[0263SOTIAV]20CO2URL httpwwwbiooneorgdoifull1016740003-0031282002291485B02633ASOTIAV5D20CO3B2

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Spread of the Invasive Alien Vine Vincetoxicum rossicumTradeoffs Between Seed Dispersability and Seed Quality

NAOMI CAPPUCCINO1 ROBIN MACKAY AND CANDICE EISNERDepartment of Biology Carleton University Ottawa Ontario K1S 5B6 Canada

ABSTRACTmdashThe relationship between seed weight dispersal ability and seed lsquolsquoqualityrsquorsquowas tested for the seeds of pale swallow-wort Vincetoxicum rossicum (Kleo) Barb an invasivealien vine In a field release small seeds traveled significantly farther than large seeds Inone of two greenhouse experiments large seeds were significantly more likely to germinatethan small seeds Seed weight also had a significant effect on seedling size at the time ofharvest but only when the seeds were grown in competition with a mixture of grassesSeeds containing more than one embryo produced greater total seedling weight in the ab-sence of competition but polyembryony was not advantageous when seedlings were grownwith grasses We argue that if long-distance dispersers are less likely to germinate or sur-vive measured seed dispersal curves could lead to overestimation of the rate of spread ofinvasive plants

INTRODUCTION

An important life-history trait of invasive alien plants is small seed size (Rejmanek andRichardson 1996) Small seeds generally disperse farther than large seeds This relation-ship between seed weight and dispersal distance is commonly observed both between(Harper et al 1970 Rydin and Borgegard 1991) and within species (Augspurger andHogan 1983 Morse and Schmitt 1985) However small seeds may be at a disadvantagein terms of germination and establishment (Stanton 1984 Morse and Schmitt 1985Hendrix et al 1991 Baskin and Baskin 1998) especially if they land in established back-ground vegetation This trade-off between dispersal and seed quality is critical to under-standing and predicting the spread of invasive species Long-distance dispersers areresponsible for founding new satellite populations or lsquolsquonascent focirsquorsquo which often contrib-ute disproportionately to the spread of invading species (Moody and Mack 1988) How-ever if the propagules that disperse farthest are unlikely to become established thennascent foci will be formed only rarely

Pale swallow-wort Vincetoxicum rossicum (Kleo) Barb (Asclepiadaceae syn Cynanchumrossicum) is a perennial herbaceous vine that was introduced to North America from theUkraine and Russia in the late 1800s and to the Ottawa area in the early 1900s (Sheeleyand Raynal 1996) It has recently become a pest of natural areas forming monoculturesin both old field and understory habitats and locally excluding native species Its spreadin recent years throughout natural areas in the Toronto and Ottawa metropolitan regionshas caused it to be considered one of the important invasive plants of natural areas inOntario (Ontario Invasive Plants Working Group 2000)

Like most species in the family Asclepiadaceae Vincetoxicum rossicum produces comoseseeds borne in follicles The first follicles open in September in the Ottawa area and theplants continue to release seeds through November Of those seeds that do not land oninhospitable terrain such as roads and parking lots most probably touch down in citylsquolsquogreenspacersquorsquo and lsquolsquogreenbeltrsquorsquo areasmdashcorridors of old-field vegetation in various stages of

1 Corresponding author Telephone (613) 520-2600 ext 7549 e-mail ncappuccccscarletonca

263

Am Midl Nat 148263ndash270

succession interspersed with woodlots The old-field habitat is dominated by a mix ofgrasses and native forbs

The objectives of the present study were to investigate the trade-off between dispers-ability and seed quality in Vincetoxicum rossicum seeds We approached this through acombination of seed-release trials in the field and greenhouse germination trials Wehypothesized that if there were an advantage to large seeds in terms of germination andgrowth it would be most important for seeds landing in established vegetation wheremore lsquolsquostarting capitalrsquorsquo might be the key to succeeding in a competitive environmentTherefore we asked whether the relationship between seed size and quality would bemore apparent when seeds were planted in competition with a mix of turfgrasses com-parable to that found in city greenspaces that had previously been mowed but were nowseveral years into succession

METHODS

Seed weight dispersal distance and germinationmdashPreliminary observations indicated thatmany dispersing Vincetoxicum rossicum seeds were thin and appeared to contain no em-bryo Dissections and tetrazolium tests (Moore 1962) on a sample of these seeds indi-cated that our success in classifying seeds as filled or unfilled exceeded 95 Only filledseeds were used in the dispersal trial and germination experiments described below

On 8 November 2000 we conducted a dispersal trial using seeds from Vincetoxicum rossi-cum vines growing along the edge of a large mowed field on the Carleton University Cam-pus The vines had climbed up some shrubs and the highest follicles occurred at a heightof about 15 m Seeds were collected from the open follicles of 15ndash20 plants placed in abag and chosen haphazardly for release from the center of the field Seeds were releasedindividually from a height of 15 m and followed until they landed We measured the dis-tance traveled by each seed and then collected each seed in an individually marked vialWindspeed at the nearby Ottawa MacDonald-Cartier International Airport was 112 kmhrduring the trial

Seeds were then weighed individually and planted in seedling-starter trays in Pro-Mix(Premier Horticulture Ltd Dorval QC) at a depth of 05 cm Each individual compart-ment of the trays contained approximately 50 ml of soil Seedling trays were kept in agreenhouse in which natural light was supplemented with artificial light (1410 light-dark) The trays were watered dailyInfluence of seed weight on germination and seedling size with and without competitionmdashOn

13 February 2000 we planted 284 weighed seeds that had been gathered the previousOctober at several sites near the Carleton University campus and kept at 4 C untilmid-January Half of these seeds were planted at a depth of 05 cm in individual compart-ments of seedling-starter trays containing Pro-mix The other half were planted individu-ally into seedling-starter trays in which a mix of grasses (Kentucky bluegrass Poa praetensisL creeping red fescue Lolium perenne L and perennial ryegrass Festuca rubra L) had beengrowing in Pro-mix for 3 wk One day before planting the grasses had been cropped to aheight of approximately 2 cm so that compartments with grass would not shade neighbor-ing compartments without Trays were watered daily and fertilized (153015 NPKMiracle-Gro All-purpose Plant Food The Scotts Company Marysville Ohio) every 3 wkAll plants were harvested on day 57 following planting then dried to constant weight at60 C and weighed Vincetoxicum rossicum is characterized by polyembryony in which a seedmay contain two or more genetically identical embryos We first consider the effectof seed size on the total weight of all seedlings issuing from a single seed Then since

264 148(2)THE AMERICAN MIDLAND NATURALIST

polyembryonic individuals might be expected to yield a higher total seedling weight for agiven seed weight we also assessed the influence of embryony on seedling weightStatistical analysesmdashAll analyses were performed using JMP version 325 (SAS Institute

Inc 1995) The Shapiro-Wilk W test (Shapiro and Wilk 1965) was used to test responsevariables for normality and appropriate transformations were applied before performingparametric statistics

Standard least-squares regression was used to test the effect of seed weight on log-trans-formed dispersal distance Logistic regression was employed to test the effect of predictorvariables (grass vs no grass seed weight) on the probability of germination

In all analyses involving final seedling weight as the response variable we first had tocorrect for the influence of date of germination since seedlings from late germinatingseeds were smaller at the time of harvest that those that germinated earlier We did thisby performing a regression of log-transformed seedling weight on the date of germina-tion and using the residuals from this regression in subsequent analyses Analysis of cova-riance with treatment (grassno grass) as the main effect and seed weight as the covariatewas performed on these residuals Regression of the residuals on seed weight was alsoperformed separately for the competition (grass) treatment

The relationship between seed weight and embryony (one embryo vs more than one)was tested with logistic regression The effect of embryony on seedling weight was eval-uated separately for the grass and no-grass treatments using residuals from the weight-date regression as the response variable

RESULTS

Seed weight dispersal and germinationmdashIn the dispersal trial with windspeed averaging112 kmh seeds traveled up to 18 m from the release point and 50 fell within 25 mof the release point Seed weight was a significant predictor of dispersal distance (Fig 1)A single seed traveled much farther than the rest appearing to get caught up in a risingair current and disappeared from view We do not know the weight of that individualWithout a cold period 446 of these seeds germinated The probability of germinationdid not depend significantly on seed weight (Table 1a)Influence of seed weight on germination and seedling dry weight with and without competitionmdash

Of 142 seeds planted in trays containing a mixture of grasses 345 germinated whereas445 of the 142 seeds planted without grass germinated This difference in germinationrate was not statistically significant (Table 1b) Seed weight was a significant predictor ofgermination (Table 1b) Although 50 of the germination took place within a 3-d period(days 11ndash13 following planting) germination continued until 47 d following the date ofplanting Linear regression revealed that larger seeds germinated later although the var-

TABLE 1mdashResults of logistic regressions of the probability of germination on the independentvariables seed weight and presenceabsence of grass (a) seeds from the field dispersal trial plantedwithout cold treatment (b) seeds gathered separately planted with or without grasses following a 3-mocold period

Independent variable Parameter estimate Odds ratio Chi-square Prob Chi-square

a Seed weight 0021 1214 0079 07792b Whole model 7807 00202

Treatment [grass - no grass] 0211 1524 2919 00876Seed weight 20174 0213 4708 00300

2002 265CAPPUCCINO ET AL INVASIVE ALIEN

iance in germination date that was explained by seed weight was low (date 5 831 1 122weight P 5 0008 R2 5 006)

Dry weight attained by a seedling was a function of how long it had been growing be-fore the date of harvest (log10 (weight) 5 5022 2 0134 (date of germination) N 5 96R2 5 0203 P 00001) The residuals from this regression of log-transformed seedlingweight on the germination date were used in further analyses Competition (grass vs nograss) had a highly significant effect on the residuals (ANOVA F 5 1248 P 00001)In fact only two seedlings from the grass treatment had positive residuals (Fig 2) dem-onstrating that with competition seedlings were smaller than expected based on timegrowing In contrast 90 of the seedlings grown without grass had positive residuals(Fig 2) An analysis of covariance with competition treatment as the main effect and seedweight as the covariate revealed no effect of seed weight (P 050) However if weconsider only the competition treatment seed weight was a significant predictor of the re-sidual from the weight-date regression Although the grass treatment had a significantnegative effect on seedling weight larger seeds in that treatment nevertheless producedhigher total seedling biomass than smaller seeds after correcting for the length of timethey had been growing (Fig 2)

Of 112 seeds that germinated 50 (446) produced a single seedling 51 (455) pro-duced two seedlings and 11 (98) produced three seedlings There was no relationshipbetween seed size and whether or not a seed was polyembryonic (logistic regressionR2 5 0008 P 5 027) In the no-grass treatment polyembryony had a significant effecton total seedling weight (ANOVA R2 5 012 F 5 336 P 5 004) The combined biomassof double seedlings was almost 50 greater than that of single seedlings In the grasstreatment however embryony did not have a significant effect on total seedling weight

FIG 1mdashDispersal distance as a function of seed weight Standard least-squares regression was per-formed on the log-transformed dispersal distance Regression equation log10 (distance 1 1) 5 0857 2

00529 (weight) N 5 192 R2 5 0151 P 5 00001

266 148(2)THE AMERICAN MIDLAND NATURALIST

either alone (ANOVA F 5 109 P 5 0302) or when included in an analysis of covariancealong with seed weight (embryos F 5 13 P 5 0254 seed weight F 5 70 P 5 001)

DISCUSSION

The invasive alien vine Vincetoxicum rossicum with its fluffy comas typical of members ofthe family Asclepiadaceae has dispersal adaptations that would seemingly predispose theplant to rapid spread across a landscape However as is often the case for other plantspecies (Howe and Westley 1986) most of the seeds we released landed close to point ofrelease Compared to dispersal distances recorded for other wind-dispersed herbs of openhabitats surveyed by Cain et al (1998) the distances we observed during our trial wereslightly above-average the mean dispersal distances for V rossicum seeds fell at appro-ximately the 75th percentile for distances recorded by Cain et al (1998) while themaximum distance we observed (18 m) ranks just above their median maximum distanceThe windspeed during our release was moderate (112 kmhr) presumably V rossicumseeds could travel farther on days with higher windspeeds

FIG 2mdashThe effect of seed weight on final seedling weight for seeds planted with grass (closedcircles) or without grass (open circles) after correcting for the effect of germination date on finalseedling weight lsquolsquoResidualrsquorsquo refers to the residuals from a standard least-squares regression of log-transformed final seedling weight on date of germination The effect of seed weight on the residualswas significant for the grass treatment Residual 5 22587 1 1576 (seed weight) N 5 41 R2 5

0142 P 0015

2002 267CAPPUCCINO ET AL INVASIVE ALIEN

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

succession interspersed with woodlots The old-field habitat is dominated by a mix ofgrasses and native forbs

The objectives of the present study were to investigate the trade-off between dispers-ability and seed quality in Vincetoxicum rossicum seeds We approached this through acombination of seed-release trials in the field and greenhouse germination trials Wehypothesized that if there were an advantage to large seeds in terms of germination andgrowth it would be most important for seeds landing in established vegetation wheremore lsquolsquostarting capitalrsquorsquo might be the key to succeeding in a competitive environmentTherefore we asked whether the relationship between seed size and quality would bemore apparent when seeds were planted in competition with a mix of turfgrasses com-parable to that found in city greenspaces that had previously been mowed but were nowseveral years into succession

METHODS

Seed weight dispersal distance and germinationmdashPreliminary observations indicated thatmany dispersing Vincetoxicum rossicum seeds were thin and appeared to contain no em-bryo Dissections and tetrazolium tests (Moore 1962) on a sample of these seeds indi-cated that our success in classifying seeds as filled or unfilled exceeded 95 Only filledseeds were used in the dispersal trial and germination experiments described below

On 8 November 2000 we conducted a dispersal trial using seeds from Vincetoxicum rossi-cum vines growing along the edge of a large mowed field on the Carleton University Cam-pus The vines had climbed up some shrubs and the highest follicles occurred at a heightof about 15 m Seeds were collected from the open follicles of 15ndash20 plants placed in abag and chosen haphazardly for release from the center of the field Seeds were releasedindividually from a height of 15 m and followed until they landed We measured the dis-tance traveled by each seed and then collected each seed in an individually marked vialWindspeed at the nearby Ottawa MacDonald-Cartier International Airport was 112 kmhrduring the trial

Seeds were then weighed individually and planted in seedling-starter trays in Pro-Mix(Premier Horticulture Ltd Dorval QC) at a depth of 05 cm Each individual compart-ment of the trays contained approximately 50 ml of soil Seedling trays were kept in agreenhouse in which natural light was supplemented with artificial light (1410 light-dark) The trays were watered dailyInfluence of seed weight on germination and seedling size with and without competitionmdashOn

13 February 2000 we planted 284 weighed seeds that had been gathered the previousOctober at several sites near the Carleton University campus and kept at 4 C untilmid-January Half of these seeds were planted at a depth of 05 cm in individual compart-ments of seedling-starter trays containing Pro-mix The other half were planted individu-ally into seedling-starter trays in which a mix of grasses (Kentucky bluegrass Poa praetensisL creeping red fescue Lolium perenne L and perennial ryegrass Festuca rubra L) had beengrowing in Pro-mix for 3 wk One day before planting the grasses had been cropped to aheight of approximately 2 cm so that compartments with grass would not shade neighbor-ing compartments without Trays were watered daily and fertilized (153015 NPKMiracle-Gro All-purpose Plant Food The Scotts Company Marysville Ohio) every 3 wkAll plants were harvested on day 57 following planting then dried to constant weight at60 C and weighed Vincetoxicum rossicum is characterized by polyembryony in which a seedmay contain two or more genetically identical embryos We first consider the effectof seed size on the total weight of all seedlings issuing from a single seed Then since

264 148(2)THE AMERICAN MIDLAND NATURALIST

polyembryonic individuals might be expected to yield a higher total seedling weight for agiven seed weight we also assessed the influence of embryony on seedling weightStatistical analysesmdashAll analyses were performed using JMP version 325 (SAS Institute

Inc 1995) The Shapiro-Wilk W test (Shapiro and Wilk 1965) was used to test responsevariables for normality and appropriate transformations were applied before performingparametric statistics

Standard least-squares regression was used to test the effect of seed weight on log-trans-formed dispersal distance Logistic regression was employed to test the effect of predictorvariables (grass vs no grass seed weight) on the probability of germination

In all analyses involving final seedling weight as the response variable we first had tocorrect for the influence of date of germination since seedlings from late germinatingseeds were smaller at the time of harvest that those that germinated earlier We did thisby performing a regression of log-transformed seedling weight on the date of germina-tion and using the residuals from this regression in subsequent analyses Analysis of cova-riance with treatment (grassno grass) as the main effect and seed weight as the covariatewas performed on these residuals Regression of the residuals on seed weight was alsoperformed separately for the competition (grass) treatment

The relationship between seed weight and embryony (one embryo vs more than one)was tested with logistic regression The effect of embryony on seedling weight was eval-uated separately for the grass and no-grass treatments using residuals from the weight-date regression as the response variable

RESULTS

Seed weight dispersal and germinationmdashIn the dispersal trial with windspeed averaging112 kmh seeds traveled up to 18 m from the release point and 50 fell within 25 mof the release point Seed weight was a significant predictor of dispersal distance (Fig 1)A single seed traveled much farther than the rest appearing to get caught up in a risingair current and disappeared from view We do not know the weight of that individualWithout a cold period 446 of these seeds germinated The probability of germinationdid not depend significantly on seed weight (Table 1a)Influence of seed weight on germination and seedling dry weight with and without competitionmdash

Of 142 seeds planted in trays containing a mixture of grasses 345 germinated whereas445 of the 142 seeds planted without grass germinated This difference in germinationrate was not statistically significant (Table 1b) Seed weight was a significant predictor ofgermination (Table 1b) Although 50 of the germination took place within a 3-d period(days 11ndash13 following planting) germination continued until 47 d following the date ofplanting Linear regression revealed that larger seeds germinated later although the var-

TABLE 1mdashResults of logistic regressions of the probability of germination on the independentvariables seed weight and presenceabsence of grass (a) seeds from the field dispersal trial plantedwithout cold treatment (b) seeds gathered separately planted with or without grasses following a 3-mocold period

Independent variable Parameter estimate Odds ratio Chi-square Prob Chi-square

a Seed weight 0021 1214 0079 07792b Whole model 7807 00202

Treatment [grass - no grass] 0211 1524 2919 00876Seed weight 20174 0213 4708 00300

2002 265CAPPUCCINO ET AL INVASIVE ALIEN

iance in germination date that was explained by seed weight was low (date 5 831 1 122weight P 5 0008 R2 5 006)

Dry weight attained by a seedling was a function of how long it had been growing be-fore the date of harvest (log10 (weight) 5 5022 2 0134 (date of germination) N 5 96R2 5 0203 P 00001) The residuals from this regression of log-transformed seedlingweight on the germination date were used in further analyses Competition (grass vs nograss) had a highly significant effect on the residuals (ANOVA F 5 1248 P 00001)In fact only two seedlings from the grass treatment had positive residuals (Fig 2) dem-onstrating that with competition seedlings were smaller than expected based on timegrowing In contrast 90 of the seedlings grown without grass had positive residuals(Fig 2) An analysis of covariance with competition treatment as the main effect and seedweight as the covariate revealed no effect of seed weight (P 050) However if weconsider only the competition treatment seed weight was a significant predictor of the re-sidual from the weight-date regression Although the grass treatment had a significantnegative effect on seedling weight larger seeds in that treatment nevertheless producedhigher total seedling biomass than smaller seeds after correcting for the length of timethey had been growing (Fig 2)

Of 112 seeds that germinated 50 (446) produced a single seedling 51 (455) pro-duced two seedlings and 11 (98) produced three seedlings There was no relationshipbetween seed size and whether or not a seed was polyembryonic (logistic regressionR2 5 0008 P 5 027) In the no-grass treatment polyembryony had a significant effecton total seedling weight (ANOVA R2 5 012 F 5 336 P 5 004) The combined biomassof double seedlings was almost 50 greater than that of single seedlings In the grasstreatment however embryony did not have a significant effect on total seedling weight

FIG 1mdashDispersal distance as a function of seed weight Standard least-squares regression was per-formed on the log-transformed dispersal distance Regression equation log10 (distance 1 1) 5 0857 2

00529 (weight) N 5 192 R2 5 0151 P 5 00001

266 148(2)THE AMERICAN MIDLAND NATURALIST

either alone (ANOVA F 5 109 P 5 0302) or when included in an analysis of covariancealong with seed weight (embryos F 5 13 P 5 0254 seed weight F 5 70 P 5 001)

DISCUSSION

The invasive alien vine Vincetoxicum rossicum with its fluffy comas typical of members ofthe family Asclepiadaceae has dispersal adaptations that would seemingly predispose theplant to rapid spread across a landscape However as is often the case for other plantspecies (Howe and Westley 1986) most of the seeds we released landed close to point ofrelease Compared to dispersal distances recorded for other wind-dispersed herbs of openhabitats surveyed by Cain et al (1998) the distances we observed during our trial wereslightly above-average the mean dispersal distances for V rossicum seeds fell at appro-ximately the 75th percentile for distances recorded by Cain et al (1998) while themaximum distance we observed (18 m) ranks just above their median maximum distanceThe windspeed during our release was moderate (112 kmhr) presumably V rossicumseeds could travel farther on days with higher windspeeds

FIG 2mdashThe effect of seed weight on final seedling weight for seeds planted with grass (closedcircles) or without grass (open circles) after correcting for the effect of germination date on finalseedling weight lsquolsquoResidualrsquorsquo refers to the residuals from a standard least-squares regression of log-transformed final seedling weight on date of germination The effect of seed weight on the residualswas significant for the grass treatment Residual 5 22587 1 1576 (seed weight) N 5 41 R2 5

0142 P 0015

2002 267CAPPUCCINO ET AL INVASIVE ALIEN

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

polyembryonic individuals might be expected to yield a higher total seedling weight for agiven seed weight we also assessed the influence of embryony on seedling weightStatistical analysesmdashAll analyses were performed using JMP version 325 (SAS Institute

Inc 1995) The Shapiro-Wilk W test (Shapiro and Wilk 1965) was used to test responsevariables for normality and appropriate transformations were applied before performingparametric statistics

Standard least-squares regression was used to test the effect of seed weight on log-trans-formed dispersal distance Logistic regression was employed to test the effect of predictorvariables (grass vs no grass seed weight) on the probability of germination

In all analyses involving final seedling weight as the response variable we first had tocorrect for the influence of date of germination since seedlings from late germinatingseeds were smaller at the time of harvest that those that germinated earlier We did thisby performing a regression of log-transformed seedling weight on the date of germina-tion and using the residuals from this regression in subsequent analyses Analysis of cova-riance with treatment (grassno grass) as the main effect and seed weight as the covariatewas performed on these residuals Regression of the residuals on seed weight was alsoperformed separately for the competition (grass) treatment

The relationship between seed weight and embryony (one embryo vs more than one)was tested with logistic regression The effect of embryony on seedling weight was eval-uated separately for the grass and no-grass treatments using residuals from the weight-date regression as the response variable

RESULTS

Seed weight dispersal and germinationmdashIn the dispersal trial with windspeed averaging112 kmh seeds traveled up to 18 m from the release point and 50 fell within 25 mof the release point Seed weight was a significant predictor of dispersal distance (Fig 1)A single seed traveled much farther than the rest appearing to get caught up in a risingair current and disappeared from view We do not know the weight of that individualWithout a cold period 446 of these seeds germinated The probability of germinationdid not depend significantly on seed weight (Table 1a)Influence of seed weight on germination and seedling dry weight with and without competitionmdash

Of 142 seeds planted in trays containing a mixture of grasses 345 germinated whereas445 of the 142 seeds planted without grass germinated This difference in germinationrate was not statistically significant (Table 1b) Seed weight was a significant predictor ofgermination (Table 1b) Although 50 of the germination took place within a 3-d period(days 11ndash13 following planting) germination continued until 47 d following the date ofplanting Linear regression revealed that larger seeds germinated later although the var-

TABLE 1mdashResults of logistic regressions of the probability of germination on the independentvariables seed weight and presenceabsence of grass (a) seeds from the field dispersal trial plantedwithout cold treatment (b) seeds gathered separately planted with or without grasses following a 3-mocold period

Independent variable Parameter estimate Odds ratio Chi-square Prob Chi-square

a Seed weight 0021 1214 0079 07792b Whole model 7807 00202

Treatment [grass - no grass] 0211 1524 2919 00876Seed weight 20174 0213 4708 00300

2002 265CAPPUCCINO ET AL INVASIVE ALIEN

iance in germination date that was explained by seed weight was low (date 5 831 1 122weight P 5 0008 R2 5 006)

Dry weight attained by a seedling was a function of how long it had been growing be-fore the date of harvest (log10 (weight) 5 5022 2 0134 (date of germination) N 5 96R2 5 0203 P 00001) The residuals from this regression of log-transformed seedlingweight on the germination date were used in further analyses Competition (grass vs nograss) had a highly significant effect on the residuals (ANOVA F 5 1248 P 00001)In fact only two seedlings from the grass treatment had positive residuals (Fig 2) dem-onstrating that with competition seedlings were smaller than expected based on timegrowing In contrast 90 of the seedlings grown without grass had positive residuals(Fig 2) An analysis of covariance with competition treatment as the main effect and seedweight as the covariate revealed no effect of seed weight (P 050) However if weconsider only the competition treatment seed weight was a significant predictor of the re-sidual from the weight-date regression Although the grass treatment had a significantnegative effect on seedling weight larger seeds in that treatment nevertheless producedhigher total seedling biomass than smaller seeds after correcting for the length of timethey had been growing (Fig 2)

Of 112 seeds that germinated 50 (446) produced a single seedling 51 (455) pro-duced two seedlings and 11 (98) produced three seedlings There was no relationshipbetween seed size and whether or not a seed was polyembryonic (logistic regressionR2 5 0008 P 5 027) In the no-grass treatment polyembryony had a significant effecton total seedling weight (ANOVA R2 5 012 F 5 336 P 5 004) The combined biomassof double seedlings was almost 50 greater than that of single seedlings In the grasstreatment however embryony did not have a significant effect on total seedling weight

FIG 1mdashDispersal distance as a function of seed weight Standard least-squares regression was per-formed on the log-transformed dispersal distance Regression equation log10 (distance 1 1) 5 0857 2

00529 (weight) N 5 192 R2 5 0151 P 5 00001

266 148(2)THE AMERICAN MIDLAND NATURALIST

either alone (ANOVA F 5 109 P 5 0302) or when included in an analysis of covariancealong with seed weight (embryos F 5 13 P 5 0254 seed weight F 5 70 P 5 001)

DISCUSSION

The invasive alien vine Vincetoxicum rossicum with its fluffy comas typical of members ofthe family Asclepiadaceae has dispersal adaptations that would seemingly predispose theplant to rapid spread across a landscape However as is often the case for other plantspecies (Howe and Westley 1986) most of the seeds we released landed close to point ofrelease Compared to dispersal distances recorded for other wind-dispersed herbs of openhabitats surveyed by Cain et al (1998) the distances we observed during our trial wereslightly above-average the mean dispersal distances for V rossicum seeds fell at appro-ximately the 75th percentile for distances recorded by Cain et al (1998) while themaximum distance we observed (18 m) ranks just above their median maximum distanceThe windspeed during our release was moderate (112 kmhr) presumably V rossicumseeds could travel farther on days with higher windspeeds

FIG 2mdashThe effect of seed weight on final seedling weight for seeds planted with grass (closedcircles) or without grass (open circles) after correcting for the effect of germination date on finalseedling weight lsquolsquoResidualrsquorsquo refers to the residuals from a standard least-squares regression of log-transformed final seedling weight on date of germination The effect of seed weight on the residualswas significant for the grass treatment Residual 5 22587 1 1576 (seed weight) N 5 41 R2 5

0142 P 0015

2002 267CAPPUCCINO ET AL INVASIVE ALIEN

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

iance in germination date that was explained by seed weight was low (date 5 831 1 122weight P 5 0008 R2 5 006)

Dry weight attained by a seedling was a function of how long it had been growing be-fore the date of harvest (log10 (weight) 5 5022 2 0134 (date of germination) N 5 96R2 5 0203 P 00001) The residuals from this regression of log-transformed seedlingweight on the germination date were used in further analyses Competition (grass vs nograss) had a highly significant effect on the residuals (ANOVA F 5 1248 P 00001)In fact only two seedlings from the grass treatment had positive residuals (Fig 2) dem-onstrating that with competition seedlings were smaller than expected based on timegrowing In contrast 90 of the seedlings grown without grass had positive residuals(Fig 2) An analysis of covariance with competition treatment as the main effect and seedweight as the covariate revealed no effect of seed weight (P 050) However if weconsider only the competition treatment seed weight was a significant predictor of the re-sidual from the weight-date regression Although the grass treatment had a significantnegative effect on seedling weight larger seeds in that treatment nevertheless producedhigher total seedling biomass than smaller seeds after correcting for the length of timethey had been growing (Fig 2)

Of 112 seeds that germinated 50 (446) produced a single seedling 51 (455) pro-duced two seedlings and 11 (98) produced three seedlings There was no relationshipbetween seed size and whether or not a seed was polyembryonic (logistic regressionR2 5 0008 P 5 027) In the no-grass treatment polyembryony had a significant effecton total seedling weight (ANOVA R2 5 012 F 5 336 P 5 004) The combined biomassof double seedlings was almost 50 greater than that of single seedlings In the grasstreatment however embryony did not have a significant effect on total seedling weight

FIG 1mdashDispersal distance as a function of seed weight Standard least-squares regression was per-formed on the log-transformed dispersal distance Regression equation log10 (distance 1 1) 5 0857 2

00529 (weight) N 5 192 R2 5 0151 P 5 00001

266 148(2)THE AMERICAN MIDLAND NATURALIST

either alone (ANOVA F 5 109 P 5 0302) or when included in an analysis of covariancealong with seed weight (embryos F 5 13 P 5 0254 seed weight F 5 70 P 5 001)

DISCUSSION

The invasive alien vine Vincetoxicum rossicum with its fluffy comas typical of members ofthe family Asclepiadaceae has dispersal adaptations that would seemingly predispose theplant to rapid spread across a landscape However as is often the case for other plantspecies (Howe and Westley 1986) most of the seeds we released landed close to point ofrelease Compared to dispersal distances recorded for other wind-dispersed herbs of openhabitats surveyed by Cain et al (1998) the distances we observed during our trial wereslightly above-average the mean dispersal distances for V rossicum seeds fell at appro-ximately the 75th percentile for distances recorded by Cain et al (1998) while themaximum distance we observed (18 m) ranks just above their median maximum distanceThe windspeed during our release was moderate (112 kmhr) presumably V rossicumseeds could travel farther on days with higher windspeeds

FIG 2mdashThe effect of seed weight on final seedling weight for seeds planted with grass (closedcircles) or without grass (open circles) after correcting for the effect of germination date on finalseedling weight lsquolsquoResidualrsquorsquo refers to the residuals from a standard least-squares regression of log-transformed final seedling weight on date of germination The effect of seed weight on the residualswas significant for the grass treatment Residual 5 22587 1 1576 (seed weight) N 5 41 R2 5

0142 P 0015

2002 267CAPPUCCINO ET AL INVASIVE ALIEN

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

either alone (ANOVA F 5 109 P 5 0302) or when included in an analysis of covariancealong with seed weight (embryos F 5 13 P 5 0254 seed weight F 5 70 P 5 001)

DISCUSSION

The invasive alien vine Vincetoxicum rossicum with its fluffy comas typical of members ofthe family Asclepiadaceae has dispersal adaptations that would seemingly predispose theplant to rapid spread across a landscape However as is often the case for other plantspecies (Howe and Westley 1986) most of the seeds we released landed close to point ofrelease Compared to dispersal distances recorded for other wind-dispersed herbs of openhabitats surveyed by Cain et al (1998) the distances we observed during our trial wereslightly above-average the mean dispersal distances for V rossicum seeds fell at appro-ximately the 75th percentile for distances recorded by Cain et al (1998) while themaximum distance we observed (18 m) ranks just above their median maximum distanceThe windspeed during our release was moderate (112 kmhr) presumably V rossicumseeds could travel farther on days with higher windspeeds

FIG 2mdashThe effect of seed weight on final seedling weight for seeds planted with grass (closedcircles) or without grass (open circles) after correcting for the effect of germination date on finalseedling weight lsquolsquoResidualrsquorsquo refers to the residuals from a standard least-squares regression of log-transformed final seedling weight on date of germination The effect of seed weight on the residualswas significant for the grass treatment Residual 5 22587 1 1576 (seed weight) N 5 41 R2 5

0142 P 0015

2002 267CAPPUCCINO ET AL INVASIVE ALIEN

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

Windspeed on the day we conducted our trial was comparable to those prevailing (2ndash8and 1ndash14 kmh) when Morse and Schmitt (1985) conducted similar dispersal trials withanother asclepiad the common milkweed Asclepias syriaca Their champion dispersersdrifted for hundreds of meters whereas none of our seeds reached even 20 m with theexception of a single individual that drifted out of sight Moreover our Vincetoxicum rossi-cum seeds were released at a height of 15 m compared to heights of 05 m and 1 m forthe A syriaca seeds released by Morse and Schmitt (1985) Vincetoxicum rossicum plantsgrowing along the edges of woodlots and hedgerows often climb to a height of 2ndash3 mwhereas individuals growing in old fields have typically fallen over in a dense tangled matby the time seeds are released Thus our release height falls within the range at whichseeds are naturally released

For wind-dispersed seeds dispersal distance generally depends on the size of the seedwith small seeds travelling farthest (eg Sheldon and Burrows 1973 Morse and Schmitt1985) Vincetoxicum rossicum was no exception to this pattern We found a significant nega-tive relationship between seed weight and dispersal distance although seed weight ex-plained only a small percentage of the variance in dispersal distance (15) No doubtoccasional gusts of wind carried some heavier seeds farther than they might have goneotherwise Insufficiently dried comas may have prevented some seeds from travelling asfar as they might have had they been released naturally from the follicles

For dispersing seed to contribute to the spread of a species it must be able to germi-nate and become established often against competition from other vegetation If smallseeds are inferior to large seeds in terms of germination growth or survival as has beenshown by several authors (eg Morse and Schmitt 1985 Stanton 1984 Gonzalez 1993Hendrix et al 1991 see review in Baskin and Baskin 1998 p 212) the rate of spread ofan invasive species will be lower than that expected by considering dispersal alone In Vin-cetoxicum rossicum larger seeds were significantly more likely to germinate in one of twotrials The seeds in the trial yielding statistical significance had undergone a 3-mo coldperiod during which smaller seeds may have been less likely to remain viable (eg Bano-vetz and Scheiner 1994)Vincetoxicum rossicum is currently spreading through the greenspaces surrounding Ottawa

and Toronto where the dominant habitat that dispersing seeds are likely to encounterwith the exception of developed areas are old fields which contain a mix of grasses simi-lar to that used in our second germination trial The presence of grasses did not signifi-cantly inhibit germination One might argue that the germination rate observed in thegrass treatment was as high as it was (34) because the grasses had been growing for on-ly 3 wk before planting the Vincetoxicum seeds However preliminary field experimentshave indicated that germination can reach similarly high levels when seeds are sown inplots dominated by well established grasses (range in germination 2ndash34)

Although it did not significantly affect germination competition with grasses caused asignificant decrease in seedling size Moreover it was only in the presence of grass thatlarger seeds had an advantage over smaller ones in terms of seedling size at the time of har-vest Eriksson (1999) working with Convallaria majalis likewise found that competitioninfluenced the advantage conferred by large seed size the strength of the positive relation-ship between seed size and seedling growth rate was enhanced in the presence of otherspecies

About half of the seeds in our experiments were polyembryonic Second and third em-bryos sometimes germinated several days after the first and were often much smaller Thenumber of seedlings germinating from a given seed did not depend on seed weight Inthe absence of competition with grasses polyembryony was advantageous the combined

268 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

weight of all seedlings issuing from a seed was greater for polyembryonic individuals Inthe presence of grasses however there was no advantage to having more than one em-bryo The success of polyembryonic seedlings in the field may thus depend on whetherthe seeds land on disturbed ground or in established vegetation The fate of polyem-bryonic individuals in the field will be the subject of future studies

To predict the potential of Vincetoxicum rossicum to spread throughout the Ottawa-arealandscape we must understand both how fast it spreads locally through the differenthabitat types in which it occurs (old fields and woodlots) and how often it forms newdisjunct colonies or lsquolsquonascent focirsquorsquo (Moody and Mack 1988) The latter will require char-acterizing the tail of the dispersal distribution those rare individuals that traverse excep-tionally long distances like the single seed in our release experiment that vanished fromsight on a rising current of air Several authors (Clark 1998 Cain et al 1998 Higginsand Richardson 1999 Cain et al 2000) have argued that without an understanding oflong-distance dispersal events we will inevitably underestimate the rate of spread of plantinvasions or migrations If however propagules making up the tail of the dispersal curveare less likely to germinate or survive then measured seed dispersal curves may lead toan overestimation of the rate of spread Estimating the effective long-distance dispersal ofV rossicum will thus be one of our next objectives

AcknowledgmentsmdashThe authors wish to thank Ed Bruggink for his help in the greenhouse EugeneSchupp members of the Carleton Landscape Ecology Laboratory and two anonymous reviewers pro-vided helpful comments on the manuscript This work was supported by grants from NSERC andCarleton University (GR-5) to the first author

LITERATURE CITED

AUGSPURGER C K AND K P HOGAN 1983 Wind dispersed fruits with variable seed number in a tropi-cal tree (Lonchocarpus pentaphyllus Leguminosae) Am J Bot 701031ndash1037

BANOVETZ S J AND S M SCHEINER 1994 The effects of seed mass on the seed ecology of Coreopsis lan-ceolata Am Midl Nat 13165ndash74

BASKIN C C AND J M BASKIN 1998 Seeds ecology biogeography and evolution of dormancy andgermination Academic Press San Diego CA 665 p

CAIN M L H DAMMAN AND A MUIR 1998 Seed dispersal and the holocene migration of woodlandherbs Ecol Monogr 68325ndash347

mdashmdashmdash B G MILLIGAN AND A E STRAND 2000 Long-distance dispersal in plant populations Am JBot 871217ndash1227

CLARK J S 1998 Why trees migrate so fast confronting theory with dispersal biology and the paleo-record Am Nat 152204ndash224

ERIKSSON O 1999 Seed size variation and its effect on germination and seedling performance in theclonal herb Convallaria majalis Acta Oecol 2061ndash66

GONZALEZ J E 1993 Effect of seed size on germination and seedling vigor of Virola koschnyi WarbForest Ecol Manag 57275ndash281

HARPER J L P H LOWELL AND K G MOORE 1970 The shapes and sizes of seeds Annu Rev EcolSyst 1327ndash356

HENDRIX S D E NIELSEN T NIELSEN AND M SCHUTT 1991 Are seedlings from small seeds always in-ferior to seedlings from large seeds effects of seed biomass on seedling growth in Pastinacasativa L New Phytol 119299ndash305

HIGGINS S I AND D M RICHARDSON 1999 Predicting plant migration rates in a changing world therole of long-distance dispersal Am Nat 153464ndash475

HOWE H F AND L C WESTLEY 1986 Ecology of pollination and seed dispersal p 185ndash216 In M JCrawley (ed) Plant Ecology Blackwell Scientific Oxford UK

2002 269CAPPUCCINO ET AL INVASIVE ALIEN

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST

MOODY M E AND R N MACK 1988 Controlling the spread of plant invasions the importance ofnascent foci J Appl Ecol 251009ndash1021

MOORE R P 1960 Tetrazolium as a universally acceptable quality test of viable seed Proceedings of theInternational Seed Testing Association 27795ndash805

MORSE D H AND J SCHMITT 1985 Propagule size dispersal ability and seedling performance in Ascle-pias syriaca Oecologia 67372ndash379

ONTARIO INVASIVE PLANTS WORKING GROUP 2000 Sustaining biodiversity a strategic plan for managinginvasive plants in Southern Ontario Office of the City Forester Toronto ON 40 p

REJMANEK M AND D M RICHARDSON 1996 What attributes make some plant species more invasiveEcology 771655ndash1661

RYDIN H AND S BORGEGARD 1991 Plant characteristics over a century of primary succession onislands Lake Hjalmaren Ecology 721089ndash1101

SAS INSTITUTE INC 1995 JMP Statistics and Graphics Guide Version 3 Cary NC 593 pSHAPIRO S S AND M B WILK 1965 An analysis of variance test for normality (complete samples)

Biometrika 52591ndash611SHEELY S F AND D J RAYNAL 1996 The distribution and status of Vincetoxicum in eastern North

America Bull Torr Bot Club 123148ndash156STANTON M L 1984 Seed variation in wild radish effect of seed size on components of seedling and

adult fitness Ecology 651105ndash1112

SUBMITTED 21 NOVEMBER 2001 ACCEPTED 16 APRIL 2002

270 148(2)THE AMERICAN MIDLAND NATURALIST