Secondary seed dispersal in Montrichardia arborescens (L.) schottdominated wetlands in laguna grande, Venezuela

Elizabeth Gordon1 and Arnold G. van der Valk2,*1Instituto de Zoología Tropical, Facultad de Ciencias, Universidad Central de Venezuela, A. P. 47058,Caracas, 1041-A, Venezuela; 2Department of Botany, Iowa State University, 141 Bessey Hall Ames,50011-1020, IA, USA; *Author for correspondence

Received 17 July 2001; accepted in revised form 26 February 2002

Key words: Hydrochory, Seed bank, Seed dispersal, Tropical lake, Tropical wetlands

Abstract

Laguna Grande, Monagas State, Venezuela, is a shallow, V-shaped lake created by the confluence of two rivers.Montrichardia arborescens (L.) Schott. dominated wetlands cover most of the north and south arms and thelittoral zone of the main body of the lake. The vegetation and seed banks of Montrichardia wetland sites weresampled in the north arm, south arm and main body five times from the end of the dry season in 1991 to begin-ning of the rainy season in 1992. The composition of the vegetation was similar and changed very little at allthree sites during the course of the study. These wetlands had 53 species. Besides M. arborescens, other commonspecies were Hamelia patens Jacq., Mikania cordifolia (L.) Wild., Sarcostemma clausum (Jacq.) Roem. & Schult.,and Vitis caribaea L. In both the vegetation and seed banks, species richness was highest during the dry season.Altogether, the seed banks contained the seeds of 61 species of which 35 were also found in the vegetation.Seeds of three tree species were found in the seed banks that did not grow anywhere in the lake. In the seedbank, seeds of Cyperus odoratus L., Eleocharis interstincta (Vahl.) R&S, Ludwigia hyssopifolia (G. Don.) Ex-cell, L. lithospermifolia (Mich.) Hara, Polygonum acuminatum H.B.K., and Sacciolepis striata (L.) Nash werethe most abundant. Mean total seed density over the entire study was 6,500, 3,800, and 6,000 seeds/m2 in thenorth arm, south arm, and main basin, respectively. Seed production and dispersal occur primarily during the dryseason, and the highest seed densities at all sites were found in the dry season when there was no or little stand-ing water. The lowest seed densities at all sites were found during the rainy season during which seed densitiesdeclined over 80% at the north and south arm sites. In the main body of the lake, however, seed densities duringthe rainy season, although lower than during the dry season, actually increased significantly from 3,600 seeds/m2

in August 1991 to 6,000 seeds/m2 in October 1991. A significant decrease in seed density in either the north orsouth arms or both and a significant increase in the main body site during the rainy season occurred for 5 of the8 species whose seeds were the most abundant, for all life-form guilds, except hydrophytes and for the entireseed bank. Secondary dispersal by water currents during the rainy season appears to be transporting seeds fromthe north and south arms into the seed bank of the main body of the lake.

Introduction

Because seeds of most wetland species float, primaryand secondary dispersal of seeds by water currents isubiquitous in wetlands (Sculthorpe 1967; Cook 1987;Middleton 1999). Although there has been, and con-tinues to be, considerably interest in studying seeddispersal by water (see Ridley (1930) for an exhaus-

tive summary of older data and also Appendix I inMiddleton (1999)), there have been few field studiesof secondary dispersal of wetland seeds. Notable ex-ceptions are studies of seed dispersal by rivers (Stan-iforth and Cavers 1976; Junk 1986; Schneider andShariitz 1986, 1988; Middleton 1995). For example,Schneider and Shariitz (1986, 1988) studied second-ary dispersal of seeds in a riverine swamp forest. In

177Plant Ecology 168: 177–190, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

the spring when water levels rose, seeds from non-flooded areas were carried away by water currents. Inflooded areas, however, rising water levels did not af-fect the size of the seed bank (Schneider and Shariitz1986).

Seed banks play an important role in the vegeta-tion dynamics of many types of wetlands (Leck 1989;Middleton 1999). Secondary seed dispersal amongseed banks can be allochthonous or autochthonous.The former, which has received the most attention, isthe secondary dispersal of seed from one wetland toanother. The latter, which has received almost no at-tention, is the secondary dispersal of seeds within awetland. Nevertheless, there has been circumstantialevidence for many years (McAtee 1925) to suggestthat autochthonous secondary dispersal occurswidely. For example in the Delta Marsh, Manitoba,Canada, the seed bank at the waterline was alwaysmuch larger than the seed banks both upslope anddownslope and it contained seeds of species that didnot occur at that elevation (Pederson and van der Valk1985). This accumulation of seeds at the shoreline(drift line) is common around lakeshores (Keddy andReznicek 1982).

Although seeds with many different kinds of pri-mary dispersal syndromes are found in these driftlines (Pederson and van der Valk 1985), it is notknown unequivocally whether these seeds were everincorporated into the seed bank prior to being trans-ported to the water’s edge. In other words, it is notknown whether these seeds were carried to the shore-line when the seeds were still buoyant or whether theywere carried to the shoreline after becoming water-logged and sinking to the bottom, i.e., after enteringthe seed bank. Data in Schneider and Shariitz (1986);Staniforth and Cavers (1976) suggest that floatingseeds picked up by rising water from unflooded areasare being re-distributed by water currents. We believe,however, that the secondary dispersal of waterloggedseeds from one seed bank to another can also occurin wetlands when they are flooded due to water cur-rents picking up and re-depositing seeds.

Changes in the number of seeds in the seed bankover a growing season can be used to estimate themagnitude of the seed rains (inputs) into or lossesfrom a seed bank. During periods when there is noprimary dispersal of seeds, this seed rain can only bedue to secondary dispersal. This was the approachused by Schneider and Shariitz (1986) to detect sec-ondary dispersal. This sampling approach will onlygive unequivocal results if there is an increase in the

size of the seed bank or the appearance of seeds of aspecies that does not grow in the wetland. A declinein the number of seeds or species in the seed bankmight be due to secondary dispersal but could also bedue to seed predation, seed germination, or seeddeath. An ideal field site for the study of secondaryseed dispersal among seed banks would be a lacus-trine or palustrine wetland into which one or moreriverine wetlands flow. If secondary dispersal is oc-curring, wetland seeds from the seed bank(s) of theriverine wetland(s) should be deposited in the seedbank of the lacustrine or palustrine wetland duringperiods of high flow. Confirming evidence would bethe simultaneous decline in the seed bank(s) of theriverine wetland(s).

Laguna Grande in Monagas State, Venezuela issuch a site (Figure 1). The main basin of thisV-shaped, shallow lake is formed by the confluenceof two rivers. Montrichardia arborescens dominatedwetlands cover both the riverine arms and the lacus-trine main basin. The climate of Monagas State isstrongly seasonal with a rainy season from May toDecember with June, July and August having thehighest rainfall. The dry season occurs from Januaryto April. Flowering and seed dispersal occurs in Ven-ezuelan wetlands primarily during the dry season(Ramírez and Brito 1987) as it does in other herba-ceous wetlands in wet-dry tropical or subtropical cli-mates (Patton and Judd 1988). Consequently, the seedbanks of both the riverine and lacustrine wetlands inLaguna Grande should have their highest seed densi-ties toward the end of the dry season. We hypothe-sized that, if autochthonous secondary dispersal ofseeds occurs among seed banks in Laguna Grandewetlands, that during the wet season the number ofseeds in the seed banks of the riverine wetlands woulddecrease and would in themain basin increase (seeFigure 2). To test our hypothesis, we collected seedbank samples periodically at two riverine wetlandsites and one main-basin wetland site from the end of1991 dry season through the rainy season to the endof the 1992 dry season. We also sampled the vegeta-tion at each site at the same time to determine if thereany changes in vegetation during the study that couldconfound the interpretation of the seed bank data.

Study area

Laguna Grande is located approximately 18 km eastof Maturin City (Monagas State, Venezuela) at 9° 45�

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N and 63° 2�30� W. Laguna Grande is a natural lakeformed by the confluence of two rivers: the Juanico(north arm) and the Manteco (south arm). After theycome together, they form the main basin of the lake(Figure 1). The north arm of Laguna Grande usually

carries more water than south arm. Water currents inthe main basin are slower and mostly wind drivenmaking this basin a depositional area for sedimentcarried into lake. Total annual precipitation in the areais 1456 mm. The precipitation is strongly seasonal

Figure 1. The vegetation of Laguna Grande (Monagas, Venezuela) based on a 1992 aerial photograph.

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with a short dry season running from January to mid-April (Gordon 1996). The onset and end of both thewet and dry seasons can vary from year to year by amonth or more. Annual mean temperature is 26 °C.

Total soil organic matter (percent loss on ignition)was 48% and 58% for the south and north arms, re-

spectively, and 69% for the main basin (Gordon un-published data). Mean soil pH is 4.6 in the Montri-chardia wetlands of the main basin and 5.2 in thenorth and south arms. Mean soil conductivity is 189�mhos/cm in the north arm, 211 �mhos/cm in southarm, and 325 �mhos/cm in the main basin. Soil anly-

Figure 2. Hypothesized changes in total seed density in the seed banks of the north and south arms and main basin of Laguna Grande withand without secondary dispersal of seeds during the rainy season from the north and south arms to the main basin. (Mid-Wet = Middle of thewet season; End-Wet = End of the wet season)

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ses were done using standard methods described inJackson (1982)

Two types of herbaceous wetland cover most ofLaguna Grande (Figure 1): one dominated by speciesof Cyperaceae and Poaceae, and the other by Montri-chardia arborescens (L.) Schott (Gordon 1996). Inthe upper reaches of the north and south arms of thelake, Eichhornia crassipes (Mart.) Solms (water hya-cinth) and Pistia stratiotes L. can become abundant(Gordon 1996). M. arborescens (an emergent herb)covers almost 80% of the wetland area of LagunaGrande. Only wetlands dominated by Montrichardiaarborescens were sampled during this study. M. ar-borescens wetlands have very dense canopies and asparse understory (Gordon 1998a). Sometimes thecanopy is interrupted by an occasional tree, such asa: Mauritia flexuosa L., Hecatostemon guazumaefo-lius (H.B.K.) Sleumer, Homalium racemosum Jacq.,and Annona sp. (Gordon 1998a). Vines can be locallycommon.

Methods

Vegetation, water depth and seed bank sampling

Sampling sites were selected along the north arm,south arm, and in the main basin of Laguna Grandein areas dominated by M. arborescens (Figure 1). Ateach site, three randomly selected plots (10 m × 10m) were laid out. In each of the 10 m × 10 m plots,the vegetation in five random 1 -m2 quadrats wassampled by counting the number of individuals ofeach species in the canopy and understory. In the cen-ter of each quadrat, water depth was also measuredwith a meter stick. The seed bank in each 10 m × 10m plot was sampled by collecting a 10 cm diametersoil core to a depth of 5 cm in each of the five quad-rats used for vegetation sampling. These cores werecombined into one composite soil sample for eachplot. In the laboratory, soil samples were air dried inthe dark for two weeks, in order to facilitate sievingand mixing the samples. Each site was sampled fivetimes: the end of the dry (April-May), rainy (August),and end of the rainy season (October) in 1991, and inthe dry (February) and beginning of the rainy (April-May) season in 1992.

The seedling emergence technique was used to es-timate seed density and species composition of theseed bank (van der Valk and Rosburg 1997). Eachcomposite soil sample was first sieved through a 2

mm mesh to separate it into two fractions: > 2 mm orcoarse fraction composed mainly of plant remains(roots and litter), and < 2 mm or fine fraction. Sub-samples of the fine and coarse fractions were placedin plastics trays (20 cm × 20 cm × 10 cm) containinga layer of sand five cm thick. The subsample in eachtray was kept moist by watering it regularly. Becausethere was more coarse than fine material, we hadthree replicate trays with coarse material and two withfine.

The seedling assay was carried out outdoors inCaracas, Venezuela. Seedling assays after each sam-pling period began a little over two weeks after thesamples were collected. Seedling emergence in eachsample was monitored for three months. Seedlingswere identified, counted, and removed from the traysevery five days. Seedlings that could not be identifiedwere transplanted into in pots and grown until theycould be identified. Seed density per m2 was esti-mated by adding the total number of seeds germinatedin each subsample of the fine and coarse fractions ofeach composite sample and dividing this total by thetotal surface area (0.03925 m2) of 5 cores. During theseedling assays, maximum daily temperature were28.1±3.9 °C; minimum daily temperature 19.5±2.1°C; maximum daily relative humidity 90.7± 6.7%;minimum daily relative humidity 50±9.8%; and glo-bal radiation 219,7 ±20 cal/cm2/day. Plant nomencla-ture is based on Velásquez (1989) for aquatics andSchnee (1984) for all other species.

Statistical tests

Spearman’s coefficient of correlation was used tocompare the degree of similarity between the speciescomposition of seed banks and vegetation of the threesites. To determine if seeds were being moved fromthe arms to the main basin, residual analysis of con-tingency tables (Siegel and Castellan 1995) for themost abundant species in the seed banks and for life-form guilds of species was used to examine seed-bankdensity patterns among the three sites at the end ofthe dry season (April- May 1991), middle of the wetseason (August 1991) and toward the end of the wetseason (October 1991). Chi-square tests and Chi-re-siduals can be used to detect seasonal differenceseven without true site replication because each ger-minating seed is considered a replicate. In otherwords, the sample size is the total number of germi-nating seeds of a species or of the species in a lifeform guild in the seed bank samples, not the total

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number of seeds per square meter. Residual values arethe difference between the observed and expected val-ues divided by the square root of the expected value.A positive residual value that is equal to or higherthan 1.96 (p=0.05) indicates that the seed bank hadhad a significantly higher seed density than expected.A negative residual value that is greater than 1.96 in-dicates that the seed bank has a significantly lowerdensity than expected. The Statistica 6.0 softwarepackage was used for all statistical analyses.

Results

Water depth

Mean water depth in the south arm and main basinrose during the rainy season to a maximum in August1991 and then began to decline (Figure 3). Maximumwater depth in the north arm, however, occurred later,in October 1991. Minimum water depths (0 to 6 cm)were found from February until the end of the dryseason (April-May). During the dry season, althoughthere was often no standing water, the substrate gen-erally remained moist, especially in the main basin.

Figure 3. Mean water depths in the north arm, south arm and main basin of Laguna Grande, Monagas, Venezuela.

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Vegetation composition

Altogether, 53 species were found in the vegetation,of which 36, 34 and 33 were found, respectively, atnorth arm, south arm, and main basin (Table 1). Sea-sonally, total species richness was lowest when waterlevels were highest (August and October) in both thecanopy and understory (Table 2).

Besides M. arborescens, abundant as both adultsand seedlings (Table 1), other species that appearedin the canopy in all sites were mostly vines (Asclepiassp., Mikania cordifolia (L.) Wild., Vitis caribaea L.,Sarcostemma clausum (Jacq.) Roem. & Schult., Pas-siflora pulchella H.B.K., Cuphea melvilla Lindl.), pe-

rennial herbaceous species (Hymenachne amplexicau-lis Rudge, Leersia hexandra Swartz, Sacciolepisstriata (L.) Nash, Thalia geniculata L.), andPolygonum acuminatum H.B.K. In the understory,common species included E. crassipes, P. stratiotes,Salvinia auriculata Aubl., Osmunda cinnamomea L.and Mauritia flexuosa.

There was little change in the seasonal composi-tion of the vegetation at any of the sites (data not in-cluded). Overall, the two most similar sites were thenorth and south arm sites (Table 3) with a Spearman’scorrelation of 0.88 for canopy species composition.The least similar sites based on canopy compositionwere the south arm and main basin (0.54).

Table 1. Mean relative density (%) of the most common species in the canopy and understory vegetation at the north arm, south arm, andmain basin sites in Laguna Grande. The number of species in each life-form guild is also presented as is the total number of species foundin the canopy and understory layers at each site.

Canopy Understory

Species/Life Form Life Form1 North Arm Main Basin South Arm North Arm Main Basin South Arm

Species

Asclepias sp. V <1 1 1 – – –

Cuphea melvilla V <1 1 2 – – –

Eichhornia crassipes H – – – 1 8 6

Hamelia patens V 2 8 2 1 1 4

Hecatostemon guazumaefolius T <1 1 <1 2 4 2

Hymenachne amplexicaulis PH 2 2 2 – – –

Leersia hexandra PH 1 1 2 – – –

Mauritia flexuosa T – – – 3 1 1

Mikania cordifolia V 2 1 6 5 2 8

Montrichardia arborescens PH 72 72 62 42 57 46

Osmunda cinnamonea PH – – – 2 <1 1

Passiflora pulchella V 3 2 3 3 4 3

Pistia stratiotes H – – – 22 7 6

Polygonum acuminatum S 1 3 2 – – –

Sacciolepis striata PH 2 3 2 – – –

Salvinia auriculata H – – – 6 1 6

Sarcostemma claussum V 4 2 4 – – –

Thalia geniculata PH <1 <1 1 – – –

Vitis caribaea V 1 1 5 3 3 4

Life Form

Annual herbs 3 3 2 1 – 1 –

Hydrophytes 4 – – – 3 4 3

Perennial herbs 19 9 7 11 5 2 4

Shrubs 1 1 1 – – 1 –

Semishrubs 8 5 3 2 1 3 4

Trees 6 2 2 – 3 4 2

Vines 12 8 10 8 5 6 6

Species richness 53 27 21 23 18 21 19

1Life forms: V – Vine; T – Tree; PH – Perennial herb; H – Hydrophytes; S – Semishrubs.

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Seed banks

A total of 61 species were found in the seed banks.As in the vegetation, the fewest species were foundat each site during the rainy season (August) and themost during the dry season. Seeds of Cyperus odora-tus L., Eleocharis interstincta (Vahl.) R.&S., Ludwi-gia hyssopifolia (G. Don.), Ludwigia lithospermifolia(Mich.) Hara, Mikania cordifolia, Polygonum acumi-natum and Sacciolepis striata accounted for about80% of the total seeds in the seed banks. Overall, seedbanks in the north and south arms (Table 3) were mostsimilar to seed banks in the main basin (Spearman’scorrelations 0.77 and 0.69, respectively).

Seeds of the most common species in the canopyand understory (H. amplexicaulis (perennial herb); H.guazumaefolius (tree); the vines Asclepias sp., P. pul-chella, S. clausum, C. melvilla, V. caribaea; and hy-drophytes E. crassipes and P. stratiotes (Table 1))

were found in low densities in the seed bank (Ta-ble 4). M. arborescens seed was recorded only onceat a low density (17 seeds/m2 in the main basin). Onthe other hand, Eleocharis interstincta, Cyperus odo-ratus, Ludwigia lithospermifolia, and Ludwigia octo-valvis (Jacq.) Raven were abundant in the seed bank,but found infrequently in the vegetation (Table 1).Low densities (13–178 seeds/m2) of seeds of severaltree species (Annona sp., Cassia alata L. and Cecro-pia peltata L.) that do not grow in Laguna Grandewere also found in its seed banks. Spearman’s corre-lations between canopy and understory vegetationcomposition and seed bank composition at a givensite ranged from 0.02 to 0.39 and are generally be-tween 0.21 and 0.26 (Table 3).

Mean overall seed density was 5,400 seeds/m2

with a mean of 6,500, 3,800 and 6,000 seeds/m2 inthe north arm, south arm and main basin, respectively(Table 4). Mean overall seed densities were highest

Table 2. Total number of species found in each guild in the canopy and understory vegetation and in the seed bank of the north arm, southarm and main basin of Laguna Grande.

North Arm Main Basin South Arm

Guild April Aug. Oct. Feb. April April Aug. Oct. Feb. April April Aug. Oct. Feb. April

May May May May May May

1991 1991 1991 1992 1992 1991 1991 1991 1992 1992 1991 1991 1991 1992 1992

CanopyAnnual herbs 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0

Perennial herbs 4 2 1 4 4 3 2 3 4 5 8 3 3 3 5

Semishrubs 1 1 0 1 1 0 0 0 1 1 0 0 0 0 1

Trees 1 0 0 0 1 0 0 1 1 0 0 0 0 0 0

Vines 3 2 4 7 9 4 4 1 6 7 5 5 3 4 7

Total 9 5 5 12 16 7 6 5 12 14 13 8 6 7 13

UnderstoryAnnual herbs 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0

Perennial herbs 3 2 1 3 2 1 1 1 1 2 2 0 1 3 1

Semishrubs 0 0 0 1 0 1 0 0 1 3 0 0 0 2 2

Trees 1 0 0 1 2 1 0 0 2 5 1 0 0 1 2

Vines 0 0 0 3 4 0 0 0 3 5 1 0 1 4 6

Hydrophytes 2 1 0 2 0 0 1 4 2 0 0 0 3 2 0

Total 6 3 1 11 8 3 2 5 9 15 5 0 5 12 11

Seed bankAnnual herbs 2 0 1 4 1 1 2 2 3 3 2 0 2 4 2

Perennial herbs 10 5 5 12 12 13 6 9 9 15 8 4 8 9 5

Semishrubs 6 2 3 5 7 7 5 6 4 6 6 3 6 4 5

Trees 0 0 1 0 1 0 0 0 0 1 1 0 0 2 0

Vines 1 1 2 2 3 2 1 3 3 3 4 2 3 3 2

Hydrophytes 1 1 0 3 3 1 1 2 2 2 1 0 1 1 1

Total 20 9 12 26 27 24 15 22 21 30 22 9 20 23 15

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(8,500 seeds/m2) at the end of the 1991 dry season(April- May 1991), were lowest (2,800 seeds/m2) dur-ing the 1991 rainy season (August and October 1991),and increased (5,600 seeds/m2) during the 1992 dryseason (February 1992). The largest decrease in seeddensities in 1991 between the dry (April-May) andrainy (August) seasons occurred in the north andsouth arms (7,500 seeds/m2) versus 5,500 seeds/m2

in the main basin. In the main basin, however, seeddensities during the rainy season in 1991 actually in-creased by 2,500 seeds/m2 from August to October1991 while they decreased or remained constant inthe south and north arms, respectively (Table 4).

Residual analyses of the seed density data in April-May 1991, August 1991, and October 1991 in thenorth arm, south arm, and main basin for the mostabundant species are summarized in Table 5. Of the 8species for which there is adequate data for this anal-ysis, seed densities of one (Eleocharis interstincta)were significantly higher than expected in the mainbasin in August and October. Seed densities of fourmore species (Cyperus odoratus, Ludwigia hyssopi-folia, Mikania cordifolia, and Sacciolepis striata,)were significantly higher than expected in the mainbasin in October. For all five species seed densities atleast one arm were significantly lower than expectedin October. Residual analysis of the total seed densi-ties of various life forms guilds (Table 5) indicatethat, with the exception of hydrophytes, seed densi-ties of all life forms were significantly higher thanexpected in the main basin during the rainy season inOctober. Again, seed densities in one or both arms in

October were significantly lower than expected, ex-cept for hydrophytes.

Discussion

Overall, the seed banks of Laguna Grande resemblethose of other wetlands. There is only a modest re-semblance between the composition of the seed banksand the vegetation (Leck 1989). As in many wetlands,seed of the dominant species in the vegetation isnearly absent from the seed banks (Leck 1989). Dif-ferences in the sampling design, environmental con-ditions to which samples are exposed, frequency ofseedling removal, and duration of a study can all af-fect the results of seed bank studies (Gerritsen andGreening 1989; Ball and Miller 1989; Gross 1990;Benoit et al. 1992; Brown 1992). Our estimates of thetotal density of seeds (1,100 to 13,100 seeds/m2) inthe seed banks of Laguna Grande are comparable tothose reported in other wetland studies (Leck 1989).For example, 1,100–3,100 seeds/m2 were reported byMiddleton et al. (1991) for the seed bank in a wet-land dominated by Paspalum distichum in the India.Baldwin et al. (1996) reported 2,400–3,500 seeds/m2

in oligohaline coastal mashes in North America. Inprairie glacial marshes in Iowa; 2,200–7,300seeds/m2 were found by van der Valk and Davis(1978). From 8,000 to 15,400 seeds/m2 were reportedby Finlayson et al. (1990) for tropical floodplain wet-lands in Northern Australia.

Table 3. Spearman’s correlation coefficient comparing the composition of the seed bank, canopy vegetation and understory vegetation at thenorth arm, south arm and main basin sites in Laguna Grande.

Seed bank Canopy Understory

North Arm Main Basin South Arm North Arm Main Basin South Arm North Arm Main Basin South Arm

Seed BankNorth Arm 1 0.77 0.57 0.26 0.51 0.16 0.22 0.22 0.36

Main Basin 1 0.69 0.05 0.39 0.09 0.04 0.21 0.02

South Arm 1 0.14 0.46 0.26 0.45 0.21 0.02

CanopyNorth Arm 1 0.72 0.88 0.54 0.65 0.61

Main Basin 1 0.54 0.26 0.41 0.48

South Arm 1 0.62 0.55 0.76

UnderstoryNorth Arm 1 0.57 0.53

Main Basin 1 0.58

South Arm 1

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Because of primary seed dispersal, densities ofseeds in the seed banks of al three sites were highestduring the 1991 and 1992 dry seasons. During the1991 rainy season, the seed banks in either the northor south arms or both lost significant quantities ofseed from August to October. Although the seed den-sity of the main basin seed bank was lower during therainy season, it increased from August to October.Our results are consistent with our hypothesis thatduring the rainy season seeds are being removed fromthe seed banks of the north and south arms and arebeing re-deposited in the seed bank of the main basin(Figure 2). Seed densities of all species guilds, excepthydrophytes, and five out of the eight species whose

seeds were most abundant showed this pattern. Dur-ing our study, no changes in the vegetation occurredthat would alter the composition of the seed banks.Likewise, field observations confirmed that seed pro-duction and dispersal were over by the end of the dryseason in 1991, and the increase in seed densities inthe main basin was not due to seed production duringthe rainy season. In Laguna Grande and probablyother wetlands, autochthonous secondary dispersal isto some extent responsible for the lack of congruencebetween the composition of seed banks and the veg-etation at a given wetland site. There is, however, astriking similarity in the composition of the seedbanks in the north and south arm and in the main ba-

Table 4. Mean seed density (seeds/m2) of the most abundant species in the seed bank and mean seed density for various guilds of species ateach sampling period at the north arm, south arm, and main basin sites in Laguna Grande.

Species/Guild North Arm Main Basin South Arm

April Aug. Oct. Feb. April April Aug. Oct. Feb. April April Aug. Oct. Feb. April

May May May May May May

1991 1991 1991 1992 1992 1991 1991 1991 1992 1992 1991 1991 1991 1992 1992

Asclepias sp. 0 0 0 0 0 0 0 0 0 94 0 0 0 0 0

Cuphea melvilla 0 0 0 8 0 0 0 0 0 0 25 25 6 0 0

Cyperus odoratus 2097 76 344 76 399 318 76 569 0 502 458 0 0 8 212

Eichhornia crassipes 0 0 0 178 8 0 0 0 0 51 0 0 0 0 0

Eleocharis interstincta 2139 509 13 666 1044 318 866 755 900 1250 751 76 25 1070 373

Hamelia patens 0 0 0 0 0 0 0 34 25 0 0 0 0 8 0

Hecatostemon guazumaefolius 0 0 13 0 0 0 0 0 0 0 0 0 0 0 0

Hymenachne amplexicaulis 0 255 0 8 8 25 0 0 0 34 0 0 76 34 0

Leersia hexandra 51 25 0 68 110 0 76 110 0 68 13 127 32 34 8

Ludwigia hyssopifolia 5713 0 153 945 127 5106 153 645 42 1131 382 0 64 136 340

Ludwigia lithospermifolia 679 560 267 815 238 1159 968 985 136 2253 471 25 51 526 42

Ludwigia octovalvis 102 0 13 0 25 0 0 153 0 502 76 0 45 0 0

Mikania cordifolia 424 0 1120 2082 119 267 76 1536 272 186 2406 433 255 985 1851

Montrichardia arborescens 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0

Passiflora pulchella 0 0 13 0 17 0 0 0 0 0 0 0 0 0 17

Pistia stratiotes 8 25 0 119 42 51 51 0 42 60 0 0 0 17 8

Polygonum acuminatum 76 25 64 978 297 968 204 315 178 332 407 789 204 373 416

Sacciolepis striata 705 409 25 1369 530 229 333 367 395 482 449 613 211 2338 416

Sarcostemma clausum 0 0 0 0 0 13 0 0 8 0 25 0 57 0 0

Thalia geniculata 0 0 0 0 17 13 0 0 0 0 0 0 6 0 0

Vitis caribaea 0 102 0 0 0 0 0 17 0 17 51 0 0 17 0

GuildsAnnual herbs 4968 0 153 1046 127 4106 280 687 127 1191 548 0 96 220 348

Perennial herbs 5576 1274 432 2414 2720 2247 1427 2190 2124 2916 1874 867 394 3584 1017

Semishrubs 2170 585 344 1835 669 2432 1757 1572 458 3214 1157 865 332 984 655

Trees 0 0 13 0 178 0 0 0 0 43 13 0 0 153 0

Vines 424 102 1133 2090 178 280 76 1587 305 297 2507 458 318 1010 1868

Hydrophytes 8 25 0 305 101 51 51 25 127 111 13 0 6 17 8

Mean total density 13146 1986 2075 7690 3973 9116 3591 6061 3141 7772 6112 2190 1146 5968 3896

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Table 5. Chi-square residuals of mean seed densities of the most common species and all species guilds in the seed banks in April-May(1991) August (1991) and October (1991). Values less than 1.96 are not significant (p=0.05). Significant values are given in bold. A negativenumber indicates fewer seeds were found than expected on a given date. A positive number indicates that more seeds were found thanexpected on a given date.

Species/Life Form Site Sampling Date

April-May 1991 (Dry Season) Aug. 1991 (Mid-Wet1) Oct. 1991 (End-Wet1)

Cyperus odoratus North arm 1.20 −0.43 −1.97South Arm 1.34 −0.83 −2.04Main Basin −2.87 1.26 9.81

Eleocharis interstincta North Arm 2.87 −1.48 −3.77South Arm 2.21 −1.98 −1.76Main Basin −4.83 3.05 5.58

Leersia hexandra North Arm 2.36 −0.46 −0.99South Arm −0.49 0.76 −0.64Main Basin −1.04 −0.44 1.25

Ludwigia hyssopifolia North Arm 0.91 −1.70 −2.56South Arm −0.26 −0.47 1.14Main Basin −0.85 1.84 2.26

Ludwigia lithospermifolia North Arm 0.044 1.00 −1.15South Arm 2.87 −2.16 −1.47Main Basin −1.23 0.21 1.41

Mikania cordifolia North Arm −2.27 −2.17 3.25South Arm 4.84 2.49 −6.01Main Basin −4.15 −1.15 4.76

Polygonum acuminatum North Arm −0.065 −0.78 1.13South Arm −1.98 2.95 −0.77Main Basin 1.95 −2.60 0.36

Sacciolepis striata North Arm 1.91 −0.66 −2.15South Arm −0.38 1.20 −1.38Main Basin −1.69 −0.59 3.92

Annuals North Arm 1.24 −2.28 −2.72South Arm −0.20 −0.81 1.07Main Basin −1.17 2.58 2.36

Perennial herbs North Arm 3.73 −1.60 −4.95South Arm 0.012 1.36 −1.54Main Basin −4.18 0.78 6.63

Semishrubs North Arm 2.87 −2.00 −2.20South Arm −0.30 −1.46 −1.28Main Basin −1.92 0.53 2.43

Vines North Arm −2.50 −0.82 2.94South Arm 4.94 1.76 −5.88Main Basin −4.12 −1.53 4.94

Hydrohytes North Arm −0.29 0.58 −0.57South Arm 0.38 0.56 0.30Main Basin 0.00 −0.079 0.13

Total seed density North Arm 4.58 −3.49 −4.81South Arm 0.54 3.83 −3.54Main basin −4.77 1.34 7.12

1* Mid-Wet = Middle of the wet season; End-Wet = End of the wet season.

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sin, especially between the seed bank of the northarm, which conveys more water than the south arm,and that of the main basin.

Interestingly, seeds of several tree species (Annonasp., Cassia alata L. and Cecropia peltata L.) werefound in the Laguna Grande seed banks, even thoughthese species were not found in its vegetation. In Ven-ezuela, these tree species are found in seasonallyflooded evergreen forest (Gordon 1996). Such ever-green forests are found upstream from LagunaGrande along both rivers that flow into the lake. Al-lochthonous secondary dispersal, although it is ordersof magnitude less important than autochthonous sec-ondary dispersal, also occurred at Laguna Grande.

As noted, hydrophytes were the only guild ofplants whose seeds did not show any evidence ofsecondary dispersal from the north or south arms tothe main basin. Few hydrophyte seeds (8–305seeds/m2) were found in the seed banks, mostly dur-ing the dry season. The low number of hydrophyteseeds could be due to many factors. It is most likely,however, simply a procedural artifact. The seed bankassay did not provide appropriate conditions for theirgermination. It had no flooded treatment (Leck andGavelline 1979; Welling et al. 1988; Finlayson et al.1990).

As is reflected in seasonal changes in seed bankdensities, the primary dispersal of these seeds occursduring the dry season when there is no or very littlestanding water in the Montrichardia arborescens wet-lands. Most of the wetland species growing in LagunaGrande have small, light seeds (Gordon 1998b) thatinitially float (Gordon 1984). Studies of how longwetland seeds float, including unpublished studies ofseeds of Venezuelan wetland species by the seniorauthor, have indicated that it varies considerablyamong species. Seeds of some species sink immedi-ately and some can float for months. For seeds andfruits without specific adaptations for floating, theytypically float for only two or three days before be-coming waterlogged and sinking (Sculthorpe 1967).Most of these studies, including the study done by thesenior author, however, were done under laboratoryconditions with seeds simply left to float in a smallcontainer of water for some time. These are very un-realistic conditions, especially in tropical wetlandsduring the rainy season. One of the few studies (Stan-iforth and Cavers 1976) in which containers were ag-itated to simulate currents demonstrated that the moreseeds are agitated by water currents, the quicker theysink. In their study, in agitated treatments, all seeds

had sunk after about three days. When these seedswere not agitated, many were still floating after 6months. We have no data on how long the seeds ofspecies in the Laguna Grande wetlands floated afterthey were initially dispersed. Under conditions dur-ing the rainy season when heavy rainfall events arecommon, it is unlikely that seeds remain afloat formore than a few days or weeks at most. During ourstudy, floating seeds were never encountered duringrainy season sampling. In Laguna Grande becauseseed bank samples were collected in standing waterduring the rainy season in 1991, all seeds had to havebecome waterlogged and sunk before these sampleswere collected, which was many months after theirprimary dispersal during the dry season.

Seeds undoubtedly began to be carried by watercurrents from the arms of Laguna Grande into themain basin as soon as water began to flow again atthe start of the 1991 rainy season. It is likely, how-ever, that autochthonous secondary dispersal is an ep-isodic event. Seeds may initially float some distancebefore they become waterlogged and sink and aresubsequently moved from seed bank to seed bankduring periods of high water velocity. Because seeddensities in the main basin for most species and guildswere higher in October than August, our data suggestthat waterlogged seeds in Laguna Grande continuedto be transported throughout the rainy season. Seedsof many species and guilds with different kinds ofseeds simultaneously increased significantly in den-sity in the seed bank of the main basin in October1991. This suggests the mass movement of seeds intothe main basin by one or more major storm events.

Cook (1987) noted, “sooner or later . . .floating di-aspores will sink but their subsequent fate has veryrarely been studied.” Future studies of secondary dis-persal of wetland seeds need to determine the lengthof time required for seeds to become waterlogged un-der field conditions and the water velocities neededto move waterlogged seeds from one seed bank toanother. What are the densities of waterlogged wet-land seeds? Are only seeds that are on the surface ofthe sediment moving? Do seeds move as individualsor as components of organic sediments that arescoured from the bottom of a wetland by high velo-city water currents?

Seasonal seed losses in the seed banks in LagunaGrande were over 80% in the north and south armand, in spite of the influx of seeds, were still about60% in the main basin. Between October 1981 andFebruary 1992, almost 50% more of the seeds in the

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seed bank of the main basin were lost. Because therewere no changes in the composition of the vegetationduring this study, these seeds losses were not due togermination. Because there are no comparable data onseasonal changes in wetland seed banks for otherkinds of tropical wetlands, it is impossible to know ifsuch marked seasonal losses are typical. Our resultssuggest that seeds in the seed banks of LagunaGrande do not persist as long as those found in tem-perate wetlands which can survive for years or evendecades (van der Valk and Davis 1978). What causesthese large seasonal losses is unknown. It is likely thatmost tropical wetland species have short-lived seedcompared to those in temperate wetlands. Seed pre-dation pressures may also be higher in tropical wet-lands compared to those in temperate wetlands. Thefinal fate of seeds in the seed banks of Laguna Grandeand other tropical wetlands also requires furtherstudy.

Acknowledgements

Thanks are due to Consejo de Desarrollo Científico yHumanístico de la Universidad Central de Venezuelafor support of the Laguna Grande Project, MonagasState and to Dr Luis Bulla of the Universidad Centralde Venezuela and Dr Phil Dixon of Iowa State Uni-versity for their help with the statistical analysis ofthe data.

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