Factors driving seed dispersal in a Neotropical river-floodplain system

ABSTRACT

Dispersal is a key process affecting the diversity of natural communities. We addressed hydrochory of wetland plant seeds in the Middle Paraná River floodplain. We first studied seed dispersal by drifting macrophytes in the Paraná River main channel (MC), in a high discharge secondary channel (HD) and in two low discharge channels (LD) during an extraordinary flood. We then experimentally analyzed the effect of standing (SW) and moving water (MW) on seed buoyancy of different plant communities. We recorded seeds of 27 taxa distributed in 12 families. Taxa richness ranged from 17 in LD to 25 in MC, and included seeds of terrestrial, palustrine and aquatic plants. River discharge did not affect seed richness and density, which was probably associated with a homogenization process caused by the flood. Seed buoyancy significantly differed between water movement treatments independently of the source community, lasting longer in SW than in MW. Our results suggest that drifting macrophytes contribute to seed dispersal of several communities in the Middle Paraná River, and probably over long distances. Furthermore, seed buoyancy might be more important for surficial dispersal in low-energy systems, where subaqueous dispersal may be difficult due to the lack of current.

Keywords:
dispersal; drifting macrophytes; hydrochory; Middle Paraná River; propagules; river discharge; water movement

Introduction

Dispersal is regarded as one of the key processes affecting the richness and composition of natural communities (Levine & Murrel 2003). Hydrochory, i.e. seed dispersal by water, is important for the transportation and deposit of freshly produced seeds (Boedeltje et al. 2003; Levine & Murrel 2003; Sarneel 2013), structuring wetland plant communities (Nilsson et al. 1991; Andersson et al. 2000; Groves et al. 2009) and maintaining high landscape diversity (Junk et al.1989; Thomaz et al. 2007). By dispersing seeds at some distance from their source community, hydrochory may extend the species dispersal period into seasons different from the vegetative ones (Boedeltje et al. 2004) and may be important for ecological and genetic continuity among disjunct populations (Andersson et al. 2000). This may be particularly important in floodplain systems, where aquatic habitats can remain isolated during long drought periods, and where the dependence on seed banks for recolonization and plant communities structuring may be high (Junk et al. 1989; Neiff 1990).

Seed buoyancy may be influenced by the hydrological features of the habitats where they are released (Hyslop & Trowsdale 2012). For instance, some seeds that are able to float for long periods in standing water may be drowned by waves in a fast-flowing river and sunk immediately (Sculthorpe 1967). Nevertheless, most studies addressing seed buoyancy, evaluated this response variable only under standing water (Coops & Velde 1995; Danvind & Nilsson 1997), which may be not representative of what may happen in nature, where climatic, morphologic and hydrographic conditions make the hydrology of each waterbody unique (Lampert & Sommer 1997).

Several studies have looked at the effects of seed buoyancy on species distribution, with varying results (Hyslop & Trowsdale 2012). Studies in temperate regions found a relation between the seeds floating ability and the position of the parental plants communities (hereafter source community) along the hydrological gradient, suggesting that seed buoyancy probably plays a role in directing seeds towards suitable sites for germination and establishment (Coops & Velde 1995; Ozinga et al. 2004; Broek et al. 2005). Nevertheless, other studies have suggested that seed floating ability is not related to source community and, therefore, it is not important for plant distribution patterns (Danvind & Nilsson 1997).

In river-floodplains, drifting macrophytes may represent an alternative means of transport for seed dispersal (Tur 1972; Lallana 1990). This special form of hydrochory (hereafter phytohydrochory, sensuLallana 1990), where drifting plants disperse seeds, may be particularly important in river-floodplain systems (Lallana 1990) where floods commonly produce the detachment and displacements of macrophyte stands that drift from the floodplain towards the main channel (Tur 1972; Neiff 1990; Junk & Piedade 1997; Sabattini & Lallana 2007).

Seed dispersal by drifting macrophytes is likely to be related to channel discharge. At low discharge, when velocity and turbulence is lower, roughness factors such as drifting vegetation, are likely to be very important in trapping seeds. Nevertheless, at higher discharge levels, it is more likely that turbulence and waves prevent seeds adherence to vegetation forcing them onto riverbanks (Andersson et al. 2000).

Dispersal by drifting plants might be important in river-floodplain systems. Although several studies have addressed phytohydrochory by animals (e.g., Neiff & Zozaya 1989; Schiesari et al. 2003; Bulla et al. 2011), the role macrophytes play as seed dispersers in a Neotropical river-floodplain has received little attention (Lallana 1990).

Aquatic macrophytes play important roles in floodplain ecosystems in terms of biomass production, habitat structuring and provision of habitat, refuge, and food for other organisms (Jeppesen et al. 1998; Chambers et al. 2008; Wood et al. 2017). Regardless of their value, freshwater macrophytes are globally threatened, what represents a risk to the conservation of both aquatic plants and the ecosystems where they are found (Chambers et al. 2008; O’Hare et al. 2017; Zhang et al. 2017). Increasing knowledge about the patterns of seed dispersal by drifting macrophytes in river-floodplain systems can improve the understanding on how these landscapes are colonized by plants and how plant communities are organized (Nilsson et al. 2010).

We aimed to understand the potential role of drifting macrophytes as dispersal agents of plant seeds along rivers with different discharge in the Middle Paraná River floodplain and to experimentally examine seed buoyancy from plants of different communities under different water movement conditions. We predicted that (1) the richness and density of seeds dispersed by drifting macrophytes is negatively related to river order, and that, (2) seed buoyancy of plants of different source communities differs among water movement conditions.

Materials and methods

To achieve the first objective (evaluate the phytohydrochory along rivers with different discharge in the Middle Paraná River floodplain) we performed a field study, and to achieve the second objective (analyze seed buoyancy from different source communities in different water movement conditions) we performed a microcosm experiment.

Field sampling and seed processing

Phytohydrochory

To evaluate the dispersal of seeds by phytohydrochory in channels with different flow conditions, samples of drifting stands of macrophytes were manually collected in rivers with three different discharge levels: in the main channel of the Paraná River (MC, n = five), in a high discharge secondary channel named Colastiné River (HD, n = six) and in two low discharge channels named Tiradero and Miní rivers (LD, n = 3, including both rivers) (31°41'48.45"S 60°37'37.79"W and 31°38'20.14"S 60°30'12.63"W). Mean annual discharges were 17.000 m³ s⁻¹ in MC, 2.200 m³ s⁻¹ in HD and 500 m³ s⁻¹ in LD (Paira & Drago 2007). During the sampling period, the hydrometric level of the Paraná River reached 14.20 m a.s.l. (Paraná Harbor staff gauge, August 2014), corresponding to an extraordinary flooding level. Field samples of macrophytes were randomly collected from a boat with nets of 0.07 m² area and 200 µm mesh size. Due to differences in plant size and stand composition, some samples consisted in a group of plants (e.g., several individuals of Salvinia spp, and Pistia stratiotes L.), and other samples were composed by only one plant [e.g., adult individual of Eichhornia crassipes (Mart.) Solms]. Nevertheless, all samples covered the same surface area (~ 0.07 m²).

In the laboratory, each sample was washed with tap water to detach all the seeds and detritus. Plant biomass was oven dried at 60 °C up to constant weight for 72 h and weighed. Seeds were separated and counted under stereoscopic microscope (C-W 10X A/22, Nikon), and identified to the lowest taxonomic level possible following Lima et al. (2018) and others (List S1 in supplementary material) and the collection of seeds available at the Instituto Nacional de Limnología (INALI-CONICET-UNL). The species nomenclature followed the standards of The International Plant Names Index (www.ipni.org).

Although the dispersal unit of several plants is a seed covered by adhering fruit structures (e.g., Polygonaceae, Cyperaceae, Poaceae), for the sake of simplicity, whenever we use the term seed is to represent all reproductive dispersal units.

Experimental design

Seed buoyancy

Seeds of 19 taxa were randomly collected from macrophyte stands before natural dispersal occurred (Tab. 1). The collection was conducted during March and April 2015 in wild populations of the Middle Paraná River floodplain.

Table 1
List of the seed taxa registered in the drifting macrophytes and used in the buoyancy experience. Source community, propagule type and main dispersal mechanisms are indicated. Abbreviations refer to phytohydrochory (P), buoyancy experience (B), anemochory (A), hydrochory (H), endozoochory (EN), epizoochory (EP) and vegetative propagation (V)..

Family	Taxa	Source community	Propagule type	Main dispersal mechanism	This study		Bibliography used to assign the main dispersal mechanism
					P	B
Araceae	Pistia stratiotes L.	Aquatic	Seed	V/H	X		Pijl 1982; Neuenschwander et al. 2009
Asteraceae	Chromolaena squarrosoramosa (Hieron.) R.M.King & H. Rob	Terrestrial	Seed	A		X	Muniappan et al. 2005
	Asteraceae sp.		Seed	A/EN/EP	X		Pijl 1982; Arbo et al. 2001
	Bidens subalternans DC.	Terrestrial	Seed	H/EP		X	Pijl 1982; Calderón et al. 2000
	Bidens laevis (L.) Britton, Stern and Poggenb.	Terrestrial	Seed	H/EP	X	X	Pijl 1982; Calderón et al. 2000
	Soliva anthemifolia (Juss.) Sweet	Terrestrial	Fruit		X
	Eclipta prostrata (L.) L	Terrestrial	Fruit		X
	Gymnocoronis spilanthoides (D. Don en Hook. Y Arn.)	Terrestrial	Fruit		X
	Enydra anagallis Gardner	Palustrine	Fruit		X
	Melanthera latifolia (Gardner) Cabrera	Terrestrial	Fruit		X
	Aspilia silphioides (Hook. & Arn.) Benth. ex Beker	Terrestrial	Fruit		X
	Ambrosia elatior L.	Terrestrial	Fruit		X
Cyperaceae	Cyperaceae 1		Fruit			X
	Cyperus sp.	Terrestrial	Fruit	H/A	X		Arbo et al. 2001
	Schoenoplectus californicus (C.A.Mey.) Soják.	Palustrine	Fruit	H/A/EP		X	Macía & Balslev 2000
	Cyperus virens Michx. virens	Palustrine	Fruit	H		X	Arbo et al. 2001
Fabaceae	Mimosa pigra L. pigra	Terrestrial	Seed	H		X	Hurrell 2002
	Sesbania virgata (Cav.) Pers.	Terrestrial	Seed	H	X	X	Hurrell 2002
Iridaceae	Sisyrinchium sp.	Terrestrial	Seed		X
Lentibulariaceae	Utricularia foliosa L.	Aquatic	Fruit	V		X	Pijl 1982
Onagraceae	Ludwigia leptocarpa (Nutt.) H. Hara	Palustrine	Seed	H/A	X	X	Calderón et al. 2000
Poacea	Poacea sp.		Fruit		X
	Setaria parviflora (Poir.) Kerguélen	Terrestrial	Fruit		X	X
	Eriochloa punctata (L.) Desv. ex Ham.	Terrestrial	Fruit			X
Polygonaceae	Polygonum acuminatum Kunth	Palustrine	Fruit	H			Pijl 1982
	Polygonum punctatum Elliott	Palustrine	Fruit	H	X	X	Pijl 1982
	Polygonum sp.	Palustrine	Fruit	H	X		Pijl 1982
	Polygonum hydropiperoides Michx.	Palustrine	Fruit	H	X	X	Pijl 1982
	Rumex sp.	Terrestrial	Fruit	H	X		Pijl 1982; Arbo et al. 2001
Pontederiaceae	Eichhornia azurea (Sw.) Kunth	Aquatic	Seed	H/V		X	Sculthorpe 1967; Cronk & Fennessy 2001
	Eichhornia crassipes (Mart.) Solms	Aquatic	Seed	H/V		X	Sculthorpe 1967; Pijl 1982; Cronk & Fennessy 2001
Salicaceae	Salix humboldtiana Willd.	Terrestrial	Fruit	H/A	X		Neiff et al. 1985; Niiyama 1990; Budke et al. 2005
Smilacaceae	Smilax campestris Griseb.	Terrestrial	Seed	EN		X	Hurrell 2000
Solanaceae	Solanum glaucophyllum Desf.	Terrestrial	Seed			X
Malvaceae	Sida sp.	Terrestrial	Seed	EP	X		Calderón et al. 2000
	Sida rhombifolia L.	Terrestrial	Seed	EP	X		Calderón et al. 2000
Menyanthaceae	Nymphoides indica (L.) Kuntze	Aquatic	Seed		X
Verbenaceae	Lippia alba (Mill.) N.E. Br. ex Britton & P. Wilson	Terrestrial	Seed		X	X
Undetermined	Morphospecie 1		Seed		X
	Morphospecie 2		Seed		X

The experiment was carried out in plastic cubic containers with sides of 15.5 cm and 9 cm in height. Each container was filled with 1.5 L of dechlorinated tap water. The experimental design consisted of two treatments: i) moving water (MW), containers with an aerator placed on a corner (Coops & Velde 1995) to generate water movement and simulate lotic environment with high discharge conditions; and ii) standing water (SW), containers with quiet water all along the experiment, except when water was carefully added to keep the level constant (see below) simulating water movement of lentic environments. All along the experiment, dechlorinated tap water was regularly added to containers to maintain water quality and to keep the water level constant at 10 cm above the bottom.

Prior to the experiment, empty or damaged seeds were removed, and uniform seeds of each taxon were selected. Seeds of each taxon were randomly assigned to the treatments (MW or SW), each with 30 seeds. For Polygonum punctatum (Elliott) Small, only 28 seeds were used, due to its availability in the field. Each seed group was simultaneously released into the water within the containers of both treatments resulting in a total of 38 containers [19 of each treatment, each container containing 30 (28 for P. punctatum) seeds of only one taxon].

The number of sunken seeds was counted 1 h and 7 h after starting the experience; at the 24^th h from days two to four, every 48 h for the following four weeks and finally, weekly until the end of the experience. With this experimental setup, the floating behaviour of each individual seed could be tracked. The whole experiment lasted for 90 days and was performed indoor, under natural photoperiod conditions (allowed by the presence of transparent surfaces connected with the external environment) and at constant environmental temperature of ~ 25 °C.

Data analyses

All the seeds used (in the field study and in the microcosm experiment) were classified according to their source community into terrestrial, palustrine and aquatic (Tab. 1). Additionally, they were classified as fruits or seeds and when possible, according to their main dispersal mechanism. For this purpose, we conducted a broad bibliographic revision (List S1 in supplementary material).

To compare richness and density of seeds dispersed by drifting macrophytes among rivers we used a one-way analysis of variance (global Kruskal-Wallis tests and Post-hoc pairwise Mann-Whitney´s tests, p ≤ 0.05). Samples were tested for significant linear correlations between seed density and richness; and between seed richness and density with river order (Pearson linear correlations, p ≤ 0.05). Data were log10 transformed.

To evaluate differences among seed buoyancy between treatments (MW and SW), the number of sunken seeds was compared for each taxon after the whole experiment was completed with a Friedman test (p ˂ 0.01).

Additionally, the relation between seed buoyancy, source community, and water movement condition was explored by a two factor ANOVA (p ˂ 0.05).

All the statistical analyses were run in Past Version 2.17c (Hammer et al. 2001).

Results

Phytohydrochory

Although extraordinary flooding occurred, no large macrophyte stands were recorded in our study (~ 0.07 m²). The drifting macrophyte stands had different species composition but were mainly constituted by Eichhornia azurea and E. crassipes (Fig. 1). The total biomass of the sampled macrophytes was 394 g (dry weight). In terms of biomass, E. azurea was dominant in the MC, whereas E. crassipes was dominant in both types of secondary channels (HD and LD). Other species such as Salvinia biloba, Limnobium laevigatum, Ludwigia sp. and Poaceae were recorded in lower proportions.

Figure 1
Composition of the total number of drifting macrophyte stands sampled at the main channel, high discharge and low discharge secondary channels (MC, HD and LD respectively) collected in the Paraná River floodplain during an extraordinary flooding (year 2014).

The mean density of the seeds found in the drifting macrophyte stands was 168 ± 566 seeds/g (seeds per gram of plant biomass), and ranged from 5.7 ± 4.4 seeds/g in the MC and 11.7 ± 12.9 seeds/g in HD to 749.6 ± 1199.5 seeds/g in LD. We recorded a total of 27 taxa of seeds distributed in 12 botanical families (Tab. 1). Taxa richness ranged from 25 in MC and 22 in HD, to 17 in LD. The most abundant families were Poaceae and Cyperaceae (i.e. Cyperus spp.) which accounted for more than 50 % of the seed abundance (Fig. 2). Seeds of Ludwigia leptocarpa and Eclipta prostrata were also common in all the drifting stands. According to the source community, the high proportion of seeds corresponded to terrestrial plants (78 %) followed in decreasing order by palustrine (15 %) and aquatic plants (7 %). These taxa are also dispersed by other mechanisms such us hydrochory, anemochory and zoochory (Tab. 1).

Figure 2
Taxonomic composition of drifting seeds at the main channel, high and low discharge secondary channels (MC, HD and LD respectively) collected in the Paraná River during an extraordinary flooding (year 2014).

Neither the density nor the richness of plant seeds had significant differences among rivers (global Kruskall-Wallis and partial Post-hoc pairwise comparisons Mann-Whitney tests, mostly > 0.01), and no significant correlations were found between these variables and with the river order (Pearson correlation, p > 0.05).

Seed buoyancy

Most taxa showed significant differences in seed buoyancy between SW and MW treatments (Friedman´s test, p < 0.01, Tab. 2, Fig. S1 in supplementary material).

Table 2
Results of Friedman's comparisons performed to assess significant differences of seed buoyancy between moving water (MW) and standing water (ST) treatments. The Friedman test was performed whenever differences between tests could be explored.

Source community	Taxa	p-values
aquatic	Eichhornia azurea	0.001
aquatic	Utricularia foliosa	0.002
aquatic	Eichhornia crassipes	0.007
palustrine	Cyperus virens	0.060
palustrine	Ludwigia leptocarpa	0.003
palustrine	Polygonum acuminatum	0.001
palustrine	Polygonum punctatum	0.001
palustrine	Schoenoplectus californicus	0.001
terrestrial	Bidens laevis	All propagules floated along the whole experiment in SW and MW
terrestrial	Lippia alba
terrestrial	Mimosa pigra	All propagules sank at the 1 h of the experiment in ST and MW
terrestrial	Smilax campestris
terrestrial	Sesbania virgata
terrestrial	Bidens subalternans	0.001
terrestrial	Cyperaceae sp.	0.002
terrestrial	Echinochloa punctata	0.002
terrestrial	Chromolaena squarrosoramosa	0.002
terrestrial	Solanum glaucophyllum	0.003
terrestrial	Setaria parviflora	0.011

The number of total sunken seeds at the end of the experiment was 443 (78 % of the initial amount of seeds) in MW and 213 (37.5 %) in SW. Seeds sank faster in the MW treatment than in the SW one (Fig. 3). In MW, 50 % of seeds sank during the first week of the experience, while in SW the 50 % of seeds sank along the first month of the experience.

Figure 3
Cumulative number of sunken seeds along the buoyancy experience in the moving water (MW) standing water (SW) treatments.

The interaction between source community (terrestrial, palustrine and aquatic) and water movement condition (SW and MW) had no significant effect on buoyancy (two ways ANOVA, F = 0.32; p = 0.73). Partial factor analysis resulted significant for water movement (F = 8.1; p = 0.01) but not for source community (F = 2.5; p = 0.10).

Discussion

Data from our field study indicated that, contrary to our expectation, richness and density of seeds dispersed by phytohydrochory were not related to river order and thus, we could not accept our first prediction. From our experimental study, seed buoyancy was significantly related to water movement condition. Therefore, our second prediction was validated.

Phytohydrochory

Rivers could act as dispersal agents transferring organisms among waterbodies and potentially across long distances (Lallana 1990; Nilsson et al. 1991; Moggridge et al. 2009; Nilsson et al. 2010; Sarneel 2013). In our study, we found that relatively small macrophyte stands (~ 0.07 m²) transported a large amount and diversity of seeds. Most of the dominant drifting plants collected in our study were perennials, and this could favour dispersal at long distances. Indeed, the average current velocities in the Middle Paraná River, range from 0.7 to 1.6 m s^-1 (Orfeo & Stevaux 2002), and under such conditions a drifting macrophyte stand could travel 60.5 - 138.25 km per day, or in other words, could cover the entire length of the Paraná River (2570 km) in 18.6 days.

Seed dispersal by water is well recognized for plants from different ecosystems (Boedeltje et al. 2003; Kaproth & McGraw 2008; Kowarik & Säumel 2008; Sarneel 2010). Our findings broaden the knowledge regarding seed dispersal by waterpaths, demonstrating that the dispersal involving drifting plants (or by phytohydrochory), is also effective for a great diversity of seeds and from different plant communities, such as aquatic, riparian and terrestrial. Furthermore, many of the seeds we found in the drifting macrophytes are commonly dispersed by other mechanisms different from phytohydrochory. For instance, several Asteraceae seeds found are typically anemochorous and Sida rhombifolia seeds are mainly zoochorous. This large number of potential dispersal vectors does not necessarily mean that dispersal is more effective; but it does, at least, indicate that on average these species have more opportunities for long-distance dispersal and are thus less dependent on the availability of a single dispersal vector (Ozinga et al. 2004). This may represent a selective advantage for those species that succeed in spreading their propagules across large parts of the landscape (Levin et al. 1984), especially in ecosystems with frequent disturbances such as river-floodplains (Junk et al. 1989; Neiff 1990).

A significant correlation between seed richness and density was not found. This could be related to the distinct strategies of species to produce offspring. While some species invest in quality and produce seeds that can persist in the soil for long periods, other species invest in quantity and produce large numbers of seeds to increase dispersal probability (Grime 1977). In our study the higher densities of seeds corresponded only to two taxa (Poaceae and Cyperaceae), while the seeds of the remaining taxa showed much lower densities (e.g. P. stratiotes, Enydra anagallis Gardner, Soliva anthemifolia). This could explain the lack of correlation between seed richness and density.

In contrast to results found in other wetlands where the number of seed species was affected by river order and discharge (Nilsson et al. 1994; Boedeltje et al. 2004), our findings indicated that there was no difference in the richness nor in the density of seeds dispersed by phytohydrochory with respect to the river order. Therefore our results do not support our first prediction (seed richness and density are negatively related to river order). Our study was conducted during an extraordinary flooding, and this could have generated a homogenization of the seeds attached to plants in the different rivers, regardless of their discharge. Indeed, floods tend to connect water bodies with distinct hydrological characteristics and, as a result, biological communities tend to be more similar among the distinct habitats within a floodplain. This is in agreement with the generalized hypothesis that floods increase similarity among habitats in river-floodplain systems (Thomaz et al. 2007).

Seed buoyancy

Knowledge about seed buoyancy ability is critical for a better understanding of dispersal patterns and species distribution in river-floodplain systems (Kubitzki & Ziburski 1994; Coops & Velde 1995). In our study, seed buoyancy of most taxa significantly differed between SW and MW conditions. Indeed, the movement of water was a strong factor in determining seed buoyancy independently of their source community.

The fast sink of E. crassipes, E. azurea and Utricularia foliosa seeds in MW conditions, could be related to the fact that the survival of aquatic plant seedlings would be successful only if germination occurs in a flooded site. Thus, many aquatic species produce seeds that sink and are transported by water as bed load towards inundated sites (Soons et al. 2017). Furthermore, in many aquatic and several palustrine plant species, vegetative regeneration and dispersal dominate over generative regeneration and dispersal (e.g., Sculthorpe 1967; Barrat-Segretain 1996). Therefore, considering the great buoyancy ability of vegetative propagules, their flotation time might be more critical than seed buoyancy for dispersal.

Seeds of terrestrial species varied in relation to buoyancy time. Indeed, several palustrine and terrestrial species subjected to frequent floods, produce seeds that float for long time (like Cyperus virens and Lippia alba in our study), so that many will have been stranded in favorable marginal or higher sites (Sculthorpe 1967; Soons et al. 2017). For instance, seeds of Bidens laevis and L. alba remained floating along the whole experiment in both treatments, whereas seeds of Smilax campestris, Mimosa pigra and Sesbania virgata sank at the first hour of the experiment in both treatments. The sudden sinking of S. campestris, M. pigra and S. virgata seeds may have been due to the fact that S. campestris is a climbing plant common in riverbank forests, but its fleshy fruits are dispersed by animals. M. pigra and S. virgata fruits are buoyant and indehiscent, and therefore the fruit is responsible for seed dispersal to suitable sites. Seeds of other terrestrial taxa presented intermediate buoyancy duration. These different species abilities to float on the water surface suggest that hydrochory might be an important process underlying patterns or aquatic and riparian plant zonation (Poiani & Johnson 1989; Grelsson & Nilsson 1991).

Seeds used in this study had different forms, sizes and possibly weights, all metrics that can affect buoyancy ability (Sculthorpe 1967). For instance, seed form varied from oblong (e.g. E. crassipes), to flattened (e.g. B. laevis) and to circular (e.g. U. foliosa) among many others. Although we did not measure seed form and weight, one might expect these characteristics to be related to the different buoyancy abilities found among taxa.

Source community had no significant effect on seed buoyancy. The reason for this may be that most wetland plants have other means of dispersal (Sculthorpe 1967; Danvind & Nilsson 1997). Multiple means of dispersal will certainly complicate the interpretation of patterns resulting from dispersal by water. For instance, anemochores seeds like those of Schoenoplectus californicus (or Chromolaena squarrosoramosa are also able to float well. Conversely, water movement was a significant factor in determining seed buoyancy and seeds of most species floated more time in standing than in moving water. These results suggest that, in the floodplain studied, seed buoyancy may be more important for surficial seed dispersal in low-energy systems, such as lakes or slow-flowing streams, where sub-aqueous dispersal is difficult due to lack of current (Soons et al. 2017). In such waterbodies, the period of buoyancy, from a few hours to a day or more, may be enough to allow the propagules to be carried well away from the competitive source community (Sculthorpe 1967). In rivers, where flow speed and turbulence are higher, non-buoyant or sunken seeds may be transported in suspension by water flow, or be re-suspended during floods, and deposited onto more suitable areas providing an opportunity to germinate and establish (Markwith & Leight 2008; Hyslop & Trowsdale 2012). Indeed, non-buoyant seeds can also be transported by drifting macrophytes as we demonstrate.

Despite some limitations in the extrapolation for our experimental results to natural conditions, our experimental study highlights the importance of using different water movement conditions to evaluate time of seed buoyancy, since in field the hydrologic features of waterbodies vary from standing waters in disconnected lakes, to flowing waters with different speed in channels.

In conclusion, in the Middle Paraná floodplain, drifting macrophytes contribute to seed dispersal of aquatic, palustrine and several terrestrial plants, and probably within large distances. Seed richness and density were not affected by river order, probably related to a homogenization of communities caused by the occurrence of an extraordinary flooding event. We evaluated seed buoyancy simulating the natural water movement conditions of lakes and rivers. However, in natural systems water movement is more complex and may vary spatially and temporally and it is probable that a different situation from the one discussed in this study may arise. Nevertheless, from our experimental study, we can state that the seed buoyancy of wetland plants can differ between habitats with different lotic and lentic conditions. This suggests that seed buoyancy may be more important for seed surficial dispersal in low-energy systems, where subaqueous dispersal may be difficult due to the lack of current.

Acknowledgements

We thank the Associate Editor and two anonymous reviewers whose constructive comments greatly improved the manuscript. This research was financially supported by Prest. BID PICT 2012 Nº 2791, FONCyT, ANPCyT and PIP 318-CONICET, MinCyT, Argentina.

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Publication Dates

• Publication in this collection
  18 July 2019
• Date of issue
  Jul-Sep 2019

History

• Received
  28 Feb 2019
• Accepted
  23 Apr 2019