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Applied Vegetation Science 9: 279-284, 2006© IAVS; Opulus Press Uppsala.

AbstractQuestion: How does seed dispersal reduce fen isolation andcontribute to biodiversity?Location: European and North American fens.Methods: This paper reviews the literature on seed dispersalto fens.Results: Landscape fragmentation may reduce dispersal op-portunities thereby isolating fens and reducing genetic ex-change. Species in fragmented wetlands may have lowerreproductive success, which can lead to biodiversity loss.While fens may have always been relatively isolated fromeach other, they have become increasingly fragmented inmodern times within agricultural and urban landscapes in bothEurope and North America. Dispersal by water, animals andwind has been hampered by changes related to development inlandscapes surrounding fens. Because the seeds of certainspecies are long-lived in the seed bank, frequent episodes ofdispersal are not always necessary to maintain the biodiversityof fens. However, of particular concern to restoration is thatsome dominant species, such as the tussock sedge Carexstricta, may not disperse readily between fens.Conclusions: Knowledge of seed dispersal can be used tomaintain and restore the biodiversity of fens in fragmentedlandscapes. Given that development has fragmented land-scapes and that this situation is not likely to change, thedispersal of seeds might be enhanced by moving hay or cattlefrom fens to damaged sites, or by reestablishing lost hydro-logical connections.

Keywords: Adhesive dispersal; Biodiversity; Cattle; Conser-vation biology; Fragmentation; Invasive plant; Island bioge-ography; Polychory; Seed bank; Zoochory.

Nomenclature: Anon. (2004).

Seed dispersal in fens

Middleton, Beth1*; van Diggelen, Rudy2 & Jensen, Kai3

1USGS National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70506 USA;2Community and Conservation Ecology Group, Biological Sciences, University of Groningen, PO Box 14,

NL-9750 AA Haren The Netherlands; E-mail r.van.diggelen@rug.nl;3Population and Vegetation Ecology Group, Biocentre Klein Flottbek, University Hamburg, Ohnhorststr. 18,

DE-22609 Hamburg, Germany; E-mail kai.jensen@botanik.uni-hamburg.de;*Corresponding author; Fax +1 3372668586, E-mail beth_middleton@usgs.gov

Introduction

Seed dispersal and biodiversity

Seed dispersal underlies the ability of fens to main-tain biodiversity. Dispersing seeds may be able toreplenish individuals in gaps after the death of adultindividuals (Harper 1977) or after the complete extirpa-tion of the species, as in restoration areas (Middleton1999). However, seed dispersal limitations due to frag-mentation may hamper the ability of seeds to dispersebetween natural areas (Galatowitsch & van der Valk1996a). Furthermore, fragmented populations may havea reduced supply of dispersing seeds because somefragmented populations suffer from edge effects, lead-ing to lower reproductive success (Lienert & Fischer2004) or to genetic impoverishment because of areduction of seed and pollen exchange betweenpopulations (Barrett & Kohn 1991; Young et al. 1996).Thus, certain wetland species may become locallyextirpated due to the reproductive problems stemmingfrom habitat isolation (Lienert et al. 2002; Hooftman etal. 2003; Lienert & Fischer 2003). For these reasons,seed dispersal within and between fens may be a keyfeature of the conservation of biodiversity in thesesystems.

Seed dispersal limitation may underlie biodiversitydecline in fens because these wetlands have becomeisolated by agricultural and urban development. Forexample, prior to agricultural development, wetlandswere more interconnected by hydrological dispersal inthe Prairie Pothole Region of North America (Galato-witsch & van der Valk 1996a) and in Europe (Trepel etal. 2003). Grazed wetlands were probably also moreconnected to one another via seed dispersal by cattle inthe past than they are nowadays. Cattle grazing of natu-ral habitats has nearly ceased in some parts of the world(Johansson 1997; Middleton 2002a), so that cattle areless likely to move seeds in modern times than they oncedid (Bruun & Fritzbøger 2002). However, animal move-

280 MIDDLETON, B. ET AL.

ment patterns via the modern cattle trade are complex,with live cattle sometimes moved across continents.Hay mowing machinery can also readily move seedsbetween mown grasslands because the seeds stick to themowers. Hay mowing may have been more effective inthe past in moving seeds between wetlands, because thelarge-scale operations of current day agriculture maynot cut all hay fields at an optimal time for seed dispersal(Strykstra et al. 1997). However, not all studies agreewith the idea that recent agricultural activities limitdispersal and isolate wetlands. Seed longevity and seeddispersal by birds may work to counteract isolation,particularly in temporary wetlands (Brose 2001).

Nevertheless, genetic and experimental ecologicalstudies support the idea that fens are becoming increas-ingly isolated by fragmentation (Hooftman et al. 2003;Lienert & Fischer 2003) and that fragmentation affectsthe performance of fen species (Lienert & Fischer 2004).In endangered populations, inbreeding and genetic driftare common, so that, in general, genetic variability ishigher in larger populations (Ellstrand & Elam 1993).Populations are becoming extinct even in intact rem-nants of (dry) calcareous grasslands in the Swiss Juramountains (Fischer & Stöcklin 1997), and it might alsobe assumed that these processes are active in smallremnants of fen grasslands. However, species havedifferent thresholds of isolation, beyond which geneticloss and extinction may occur because of the inherentdifferences in species abilities to cope with fragmenta-tion (Young et al. 1996; see also Oostermeijer 2003).High levels of genetic diversity were found in small andrecently (30 years ago) fragmented populations ofPedicularis palustris (Schmidt & Jensen 2000), whilereduced genetic variability of Swertia perennis wasfound in small and isolated fens (Lienert et al. 2002).Thus, the effects of fragmentation on genetic variationand on fitness components of fen species differ amongspecies. More research on the role of genetic isolationand fragmentation in the extirpation of specific speciesis essential for managers to develop strategies forbiodiversity conservation.

The persistence of fen species or their regenerationpotential in restored wetlands further depends on seedlongevity in seed banks (Thompson et al. 1997;Middleton 1999, 2002a). For all plant communities, themost common categorization of seed longevity comesfrom Thompson et al. (1997), and groups seed longevitytypes as transient (viability of less than 1 a), short-termpersistent (1-4 a) and long-term persistent (more than 4a), although other classification systems also exist (seeCsontos & Tamás 2003). At least some species in fensof the Prairie Pothole Region of North America havelong-term seed banks, which can survive farming formany decades (Wienhold & van der Valk 1989). Sev-

eral European fen species have persistent seed banks,with seeds still viable after five years or more (Jensen2004) but many other species do not have persistentseed banks (Bekker et al. 1997). An example of the lattergroup is Carex stricta in North American fens (Galato-witsch & van der Valk 1996a), so that dispersal may beeven more important for the maintenance of biodiver-sity in these wetlands (Wetzel et al. 2001; Middleton2003).

Dispersal strategies

All dispersal strategies are capable of moving seedsaway from the parent plant, where regeneration is morelikely to be successful (Harper 1977). Animals, windand water can aid the movement of seeds, and thesedispersal mechanisms are capable of moving seeds overlong distances (Cain et al. 2000). Here, we define long-distance dispersal as dispersal beyond the site of themother plant (Nathan et al. 2003). Long-distancedispersal has been little studied in comparison to localdispersal, but it is a key factor in biological migrationssupporting species distribution across geographic ranges(Nathan 2001), the movement of species between islands(Sengupta et al. 2005), the maintenance of species incommunities, and the re-establishment of biodiversityin disturbed and restored sites (Middleton 1999).

Of the various types of dispersal, water (hydrochory)is likely the most important mechanism for the dispersalof seeds in fens (Vogt et al. 2004), but seeds alsodisperse by gravity, animals (zoochory) and wind(anemochory). This paper will focus on all dispersalmechanisms except dispersal by wind, which will becovered elsewhere in this volume. The majority of speciesdisperse in more than one way (polychory), even thoughthe seed morphology of a species may seem best adaptedfor one type of dispersal. For example, the pappus ofmany seeds in the Asteraceae seems designed for winddispersal, but a seed with a pappus may actually travelefficiently and possibly farther on the water (Bakker etal. 1996; Barrat-Segretain 1996; Bill et al. 1999;Middleton 1999). Movement by means other than thatsuggested by the seed’s morphology is known as ‘non-standard dispersal’ (Higgins et al. 2003). This definitionsuggests that the ‘alternative’ dispersal mode is lessimportant or less common than the morphologicallysuggested mode, but non-standard dispersal may be themost common route of dispersal for a particular plantspecies.

The seeds and propagules of many species of wetlandsdisperse in the water and have morphological adapta-tions for floating (Cook 1990; Middleton 1999), and afew older works focus on fen species (Ohlendorf 1907;Ulbrich 1928). More attention has been paid to

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hydrochory in rivers (Barrat-Segretain 1996; Cellot etal. 1998; Merritt & Wohl 2002; Middleton 1999; Nilssonet al. 2002) and oceans (Murray 1986; Sengupta et al.2005) than in fens (Vogt et al. 2004; see also Boedeltjeet al. 2003). Nevertheless, the ability of seeds to dispersein water is a critical issue in all types of wetlands,especially because the level of hydrologic re-establish-ment ultimately may determine the success of naturalrestoration, i.e., the ability of species to re-establishthemselves in restoration sites without any direct inter-vention such as re-planting (Middleton 1999, 2002b).For example, hydrochory may be critical in the mainte-nance of baldcypress swamps (Schneider & Sharitz1988; Middleton 2000) because some of the dominantspecies have seeds that live for one year or less inswamp (Middleton 2000). Similarly, hydrochory mayserve as an important link to supply live seeds to theseed banks of fens, particularly since so many fen speciesdisperse in the water (Vogt et al. 2004).

Animals have the potential of moving seeds overgreat distances (Pakeman 2001; Mouissie et al. 2005a).Before Europe and North America were extensivelydeveloped, seeds were probably moved by the largegrazing ungulates that used fens, such as wild Auroch(Bos taurus primigenius) and Red deer (Cervus elaphus)in Europe (Vera 2001), and Moose (Alces alces) orbison (Bison bison bison) in North America (Jackson1961). In addition, waterfowl are likely to move seedsbetween fens, however, most of the available informa-tion pertains to other wetland types (Bakker et al. 2002;Figuerola et al. 2003). We know little about the specificsof seed transmission by these animals during the timebefore the extensive development of these landscapes.More recently, cattle may have moved seeds readilybetween fens, so that seeds of fen species had an oppor-tunity to be eaten, moved and defecated into distantwetlands by cattle. Studies in the cultural landscapeshowed that cows moved between pastures for distancesof at least 10-50 km. Sheep movement between summergrazing habitats and wintering areas is quantitativelyprobably even more important for seed dispersal. Theanimals walked along so called ‘trans humances’ overdistances of up to several 100s of km (Poschlod &WallisdeVries 2002). These agriculturally related ac-tivities had a tremendous potential of moving seedsacross large distances (Bruun & Fritzbøger 2002), andcattle movement may also have moved fen seeds inthese landscapes.

Some plant species are not readily moved by animals.Mitlacher et al. (2002) found that only 15% of grasslandmeadow species were moved by cattle and sheep whereasMouissie et al. (2005a) showed experimentally that onlyspecific seeds had a high probability of being dispersedby animals. Small, round seeds performed well whereas

the opposite was true for large seeds. The study byMouissie et al. (2005a) found a good correlation be-tween survival in the animal digestive tract and survivalin the soil seed bank. This suggests that internal disper-sal by animals favors the same species that survive in theseedbank.

Cattle may also move seeds that stick to their fur(adhesive dispersal; Murray 1986; Couvreur et al. 2004;Mouissie et al. 2005a). Even seeds without specializedstructure for attaching to the fur may adhere to animals(Kiviniemi & Eriksson 1999). We know of no specificstudies about the ability of fen species to attach toanimals, however, a number of them have such adhesiveabilities (e.g. Eleocharis spp., Galium spp.; Middleton1999).

According to some authors, animal dispersal has thepotential of moving seeds much farther than other typesof dispersal such as wind (Vegelin et al. 1997). Therefore,domestic cattle are likely to have played a role in themovement of seeds of fen species, so that the recentabandonment of these wetlands for cattle grazing(especially in North America) could have a negativeeffect on the genetic exchange of these species.

However, seed dispersal by cattle may also affectspecies composition negatively. In North America, cattlespread non-indigenous species that are fed to cattle inbarns (Mt. Pleasant & Schlather 1994) and in Europe,cattle may spread undesirable (non-target species) fromeutrophic communities to restored sites (Mouissie et al.2005b). These species remain in the seed banks of fensfor many decades after cattle were removed from pas-tures (Middleton 2002a).

At least one study has suggested that wind dispersalis more important than water, animals or gravity inmoving seeds because flooding, hay-making and autumngrazing did not appreciably move seeds (Donath et al.2003). However, this study may be inconclusive becauseit was conducted on a flood-plain separated from thechannel by a levee (dyke) and with no flood pulsing.Other authors contest the idea that wind dispersal isimportant in long-distance dispersal and instead con-clude that wind-dispersed seeds may travel only shortdistances (up to 10 m; Vegelin et al. 1997), so thatgrazing animals or hay mowing equipment may have arelatively large influence on seed dispersal (Bakker1989). Water certainly has the potential of moving seedsgreat distances; some seeds travel more than 100 km inthe ocean (Murray 1986). At the same time that highlyinterconnected sites may be desirable for biodiversityconservation, the most interconnected disturbedlandscapes are most likely to be invaded by non-indig-enous species (With 2003).

282 MIDDLETON, B. ET AL.

Dispersal in restoration and management

Knowledge of how seeds disperse is useful for themanagement and restoration of fens. Well-connectedfens and restoration sites are likely to receive higheramounts of dispersed seed, and thus, have better oppor-tunities for the establishment of new species and thelong term maintenance of biodiversity (Taylor et al.1993). Alternatively, the poor interconnection ofrestoration sites may hinder the restoration of fens,although fens may never have been as interconnected ascertain other wetland types (Galatowitsch & van derValk 1996a; van Diggelen & Grootjans 1999; Jansen etal. 2000).

Besides promoting seed dispersal by improvingconnectivity between wetlands, the native biodiversityof restored fens can be increased through the directphysical delivery of seeds to restoration sites. Hay cutfrom intact fens can be moved to restoration sites. Haytransfer from a donor fen to a restoration site re-established 70% of the species of the donor fen, eventhough the restoration site had been drained and usedintensively in agriculture for 200 years (Patzelt et al.2001). This procedure is useful in restoration sites wherethe top soil has been removed to reduce nutrient loads(e.g. in the central part of The Netherlands), but also inalluvial floodplain grasslands where the topsoil has notbeen removed (Hölzel & Otte 2003). This technique ofhay transfer to increase species richness is not widelypracticed in North America but could have great prom-ise in improving the biodiversity of restored or degradedfens.

A problem in restoration is that some key fen speciesdo not only have a limited dispersal ability, their seedsmay also have very restrictive germination require-ments, which may not be met by the environment in therestoration site. For example, Carex stricta neitherdisperses (Galatowitsch & van der Valk 1996a), norgerminates readily in North America (Budelsky &Galatowitsch 1999; Schütz 2000), yet its establishmentis essential for restoration because the species formstussocks, which determine the structure of the wetland.The proper regeneration environment on restorationsites is therefore critical to restoration success(Galatowitsch & van der Valk 1996b; Zedler & Callaway1999; Middleton 1999, 2002b; Warren et al. 2002).Thus, it is lack of seed dispersal as well as inappropriateregeneration environment that may hinder the successof fen restoration.

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Received 22 February 2005;Accepted 7 July 2005;

Co-ordinating Editor: R. Pakeman.