Applied Vegetation Science 9: 279-284, 2006 © IAVS; Opulus Press Uppsala.
- SEED DISPERSAL IN FENS -
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Seed dispersal in fens Middleton, Beth1*; van Diggelen, Rudy2 & Jensen, Kai3 1USGS
National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70506 USA; and Conservation Ecology Group, Biological Sciences, University of Groningen, PO Box 14, NL-9750 AA Haren The Netherlands; E-mail
[email protected]; 3Population and Vegetation Ecology Group, Biocentre Klein Flottbek, University Hamburg, Ohnhorststr. 18, DE-22609 Hamburg, Germany; E-mail
[email protected]; *Corresponding author; Fax +1 3372668586, E-mail
[email protected] 2Community
Abstract Question: How does seed dispersal reduce fen isolation and contribute to biodiversity? Location: European and North American fens. Methods: This paper reviews the literature on seed dispersal to fens. Results: Landscape fragmentation may reduce dispersal opportunities thereby isolating fens and reducing genetic exchange. Species in fragmented wetlands may have lower reproductive success, which can lead to biodiversity loss. While fens may have always been relatively isolated from each other, they have become increasingly fragmented in modern times within agricultural and urban landscapes in both Europe and North America. Dispersal by water, animals and wind has been hampered by changes related to development in landscapes surrounding fens. Because the seeds of certain species are long-lived in the seed bank, frequent episodes of dispersal are not always necessary to maintain the biodiversity of fens. However, of particular concern to restoration is that some dominant species, such as the tussock sedge Carex stricta, may not disperse readily between fens. Conclusions: Knowledge of seed dispersal can be used to maintain and restore the biodiversity of fens in fragmented landscapes. Given that development has fragmented landscapes and that this situation is not likely to change, the dispersal of seeds might be enhanced by moving hay or cattle from fens to damaged sites, or by reestablishing lost hydrological connections.
Keywords: Adhesive dispersal; Biodiversity; Cattle; Conservation biology; Fragmentation; Invasive plant; Island biogeography; Polychory; Seed bank; Zoochory.
Nomenclature: Anon. (2004).
Introduction Seed dispersal and biodiversity Seed dispersal underlies the ability of fens to maintain biodiversity. Dispersing seeds may be able to replenish individuals in gaps after the death of adult individuals (Harper 1977) or after the complete extirpation of the species, as in restoration areas (Middleton 1999). However, seed dispersal limitations due to fragmentation may hamper the ability of seeds to disperse between natural areas (Galatowitsch & van der Valk 1996a). Furthermore, fragmented populations may have a reduced supply of dispersing seeds because some fragmented populations suffer from edge effects, leading to lower reproductive success (Lienert & Fischer 2004) or to genetic impoverishment because of a reduction of seed and pollen exchange between populations (Barrett & Kohn 1991; Young et al. 1996). Thus, certain wetland species may become locally extirpated due to the reproductive problems stemming from habitat isolation (Lienert et al. 2002; Hooftman et al. 2003; Lienert & Fischer 2003). For these reasons, seed dispersal within and between fens may be a key feature of the conservation of biodiversity in these systems. Seed dispersal limitation may underlie biodiversity decline in fens because these wetlands have become isolated by agricultural and urban development. For example, prior to agricultural development, wetlands were more interconnected by hydrological dispersal in the Prairie Pothole Region of North America (Galatowitsch & van der Valk 1996a) and in Europe (Trepel et al. 2003). Grazed wetlands were probably also more connected to one another via seed dispersal by cattle in the past than they are nowadays. Cattle grazing of natural habitats has nearly ceased in some parts of the world (Johansson 1997; Middleton 2002a), so that cattle are less likely to move seeds in modern times than they once did (Bruun & Fritzbøger 2002). However, animal move-
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ment patterns via the modern cattle trade are complex, with live cattle sometimes moved across continents. Hay mowing machinery can also readily move seeds between mown grasslands because the seeds stick to the mowers. Hay mowing may have been more effective in the past in moving seeds between wetlands, because the large-scale operations of current day agriculture may not cut all hay fields at an optimal time for seed dispersal (Strykstra et al. 1997). However, not all studies agree with the idea that recent agricultural activities limit dispersal and isolate wetlands. Seed longevity and seed dispersal by birds may work to counteract isolation, particularly in temporary wetlands (Brose 2001). Nevertheless, genetic and experimental ecological studies support the idea that fens are becoming increasingly isolated by fragmentation (Hooftman et al. 2003; Lienert & Fischer 2003) and that fragmentation affects the performance of fen species (Lienert & Fischer 2004). In endangered populations, inbreeding and genetic drift are common, so that, in general, genetic variability is higher in larger populations (Ellstrand & Elam 1993). Populations are becoming extinct even in intact remnants of (dry) calcareous grasslands in the Swiss Jura mountains (Fischer & Stöcklin 1997), and it might also be assumed that these processes are active in small remnants of fen grasslands. However, species have different thresholds of isolation, beyond which genetic loss and extinction may occur because of the inherent differences in species abilities to cope with fragmentation (Young et al. 1996; see also Oostermeijer 2003). High levels of genetic diversity were found in small and recently (30 years ago) fragmented populations of Pedicularis palustris (Schmidt & Jensen 2000), while reduced genetic variability of Swertia perennis was found in small and isolated fens (Lienert et al. 2002). Thus, the effects of fragmentation on genetic variation and on fitness components of fen species differ among species. More research on the role of genetic isolation and fragmentation in the extirpation of specific species is essential for managers to develop strategies for biodiversity conservation. The persistence of fen species or their regeneration potential in restored wetlands further depends on seed longevity in seed banks (Thompson et al. 1997; Middleton 1999, 2002a). For all plant communities, the most common categorization of seed longevity comes from Thompson et al. (1997), and groups seed longevity types as transient (viability of less than 1 a), short-term persistent (1-4 a) and long-term persistent (more than 4 a), although other classification systems also exist (see Csontos & Tamás 2003). At least some species in fens of the Prairie Pothole Region of North America have long-term seed banks, which can survive farming for many decades (Wienhold & van der Valk 1989). Sev-
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eral European fen species have persistent seed banks, with seeds still viable after five years or more (Jensen 2004) but many other species do not have persistent seed banks (Bekker et al. 1997). An example of the latter group is Carex stricta in North American fens (Galatowitsch & van der Valk 1996a), so that dispersal may be even more important for the maintenance of biodiversity in these wetlands (Wetzel et al. 2001; Middleton 2003). Dispersal strategies All dispersal strategies are capable of moving seeds away from the parent plant, where regeneration is more likely to be successful (Harper 1977). Animals, wind and water can aid the movement of seeds, and these dispersal mechanisms are capable of moving seeds over long distances (Cain et al. 2000). Here, we define longdistance dispersal as dispersal beyond the site of the mother plant (Nathan et al. 2003). Long-distance dispersal has been little studied in comparison to local dispersal, but it is a key factor in biological migrations supporting species distribution across geographic ranges (Nathan 2001), the movement of species between islands (Sengupta et al. 2005), the maintenance of species in communities, and the re-establishment of biodiversity in disturbed and restored sites (Middleton 1999). Of the various types of dispersal, water (hydrochory) is likely the most important mechanism for the dispersal of seeds in fens (Vogt et al. 2004), but seeds also disperse by gravity, animals (zoochory) and wind (anemochory). This paper will focus on all dispersal mechanisms except dispersal by wind, which will be covered elsewhere in this volume. The majority of species disperse in more than one way (polychory), even though the seed morphology of a species may seem best adapted for one type of dispersal. For example, the pappus of many seeds in the Asteraceae seems designed for wind dispersal, but a seed with a pappus may actually travel efficiently and possibly farther on the water (Bakker et al. 1996; Barrat-Segretain 1996; Bill et al. 1999; Middleton 1999). Movement by means other than that suggested by the seed’s morphology is known as ‘nonstandard dispersal’ (Higgins et al. 2003). This definition suggests that the ‘alternative’ dispersal mode is less important or less common than the morphologically suggested mode, but non-standard dispersal may be the most common route of dispersal for a particular plant species. The seeds and propagules of many species of wetlands disperse in the water and have morphological adaptations for floating (Cook 1990; Middleton 1999), and a few older works focus on fen species (Ohlendorf 1907; Ulbrich 1928). More attention has been paid to
- SEED DISPERSAL IN FENS hydrochory in rivers (Barrat-Segretain 1996; Cellot et al. 1998; Merritt & Wohl 2002; Middleton 1999; Nilsson et al. 2002) and oceans (Murray 1986; Sengupta et al. 2005) than in fens (Vogt et al. 2004; see also Boedeltje et al. 2003). Nevertheless, the ability of seeds to disperse in water is a critical issue in all types of wetlands, especially because the level of hydrologic re-establishment ultimately may determine the success of natural restoration, i.e., the ability of species to re-establish themselves in restoration sites without any direct intervention such as re-planting (Middleton 1999, 2002b). For example, hydrochory may be critical in the maintenance of baldcypress swamps (Schneider & Sharitz 1988; Middleton 2000) because some of the dominant species have seeds that live for one year or less in swamp (Middleton 2000). Similarly, hydrochory may serve as an important link to supply live seeds to the seed banks of fens, particularly since so many fen species disperse in the water (Vogt et al. 2004). Animals have the potential of moving seeds over great distances (Pakeman 2001; Mouissie et al. 2005a). Before Europe and North America were extensively developed, seeds were probably moved by the large grazing ungulates that used fens, such as wild Auroch (Bos taurus primigenius) and Red deer (Cervus elaphus) in Europe (Vera 2001), and Moose (Alces alces) or bison (Bison bison bison) in North America (Jackson 1961). In addition, waterfowl are likely to move seeds between fens, however, most of the available information pertains to other wetland types (Bakker et al. 2002; Figuerola et al. 2003). We know little about the specifics of seed transmission by these animals during the time before the extensive development of these landscapes. More recently, cattle may have moved seeds readily between fens, so that seeds of fen species had an opportunity to be eaten, moved and defecated into distant wetlands by cattle. Studies in the cultural landscape showed that cows moved between pastures for distances of at least 10-50 km. Sheep movement between summer grazing habitats and wintering areas is quantitatively probably even more important for seed dispersal. The animals walked along so called ‘trans humances’ over distances of up to several 100s of km (Poschlod & WallisdeVries 2002). These agriculturally related activities had a tremendous potential of moving seeds across large distances (Bruun & Fritzbøger 2002), and cattle movement may also have moved fen seeds in these landscapes. Some plant species are not readily moved by animals. Mitlacher et al. (2002) found that only 15% of grassland meadow species were moved by cattle and sheep whereas Mouissie et al. (2005a) showed experimentally that only specific seeds had a high probability of being dispersed by animals. Small, round seeds performed well whereas
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the opposite was true for large seeds. The study by Mouissie et al. (2005a) found a good correlation between survival in the animal digestive tract and survival in the soil seed bank. This suggests that internal dispersal by animals favors the same species that survive in the seedbank. 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 specialized structure for attaching to the fur may adhere to animals (Kiviniemi & Eriksson 1999). We know of no specific studies about the ability of fen species to attach to animals, however, a number of them have such adhesive abilities (e.g. Eleocharis spp., Galium spp.; Middleton 1999). According to some authors, animal dispersal has the potential of moving seeds much farther than other types of dispersal such as wind (Vegelin et al. 1997). Therefore, domestic cattle are likely to have played a role in the movement of seeds of fen species, so that the recent abandonment of these wetlands for cattle grazing (especially in North America) could have a negative effect on the genetic exchange of these species. However, seed dispersal by cattle may also affect species composition negatively. In North America, cattle spread non-indigenous species that are fed to cattle in barns (Mt. Pleasant & Schlather 1994) and in Europe, cattle may spread undesirable (non-target species) from eutrophic communities to restored sites (Mouissie et al. 2005b). These species remain in the seed banks of fens for many decades after cattle were removed from pastures (Middleton 2002a). At least one study has suggested that wind dispersal is more important than water, animals or gravity in moving seeds because flooding, hay-making and autumn grazing did not appreciably move seeds (Donath et al. 2003). However, this study may be inconclusive because it was conducted on a flood-plain separated from the channel by a levee (dyke) and with no flood pulsing. Other authors contest the idea that wind dispersal is important in long-distance dispersal and instead conclude that wind-dispersed seeds may travel only short distances (up to 10 m; Vegelin et al. 1997), so that grazing animals or hay mowing equipment may have a relatively large influence on seed dispersal (Bakker 1989). Water certainly has the potential of moving seeds great distances; some seeds travel more than 100 km in the ocean (Murray 1986). At the same time that highly interconnected sites may be desirable for biodiversity conservation, the most interconnected disturbed landscapes are most likely to be invaded by non-indigenous species (With 2003).
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Dispersal in restoration and management
References
Knowledge of how seeds disperse is useful for the management and restoration of fens. Well-connected fens and restoration sites are likely to receive higher amounts of dispersed seed, and thus, have better opportunities for the establishment of new species and the long term maintenance of biodiversity (Taylor et al. 1993). Alternatively, the poor interconnection of restoration sites may hinder the restoration of fens, although fens may never have been as interconnected as certain other wetland types (Galatowitsch & van der Valk 1996a; van Diggelen & Grootjans 1999; Jansen et al. 2000). Besides promoting seed dispersal by improving connectivity between wetlands, the native biodiversity of restored fens can be increased through the direct physical delivery of seeds to restoration sites. Hay cut from intact fens can be moved to restoration sites. Hay transfer from a donor fen to a restoration site reestablished 70% of the species of the donor fen, even though the restoration site had been drained and used intensively in agriculture for 200 years (Patzelt et al. 2001). This procedure is useful in restoration sites where the top soil has been removed to reduce nutrient loads (e.g. in the central part of The Netherlands), but also in alluvial floodplain grasslands where the topsoil has not been removed (Hölzel & Otte 2003). This technique of hay transfer to increase species richness is not widely practiced in North America but could have great promise in improving the biodiversity of restored or degraded fens. A problem in restoration is that some key fen species do not only have a limited dispersal ability, their seeds may also have very restrictive germination requirements, which may not be met by the environment in the restoration site. For example, Carex stricta neither disperses (Galatowitsch & van der Valk 1996a), nor germinates readily in North America (Budelsky & Galatowitsch 1999; Schütz 2000), yet its establishment is essential for restoration because the species forms tussocks, which determine the structure of the wetland. The proper regeneration environment on restoration sites is therefore critical to restoration success (Galatowitsch & van der Valk 1996b; Zedler & Callaway 1999; Middleton 1999, 2002b; Warren et al. 2002). Thus, it is lack of seed dispersal as well as inappropriate regeneration environment that may hinder the success of fen restoration.
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Received 22 February 2005; Accepted 7 July 2005; Co-ordinating Editor: R. Pakeman.
