- BIODIVERSITY MANAGEMENT OF FENS AND FEN MEADOWS BY GRAZING, CUTTING AND BURNING - 307

Applied Vegetation Science 9: 307-316, 2006© IAVS; Opulus Press Uppsala.

AbstractQuestion: Can the biodiversity of fens in Europe and NorthAmerica be maintained through the use of grazing (especiallycattle grazing), fire, and/or cutting?Location: European and North American fens.Methods: This paper is a review of the literature on the effectsof grazing, fire and cutting on fens, to explore the relationshipbetween management and biodiversity in fens.Results: A reduction of cattle grazing, mowing and burning infens has led to a reduction in biodiversity in fens. The vegeta-tion of abandoned fens shifts to trees and shrubs after 10-15years, which shade the smaller and rarer species of thesewetlands. While careful use of fire is used to manage fens inNorth America, it is not widely used in European fens, perhapsbecause the peat of drained fens may catch fire. Cattle grazingcannot be considered a natural disturbance in North America,since cattle did not evolve on that continent. In Europe, cattledo not generally graze in unaltered fens, but they do useslightly drained fen meadows.Conclusions: Three approaches have been used to control thedominance of tall woody and herbaceous species in aban-doned fens, including the re-introduction of cattle, mowing,and burning. Overgrazing results in a permanent reduction inbiodiversity, therefore cattle re-introduction must be ap-proached cautiously. In Europe, but not in North America,mowing has been an important management tool, and mowinghas been successful in maintaining species richness, particularlyin fens that have been mowed annually for centuries. Fire hasbeen the most common and successful management tool inNorth America although it is not effective in removing shrubsthat have become large. Because the problems and solutionsare similar, the literature of both European and North Ameri-can fen management can be analyzed to better assess themanagement of fens on both continents. Many managementquestions require further study and these are listed in thepaper.

Keywords: Cattle grazing; Fire; Mowing; Plant species rich-ness; Sedge meadow.

Nomenclature: Anon. (2004).

Biodiversity management of fens and fen meadowsby grazing, cutting and burning

Middleton, Beth A.1*; Holsten, Bettina2 & van Diggelen, Rudy3

1USGS National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70506, USA;2Ökologie-Zentrum, University of Kiel, Olshausenstr. 75, DE-24118 Kiel, Germany; E-mail bholsten@ecology.uni-kiel.de;

3Community and Conservation Ecology Group, University of Groningen, PO Box 14, NL-9750 AA Haren, The Nether-lands; E-mail r.van.diggelen@rug.nl; *Corresponding author; Fax +1 3372668586; E-mail beth_middleton@usgs.gov

Introduction

The loss of biodiversity of plant species in fens hasbeen reported from many regions of the world (Middleton2002a; Bragg & Lindsay 2003), and one reason forspecies loss in fens may be recent changes in land usepractices related to agriculture (Vermeer & Joosten1992; Jensen & Schrautzer 1999). Re-establishing formerland management practices such as grazing, mowingand burning is often seen as a promising restorationstrategy to increase biodiversity but these practices havea different historical context in various regions of theworld. The question arises whether the regional historyof management in fens has an influence on currentmanagement options because species are adapted tolocal management practices. Alternatively, the effectsof mowing, grazing and burning may not be all thatregionally specific so that the relationships of speciesrichness, disturbance, productivity and spatial heteroge-neity may be independent of land use history (Pollock etal. 1998).

Fens are defined as peat accumulating systems, whichare influenced by groundwater (Joosten & Clarke 2002)but even after slight drainage, many characteristic veg-etation types remain on altered fens if low intensity landuse is practiced (Ellenberg 1996). In this article, wecover peat forming tall sedge and sedge moss communi-ties but not reeds and carr vegetation types, which are ofless interest from a biodiversity perspective (Wheeler &Shaw 1995). Additionally we deal with species-richvegetation types that develop under low intensity landuse on somewhat drained fens such as fen meadows.

Cattle grazing has historically been an importantland use in slightly drained fens and fen meadows(Middleton 1999). General views on the effects of cattlegrazing range from the idea that low to moderate levelsof grazing are beneficial for plants species diversity(Hald & Vinther 2000; Bullock et al. 2001) to the ideathat grazing is nearly always detrimental (Noss 1994).The specific effects of grazing may depend on site

308 MIDDLETON, B.A. ET AL.

productivity; with grazing, the species richness of nutri-ent-rich sites will increase whereas in nutrient-poorones, species richness will decrease (Proulx & Mazumder1998). The relationship between species richness andcattle grazing in fens is especially relevant for manage-ment and conservation purposes. While this paper fo-cuses on the relationship of cattle grazing and plantspecies richness, many other studies have examined therelationship of cattle grazing and faunal communities,e.g., birds (McCoy & Rodriguez 1994; Norris et al.1998) and insects (Gibson et al. 1992; Painter 1999;Pöyry et al. 2004).

Many characteristic fen species are increasingly rareat the same time that cutting, burning and cattle grazinghave become less common in fens. A relevant questionin this context is whether fens can be restored by the re-establishment of grazing once woody species haveencroached or species richness has declined (van Wieren1995). One question addressed in this paper is whethercutting, burning or cattle grazing is warranted asmanagement tool in fens, and if so, under whatcircumstances it is appropriate in either North Americaor Europe. The role of each as a natural disturbancediffers, as does the historical and cultural context ofthese management techniques.

Historically, the nature of burning, grazing andcutting, as well as the fens themselves, have differedbetween North America and Europe, but in this paperwe seek to find some common ground for management.Fens used by humans for grazing were often drained tosome extent, which adds a further complication to thediscussion, because the hydrology of many fens hasbeen altered. Slight drainage turned fens into fen mead-ows (Grootjans & van Diggelen 1995). Especially afterfens have been grazed and/or drained, shrubs may invadeand reduce biodiversity (Middleton 2002a), so that it isimportant to find some management alternatives tomaintain the biodiversity of formerly grazed fens.

In the present paper, we will review literature aboutthe effects of cattle grazing on fens in Europe and NorthAmerica and management alternatives to conserve bio-diversity. We specifically examine whether mowingand burning are more suitable to manage fens for nativeplant biodiversity than cattle grazing. First, the historyof fire and cattle grazing will be reviewed to helpdetermine if these are natural disturbances of inherentimportance to the management of fens. Second, specificbiodiversity responses of fen species to managementusing fire, grazing or cutting will be explored, and a fewpreliminary management recommendations will bedrawn from those studies. Lastly, priority research to fillin gaps in knowledge related to fen management will beoutlined.

Cattle grazing and cutting history in fens

To evaluate whether or not cattle grazing can beconsidered a natural disturbance in fens, we need toreview the history of cattle grazing in fens. Before theera of livestock farming (Neolithic times), no domesticcattle grazed in wetlands. Instead, fens and/orneighboring woodlands in Europe were grazed by largeungulates such as Elk (Alces alces) and Red deer (Cervuselaphus; Cromsigt 2000), and in North America, Moose(Alces alces) and Wapiti (Cervus elaphus canadensis;Jackson 1961).

In about 6800 BP, domestic cattle (Bos taurus) weremoved into central Europe. Free ranging domestic cattlewere able to graze on winter cereals during autumn andearly spring as well as on rotational abandoned fields,and they were fed with dried leaves during the winter(Ellenberg 1996). The first sites mowed by hand byhumans for cattle forage were reportedly developedfrom natural fens and alder woodlands that were domi-nated by Carex species (Ellenberg 1996). Mowing wasthe major land use type responsible for keeping thewetlands open and for the development of species-richfen meadow vegetation in Europe (Bignal & McCracken1996; Ellenberg 1996). The quality of the fodder cutfrom undrained fens was so low that hand-cut hay wasused only as bedding in animal barns and sheds (Stammelet al. 2003). Free-ranging cattle avoid very wet, undrainedfens, whereas drained fens are grazed intensively insummer (J.P. Bakker & Grootjans 1991). The preferredgrazing habitats for cattle are productive ones such asflood-plains, salt marshes, temporary marshes and othersimilar systems, so that fens may never have been asimportant as other wetland types for cattle grazing.

The history of cattle grazing in fens in North Americabegan with the influx of settlers in North America.Cattle were introduced in the 1700s from the east coastwestward; some cattle grazed in Kentucky as early as1784 (Prince 1997). At first, cattle grazing intensity waslight because the cattle were not constrained by fencesduring the early settlement period. For example, inWisconsin, cattle roamed freely before the 1870 lawsrequiring cattle to be fenced within property boundaries(Liegel 1988). After 1880, with the expansion of therailroads and better access to markets, the dairy industryexpanded in North America (Prince 1997). After thisexpansion and with fencing, herd size increased gradu-ally over time, so that the cattle grazing pressure on fensincreased (Middleton 2002a).

In both Europe and North America, fen utilizationfollowed economic developments in two directions af-ter World War II. Most fens were drained to increasetheir productivity in densely populated regions. Pasturedfens were abandoned in less densely populated areas in

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mountain regions and areas far away from populationcenters, and these abandoned fens often lost biodiversitybecause of encroachment by shrubs (Middleton 2002a;Matějková et al. 2003). After 1945, pasture abandon-ment was also associated with a reduction of handcutting and hay making in fens (Slotte 2001). The cut-ting of fens had already ceased in North America beforeWorld War II (Prince 1997; Middleton 2002a). Thus,both intensification and abandonment led to a decreasein species richness in fens (Bekker et al. 1997).

In the context of this paper, we can ask the questionof whether or not cattle grazing can be considered to bea natural disturbance in fens, and therefore used tomaintain biodiversity in fens by mimicking prehistoricconditions. The answer in North America is an unquali-fied ‘no’, since cattle originated in the Old World. Theanswer also is ‘no’ in Europe, but the situation is lessclear. While the aurochs (Bos taurus primigenius) origi-nated in Europe, aurochs are unlikely to have ever usedfens extensively. Nevertheless, we will consider later inthe paper if there may be situations under which it isjustifiable to use cattle to maintain the biodiversity offens, particularly drained fens.

Fire history of fens

In North America, fire was probably a much morestructuring factor than grazing during the presettlementperiod. Before the defeat of Chief Black Hawk in 1832,American Indians and lightning fires annually burnedfens in Wisconsin and other parts of the Middle WesternUnited States (Curtis 1959; Prince 1997). Also, farmershad once burned pastured fens to improve forage quality,but in places such as southern Wisconsin, this practiceabruptly ceased after World War II (Middleton 2002b).After that time, many fens became less species-rich and/or encroached by woody species. Fire in North-Ameri-can fens can be considered a natural disturbance, andshould be considered to help maintain the biodiversityof fens.

Fire is generally considered destructive in peatlandsin Europe (Klimkowska 2004), mainly because the peatmay catch fire, particularly in drained fens. Once set, thefire may smolder for long periods of time (Anon. 2005).Nevertheless, fire set by lightning strike must have onceburned undrained fens in the European landscape as it isreported for mires in Finland (Pitkänen et al. 1999), andso could be considered as a natural disturbance at leastfor the management of undrained fens. The burning ofsedge meadows was regularly practiced in eastern Eu-rope (Poland and Belarus) until recently (Kloskowski &Krogulec 1999; Kozulin & Flade 1999).

Effects of cattle grazing on fens

Effects on vegetation composition

While one might conclude that the loss of biodiversityin fens may be attributable primarily to land use changesassociated with shifts away from cattle grazing, and thatsomehow re-instigating the practices of small farmerswould solve the problem, the situation is actually morecomplicated than that. Unfortunately, there is little re-search available to illuminate some of the questions. Forexample, except for a few studies in calcareous fens(Middleton 2002a; Stammel et al. 2003), we are notaware of any studies on the effects of grazing in undrainedfens. Also, the effects of establishing a grazing regimein abandoned fen meadows that were previously mownhave been rarely studied (Arnesen 1999; Hald & Vinther2000). We will therefore extrapolate knowledge on graz-ing from other systems to fens in some of our followingexamples.

Cattle grazing favors certain plant traits (Hald &Vinther 2000; Stammel et al. 2003), and thus the activi-ties of cattle lead to shifts in biodiversity and commu-nity composition. In North American fens, Poa pratensisincreases in dominance with grazing (Schulz & Leininger1990; Martin & Chambers 2001; Middleton 2002a)because the species invades gaps more quickly thanCarex nebrascensis, Poa pratensis has a phalanx growthstrategy (clumped individuals with short rhizomes) andCarex nebrascensis has a guerilla growth strategy(unclumped individuals with long rhizomes; Martin &Chambers 2001). Once Poa pratensis has become es-tablished in grazed fens and sedge meadows, the speciescan remain in the community for decades after cattlegrazing has ceased (Middleton 2002a).

Cattle defoliate plants, trample vegetation (Arnesen1999), alter soil structure (Murray 1997), and crushsedge tussocks with their hooves, which can allow theinvasion of shrubs. Cattle maintain palatable shrubspecies in a state of small stature while grazing, but ifthe cattle subsequently are removed from the fen, theshrubs may grow prolifically to form a dense stand ofshrubs (Middleton 2002a). Unpalatable tree species suchas Alder (Alnus glutinosa) may even grow up underconstant grazing management with low animal numbersbecause cattle largely avoid eating the saplings (Bunzel-Drücke 1996; Holsten 2003). From a conservation per-spective, the invasion of trees in fen areas is mostlyregarded as a problem because the biodiversity of char-acteristic, low-stature fen species is reduced because oflow light availability (Kotowski & Van Diggelen 2004).At the same time, cattle create ‘lawns’ of regularlygrazed sites and ‘tussocks’ that are rarely grazed(McNaughton 1984; J.P. Bakker et al. 1984), so that

310 MIDDLETON, B.A. ET AL.

cattle increase the spatial heterogeneity of habitats forplants and animals. In wet meadows, small mammaldensity was positively affected by cattle at intermediategrazing intensities (ca. 0.5 cattle/ha) and negatively bymuch higher or lower stocking densities (Schmidt et al.2005). The species number and abundances of grass-hoppers, butterflies, solitary bees and wasps was higherin pastures with lower stocking rates than in intensivelygrazed pastures (0.7 cattle/ha vs. 3.2 cattle/ha, respec-tively; Kruess & Tscharntke 2002). However, otherstudies of freshwater wetlands indicate that grazingintensity hardly affects the species richness of inverte-brate communities (Steinman et al. 2003).

Effects on seed dispersal

Large grazers are known to disperse many seeds,both in their manure (Burton 1948; Middleton & Mason1992; Malo & Suarez 1995; Fischer et al. 1996; Pakemanet al. 2002; E.S. Bakker & Olff 2003; Myers et al. 2004;Mouissie et al. 2005c) and in their fur (Fischer et al.1996; Middleton 2002a; Mouissie et al. 2005b). Thedispersal of many plant species including fen speciesmay once have benefited from the movements of cattlebetween mountainous summer grazing areas and low-land winter habitats in Europe, the so-called trans-humance (Poschlod & WallisDeVries 2002). However,only very few characteristic species of montanegrasslands were found as viable seeds in cattle dungwhile being absent from the local vegetation, eitherbecause these species were not consumed or becauseseeds did not survive (Matějková et al. 2003). Herbiv-ores select productive patches for grazing but may causea net transport of seeds and nutrients from sites of highto those of low productivity if the latter are used forresting (Mouissie et al. 2005a). Therefore, seed disper-sal via cattle dung may be of limited value for restora-tion because seeds of novel species may not be trans-ported between appropriate systems. Some studies indi-cate that the seed species carried by cattle are the princi-pal species that invade hoof print gaps (Malo & Suárez1995), while another study indicates that the establish-ment of species from seeds is prevented in hoof printgaps because nearby rhizomes quickly invade gaps(Stammel & Kiehl 2004). Nevertheless, grazers such ascattle and rabbits may have a dual role in seed dispersaland gap creation, respectively, and together maintainthe species richness of subordinate herbs (floodplaingrasslands; E.S. Bakker & Olff 2003). Clearly, notenough research is available to make the case that seeddispersal by cattle can increase fen biodiversity.

The role of cattle in introducing non-native speciesinto wetlands also needs to be considered in the discussionof biodiversity conservation in fens. Especially in North

America, cattle have a significant role in introducingnon-native species into fens. Some of the non-nativespecies of seeds that are found in the manure of cattle(Mt. Pleasant & Schlather 1994) are maintained in theseed banks of fens for decades after abandonment bycattle (Middleton 2002a). We are not aware of anystudies that examine the role of cattle in maintainingnative biodiversity in fens in North America.

Effects on nutrient availability

The idea that grazers can transport nutrients fromfertile foraging sites to nutrient-poor sites (e.g. restingsites) requires further evaluation from the perspective ofthe maintenance of fen biodiversity (Bokdam &Gleichmann 2000; Mouissie et al. 2005c). Grazing cat-tle can add nutrients via manure to the otherwise nutri-ent-poor environment of a fen community (deMazancourt et al. 1998). If supplemental feed is fed todairy cattle, these additional nutrients may further changethe status of fens (Middleton 2003).

The relationship of grazing to N-mineralization ratesdepends on the intensity of cattle grazing, the amount offlooding and the nature of the soil (organic vs. mineral).Because of the importance of local site conditions, graz-ing has been related to enhanced (McNaughton et al.1997; Frank & Groffman 1998), decreased (Pastor et al.1993; Ritchie et al. 1998; van Wijnen et al. 1999) andunaltered N-mineralisation rates (Van Hoewyk et al.2000; C. Bakker et al. 2003). The mineralization ratemay depend on the size of the herbivore, the degree towhich the vegetation is removed and dung returned, andthe subsequent temperature and humidity of the gapscreated. If mineralization is affected by cattle grazing,then not only the size of the grazer but also stockingdensity could affect the mineralization rate (E.S. Bakkeret al. 2004). Recent evidence suggests that prolongedgrazing also stimulates P-mobilisation and results inelevated P-levels in the upper soil layers of fens (pers.comm. J.P. Bakker), however, the mechanism of thisphenomenon is not known.

At the same time that nutrient availability affectsgrazing, high amounts of nutrients may reduce the for-aging efficiency of herbivores by supporting a highplant standing crop (van de Koppel et al. 1996). Relatedto this idea, after cattle were removed in various types ofwetlands, there is often a drop in the number of water-fowl reported related to reduced foraging efficiency ofthe geese in tall vegetation (Olff 1992; McCoy &Rodriguez 1994). However, after cattle were removedfrom a monsoonal wetland in India, larger species ofgeese, e.g. Greylag geese, were more able to handle tallgrasses than the smaller Barheaded geese (Middleton1993; Middleton et al. 1991). The geese preferred to

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graze in parts of the wetland that had been previouslygrazed by herbivores (Middleton 1993). In a Canadiansalt marsh, standing crop increased when geese werefenced out of plots, but when the exclosures were re-moved, the geese did not graze the exclosed areas (Bazely& Jefferies 1986). Cattle may therefore facilitate graz-ing for other herbivores by reducing the size of plants,and this facilitation by cattle might be especially true inhigh-nutrient settings with high standing crops of plants(van de Koppel et al. 1996; Bos 2002).

Management conclusions

In both North America and Europe, a variety ofmanagement approaches have been used to maintainbiodiversity in fens, including cattle grazing, burning,mowing and herbicide treatments. Not all of these ap-proaches can be applied in all situations, and the re-search that is available indicates that the results ofparticular management strategies can be highly situa-tion dependent. We have reviewed the available litera-ture to convey the complicated nature of the issue, andaddress as many scenarios as possible. Much is notknown, and gaps in knowledge are outlined in the finalsection of this paper.

Cattle vs. native ungulates

The re-introduction of appropriate native ungulatesin fens could be beneficial, especially in relatively un-populated regions. Native ungulates require large areasin which to range, and so it is not likely that these couldbe re-introduced successfully in urbanized areas. Othernative ungulates could be re-introduced to fens in bothEurope and North America. In North America, moosemay once have been important herbivores in fens, andcould be reintroduced in remote parts of North America.Moose are dangerous animals and not especially welcomein urbanized areas. Elk eat woody vegetation in Euro-pean fens (Bokdam & van Braekel 2002), so that thesecould be useful to manage woody species in fens. Freeranging wild animals such as red deer in Central Europehave little influence on sedge-dominated fen vegetationbecause hunting pressure and disturbance by peoplekeep the animals in the forests during daytime. Whenred deer enter open habitat at night, they prefer food ofhigh energy and protein content (Wagenknecht 2000)and avoid vegetation dominated by sedges because oftheir low nutritional value.

With the simultaneous decline of cattle grazing andbiodiversity in fens, it seems logical that a reintroduc-tion of cattle in fens would lead to an increase in biodi-versity. While Aurochs and wild horses no longer exist,

Heck cattle and Polish Konik horses are available. Heckcattle were bred in an attempt to redevelop the aurochsby mixing various European cattle races. Polish Konikhorses are a primitive horse breed from Poland and aresometimes used to graze fens in Europe (e.g. WickenFen, Norfolk and Suffolk Broads, England). Both Heckcattle and Polish Konik horses should be regarded asprimitive breeds only, and not directly equivalent totheir wild ancestors. Both Heck cattle and Konik horsesavoid nutrient-poor sedge vegetation in the same way asred deer when nutrient-rich food is available (Vulink2001). However, both of these large ungulate speciesresemble their extinct wild ancestors, and so have beenintroduced to some nature reserves in Europe (J.P. Bakker1989).

Cattle have been re-introduced experimentally intonature reserves in Europe in an attempt to reduce domi-nant species and increase species richness; however,species-rich meadows did not redevelop in the SumavaNational Park, Czech Republic even several years aftercattle were re-introduced. In fact, cattle re-introductionhad a negative effect on species richness there, probablybecause of small-scale differences in grazing pressure(Matějková et al. 2003). Such effects were found also ina mosaic landscape with wet sedge meadows and driergrasslands. The dry sites were overgrazed and the wetsites were avoided and remained ungrazed (J.P. Bakker& Grootjans 1991). Cattle re-introduction may be ofquestionable value because cattle may reduce speciesrichness. However, over time shrubs sometimes prolif-erate in abandoned fens (after 10-15 years), so that inthat sense, excluding cattle in former pastures mayfurther reduce species richness as shrubs develop andshade herbaceous species (Preiss 1982; Jensen &Schrautzer 1999, Diemer et al. 2001; Middleton 2002a;Matějková et al. 2003). Following the Intermediate Dis-turbance Hypothesis (Connell 1978), low to moderatelevels of cattle grazing could be beneficial to thebiodiversity of lightly drained fens that have not beenovergrazed in the past. In such cases, the cattle may beable to maintain rare species of herbs within shorterstands of fen vegetation.

Any benefits of cattle grazing to the biodiversity offens would be highly dependent on the level of grazinginduced by the cattle, and this in itself is a complicatedissue. The intensity of cattle grazing depends on theproductivity and food requirements of the cattle breed,number of cattle, the life stage of the animal, duration ofthe grazing period, and the productivity and energy con-tent of the vegetation (Proulx & Mazumder 1998). Toavoid health problems in the cattle, these cattle pastureswould require a considerable amount of mineral soil(Elsäßer 2000), but soil heterogeneity could lead to localunder- and overgrazing (J.P. Bakker & Grootjans 1991;

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Matějková et al. 2003). Therefore, we only advocateusing grazing as a management tool in the case of aban-doned nutrient-rich fens (not nutrient-poor fens) evenfrom the perspective of animal health. High amounts ofgrazing pressure results in biodiversity increase in somecases, and this would suggest applying high grazingpressure, except that high grazing pressure increases soildegradation due to cattle trampling. If there are anybenefits to the biodiversity of fens from cattle grazing,these benefits would be realized only at very low stockingdensities. Because of the irreversibility of soil degrada-tion, we can only advocate using very low grazing densi-ties. Such low grazing pressure is often not sufficient tohalt or reverse biodiversity losses (Schrautzer et al. 2004)so that additional measures to remove excess biomasswould remain necessary (Kleyer 2004).

Cutting and mowing

Considering the problems inherent to cattle grazing,mowing or cutting may be more appropriate to managefor biodiversity in fens. Two studies have compared theeffectiveness of grazing versus mowing in undrainedcalcareous fens (Stammel et al. 2003) and in anabandoned fen meadow (Hald & Vinther 2000). Bothstudies showed that mowing resulted in somewhat higherspecies richness than grazing. In a study of montanegrasslands in Switzerland designed to tease apart theeffects of vegetation removal vs. other aspects of cattledisturbance, herbage removal was found to be the mostimportant element of grazing in the maintenance ofvegetation (Kohler et al. 2004). These studies suggestthat it is the removal of the biomass that is important inthe maintenance of biodiversity, and not necessarily thepresence of the cattle in the process of biomass removal.

In Europe, hand mowing has been used successfullyin the management of fens, sedge meadows, and wetgrasslands (Hansson & Fogelfors 2000; Øien & Moen2001; Mitlacher et al. 2002; Billeter et al. 2003). Mowingreduces litter, increases light availability at the groundlevel, and so therefore enhances seedling recruitmentand species performance (Kotowski et al. 2001; Jensen& Gutekunst 2003); for instance, re-instating a mowingregime with hay increased the population size of the rarefen orchid Nigritella nigra ssp. naira in Norway (Moen& Øien 2002). Species such as Viola palustris performbetter at higher light availability (Jensen & Meyer 2001;Kotowski et al 2001); however, mowing only increasesspecies richness if the species are still part of the standingcrop of the community, are in the seed bank, or candisperse to the site (Billeter et al. 2003). While mowingis a widely practiced way of maintaining biodiversity infens in Europe, we do not know of any projects in NorthAmerica that use mowing for managing fens.

Burning

In North America, burning is used to manage thebiomass of dominant species in fens. Without fire, evenfens that have never been grazed in the past begin to losebiodiversity, not because of the proliferation of domi-nating woody species but because of shading by litter(Middleton 2002b). After a fire in a fen not damaged bygrazing, a flush of native herbaceous species appears inthe fen for about two seasons (Kost & De Steven 2000;Middleton 2002b). Overgrazed wetlands, however, donot necessarily increase in biodiversity after prescribedfire; instead they may have an increase of non-nativespecies (Maret & Wilson 2000; Middleton 2002b). Incases where woody species are established, burning,cutting, and herbicide treatments may be necessary toremove woody species (Anderson 1990; Bowles et al.1996).

In North America, prescribed fires are generally setin wet conditions during the spring, after the snow meltsbut before the growth of vegetation (Doris Ruesch,Wisconsin Department of Natural Resources, pers.comm.). Winter burning does not seem to be successfulin reducing the cover of shrubs, even though it does playa role in temporarily increasing the biodiversity of her-baceous species (Middleton 2002b). In Europe, burningis generally considered destructive in peatlands (Klim-kowska 2004) and is not commonly used to managefens. If fire is used, it is recommended for the winter orearly spring season, so that the peat does not catch fire(Anon. 2005).

Priority research questions for fen management

Several research questions emerge after a review ofthe literature of the maintenance of fen biodiversity inEurope versus North America. We list several researchareas that have been inadequately addressed in pastresearch, and deserve further exploration in the future.

1. Native ungulate research to determine the poten-tial for native herbivores to increase the biodiversity offen species, e.g. Moose in northern fens in North America,and Elk in Europe. Research is necessary to determine ifnative herbivores maintain biodiversity, and in particu-lar, if they reduce the amount of shrubby species in fensabandoned for cattle grazing. Also, the suitability ofspecies of herbivores needs to be examined within thelandscape setting, particularly because very large ungu-lates, e.g. North American moose are undesirable alonghighways and in urban settings.

2. Research on the re-introduction of cattle intoabandoned fen pastures with shrub proliferation todetermine if cattle can reduce the amount of shrubcover, and increase the biodiversity of fen species either

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from the seed bank or dispersal via the animals or viarestoration of species via re-seeding or re-planting.

3. Interaction between the effects of grazing densityand water table. As stated before, there is a positiverelationship between stocking rate and the amount ofbiomass reduction. At the same time, high animal densitypromotes soil degradation, especially for organic soilswith high water tables.

4. Cattle are not native in North America, but theycould play some positive role in the restoration of drainedor formerly overgrazed fens. Research looking at therole of cattle in maintaining native biodiversity and inthe dispersal of seeds of fen species is recommended.

5. Mowing and shrub removal studies in NorthAmerica to determine if these procedures can increasethe biodiversity of fens.

6. Research on the historical role of fire in fens,particularly in Europe. Research on the potential role offire in maintaining biodiversity is necessary in fens ofvarious hydrology (e.g. drained, undisturbed). Detailedinformation is required on the desirable season andreturn interval of fire (also useful for North America).Strategies to prevent wildfires in urban areas, as well asto avoid peat fires need to be developed.

Acknowledgements. We wish to thank Jos Verhoeven andother organizers of the INTECOL conference in Utrecht (TheNetherlands) in July 2004 for making possible the symposiumand discussion on which this paper is based. Also, manythanks for comments on earlier drafts of the paper from WadeHarrell, Rassa Dale, Tracy Elsey, Beth Vairin and VictoriaJenkins. Any use of trade, products, or firm names is fordescriptive purposes only, and does not constitute endorse-ment by the U.S. Government.

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