hedgerows. an international perspective on their origin, function and management

7/27/2019 Hedgerows. an International Perspective on Their Origin, Function and Management

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Journal of Environmental Management (2000) 60, 722doi:10.1006/jema.2000.0358, available online at http://www.idealibrary.com on

Hedgerows: An international perspectiveon their origin, function and

management

J. Baudry*1, R. G. H. Bunce and F. Burel

A framework for the ecological study of hedgerows is given, before a short review of problems relating totheir definition. Hedgerows have developed in response to the inherent physical and biological character ofthe region concerned, in conjunction with cultural factors. Traditionally, hedgerows were an important sourceof wood as well as other products, although such functions are in decline in many regions. They still have an

important role in the landscape for soil protection and act as barriers and boundaries between management

units. Although the closed landscapes of the Atlantic fringe, termed bocage are often considered as thecore of hedgerow distribution, there are many other regions, not only in Europe, but also elsewhere in theworld with equally dense networks. It is concluded that a review of their distribution would be of great interestfor the design of appropriate policies for landscape management and conservation of hedgerows. Such

regions show marked differences in hedgerow character, species composition and the type of managementand there is a degree of convergence between them. The diversity of hedgerows stems from their multipleorigins and includes their role in the landscape, as well as their contribution to biodiversity and function. It isemphasised that hedgerows have often changed roles in history and current regulations for their protection

must be based on sound science and a precise definition of their role and contribution to social objectives.

2000 Academic Press

Keywords: hedgerows, landscape, diversity, management, products, distribution, functions,biodiversity.

Introduction

Hedgerows, or related structures such asrows of trees or shrubs, are conspicuouslandscape elements in many parts of theWorld. They are human-made features andhave contrasting roles recognised by differentpeople, from those who plant or let themgrow, to casual visitors and ecologists. Today,hedgerows are highly regarded primarilyfor their ecological and cultural values, for

example as recognised by the HedgerowsRegulation 1997 legislation in the UK. Inthe past other functions were dominant e.g.boundaries, shelter, sources of products andeven as snow breaks by railways in Poland.The change and subsequent loss of pastfunctions are the main reasons for the threat

1 Email address of corresponding author: [email protected]

Corresponding author

Institute National de la

Recherche Agronomique,

SAD-Armorique research

unit, 65, rue de

Saint-Brieuc, 35042

Rennes Cedex, France

Centre for Ecology and

Hydrology, Merlewood

Research Station,

Grange-over-Sands,

Cumbria, LA11 6JU, UK

Centre National de la

Recherche Scientifique,

Universit e de Rennes 1,

UMR ECOBIO, Campus de

Beaulieu, 35042 Rennes

Cedex, France

Received 11 October

1999; accepted 26 June

2000

and loss of hedgerows. This paper exploresthe geographical distribution of hedgerowsand provides a classification according totheir function in the landscape. It thereforecould provide a stimulus for the definitionof regulations for protection and manage-ment.

Agriculturists have praised hedgerows fortheir utility for centuries, (see Rozier, 1784).Geographers and ecologists have studiedhedgerows for almost a century (Alexan-der, 1932; Bates, 1937). Geographers, ecol-

ogists and conservationists showed increasedinterest in hedgerows and bocage land-scapes in the 1950s and 1960s, when muchremoval occurred. In Great Britain Pollardet al. (1974) and France INRA et al. (1976)described important research programmesthat had been carried out. Although thepresence of hedgerows has long been recog-nised in many parts of Europe, for example

03014797/00/090007C16 $35.00/0 2000 Academic Press


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8 J. Baudryet al.

in Germany and Switzerland, the majorityof research in the last 2 or 3 decadesconcerns the Atlantic zone of western Franceand Britain. Despite this narrow perception,hedgerows are actually important landmarksof human settlements and agricultural land-scapes throughout the world. However, thedescription anddistributionof hedgerowshasnot yet been widely carried out, althoughRackham (1988) has already pointed out thathedgerows do have a much wider distribu-tion than many people realised. Shelterbeltsin prairie landscapes and fences in the trop-ics are well known linear features sharingmany characteristics with temperate zonehedgerows, and are composed of trees andshrubs. They are adjacent to agriculturalfields and form networks in a similar struc-ture to that seen in Europe, although theyare of different origin.

The continued and expanding interest in

hedgerows stems from their ecological andcultural value andbecause they actas refugesfor species unable to exist elsewhere in farm-land. They are also artefacts of vanishingrural cultures. From a scientific point of view,the development of landscape ecology pro-vides new concepts, methods and theories togain insight on hedgerow function and theirecological status (Forman and Baudry, 1984;Burel, 1996). Studies of cultural landscapes(Rackham, 1986) stress the diversity of treemanagement, related to technical, and socialfactors of hedgerows, as well as other func-

tions e.g. protection from erosion.

What is a hedgerow?

Definition

Few studies attempt to give a formal defini-tion of what constitutes a hedgerow beyondthe statement that it is a row of shrubs and/ortrees managed in various ways (althoughPollard et al. (1974) do devote a chapter

to hedgerow description). We propose thatalthough any row of woody vegetation can bea corridor or a barrier for ecological processes,it may not bea hedgerow if it isnot managed.A hedgerow, whether planted or spontaneous,always has a human component and is moreor less managed to provide control and to pre-vent its expansion into adjacent fields. Most

of the time a hedgerow is a field boundaryand its management forms part of farmingactivities. As we shall see, this human com-ponent is essential in shaping hedgerows. So,we define a hedgerow as a linear feature com-posed or shrubs and/or trees that forms partof a management unit. Different definitionsmay be used over the minimum length con-sidered to be a hedgerow depending upon theobjectives of study e.g. the minor differencesbetween Barr et al. (1993) and Cooper andMcCann (1997). Hedgerow is a generic termscientists use, but it also exists as in a vari-ety of local names that are the product of thecultural, technical and social values of theregion concerned. This subject has been anal-ysed and debated in several social groups,including scientists and others who are inter-ested in the perception and language usedto describe hedgerows (Oreszczyn and Lane,1999).

The landscape elements we define ashedgerow are, most of the time intercon-nected into networks. When it comes toanalyse hedgerows, or to make decisionson individual hedgerows, the question thatarises is where are the limits?. For sur-veys, evaluation and mapping for definingthe resource in a given area, one needs todefine the hedgerow unit. Hedgerow inter-sections are identifiable and provide tangiblepoints (a hedgerow unit is defined as thesegment of hedgerows between two intersec-tions), but other measurements are possible.

For example, Baudry and Thenail (1999)have used the limits of land-use units, i.e.there must be a single type of land useon one side of a hedgerow. This definitionenabled them to make links between landuse, land managers and hedgerow character-istics. In contrast, the Institute of TerrestrialEcology (ITE) use homogeneous units forthe Countryside Survey with a hedge beingdescribed as a boundary, or part of a bound-ary which comprises a row of bushes or lowtrees growing close together, and which havebeen managed through cutting to maintain a

more-or-less dense linear barrier (Barr et al.,1993). A minimum length of 20m is usedtogether with description in terms of codecombinations to define the character of thehedge for example, height and proportion ofgaps. The length of each boundary segmentis determined by the constancy of a combi-nation of codes along the length; where any


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Hedgerows 9

one description differs, then a new length isdemarcated, and a new combination of codesapplied. Other definitions have been used e.g.Cooper and McCann (1997) but work in ITEhas shown that a precise definition of thedelimitation of hedgerows can enable com-parisons to be made between independentstudies.

Origin and distribution

French geographers use the word bocageto mean a landscape where hedgerows arecharacteristic features. Bocage has beenused some time in English in the past,and here we use it because the expressionhedgerow network landscape is inelegant.We approach the distribution of bocage,rather than hedgerows, which are of minorimportance if few are present in a land-

scape.European rural geographers of the Atlanticfringe have worked for decades on bocage, asopposed to open landscapes. In France workby Bloch (1931), Meynier (1970), and Flatres(1979) described regional differentiation. InBritain, Pollard et al. (1974) produced thedefinitive work on British hedges describ-ing their history, status and ecological rolein the British landscape. Rackham (1986)provided further details on the historicalbackground to hedges, even dating them backto Roman times, with two chapters on hedges

and hedgerow trees. Less detailed studiesdescribe the bocage landscapes of Galicia(north-west Spain) (Bouhier, 1979) and Jut-land (western Denmark) (Bazin, 1994). Wecan also find such landscapes in north-ern Spain, Italy, Switzerland, Belgium andGermany. Shelterbelts are present in someparts of Poland and Ukraine. In their his-tory of the Cretan landscape, Rackham andMoody (1996) describes a variety of hedges,some being almost English in character.Hoskins (1955) describes how in medievaltimes the ancient enclosures were often

taken out of woodland. He mentions thathawthorn (Crataegus monogyna) is the old-est species used for hedgerows and pointsout that it even gets its name from theold word haga, a hedge or enclosure. Atthe beginning of the eighteenth centuryin Britain the landscapes were still dom-inated by the medieval open-field system

but the parliamentary Enclosure Acts grad-ually replaced these by a chequer-board ofsmall fields enclosed by hedges of hawthorn.These hedges were planted, usually withditch alongside in the midlands, but oftenon a low bank in the north and west ofEngland. Trees were also often planted inthe new hedges. Rackham (1986) gives fur-ther details of how the Enclosures affectedthe landscape, and also points out that innorth and west Britain the process of enclo-sure is not as well documented as the mid-lands and the south. In north-west Englandhedges by old roads often have many woodyspecies, whilst those across fields are virtu-ally pure hawthorn, suggesting that theseare the planted Enclosure hedges (pers.observ. R.G.H.B.). Therefore, the view thatBritain has one of the most dense ancienthedge networks in Europe is not strictlytruerather the majority of hedges are rela-

tively recent.The distribution of hedgerow types isdependent upon a combination of the inher-ent environmental and cultural factors.There is therefore convergence between thecharacteristics of the hedgerow in similarcombinations of circumstances e.g. gorse(Ulex europaeus) occurs on hedge banks inthe celtic regions of western Ireland, westCumbria, west Wales, Brittany and Gali-cia, all of which have poor soils and areexposed to Atlantic gales. Elsewhere, whilstsome regions as mentioned above, have well-

defined networks, other areas are less wellknown but have equally dense networks,for example, the northern slopes of the cen-tral Pyrennees. Another distinctive feature ofhedgerows in Europe is in the developmentof the local use of planted exotic species thathas led to the creation of locally distinctivehedgerows that have become well knownforexample, the Fuschia (Fuschia magellanica)hedges of Ireland and the beech (Fagus syl-vatica) hedges of south-west England. Anindependent assessment of hedgerows notonly in Europe but also elsewhere is therefore

long overdue.OutsideEurope, studies arescarce. Despois

provides the first description of tropicalbocages in Africa (Lauga-Sallenave, 1997),where villages are surrounded by hedgerowsto protect gardens. Such situations are alsofound in Northern Ireland where there isno discontinuity between rural hedgerows


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10 J. Baudryet al.

and those surrounding the gardens of housesset in the countryside. Hedgerows arealso extensive in Ecuador and in Boliviawhere they separate paddocks, or gar-dens. In China, hedgerows are also foundmostly around villages, for example, Zhen-rong et al. (1999) describe field bound-aries in two subtropical Chinese villages.Twenty-three percent of field boundarieswere hedgerows. Most of the time a sin-gle species of tree is planted e.g. dawnredwood (Metasequoia glyptostroboides) orpoplar (Populus spp.).

In North America, hedgerows and shelter-belts were planted by early European settlerswho wanted to recreate their homeland land-scapes (Hewes, 1981; Sutton, 1985), and alsoas a means to manage and protect soils(Nabhan and Sheridan, 1977). Baltensperger(1987) describes considerable changes inhedgerow distribution in the Midwest since

World War II and the development of irri-gation systems and roads. Osage Orange(Maclura pomifera) was the principal treeused for planting. In eastern North Americaand Canada, colonisation of field boundariesby bird dispersed species is widespread (For-man and Baudry, 1984; Wegner and Mer-riam, 1990). Such fencerows can form densenetworks, but are now often under threatfrom mechanised farming and have oftenbeen removed.

Management is an integral feature ofhedgerows, as opposed to lines of shrubs,

developing spontaneously along terrace wallsin the Mediterranean region, and there aremany styles, which have developed duringthe evolution of cultural landscapes. Virtu-ally all hedges are cut back in some way,whether using traditional implements asdescribed by Rackham (1986) or by mod-ern machines. These long-term methods ofmanagement also show convergence e.g. cop-picing (Britain and France) and interweaving(Britain and Spain). Various forms of lay-ering are present in many areas but thedetails of the practice are invariably unique

to particular regions e.g. the distinctive man-agement of hedgerow trees in Normandy andthe construction of hedgerows of dead mate-rial in north-west Spain. This short reviewshows that hedgerows are diverse in struc-ture, origin and arrangement within land-scapes. Below, we analyse the reasons forsuch diversity.

Factors contributing to thediversity of hedgerows

We propose an analysis based upon an empir-ical approach resulting from long experienceof field observation of hedgerows and studyof their ecological characteristics in differentcountries. This analysis should be helpfulto propose management and conservationpolicies to both sustain their cultural andecological heritage, and to fulfill new roles inresponse to changing functions. The fact thattrees live about twice as long as the averagefarm business emphasises their importanceas indicators of historical change as empha-sised by Rackham (1986) who describes suchfeatures as landscape ghosts.

Primary functions

Hedgerows are limits, boundaries and bor-ders. The boundary function can be bothphysical and symbolic, as a row of shrubsaround a garden with an open gate or afence or as stock-proof fences originatingfrom the Enclosure Acts. Many historiansand geographers e.g. Rackham (1986) haveemphasised the function of hedgerows asphysical property limits back to as far asRoman times. Fences can be enclosures toprevent cattle mixing with their neighboursherds: the reason for the establishment of

Enclosure hedgerows in Britain. In this case,planting hedgerows meant that new farm-ing systems were coming into being. Fencesare generally made of non-palatable speciessuch as hawthorn (Crataegus monogyna)in Europe or spurges (Euphorbia spp.) inEcuador, or shrubs e.g. willow (Salix spp.)and hazel (Corylus avellana) that are man-aged by laying. The establishment of thistype of hedgerows historically has been whenthere is a change from collective to individualuse of the land. Examples of such conver-sions are found in Brittany, France, dur-

ing the eighteenth and nineteenth centuries;in Schleswig-Holstein (northern Germany),land-consolidation programmes led to theedification of hedgerows (Luhning, 1984).Conversely, when this role is reversed,as dur-ing re-allotment programs, hedgerows tendto be removed or are under threat. Theamenity role of hedgerows is often due to


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Hedgerows 11

this shelter/limit effect, the sense of feel-ing at home (Oreszczyn and Lane, 2000)and explains why European settlers plantedhedgerowswhen they arrived in the Midwest.

Products

Hedgerows are a source of products. Woodis the most important; many forms of woodcan be produced in hedgerows e.g. firewood,timber, fenceposts. The different usages ofthe wood lead to contrastingly different formsof trees (Figure 1). For timber, branches mustbe removed as much as possible from thetrunk to produce logs and boards. A goodphysiological balance is where 30% of thelower trunk is without branches and the top70% retains its foliage. This creates the typi-cal shape of a tree, with a much larger crownthan in forest. For firewood, it is necessary to

enhance the production of branches. The typ-ical shape is a trunk from which branchesare cut every few years. A great varietyof shapes are derived from pollarded trees(a short trunk and a crown of branches),shredded trees (a tall trunk with branchesalong the whole length) and coppice (a basalstump with several growing stems). The lat-ter is used for sweet chestnut (Castaneasativa) to produce fenceposts. In Europe, eachregion had only one dominant type of treemanagement, often strictly enforced by localregulations. This contributes to the cultural

characterof each regione.g. thepollardedwil-lows (Salix spp.) of Somerset in south-westEngland and the candelabra ashes (Fraxi-nus excelsior) in northwest Spain. In tra-ditional rural societies wood was the onlysource of energy for cooking and heating andpeasants therefore needed to have access tosuch a resource. Often farm hedgerows werethe only available source of wood to them,as woodlands were often owned by largelandowners. The use of wood is still impor-tant in some areas. In northeastern Brittany(France), Thenail interviewed 65 farmers

regarding wood consumption (Baudry andJouin, unpublished data). She found thatalmost all of them utilise wood from theirhedgerows and annual consumption as fire-wood varies from 130m3, with a medianaround 15 m3. Half of the farmers use woodas the main source of energy for heating theirhouse, half have a fireplace as an extra source

ofheat. The same farmersproduce anaverageof 135 fenceposts per annum. This exampledemonstrates that hedgerows still have aneconomic value, though difficult to evaluate.Fodder production from elm (usually Ulmusglabra) and ash was important in severalregions and especially in mountains whereash leaves were kept as winter forage e.g. inNorway, in the Picos de Europa in north-westSpain, and in the French Alps (Martin, 1999).This function led to the planting of manytrees of these two species. These species canbe mixed with each other but are more abun-dant near farmsteads. Other products maybe fruits e.g. blackberry (Rubus fruticosus),medicinal plants e.g. nettles (Urtica dioica),material for tools e.g. ash, baskets e.g. willow(Salix spp.) and even vegetables e.g. blackbryony (Tamus communis) in central Spain.

Functions in a landscape

At the landscape level, hedgerows have amajor role in controlling physical, chemi-cal and biological fluxes, as well as beingcultural indicators. Hedgerows and associ-ated elements (ditches and earth banks) havebeen made to control physical fluxes, such aswater, for drainage or irrigation, soil parti-cles, to diminish erosion or wind. This latterfunction gave rise to the name windbreak,which are widely used in steppe areas ofEurope and northern America. In this case,

efficiency requires that hedgerows be set upas networks through the landscape. To con-trol water, they must be along major soilboundaries. This role is still important todiminish the speed of water runoff and pre-vent uneven stream flow through the year(Merot, 1999). In regions where subsurfaceflow is important, control of water has beena major impetus in landscape design, usingboth ditches and hedgerows. In Brittany,many hedgerows are used for water flowcontrol and are apparent in the landscape,as well as on cadastral maps. On moorland

cleared one century ago, such patterns do notexist. It seems that the evolutionary processof landscape design took a long time to adaptthe ditch networks to soil and topographicconditions. Strong patterns can therefore beseen in older landscapes. Even in open land-scapes former functions may still be visiblesince, even after they have been filled with


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Figure 1. Six examples of the diversity of hedgerow structures. (a) typical pruned (shredded) oaks inBrittany, France (height: 57 m); (b) pollarded oaks (height: 152m); (c) a dense hedgerow of shrubsoriginating from coppiced stools; (d) a coppiced hedgerow before and after wood harvesting; (e) a doublehedgerow on a bank, Normandy, France with each row of trees belonging to the adjacent field; (f) ahedgerow of shredded oaks with associated ground flora.


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Hedgerows 13

earth, formerditches continue to collect waterbecause of the accumulation of clay parti-cles along the sides and bottom. Researchon the role of hedgerows as a buffer againstnitrates and for water protection is growing,along the lines of hydrology (Caubel-Forgetand Grimaldi, 1999).

Hedgerows control biodiversity being habi-tats, refuges, corridors or barriers. Thesefunctions are critical for many plants andanimals that otherwisecould notexist in agri-cultural landscapes (Burel, 1996). Predatorsin hedgerows can also buffer cyclic-pest out-breaks, such as the ones of voles (Delattreet al., 1999).

The cultural and ecological values ofhedgerows stem from the uniqueness ofthe integration between woody species, localrules, techniques and cultural history of agiven region, in conjunction with its inherent

ecological character. The diversity or the uni-formity of roles as seen by a single owneror the local society produced the diversityof hedgerow. This historic legacy constitutesthe landscapes that must now be man-aged.

Table 1 gives a summary of hedgerow func-tions and associated hedgerow features andplace in the landscape. Figure 2 providesa visual assessment of a bocage landscape.Hedgerows are assessed for their own prop-erties and for their place in the landscape.Figure 3 gives two examples of the role ofhedgerows in contrasting British landscapes.In Figure 3(a), hedgerows have almost losttheir original function because of the domi-nance of crop production in the landscape. InFigure 3(b), hedgerows are still important forshelter for stock and as boundaries but arethreatened by lack of management.

Table 1. Hedgerow functions at site and landscape level, and associated hedgerow features and place inthe landscape

Level Physical functions Biological functions Cultural andHabitat, corridor, amenity functions

refuge, barrier

Hedgerowlevel

Water fluxes: consolidatedditch bordering hedgerow

According to speciesbiological traits andecological requirements

Age and structure: Cearthenor stone bank ditch

Soil conservation: importanceof deep-rooted trees andshrubs

Density of vegetation cover Management techniques ofheritage species e.g.Taxus baccata on religioussites

Wind: dense and high tree

layer providing asemi-permeable barrier towinds

Complexity of hedgerow

structure

Diversity of plant species Presence of dead trees and

rocks Management techniques

and regimes

Landscapelevel

Water fluxes and soilconservation: location ofthe hedgerow on the slope,key points are major limitsof soil, mainly at the borderof small valleys, andperpendicular to the steep

slopes

Relationship with adjacentland use

Historical links e.g.monastic and regionallydistinct sites

Water, soil and wind:connectivity of thehedgerow network ensurescontinuity and control offluxes

Isolation of habitat patchesand linear features

Adjacency to footpaths andcountryside access

Connectivity of thenetworks

Grain size of the landscape

Grain size of the landscape


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14 J. Baudry et al.

Hedgerowattheedgeoftheplateau,

impedingrunoff

Hedgerowsperpendiculartotheslope,

impedingerodedparticlesandwaterrunoff

Villageandhousesprotected

byhe

dgerows

Refugeforperennialspecies

ofopenconditions

Hedgerowborderingtheriparian

zone,controllingfloodsandnutrients

Earthenbankwheretreesare

cutandnotreplac

ed

Hedgerowwithditch,

channellingwaterto

thestream

Corridorfor

forestdwelling

species

Figure 2. A visual assessment of some different functions of hedgerows in a model landscape.


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Hedgerows 15

(a)

Generally large fields withfew hedges/trees Open 'prairie'

landscape

Hedge network withfew connections

Hedges with limitedfunction

Hedgesoften heavilycut

Narrow headlands affectedby adjacent crops

Ground flora species dominated by competitors e.g. Galium aparine (weed),Arrhenatherum elatius (grassland),Heracleum sphondylium (tall herb)

Except forwoodyspecies,usuallyfew woodlandplants

(b)

Generally small fieldsmany hedges/trees

Closed 'bocage'landscape

Hedge network withmany connections

Hedges function asstock shelter

Hedges notusuallyovermanaged

Oftenrich inwoodlandspecies

Relatively wide headlands

sometimes with infertilegrassland species

Ground flora often rich with species from different habitats e.g.Primula vulgaris

(woodland),Stachys betonica (grassland), andAngelica sylvestris (woodland edge)

Figure 3. Two examples of model hedgerow landscapes in Britain. (a) An open landscape in East Anglia,England; (b) a bocage landscape in the West Country, England.


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Shifting functions andprotection

Nowadays, the number of conservationgroups with an interest in the management ofhedgerows continues to expand. Regulationsno longer come from local stakeholders andmanagers, with governments making lawsfor controlling the removal of hedgerows e.g.the Hedgerows Regulation 1997 legislationin the UK. The maintenance, restoration andplanting of hedgerows has been supportedin agrienvironment programs such asthe Environmentally Sensitive Area andCountryside Stewardshipschemes in Britain.The main consequence is that those whomanage hedgerows may no longer have thefull control of the decisions on them. Astheir function as boundaries or the source ofproducts declines, so novel functions increase

e.g. as reservoirs of biodiversity or as amenityfeatures. Formerly, there were local rules tomanage and plant or remove hedgerows. Inmany places hedgerows are now protected, asthey have been in historical times. The gapbetween former and novel uses, as well as thelag between recognition of new functions andthe implementation of those functions in law,is a source of concern since there has beenmuch removal.

Several issues must be addressed in thiscontext. Stakeholders and managers are dif-ferent persons. Potential conflicts therefore

exist between environmental goals e.g. thedefinition of target species for conservationand the maintenance of landscape diver-sity, where rules for notification need to be

developed. For example, the hedgerow legis-lation in Britain states that priority will begiven to lengths of hedgerow that have overfive woody species in a 30-m length, which isconsidered by many to over-simplify a morecomplex situation e.g. the historical status ofa hedgerow might be ignored.

Conservation of biodiversityIt is worth examining the biodiversity func-tion in the context of hedgerow structuraldiversity and multifunctions. Even if land-scape-scale studies are now frequent, the sin-gle hedgerow approach is still common. Thehedgerow approach seeks to define the bestvegetation structure for various groups ofspecies, biodiversity being measured as sim-ple number of species in particular taxonomicgroups. These approaches have several short-comings, even for the biodiversity function.

First, no single hedgerow can harbour allthe local species pool of a given group e.g.plants, birds or insects. This is exempli-fied in Figure 4 which shows the cumu-lative number of species recorded in theherb layer of 860 hedgerow sides in Brit-tany (France). Species are grouped in fre-quency in releves. It is worth noting that50% of the species are only present in 5%of hedgerows. The structural diversity ofhedgerows plays an important role in thesepatterns, as shady and sunny conditionsas opposed to wet and dry situations may

be present. The single hedgerow approachalso does not generally include the cyclicmanagement of hedgerows, which is partlyresponsible for the diversity of conditions.

0851

300

Herb layer relevs from the richest to the poorest

Cumu

latednumberofspecies

1

250

200

150

100

50

Total

15%

515%1530%30100%

80175170165160155150145140135130125120115110151

Figure 4. The cumulative number of species recorded in the herb layer of 860 hedgerow sides in Brittany,France. Species are grouped in % presence in relev es (D. Le Cur, unpublished data). The five curvesrepresent: species present in 15% of hedgerows; species in 515% hedgerows; species in 1530%hedgerows; species in 30100% hedgerows; and all species.


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Hedgerows 17

The implementation of an ideal managementscheme would certainly decrease the num-ber of species present. Although tree covermay decrease from time to time due to pol-larding, most authors (e.g. Petit and Burel,1998) assume that dense hedgerows, at agiven time, are efficient corridors for forestspecies movement. Furthermore, hedgerowsmay remain in the landscape but still losespecies, as described by Barr et al. (1993),and even the number of woody species candecline (Garbutt and Sparks, 1999).

Biodiversity must be assessed at the land-scape scale not only because of the diversityof hedgerows, but also because of landscape-scale processes. The corridor function of net-works of hedgerows has been demonstratedin various conditions: for plants by Baudry(1988) and Marshall and Arnold (1995); forinsects by Burel (1989) and Duelli et al.(1990); for birds by Dmowski and Kozi-akiewicz (1990) and Clergeau and Burel(1997). The effect of grain size (the aver-age size of a field surrounded by hedgerows)has been less well studied. Burel et al. (1998)compared four adjacent landscapes differingmainly in their hedgerow density. They foundthat species number decreased from denseto open landscapes for only some groups ofspecies e.g. diptera (Dolichopodidae, Empi-didea, Chiropodidae). For carabids the num-ber of species did not vary significantly,however although species composition wasdifferent, the average body size of the species

decreased from dense to open landscapes.Garbutt and Sparks(1999) pointed out thatregionally widespread species canalso invadewhen landscapes often do not support rarerspecies any more because of the expansionof common plants. Therefore, the numberof species may be a misleading objective interms of nature conservation because sometypes of hedgerow e.g. those dominated bybeech (Fagus sylvatica) may be inherentlypoor in species but nevertheless, of consider-able ecological interest.

Directions for future researchfor landscape and hedgerowmanagement

Over 15 years ago, Forman and Baudry(1984)setup a research agenda forhedgerows

based upon the emerging principles of land-scape ecology. Concepts such as corridors,connectivity and barriers have proved tobe useful in designing landscapes and theplanting of hedgerows. Research has focusedon the ecological functioning of hedgerowsand hedgerow networks. Landscape planningand management could still be improved byfurther research. Landscape ecology, land-scape planning, public policies, and farm-ing system research are the main areasof research that need to be followed infuture.

Landscape ecology represents a moveupscale from individual hedgerows to afew km2; a regional approach (several km2)should be the next move up. Bocage land-scapes are diverse and this diversity is notrecognised in planning. Does this diversitymean that different management objectivesshould be defined for each landscape? Some

landscapes need planning against soil ero-sion (wind or water erosion), others are ofhigh cultural and historical value, and othersharbour many rare species. Different objec-tives must thereforebe setforeach landscape,in order to conserve regional diversity. Theuse of a single criterion to define importanthedgerows (or any landscape feature) will bedetrimental in the long run, as hedgerowswith secondary functions may be removed,though these functions may appear impor-tant later, or in a different context.

The sustainable use of agricultural land-scapes requires that these landscapes shouldbe multifunctional, not only devoted tofood/fibre production, but also to hydrol-ogy, amenity and biodiversity. Formerly,land-consolidation programmes were aimedspecifically at regrouping the parcels fromdifferent farms in order to facilitate the use ofmachines and save time, leading to hedgerowremoval and landscape change (Braekevelt,1988). The policy, in itself, was not the cause,rather the use of land solely for production.However, hedgerows have been removed out-

side planning operations (Burel and Baudry,1990). Land-consolidation programmes canhowever, also be used to restore landscapes(Pointereau and Bazile, 1995). The imple-mentation of new policies necessitate thedefinition of new planning principles, sonew knowledge on the ecological function-ing of landscapes is necessary. Among the


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remaining questions, the following need to beconsidered:

(1) What should be the spatial componentsof a bocage (size of the landscape, connec-tivity, grain) to sustain various types ofspecies? Few studies on hedgerows andbiodiversity make reference to metapop-ulation theory (but, see Petit and Burel,

1998), although this concept can be use-ful to define the geometry of a landscape,in conjunction with species life historytraits.

(2) Is it better to have a bocage with a homo-geneous grain (as most studies assume),compared with a landscape where finegrain and coarse grain areas are bothpresent? For example, Burel et al. (1998)showed that a diversity of adjacentlandscapes (a few thousand hectares)increased the total number of speciespresent. Such complexity makes it dif-

ficult to provide insight for landscapeplanning which needs to take account ofthe scale of landscapes and their arrange-ment in space.

(3) Do hedgerow and landscape structurerequirements for a given group of species(i.e. passerine birds) complement orconflict with those of other groups (i.e.butterflies)?

Hedgerows are usually on farms, but stud-ies on how hedgerow types or landscapestructure relate to farming-system types are

scarce. The conflict between large-scale cash-crop farms and small scale mixed farms interm of landscape is not very useful for

policy-makers or planners; it would be moreuseful to know, within a type of farming sys-tems, how much and why hedgerows areretained or removed. The common assump-tion is that larger fields are easier to farm,so field enlargement diminishes the cost offarming. This leads to the cause and effectsrelationships depicted on Figure 5.

Findings from a recent study in France bya national association of farmers (FederationNationale des Cooperatives dUtilisation deMateriel Agricole) challenge this assumption.Measures in fields and simulation demon-strate that above six/eight hectares, the gainof increasing field size is very low. In contrast,travel from farm to fields is very expensive(Francart et al., 1998).

Decisions on hedgerow management aregenerally made by farmers. They may dothe work themselves or hire workers. Thehedgerow-management regime in a given

place depends on its function. Whilst firewoodis harvested in cycles of several years (6 to12), some hedgerows are trimmed each year(e.g. hawthorn hedgerows). In the formercase not all hedgerows of the farm, or thelandscape, are managed in any given year.How therefore does the spatial distributionof management regimes from year to yearaffect the visual appearance of landscapes orthe population dynamics of species?

The general public has a positive viewof hedgerows and of their role in conserv-ing biodiversity. Farmers have mixed feel-

ing, as hedgerows may harbour both pestsand beneficial organisms (Lemaire, 1980;Thresh, 1981). Although the importance of

Decrease offarmers number

Increase ofmean farm size

Abandonment

Increase of fieldsize

Increase ofmanagement

intensity aroundarable fields

Decrease of thedensity of tree andshrub layers

Removal ofhedgerow

Hedgerows become parts offallow lands or earlysuccessional stages of forest

Figure 5. Threats to hedgerows likely to be caused by diminishing farming communities.


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Hedgerows 19

(a)

(b)

(c)

Figure 6. Three examples of hedgerow landscapes. (a) An aerial view of a bocage landscape in Quebec,Canada; with spontaneous hedgerows (photo S. De Blois); (b) a distant view of a hedgerow landscape inthe Andes, Ecuador (photo J. Baudry); (c) a view of a Brittany, France landscape (photo J. Baudry).


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20 J. Baudryet al.

hedgerows, and field margins in general,for biological control has been claimed andassessed for decades (Altieri, 1999), it is notyet known how beneficial the presence of pestpredators in hedgerows is for crop protection.

Policy-makers argue that agricultural poli-cies are becoming more environmentallyfriendly. In fact, market forces are stillvery strong and can overwhelm policy objec-tives (Winter, 2000). In the field of socialsciences the policy approach must be com-bined with studies at the level of individualfarmers and the reason for their behaviourtoward hedgerows. It is important that farm-ers accept hedgerows and maintain them. Asfor other environmental schemes, knowledgeof the restrictions for hedgerow acceptanceare probably a mix of perception and withinfarm constraints (Battershill and Gilg, 1997;Beedel and Rehman, 2000).

Conclusion

Hedgerows are important elements of manylandscapes around the word. They have ful-filled many functions in the past and stillhave a major role in the ecological healthof the countryside. Therefore, regulationsregarding hedgerows must now not onlyconsider the various social, historical, eco-logical and production functions, but alsotheir diversity of structure and species com-position (Figure 6). However, these issuesare seldom addressed in most of the regu-lations protecting hedgerows. The range ofvariation of hedgerow characteristics andfunctions still need to be fully described atEuropean and inter-continental levels, whichshould then actas a stimulus for conservationmeasures by identifying rare and threatenedlandscapes. This is a major challenge for sci-entists, policy-makers and planners. Bocagelandscapes are complex with a diversity offunctions, which have developed and changedover centuries, but nowadays they should

be regarded as sustainable landscapes andshould be managed as such.

Acknowledgements

J.B. and F.B. thank the research department ofThe Ministere de lAmenagement du Territoire

et de lEnvironnement for its continuous support.They also benefited from the financial support ofseveral European projects: Field boundaries forwildlife and environmental protection (AIR3-CT920476/920477), Impacts of agriculture intensifi-cation on resources use sustainability and foodsafety and measures for its solution in highly pop-ulated subtropical areas in China (STD TS3 CT920065) and the ALFA project B7-3011, Sustain-able Use and Promotion of Natural Resources and

Biodiversity. We thank Duncan McCollin and ananonymous reviewer for comments.

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