breeding birds in short-rotation coppices on farmland in central sweden the= importance of salix hei

Agriculture, Ecosystems and Environment 90 (2002) 265–276

Breeding birds in short-rotation coppices on farmland in centralSweden—the importance ofSalixheight and adjacent habitats

Åke Berg∗Department of Conservation Biology, The Swedish University of Agricultural Sciences, P.O. Box 7002, S-750 07 Uppsala, Sweden

Received 23 June 2000; received in revised form 13 March 2001; accepted 21 March 2001

Abstract

Plantations of short-rotation coppice (SRC) have a potential for being a useful measure to stop the ongoing impoverishment offarmland biodiversity by increasing structural diversity, and decreasing cultivation intensity and use of pesticides in intensivelymanaged farmland landscapes. The aim of this study was to investigate the relative importance of the structure of the plantationand composition of adjacent habitats for breeding birds in 41 SRCs (mean size±S.E. = 9.4±1.6 ha). Mean number of speciesper ha in the SRCs was 2.8±0.3 species and there were more farmland birds (32 species, 808 territories) than forest birds (22species, 400 territories). A major result of the present study was the strong influence of adjacent habitats on bird communitycomposition (18 of 22 analysed species affected). There were large differences in bird communities between forest-borderedand open-bordered sites, but occurrence of residual habitats (i.e. other habitats than forest, pastures, shrub areas and arablefields) was also associated with occurrence of several species. The second factor of major importance for the bird fauna wasthe height of the plantations. Most species (14) were associated with tall plantations (>2 m), seven species were associatedwith plantations of intermediate (1–2 m) height, and six species were associated with plantations of low height (<1 m). Acomparison of ecological traits between species classified as preferring SRC and species classified as avoiding SRC suggeststhat nest height was the only ecological character associated with a preference forSalixhabitats, i.e. species with nests on theground or in shrubs were more abundant inSalixhabitats than in farmland landscapes in general. However, a broad spectrum ofspecies was found in the SRCs and many of these seemed to be dependent on habitat features outside the plantations. Plantingof Salixin intensively managed farmland plains will have positive effects on bird diversity by increasing the structural diversityof the landscape. In contrast, plantations on infields in forest-dominated landscapes will have negative effects, since the mosaicstructure (i.e. mixture of open and forested habitats) positive for most farmland birds will disappear, and Salix plantationsfavour relatively few forest species. Additionally,Salixcould be planted along sharp edges between coniferous plantationsand open farmland in order to increase the complexity of the ecotone in intensively managed forest-farmland landscapes.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Energy crop; Vegetation structure; Forest border; Open-field border; Residual habitats; Bird community; Central Sweden

1. Introduction

In Sweden large areas of cereal fields have beentaken out of traditional agricultural production since

∗ Fax +46-18-673537.E-mail address:[email protected] (Å. Berg).

1985 due to changes in farming policy and overpro-duction of cereals (Kumm, 1992; Official Statisticsof Sweden, 1993). As a result, the area of set-asides(permanent fallows) has increased and new forms ofland-use, e.g. short rotation coppice (SRC), in Swe-den mainlySalix, have been introduced. The numberof Salix plantations increased considerably between

0167-8809/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-8809(01)00212-2

266 Å. Berg / Agriculture, Ecosystems and Environment 90 (2002) 265–276

1991 and 1996, due to subsidies for SRC, higher taxesfor fossil fuels and the existence of a biofuel marketfor forest fuels in Sweden (Rosenqvist et al., 2000). Atpresent SRCs cover approximately 0.6% of the farm-land area in Sweden (Official Statistics of Sweden,1998), and a high concentration ofSalix growers isfound in central Sweden.

During the same period, researchers and conserva-tion biologists have aimed at identifying measures tostop the ongoing impoverishment of farmland biodi-versity in Europe (Marchant et al., 1990; Robertsonand Berg, 1992; Svensson et al., 1992; Tucker andHeath, 1994; Fuller et al., 1995). Restoration andmanagement of natural grassland have been given pri-ority in several countries, including Sweden (Götmarket al., 1998; Pärt and Söderström, 1999). Other mea-sures have included conservation headlands (Boatmanand Wilson, 1988), use of set-asides (Sotherton, 1998)and conservation and management of hedgerows andother non-crop habitats (Osborne, 1984; Parish et al.,1994). Use of SRCs could potentially have similarbeneficial effects on farmland biodiversity, since theyincrease structural diversity in intensively managedfarmland landscapes. Most SRCs are harvested everythird to fourth year. Furthermore, management ofSRCs is more extensive (i.e. restricted cultivation andrestricted use of pesticides) than that of intensivelymanaged cereal crops, although weeds are controlledfor mechanically or chemically in newly establishedstands and fertilisers are mostly applied to growingstands (Gustafsson, 1987). Intensive management ofannual crops has been viewed as a contributing causeto the decline in abundance of several agriculturalbird species (e.g. O’Connor and Shrubb, 1986). Fur-thermore, modern forestry (i.e. forest plantations)have changed farmland landscapes by sharpeningof forest edge habitats and loss of broad and struc-turally complex edges, which are important habitatsfor many farmland plants and animals (e.g. Fry andSarlöv-Herlin, 1998).

Several investigations have pointed out positiveeconomic and environmental effects of SRCs inrelation to annual food crops (Perttu, 1998, 1999;Börjesson 1999a,b). Suggested environmental bene-fits of SRCs and use of biomass fuels have included,e.g. nutrient and heavy metal uptake, possibilitiesto use SRCs as wastewater filters (Perttu, 1998),reduced wind erosion and reduced net flow of CO2 to

the atmosphere. Coppices may therefore play a rolein a society based on recycling principles (Börjesson,1999a). Few studies have investigated the bird faunain different types of SRCs (Sage and Robertson, 1996;Christian et al., 1997), and Swedish studies have beenrestricted to a few sites in the southernmost parts ofthe country (Göransson, 1990, 1994). These studieshave focused on effects of structure and height ofthe SRCs (i.e. local habitat quality). However, manyvertebrates, especially in farmland, seem to use mul-tiple habitats (With et al., 1997; Law and Dickman,1998) and are therefore dependent on certain com-binations of habitats for their breeding (Green et al.,1994). The importance of adjacent habitat type hasbeen emphasised in studies of farmland habitats withrestricted areas, such as set-asides (Berg and Pärt,1994), seminatural pastures (Pärt and Söderström,1999) and poplar plantations (Hanowski et al., 1996).In small habitat patches, edge length and compositionof surrounding habitats are likely to have a stronginfluence on the communities of different organisms(e.g. Saunders et al., 1991; Andrén, 1994; Hanssonet al., 1995). Thus, additional studies of the bird faunain SRC that include these aspects are needed.

The aim of the present study was to investigate therelative importance of local habitat (i.e.Salix heightand occurrence of residual habitats, such as ditchesand small roads) and structure of adjacent habitats(amount of different field types, seminatural pastures,shrub habitats and forest) for birds breeding in SRCs.Furthermore, possibilities to increase the diversity ofbirds in farmland landscapes by planting SRCs arediscussed.

2. Methods

2.1. Study sites and habitat mapping

About 41 SRCs (i.e.Salix plantations planted forfuel production) in Uppland and Västmanland (ap-proximately 59◦40′–60◦7′N and 16◦30′–18◦10′E) incentral Sweden were surveyed for breeding birds byterritory mapping in 1997. The sites were situated ina landscape gradient from farmland plains to smallfarmland areas in forest-dominated landscapes. Fur-thermore, SRC area, habitat composition within sites(i.e. shrub height) and adjacent to the coppices varied

Å. Berg / Agriculture, Ecosystems and Environment 90 (2002) 265–276 267

greatly between sites. The sites were, as far as possible,selected to include sites with different shrub height,situated in landscapes with different amount of forestand crop fields in the surroundings.

A total of 384 ha of SRC was surveyed (mean±S.E.)area was 9.4± 1.6 ha (range 1.4–43.6 ha). The heightof the Salix shrubs was classified into three cat-egories (<1 m, 1–2 m and >2 m), and the area ofdifferent height categories at each site was measuredon a digitising table. The lengths of different ad-jacent habitats were measured at the border of theSRC. The following habitats were separated; cropfields (mainly spring-sown cereal crops), cultivatedgrassland (leys and cultivated pastures), set-aside,seminatural pastures, shrub areas outside seminatu-ral pastures (mainly withJuniperus communis, Rosaspp. andPrunus spinosa), and forest. Residual habi-tats included houses and gardens, roads, ditches and,streams or rivers within or at the border of the site.Occurrence of within-field islands (small patcheswith natural vegetation) and barns in the site wasalso noted.

2.2. Bird censuses

Each site was surveyed for breeding birds by ter-ritory mapping (Svensson, 1975; Bibby et al., 1992)seven times during early mornings (mainly from sun-rise to 10.00 h), once in each of the periods 15–30April, 1–10 May, 11–20 May, 21–31 May, 1–10 June,11–20 June and 21–30 June. All sites were also vis-ited once at night (mainly 23.00–02.00 h) during theperiod 15–30 June. No counts were made on morn-ings with strong wind or rain. All bird observationswere denoted with symbols on maps of 1:3000–5000scale. Birds observed just outside the plots were in-cluded in order to decide whether a territory was sit-uated within or outside a plot. Territories across theborder of a site were counted as belonging to thesite when more than half of the observations fell in-side the borders. The sites were surveyed in differ-ent order at the above seven mornings in order toavoid bias due to visits at the same time, since mostbirds are more easily detected early in the morningwhen the singing activity is high. Details of methodsand criteria used to determine the number of breed-ing territories are given in Robertson and Skoglund(1985).

All bird species were classified either as for-est birds (i.e. species common in forest landscapeswithout farmland habitats), or farmland species (i.e.species restricted to farmland habitats or speciessimultaneously using both farmland and forest habi-tats), see Table 2, according to results from earlierstudies in the same region (Robertson and Berg,1992; Berg and Pärt, 1994; Berg, 1997; Pärt andSöderström, 1999). Several ecological characteris-tics of the observed species, i.e. home range size,nest site and type, food preferences and migratoryhabits were compiled from the literature. These datawere used in analyses of factors associated to pref-erence or avoidance of SRC for different species(Table 2).

2.3. Statistical analyses

A canonical correspondence analysis (CCA) wasperformed to investigate relationships between themeasured environmental variables and all bird speciesoccurring at more than five sites (ter Braak, 1987; terBraak and Prentice, 1988). The CCA identifies majorgradients in community composition that are associ-ated with the environmental variables. However, thesuitability of different ordination methods dependson the length of the gradients and a detrended cor-respondence analysis (DCA) was used in order toestimate the length of the compositional gradients.The relatively long gradients in the data set (2.5–3S.D.) verified that unimodal models (i.e. CCA)could be used (Jongman et al., 1995; ter Braak andSmilauer, 1998).

Associations between the abundance (number of ter-ritories) of 19 of the 22 most common bird speciesand habitat variables were analysed with multiple lin-ear regression with backward selection (i.e. all speciesoccurring with ≥10 territories, except lapwing thatonly occurred at two sites, Table 2). Three species(pheasant, robin and song thrush) were analysed withnominal logistic regression (SPSS, 1994), since mostobservations (98%) of these species were categorical(i.e. occurrence or non-occurrence). The level of sig-nificance (P) to enter and to be removed were set to0.05 and 0.1, respectively. Area and lengths of bordersto different adjacent habitats were log-transformed.Furthermore, the squared term of all habitat variableswas included in order to test for non-linear associa-


tions. The squared habitat variables were allowed tobe entered both alone and together with the original(not squared) variables. Furthermore, two-way inter-actions between (1) area ofSalixhabitats, and (2) ad-jacent habitats and residual habitats were included inthe models.

3. Results

Data on habitat composition at the sites are pre-sented in Table 1, including SRC area, length ofbordering habitats, length of linear elements and oc-currence of other residual habitats. A total of 54 birdspecies and 1208 territories were recorded within theSRCs (Table 2). Mean number of species (±S.E.) perha in the censused SRCs was 2.8±0.3 species. TheSRCs harboured more farmland birds (32 species and808 territories) than forest birds (22 species and 400territories). For details and classification of farmlandand forest birds (Table 2).

Table 1Descriptive statistics (mean± S.E., maximum and minimum) for the 17 habitat variables at the 41 SRCs in central Sweden

Variable Mean S.E. Minimum Maximum

Area Salix (ha)S1a 2.3 5.3 0 26S2a 1.9 3.7 0 16S3a 5.2 7.2 0 33Total area 7.6 9.1 0 40

Adjacent habitats (m)Crop fields 360.9 546.2 0 2810Cultivated grassland 89.7 136.2 0 500Set-aside 79.2 149.3 0 700Pasture 35.8 76.4 0 315Shrub areas 49.2 70.8 0 250Forest 194.1 260.1 0 1063

Residual habitats (linear elements, m)Large roads 64.3 125.6 0 643Small roads 99.5 201.5 0 1240Ditches 83.8 124.4 0 450Rivers/streams 115.1 207.0 0 830Houses and gardens (at border) 43.4 86.4 0 480

Other residual habitatsWithinfield islands (patches with natural vegetation)b 0.3 – 0 1Barnb 0.4 – 0 1

a The height of theSalix shrubs was classified into three categories (S1<1 m, S2= 1–2 m and S3 >2 m).b Mean represents proportion of sites with habitat islands and barns.

3.1. Multivariate species analysis

A CCA, including all species occurring at≥5 sites,emphasised the importance of adjacent habitats for thebird fauna in SRC. The first axis was associated withthe length of crop field borders and rivers (Fig. 1), andwas interpreted as a gradient from open-border siteswith rivers to forest-border dominated (and dry) sites,since forest border length was negatively correlatedwith the length of crop field borders (r = −0.34, P <

0.05). The second axis was negatively associated withthe area of tall SRC (S3). However, also the length ofditches and amount of shrub habitats were associatedwith the second axis (Fig. 1). Furthermore, species as-sociated with wet habitats seemed to occur together (atthe right bottom part) in the biplot, while species as-sociated with dry and tallSalixoccurred at the bottomleft part of the biplot (Fig. 1). Thus, the major deter-minants of the bird community in the SRCs seemed tobe type of bordering habitats, height of the SRCs andwetness. However, the occurrence of residual habitats


Table 2Estimated number of territories and characteristics for all species observed at the 41 sites

Species No. ofterritories

No. ofsites

Typea Preferencefor Salixb

Nesttypec

Nestheightd

Foode Homerangef

Migratoryhabitse

Blackbird Turdus merula 22 17 FO − O 3 HI I SBlue tit Parus caeruleus 21 17 FO + H 3 HI I RBlyth’s reed warbler Acrocephalus dumetorum 1 1 FA + O 2 I S LCarrion crow Corvus corone 4 4 FA − O 3 O L RChaffinch Fringilla coelebs 73 34 FO + O 3 I S SCuckoo Cuculus canorus 1 1 FO − I L LDunnock Prunella modularis 2 2 FO − O 2 I S SFieldfare Turdus pilaris 16 10 FA − O 3 HI L SGarden warbler Sylvia borin 37 19 FO + O 2 I S LGoldcrest Regulus regulus 2 2 FO + O 3 I S RGoldfinch Carduelis carduelis 1 1 FA − O 3 H L SGrasshopper warbler Locustella naevia 42 24 FA + O 1 I S LGreat spotted woodp.Dendrocopus major 1 1 FO − H 3 O L RGreat tit Parus major 35 25 FO + H 3 HI I RGreenfinch Carduelis chloris 67 33 FA + O 2 HI I RHobby Falco subbuteo 1 1 FA − O 3 P L LIcterine warbler Hippolais icterina 1 1 FO − O 2 I S LLapwing Vanellus vanellus 12 2 FA − O 1 I I SLesser whitethroat Sylvia curruca 3 3 FO − O 2 I S LLinnet Carduelis cannabina 35 25 FA + O 2 H L SMagpie Pica pica 7 7 FA − O 3 O L RMarsh harrier Circus aeruginosus 1 1 FA − O 1 P L LMarsh tit Parus palustris 2 2 FO − H 3 HI I RMarsh warbler Acrocephalus palustris 6 6 FA + O 1 I S LNuthatch Sitta europaea 1 1 FO − H 3 I I ROrtolan bunting Emberiza hortulana 19 8 FA + O 1 HI I LPartridge Perdix perdix 3 3 FA + O 1 H L RPheasant Phasianus colchicus 15 15 FA + O 1 O L RPied flycatcher Ficedula hypoleuca 2 2 FO − H 3 I S LQuail Coturnix coturnix 2 2 FA − O 1 O I LRaven Corvus corax 1 1 FO − O 3 O L RRed-backed shrike Lanius collurio 8 7 FA − O 2 I I LRedwing Turdus iliacus 18 12 FO + O 3 HI I SReed bunting Emberiza schoeniclus 88 30 FA + O 1 HI S SReed warbler Acrocephalus scirpaceus 8 4 FA − O 2 I S LRobin Erithacus rubecula 11 9 FO − H 1 I S SSedge warbler Acrocephalus schoenob 44 10 FA + O 1 I S LSiskin Carduelis spinus 4 4 FO − O 3 HI L SSkylark Alauda arvensis 142 27 FA − O 1 HI S SSong thrush Turdus philomelos 11 10 FO + O 3 HI I SStarling Sturnus vulgaris 3 3 FA − H 3 HI L SStock dove Columba oenas 1 1 FA − H 3 H L SThrush Nightingale Luscinia luscinia 9 6 FA + O 1 I S LTree pipit Anthus trivialis 30 15 FO + O 1 I S LTree sparrow Passer montanus 4 2 FA − H 3 HI I RWheatear Oenanthe oenanthe 3 3 FA − H 1 I I LWhinchat Saxicola rubetra 69 27 FA + O 1 HI S LWhite wagtail Motacilla alba 8 8 FA − H 3 I I LWhitethroat Sylvia communis 97 30 FA + O 2 I S LWillow warbler Phylloscopus trochilus 118 33 FO + O 1 I S L


Table 2 (Continued)

Species No. ofterritories

No. ofsites

Typea Preferencefor Salixb

Nesttypec

Nestheightd

Foode Homerangef

Migratoryhabitse

Wood pigeon Columba palumbus 3 3 FA − O 3 H L SWoodlark Lullula arborea 4 3 FO + O 1 HI I SYellow wagtail Motacilla flava 6 1 FA − O 1 I S LYellowhammer Emberiza citrinella 83 31 FA + O 1 HI I R

a Forest species (FO) include species mainly associated to forest habitats and farmland species (FA) include species restricted tofarmland habitats and species using both forest (nest sites) and farmland habitats (foraging areas).

b Preference forSalix (+) or avoidance ofSalix (−) is based on comparisons of abundances with a census of farmland landscapes inthe same region (Berg, unpublished).

c Nest types are classified as open (O) or hole (H).d nest height as located on ground (1), in shrubs (2), in trees or in buildings (3).e Preferred food is classified as invertebrates (I), invertebrates and plants (IH), vertebrates (P) or as omnivore (O).f Homerange size is classified as small, i.e. movements within 100 m radius (S), intermediate, i.e. movements within 200 m radius (I)

or as large (L). Migratory habits are classified as resident (R), short-distance migrants (S) or long-distance migrants (L), all according toCramp and Simmons (1977–1993).

(e.g. ditches, small gravel roads and houses and gar-dens) also influenced bird community composition.

3.2. Single species analyses

A similar pattern was found in the single speciesanalyses (Table 3). Eighteen of the 22 analysedspecies were affected by adjacent habitat composi-tion. Moreover, 11 of the species were positivelyor negatively associated with the length of forestedborders (e.g. willow warbler, chaffinch, great tit, treepipit, blackbird, and blue tit, for latin names Table 2),see table 3. In contrast, only three species preferredSRCs in open areas (only associated with the ad-jacent habitats crop fields, cultivated grassland andset-asides), namely whinchat, grasshopper warblerand reed bunting (Table 3). The area of the differentSalixheight categories affected 18 species. Six specieswere positively associated with the area of shortSalix(S1), seven species were positively associated with thearea of intermediateSalix (S2) and 14 species werepositively associated with the area of tallSalix (S3).Ortolan bunting and linnet were only positively asso-ciated with shortSalix. Whitethroat, greenfinch, sedgewarbler and blackbird were positively associated bothwith intermediate and tallSalix. Grasshopper warbler,willow warbler, chaffinch, yellowhammer, gardenwarbler and redwing were only positively associatedto tall Salix. Skylark, reed bunting, whinchat, andblue tit were all positively associated with both low

(S1) and tallSalix (S3). A probable explanation isthat some of these species (skylark, chaffinch andblue tit) prefer other habitats thanSalix, and siteswith a large area of SRC also have large amounts ofedge habitats and other adjacent habitats. However,for reed bunting and whinchat this probably reflects ageneral preference for SRC habitats.

About 16 species were affected by occurrenceof residual habitats (for classification of habitats,Table 1) within the SRCs. Generally, most associa-tions (65%) between bird abundance and occurrenceof residual habitats were positive (Table 3). A com-parison of bird species preferring tall and low SRCssuggests that a somewhat larger proportion of thespecies preferring shortSalix were positively associ-ated with residual habitats (74 and 56%, respectively)(Table 3). Among the residual habitats, the occurrenceof houses and gardens affected the largest number ofspecies (seven species all positively associated withthis variable). The most important of the remain-ing residual habitat variables were ditches and smallgravel roads (Table 3).

3.3. Ecological characteristics

Several ecological characteristics of the observedspecies, i.e. home range size, nest site and type, foodpreferences and migratory habits were compiled fromthe literature (Table 2). A comparison of ecologicaltraits of 24 species classified as preferring SRC with


Fig. 1. Species-environmental variable biplot from a canonical correspondence analysis of all species occurring in≥5 territories and the17 habitat variables (only significant habitat variables shown in the biplot). Rare species were downweighted. Biplot scaling with focus oninter-species distances was selected.

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29 species classified as avoiding SRC (Cuckoo omit-ted, classification ofSalix, Table 2) suggests that nestheight was the only ecological character associatedwith a preference forSalix habitats (Logistic regres-sion, Wald Chi-square= 6.2, P < 0.05). About 65%of the species with nests on the ground, 50% of thespecies with nests in shrubs and 26% of the specieswith their nests placed at tree height showed a pref-erence forSalix habitats. However, a broad spectrumof species was found in the censused SRC and manyof these seemed to be dependent on habitat featuresoutside the plantations for nesting. Thus, ecologicaltraits associated with preferences forSalixplantationswere probably partly concealed by occurrence of, e.g.suitable nest sites in adjacent habitats.

4. Discussion

4.1. Diversity of birds in SRC comparedwith other farmland habitats

Bird species-richness in the SRCs was high com-pared with open farmland sites dominated by othercrop-fields, comparable with that within large forestareas, but lower than that in forest edge habitats andseminatural pastures in the same region (Table 4). Fewother studies have directly compared the bird faunain Salix plantations with other farmland habitats. Forexample, Göransson (1990) compared densities ofdifferent species in a SRC area with densities in adja-cent field habitats over a 3-year period after planting.It was concluded thatSalixplantations increased birddiversity, and that they were positive for most war-blers and pheasants, while a few open-field speciesdecreased in numbers. Berg (unpublished data) found

Table 4Mean number of species per ha (±S.E.) in the present study compared to species number in other farmland habitats within the same region

Species number Habitat Reference

2.8 ± 0.3 Salix This study0.7± 0.07a Cereal crops at forest edges Berg and Pärt, 19941.1 ± 0.1a Set-asides at forest edges Berg and Pärt, 19944.5 ± 0.4 Seminatural pastures Söderström et al., 20012.8 ± 0.4 Continuous forest Berg, 19973.6 ± 0.4 Forest fragments in farmland Berg, 1997

a Common forest species not included, i.e. species number for field habitats. All studies used territory mapping and the sites were ofsimilar sizes (mean sizes 6–12 ha).

that species-richness of farmland birds was associ-ated with the occurrence ofSalix plantations, whilespecies-richness of forest birds in the same farm-land landscapes did not show this pattern. Similarly,Christian et al. (1997) showed that bird diversity andabundance was higher in short rotation hybrid poplarplantations (similar toSalixplantations) than in cerealcrop fields.

Most species registered in the present study areabundant in Swedish farmland landscapes (for roughestimates of population sizes, Berg and Tjernberg,1996). However, a few species with relatively small ordeclining populations occurred regularly in the SRCs:ortolan bunting, red-backed shrike, marsh warbler andwoodlark. A similar observation was made for the na-tionally rare Blyth’s reed warbler (Berg, unpublished).Thus, SRCs might be a preferred habitat also for somerare or threatened species. Furthermore, SRCs seem tohave a potential as an important habitat for gamebirds(mainly pheasant and partridge). The results of thisstudy suggest that pheasants have a higher abundancein SRCs than in farmland landscapes in general. Bothpheasant and partridge have relatively high abundancein British SRCs (Sage and Robertson, 1994; Baxteret al., 1996). Thus, SRCs offer an opportunity to in-crease the area of suitable habitat for gamebirds inintensively managed farmland, which might increasethe recreational and economic values of SRCs.

4.2. Factors important for bird communitycomposition in SRCs

A major result in the present study was the stronginfluence of adjacent habitats on bird communitycomposition in the SRCs. The statistical analysessuggest that this factor was even more important


than the structure of the SRC itself, with large dif-ferences in bird communities in forest-bordered andopen-bordered sites. Similarly, the bird fauna inopen-bordered and forest-bordered seminatural pas-tures in the same region has been shown to differconsiderably (Pärt and Söderström, 1999). Thus, theposition in the landscape is a major factor associatedwith bird community composition inSalix planta-tions, especially since the occurrence of residualhabitats was also shown to influence the bird fauna.The second major factor of importance for the birdfauna was the height of the SRCs. In line with earlierstudies (Göransson, 1990, 1994; Sage and Robertson,1996), most species (14 species) were associated withtall Salix plantations, fewer species were associatedwith plantations of intermediate (seven species) orlow height (six species). Most observations duringthe census period were of singing birds, and thereforethey found patterns of preferences for differentSalixheights are likely to be real. However, it is possible thata few additional species are associated with tallSalix,but these associations were not established due to dif-ficulties in observing “non-calling” birds in tallSalix.

Thus, based on the above observations, the birdcommunity in a SRC will change considerably overthe 3–4-year period between harvests (Göransson,1990). Furthermore, species-richness and total abun-dance of birds have been shown to be associatedwith structural density and complexity of the vegeta-tion (Sage and Robertson, 1996). Dense and weedyplantations seem to be preferred by several species(mainly migrant warblers). However, the occurrenceof species usually preferring open field habitats (e.g.ortolan bunting and skylark) suggests that some plan-tations are suitable also for ground foraging species,that usually prefer open fields with short or sparsevegetation. More detailed evaluations of effects ofhabitat structure withinSalix plantations and the im-portance of open (not planted) field edge-zones fordifferent species are needed.

4.3. Management recommendations for plantationof Salix in farmland landscapes

Overall, SRCs in farmland are used by manyspecies, although relatively few species seem to showstrong preferences for this habitat (i.e. many speciesare more abundant in forest habitats, seminatural

pastures etc.). However,Salix plantations on arablefields increase the abundance and diversity of birds,especially in intensively managed farmland domi-nated by cereal crops. Relatively long periods betweenharvest would be beneficial for most bird species (tallSalix preferred), although this is in conflict with thegoals for high yearly production and harvesting overa short time frame.

The effects ofSalix plantations on biodiversitylargely depend on the position of the SRC in the land-scape. Planting ofSalix in open farmland plains willhave positive effects on bird diversity by increasing thestructural diversity of the landscape. However, severalopen-field species (e.g., waders) avoidSalix planta-tions. Thus, it is suggested that plantations should beavoided close to habitats of high conservation values,such as wet meadows. Similarly, the use ofSalix asbuffer zones along streams (Börjesson, 1999a) callsfor caution, since such sites often have a conservationvalue linked to openness of the habitat. In contrast,plantations on infields in forest-dominated landscapeswill have negative effects, since the mosaic structure(i.e. mixture of open and forested habitats) positivefor most farmland birds (Berg, unpublished data)will disappear, and relatively few forest species arefavoured bySalix plantations. However, the presentstudy suggests thatSalix should be planted alongsharp edges at borders between coniferous forestplantations and open farmland to increase the com-plexity of the ecotone between farmland and forest.However, at mixed edges with a well developed andstructurally complex shrub layer, the effects on birddiversity could even be negative.

5. Conclusion

The present study has shown that there werelarge differences in bird communities betweenforest-bordered and open-bordered SRCs, thereforelocation of Salix plantations in the landscape wouldhave a large effect on the bird fauna. Furthermore,most bird species were associated with tall planta-tions (>2 m), thus relatively long periods betweenharvest would be positive for bird diversity. Over-all, SRCs harboured more farmland birds than forestbirds, and planting ofSalix in intensively managedfarmland plains would have positive effects on bird


diversity by increasing the structural diversity of thelandscape. In contrast, planting in forest-dominatedlandscapes might have negative effects on bird abun-dance and diversity, since the mosaic structure (i.e.mixture of open and forested habitats) that is positivefor most farmland birds will disappear, and relativelyfew forest species are favoured bySalixplantations.

Acknowledgements

Thanks to Mats Edholm, Kalle Källebrink, HåkanJohansson and Anders F. Andersson for help withbird surveys. Thanks to Bo Söderström for commentson earlier versions of the manuscript. The NationalSwedish Environmental Protection Agency funded thestudy.

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