Breeding birds in short-rotation coppices on farmland in central Sweden—the importance of Salix height and adjacent habitats

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

Breeding birds in short-rotation coppices on farmland in central Sweden—the importance of Salix height 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 of farmland biodiversity by increasing structural diversity, and decreasing cultivation intensity and use of pesticides in intensively managed farmland landscapes. The aim of this study was to investigate the relative importance of the structure of the plantation and composition of adjacent habitats for breeding birds in 41 SRCs (mean size±S.E. = 9.4±1.6 ha). Mean number of species per ha in the SRCs was 2.8 ± 0.3 species and there were more farmland birds (32 species, 808 territories) than forest birds (22 species, 400 territories). A major result of the present study was the strong influence of adjacent habitats on bird community composition (18 of 22 analysed species affected). There were large differences in bird communities between forest-bordered and open-bordered sites, but occurrence of residual habitats (i.e. other habitats than forest, pastures, shrub areas and arable fields) was also associated with occurrence of several species. The second factor of major importance for the bird fauna was the height of the plantations. Most species (14) were associated with tall plantations (>2 m), seven species were associated with plantations of intermediate (1–2 m) height, and six species were associated with plantations of low height (<1 m). A comparison of ecological traits between species classified as preferring SRC and species classified as avoiding SRC suggests that nest height was the only ecological character associated with a preference for Salix habitats, i.e. species with nests on the ground or in shrubs were more abundant in Salix habitats than in farmland landscapes in general. However, a broad spectrum of species was found in the SRCs and many of these seemed to be dependent on habitat features outside the plantations. Planting of Salix in intensively managed farmland plains will have positive effects on bird diversity by increasing the structural diversity of the landscape. In contrast, plantations on infields in forest-dominated landscapes will have negative effects, since the mosaic structure (i.e. mixture of open and forested habitats) positive for most farmland birds will disappear, and Salix plantations favour relatively few forest species. Additionally, Salix could be planted along sharp edges between coniferous plantations and 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 been taken 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 overproduction of cereals (Kumm, 1992; Official Statistics of Sweden, 1993). As a result, the area of set-asides (permanent fallows) has increased and new forms of land-use, e.g. short rotation coppice (SRC), in Sweden mainly Salix, have been introduced. The number of Salix plantations increased considerably between

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1991 and 1996, due to subsidies for SRC, higher taxes for fossil fuels and the existence of a biofuel market for forest fuels in Sweden (Rosenqvist et al., 2000). At present SRCs cover approximately 0.6% of the farmland area in Sweden (Official Statistics of Sweden, 1998), and a high concentration of Salix growers is found in central Sweden. During the same period, researchers and conservation biologists have aimed at identifying measures to stop the ongoing impoverishment of farmland biodiversity in Europe (Marchant et al., 1990; Robertson and Berg, 1992; Svensson et al., 1992; Tucker and Heath, 1994; Fuller et al., 1995). Restoration and management of natural grassland have been given priority in several countries, including Sweden (Götmark et al., 1998; Pärt and Söderström, 1999). Other measures have included conservation headlands (Boatman and Wilson, 1988), use of set-asides (Sotherton, 1998) and conservation and management of hedgerows and other non-crop habitats (Osborne, 1984; Parish et al., 1994). Use of SRCs could potentially have similar beneficial effects on farmland biodiversity, since they increase structural diversity in intensively managed farmland landscapes. Most SRCs are harvested every third to fourth year. Furthermore, management of SRCs is more extensive (i.e. restricted cultivation and restricted use of pesticides) than that of intensively managed cereal crops, although weeds are controlled for mechanically or chemically in newly established stands and fertilisers are mostly applied to growing stands (Gustafsson, 1987). Intensive management of annual crops has been viewed as a contributing cause to the decline in abundance of several agricultural bird species (e.g. O’Connor and Shrubb, 1986). Furthermore, modern forestry (i.e. forest plantations) have changed farmland landscapes by sharpening of forest edge habitats and loss of broad and structurally complex edges, which are important habitats for many farmland plants and animals (e.g. Fry and Sarlöv-Herlin, 1998). Several investigations have pointed out positive economic and environmental effects of SRCs in relation to annual food crops (Perttu, 1998, 1999; Börjesson 1999a,b). Suggested environmental benefits of SRCs and use of biomass fuels have included, e.g. nutrient and heavy metal uptake, possibilities to use SRCs as wastewater filters (Perttu, 1998), reduced wind erosion and reduced net flow of CO2 to

the atmosphere. Coppices may therefore play a role in a society based on recycling principles (Börjesson, 1999a). Few studies have investigated the bird fauna in different types of SRCs (Sage and Robertson, 1996; Christian et al., 1997), and Swedish studies have been restricted to a few sites in the southernmost parts of the country (Göransson, 1990, 1994). These studies have focused on effects of structure and height of the SRCs (i.e. local habitat quality). However, many vertebrates, especially in farmland, seem to use multiple habitats (With et al., 1997; Law and Dickman, 1998) and are therefore dependent on certain combinations of habitats for their breeding (Green et al., 1994). The importance of adjacent habitat type has been emphasised in studies of farmland habitats with restricted 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 composition of surrounding habitats are likely to have a strong influence on the communities of different organisms (e.g. Saunders et al., 1991; Andrén, 1994; Hansson et al., 1995). Thus, additional studies of the bird fauna in SRC that include these aspects are needed. The aim of the present study was to investigate the relative importance of local habitat (i.e. Salix height and occurrence of residual habitats, such as ditches and 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 of birds in farmland landscapes by planting SRCs are discussed.

2. Methods 2.1. Study sites and habitat mapping About 41 SRCs (i.e. Salix plantations planted for fuel production) in Uppland and Västmanland (approximately 59◦ 40 –60◦ 7 N and 16◦ 30 –18◦ 10 E) in central Sweden were surveyed for breeding birds by territory mapping in 1997. The sites were situated in a landscape gradient from farmland plains to small farmland areas in forest-dominated landscapes. Furthermore, SRC area, habitat composition within sites (i.e. shrub height) and adjacent to the coppices varied

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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 forest and 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 height of the Salix shrubs was classified into three categories (<1 m, 1–2 m and >2 m), and the area of different height categories at each site was measured on a digitising table. The lengths of different adjacent habitats were measured at the border of the SRC. The following habitats were separated; crop fields (mainly spring-sown cereal crops), cultivated grassland (leys and cultivated pastures), set-aside, seminatural pastures, shrub areas outside seminatural pastures (mainly with Juniperus communis, Rosa spp. and Prunus spinosa), and forest. Residual habitats included houses and gardens, roads, ditches and, streams or rivers within or at the border of the site. Occurrence of within-field islands (small patches with natural vegetation) and barns in the site was also noted. 2.2. Bird censuses Each site was surveyed for breeding birds by territory mapping (Svensson, 1975; Bibby et al., 1992) seven times during early mornings (mainly from sunrise to 10.00 h), once in each of the periods 15–30 April, 1–10 May, 11–20 May, 21–31 May, 1–10 June, 11–20 June and 21–30 June. All sites were also visited once at night (mainly 23.00–02.00 h) during the period 15–30 June. No counts were made on mornings with strong wind or rain. All bird observations were denoted with symbols on maps of 1:3000–5000 scale. Birds observed just outside the plots were included in order to decide whether a territory was situated within or outside a plot. Territories across the border of a site were counted as belonging to the site when more than half of the observations fell inside the borders. The sites were surveyed in different order at the above seven mornings in order to avoid bias due to visits at the same time, since most birds are more easily detected early in the morning when the singing activity is high. Details of methods and criteria used to determine the number of breeding territories are given in Robertson and Skoglund (1985).

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All bird species were classified either as forest birds (i.e. species common in forest landscapes without farmland habitats), or farmland species (i.e. species restricted to farmland habitats or species simultaneously using both farmland and forest habitats), see Table 2, according to results from earlier studies in the same region (Robertson and Berg, 1992; Berg and Pärt, 1994; Berg, 1997; Pärt and Söderström, 1999). Several ecological characteristics of the observed species, i.e. home range size, nest site and type, food preferences and migratory habits were compiled from the literature. These data were used in analyses of factors associated to preference or avoidance of SRC for different species (Table 2). 2.3. Statistical analyses A canonical correspondence analysis (CCA) was performed to investigate relationships between the measured environmental variables and all bird species occurring at more than five sites (ter Braak, 1987; ter Braak and Prentice, 1988). The CCA identifies major gradients in community composition that are associated with the environmental variables. However, the suitability of different ordination methods depends on the length of the gradients and a detrended correspondence analysis (DCA) was used in order to estimate the length of the compositional gradients. The relatively long gradients in the data set (2.5–3 S.D.) verified that unimodal models (i.e. CCA) could be used (Jongman et al., 1995; ter Braak and Smilauer, 1998). Associations between the abundance (number of territories) of 19 of the 22 most common bird species and habitat variables were analysed with multiple linear regression with backward selection (i.e. all species occurring with ≥10 territories, except lapwing that only occurred at two sites, Table 2). Three species (pheasant, robin and song thrush) were analysed with nominal logistic regression (SPSS, 1994), since most observations (98%) of these species were categorical (i.e. occurrence or non-occurrence). The level of significance (P) to enter and to be removed were set to 0.05 and 0.1, respectively. Area and lengths of borders to different adjacent habitats were log-transformed. Furthermore, the squared term of all habitat variables was included in order to test for non-linear associa-

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tions. The squared habitat variables were allowed to be entered both alone and together with the original (not squared) variables. Furthermore, two-way interactions between (1) area of Salix habitats, and (2) adjacent habitats and residual habitats were included in the models.

3.1. Multivariate species analysis A CCA, including all species occurring at ≥5 sites, emphasised the importance of adjacent habitats for the bird fauna in SRC. The first axis was associated with the length of crop field borders and rivers (Fig. 1), and was interpreted as a gradient from open-border sites with rivers to forest-border dominated (and dry) sites, since forest border length was negatively correlated with the length of crop field borders (r = −0.34, P < 0.05). The second axis was negatively associated with the area of tall SRC (S3). However, also the length of ditches and amount of shrub habitats were associated with the second axis (Fig. 1). Furthermore, species associated with wet habitats seemed to occur together (at the right bottom part) in the biplot, while species associated with dry and tall Salix occurred at the bottom left part of the biplot (Fig. 1). Thus, the major determinants of the bird community in the SRCs seemed to be type of bordering habitats, height of the SRCs and wetness. However, the occurrence of residual habitats

3. Results Data on habitat composition at the sites are presented in Table 1, including SRC area, length of bordering habitats, length of linear elements and occurrence of other residual habitats. A total of 54 bird species and 1208 territories were recorded within the SRCs (Table 2). Mean number of species (±S.E.) per ha in the censused SRCs was 2.8±0.3 species. The SRCs harboured more farmland birds (32 species and 808 territories) than forest birds (22 species and 400 territories). For details and classification of farmland and forest birds (Table 2).

Table 1 Descriptive statistics (mean ± S.E., maximum and minimum) for the 17 habitat variables at the 41 SRCs in central Sweden Variable Area Salix (ha) S1a S2a S3a Total area

Mean

S.E.

Minimum

Maximum

2.3 1.9 5.2 7.6

5.3 3.7 7.2 9.1

0 0 0 0

26 16 33 40

Adjacent habitats (m) Crop fields Cultivated grassland Set-aside Pasture Shrub areas Forest

360.9 89.7 79.2 35.8 49.2 194.1

546.2 136.2 149.3 76.4 70.8 260.1

0 0 0 0 0 0

2810 500 700 315 250 1063

Residual habitats (linear elements, m) Large roads Small roads Ditches Rivers/streams Houses and gardens (at border)

64.3 99.5 83.8 115.1 43.4

125.6 201.5 124.4 207.0 86.4

0 0 0 0 0

643 1240 450 830 480

– –

0 0

1 1

Other residual habitats Withinfield islands (patches with natural vegetation)b Barnb a b

0.3 0.4

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

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Table 2 Estimated number of territories and characteristics for all species observed at the 41 sites Species Blackbird Blue tit Blyth’s reed warbler Carrion crow Chaffinch Cuckoo Dunnock Fieldfare Garden warbler Goldcrest Goldfinch Grasshopper warbler Great spotted woodp. Great tit Greenfinch Hobby Icterine warbler Lapwing Lesser whitethroat Linnet Magpie Marsh harrier Marsh tit Marsh warbler Nuthatch Ortolan bunting Partridge Pheasant Pied flycatcher Quail Raven Red-backed shrike Redwing Reed bunting Reed warbler Robin Sedge warbler Siskin Skylark Song thrush Starling Stock dove Thrush Nightingale Tree pipit Tree sparrow Wheatear Whinchat White wagtail Whitethroat Willow warbler

No. of territories Turdus merula 22 Parus caeruleus 21 Acrocephalus dumetorum 1 Corvus corone 4 Fringilla coelebs 73 Cuculus canorus 1 Prunella modularis 2 Turdus pilaris 16 Sylvia borin 37 Regulus regulus 2 Carduelis carduelis 1 Locustella naevia 42 Dendrocopus major 1 Parus major 35 Carduelis chloris 67 Falco subbuteo 1 Hippolais icterina 1 Vanellus vanellus 12 Sylvia curruca 3 Carduelis cannabina 35 Pica pica 7 Circus aeruginosus 1 Parus palustris 2 Acrocephalus palustris 6 Sitta europaea 1 Emberiza hortulana 19 Perdix perdix 3 Phasianus colchicus 15 Ficedula hypoleuca 2 Coturnix coturnix 2 Corvus corax 1 Lanius collurio 8 Turdus iliacus 18 Emberiza schoeniclus 88 Acrocephalus scirpaceus 8 Erithacus rubecula 11 Acrocephalus schoenob 44 Carduelis spinus 4 Alauda arvensis 142 Turdus philomelos 11 Sturnus vulgaris 3 Columba oenas 1 Luscinia luscinia 9 Anthus trivialis 30 Passer montanus 4 Oenanthe oenanthe 3 Saxicola rubetra 69 Motacilla alba 8 Sylvia communis 97 Phylloscopus trochilus 118

No. of sites

Typea Preference for Salixb

17 17 1 4 34 1 2 10 19 2 1 24 1 25 33 1 1 2 3 25 7 1 2 6 1 8 3 15 2 2 1 7 12 30 4 9 10 4 27 10 3 1 6 15 2 3 27 8 30 33

FO FO FA FA FO FO FO FA FO FO FA FA FO FO FA FA FO FA FO FA FA FA FO FA FO FA FA FA FO FA FO FA FO FA FA FO FA FO FA FO FA FA FA FO FA FA FA FA FA FO

− + + − + − − − + + − + − + + − − − − + − − − + − + + + − − − − + + − − + − − + − − + + − − + − + +

Nest typec

Nest Foode d height

Home rangef

Migratory habitse

O H O O O

3 3 2 3 3

O O O O O O H H O O O O O O O O H O H O O O H O O O O O O H O O O O H H O O H H O H O O

2 3 2 3 3 1 3 3 2 3 2 1 2 2 3 1 3 1 3 1 1 1 3 1 3 2 3 1 2 1 1 3 1 3 3 3 1 1 3 1 1 3 2 1

I I S L S L S L S S L S L I I L S I S L L L I S I I L L S I L I I S S S S L S I L L S S I I S I S S

S R L R S L S S L R S L R R R L L S L S R L R L R L R R L L R L S S L S L S S S S S L L R L L L L L

HI HI I O I I I HI I I H I O HI HI P I I I H O P HI I I HI H O I O O I HI HI I I I HI HI HI HI H I I HI I HI I I I

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Table 2 (Continued) Species Wood pigeon Woodlark Yellow wagtail Yellowhammer

No. of territories Columba palumbus Lullula arborea Motacilla flava Emberiza citrinella

3 4 6 83

No. of sites

Typea

Preference for Salixb

Nest typec

Nest heightd

Foode

Home rangef

Migratory habitse

3 3 1 31

FA FO FA FA

− + − +

O O O O

3 1 1 1

H HI I HI

L I S I

S S L R

a Forest species (FO) include species mainly associated to forest habitats and farmland species (FA) include species restricted to farmland habitats and species using both forest (nest sites) and farmland habitats (foraging areas). b Preference for Salix (+) or avoidance of Salix (−) is based on comparisons of abundances with a census of farmland landscapes in the 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 to Cramp and Simmons (1977–1993).

(e.g. ditches, small gravel roads and houses and gardens) also influenced bird community composition. 3.2. Single species analyses A similar pattern was found in the single species analyses (Table 3). Eighteen of the 22 analysed species were affected by adjacent habitat composition. Moreover, 11 of the species were positively or negatively associated with the length of forested borders (e.g. willow warbler, chaffinch, great tit, tree pipit, blackbird, and blue tit, for latin names Table 2), see table 3. In contrast, only three species preferred SRCs in open areas (only associated with the adjacent habitats crop fields, cultivated grassland and set-asides), namely whinchat, grasshopper warbler and reed bunting (Table 3). The area of the different Salix height categories affected 18 species. Six species were positively associated with the area of short Salix (S1), seven species were positively associated with the area of intermediate Salix (S2) and 14 species were positively associated with the area of tall Salix (S3). Ortolan bunting and linnet were only positively associated with short Salix. Whitethroat, greenfinch, sedge warbler and blackbird were positively associated both with intermediate and tall Salix. Grasshopper warbler, willow warbler, chaffinch, yellowhammer, garden warbler and redwing were only positively associated to tall Salix. Skylark, reed bunting, whinchat, and blue tit were all positively associated with both low

(S1) and tall Salix (S3). A probable explanation is that some of these species (skylark, chaffinch and blue tit) prefer other habitats than Salix, and sites with a large area of SRC also have large amounts of edge habitats and other adjacent habitats. However, for reed bunting and whinchat this probably reflects a general preference for SRC habitats. About 16 species were affected by occurrence of residual habitats (for classification of habitats, Table 1) within the SRCs. Generally, most associations (65%) between bird abundance and occurrence of residual habitats were positive (Table 3). A comparison of bird species preferring tall and low SRCs suggests that a somewhat larger proportion of the species preferring short Salix were positively associated with residual habitats (74 and 56%, respectively) (Table 3). Among the residual habitats, the occurrence of houses and gardens affected the largest number of species (seven species all positively associated with this variable). The most important of the remaining residual habitat variables were ditches and small gravel roads (Table 3). 3.3. Ecological characteristics Several ecological characteristics of the observed species, i.e. home range size, nest site and type, food preferences and migratory habits were compiled from the literature (Table 2). A comparison of ecological traits of 24 species classified as preferring SRC with

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Fig. 1. Species-environmental variable biplot from a canonical correspondence analysis of all species occurring in ≥5 territories and the 17 habitat variables (only significant habitat variables shown in the biplot). Rare species were downweighted. Biplot scaling with focus on inter-species distances was selected.

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

29 species classified as avoiding SRC (Cuckoo omitted, classification of Salix, Table 2) suggests that nest height was the only ecological character associated with a preference for Salix habitats (Logistic regression, Wald Chi-square = 6.2, P < 0.05). About 65% of the species with nests on the ground, 50% of the species with nests in shrubs and 26% of the species with their nests placed at tree height showed a preference for Salix habitats. However, a broad spectrum of species was found in the censused SRC and many of these seemed to be dependent on habitat features outside the plantations for nesting. Thus, ecological traits associated with preferences for Salix plantations were probably partly concealed by occurrence of, e.g. suitable nest sites in adjacent habitats.

4. Discussion 4.1. Diversity of birds in SRC compared with other farmland habitats Bird species-richness in the SRCs was high compared with open farmland sites dominated by other crop-fields, comparable with that within large forest areas, but lower than that in forest edge habitats and seminatural pastures in the same region (Table 4). Few other studies have directly compared the bird fauna in Salix plantations with other farmland habitats. For example, Göransson (1990) compared densities of different species in a SRC area with densities in adjacent field habitats over a 3-year period after planting. It was concluded that Salix plantations increased bird diversity, and that they were positive for most warblers and pheasants, while a few open-field species decreased in numbers. Berg (unpublished data) found

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that species-richness of farmland birds was associated with the occurrence of Salix plantations, while species-richness of forest birds in the same farmland landscapes did not show this pattern. Similarly, Christian et al. (1997) showed that bird diversity and abundance was higher in short rotation hybrid poplar plantations (similar to Salix plantations) than in cereal crop fields. Most species registered in the present study are abundant in Swedish farmland landscapes (for rough estimates of population sizes, Berg and Tjernberg, 1996). However, a few species with relatively small or declining populations occurred regularly in the SRCs: ortolan bunting, red-backed shrike, marsh warbler and woodlark. A similar observation was made for the nationally rare Blyth’s reed warbler (Berg, unpublished). Thus, SRCs might be a preferred habitat also for some rare or threatened species. Furthermore, SRCs seem to have a potential as an important habitat for gamebirds (mainly pheasant and partridge). The results of this study suggest that pheasants have a higher abundance in SRCs than in farmland landscapes in general. Both pheasant and partridge have relatively high abundance in British SRCs (Sage and Robertson, 1994; Baxter et al., 1996). Thus, SRCs offer an opportunity to increase the area of suitable habitat for gamebirds in intensively managed farmland, which might increase the recreational and economic values of SRCs. 4.2. Factors important for bird community composition in SRCs A major result in the present study was the strong influence of adjacent habitats on bird community composition in the SRCs. The statistical analyses suggest that this factor was even more important

Table 4 Mean 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 0.7± 0.07a 1.1 ± 0.1a 4.5 ± 0.4 2.8 ± 0.4 3.6 ± 0.4

Salix Cereal crops at forest edges Set-asides at forest edges Seminatural pastures Continuous forest Forest fragments in farmland

This study Berg and Pärt, 1994 Berg and Pärt, 1994 Söderström et al., 2001 Berg, 1997 Berg, 1997

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

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than the structure of the SRC itself, with large differences in bird communities in forest-bordered and open-bordered sites. Similarly, the bird fauna in open-bordered and forest-bordered seminatural pastures in the same region has been shown to differ considerably (Pärt and Söderström, 1999). Thus, the position in the landscape is a major factor associated with bird community composition in Salix plantations, especially since the occurrence of residual habitats was also shown to influence the bird fauna. The second major factor of importance for the bird fauna was the height of the SRCs. In line with earlier studies (Göransson, 1990, 1994; Sage and Robertson, 1996), most species (14 species) were associated with tall Salix plantations, fewer species were associated with plantations of intermediate (seven species) or low height (six species). Most observations during the census period were of singing birds, and therefore they found patterns of preferences for different Salix heights are likely to be real. However, it is possible that a few additional species are associated with tall Salix, but these associations were not established due to difficulties in observing “non-calling” birds in tall Salix. Thus, based on the above observations, the bird community in a SRC will change considerably over the 3–4-year period between harvests (Göransson, 1990). Furthermore, species-richness and total abundance of birds have been shown to be associated with structural density and complexity of the vegetation (Sage and Robertson, 1996). Dense and weedy plantations seem to be preferred by several species (mainly migrant warblers). However, the occurrence of species usually preferring open field habitats (e.g. ortolan bunting and skylark) suggests that some plantations are suitable also for ground foraging species, that usually prefer open fields with short or sparse vegetation. More detailed evaluations of effects of habitat structure within Salix plantations and the importance of open (not planted) field edge-zones for different species are needed. 4.3. Management recommendations for plantation of Salix in farmland landscapes Overall, SRCs in farmland are used by many species, although relatively few species seem to show strong preferences for this habitat (i.e. many species are more abundant in forest habitats, seminatural

pastures etc.). However, Salix plantations on arable fields increase the abundance and diversity of birds, especially in intensively managed farmland dominated by cereal crops. Relatively long periods between harvest would be beneficial for most bird species (tall Salix preferred), although this is in conflict with the goals for high yearly production and harvesting over a short time frame. The effects of Salix plantations on biodiversity largely depend on the position of the SRC in the landscape. Planting of Salix in open farmland plains will have positive effects on bird diversity by increasing the structural diversity of the landscape. However, several open-field species (e.g., waders) avoid Salix plantations. Thus, it is suggested that plantations should be avoided close to habitats of high conservation values, such as wet meadows. Similarly, the use of Salix as buffer zones along streams (Börjesson, 1999a) calls for caution, since such sites often have a conservation value linked to openness of the habitat. In contrast, plantations on infields in forest-dominated landscapes will have negative effects, since the mosaic structure (i.e. mixture of open and forested habitats) positive for most farmland birds (Berg, unpublished data) will disappear, and relatively few forest species are favoured by Salix plantations. However, the present study suggests that Salix should be planted along sharp edges at borders between coniferous forest plantations and open farmland to increase the complexity of the ecotone between farmland and forest. However, at mixed edges with a well developed and structurally complex shrub layer, the effects on bird diversity could even be negative.

5. Conclusion The present study has shown that there were large differences in bird communities between forest-bordered and open-bordered SRCs, therefore location of Salix plantations in the landscape would have a large effect on the bird fauna. Furthermore, most bird species were associated with tall plantations (>2 m), thus relatively long periods between harvest would be positive for bird diversity. Overall, SRCs harboured more farmland birds than forest birds, and planting of Salix in intensively managed farmland plains would have positive effects on bird

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