Biodiversity Data Journal : General research article
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General research article
Salix transect of Europe: latitudinal patterns in willow diversity from Greece to arctic Norway
expand article infoQuentin Cronk, Enrico Ruzzier§, Irina Belyaeva|, Diana Percy§
‡ Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
§ Department of Life Sciences, Natural History Museum, London SW7 5BD, United Kingdom
| Department of Bioinformatics and Spatial Analysis, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, United Kingdom
Open Access

Abstract

Background

Willows (Salix spp.) are ecosystem "foundation species" that are hosts to large numbers of associated insects. Determining their patterns of distribution across Europe is therefore of interest for understanding the spatial distribution of associated fauna. The aim of this study was to record species composition at multiple sites on a long latitudinal gradient (megatransect) across Europe as a baseline for the future detailed analysis of insect fauna at these sites. In this way we used willow stands as comparable mesocosms in which to study floristic and faunistic changes with latitude across Europe.

New information

To determine spatial patterning of  an ecologically important group on a latitudinal gradient across Europe, we sampled willows at the stand level in 42 sites, approximately 100 km apart, from the Aegean (38.8°N) to the Arctic Ocean (70.6°N), but at a similar longitude (21.2 to 26.1°E). The sites were predominantly lowland (elevations 1 to 556 metres amsl, median = 95 m) and wet (associated with rivers, lakes, drainage ditches or wet meadows). The median number of willow taxa (species and hybrids) per stand was four, and varied from one to nine. There is a progressive increase in willow diversity from south to north with the median number of taxa per stand in southern Europe being three, and in northern Europe six. A total of 20 willow species were recorded, along with 12 hybrids. The most widespread willow in the transect was Salix alba L. (occurring in 20 sites out of 42) followed by S. triandra L. (15 sites), S. caprea L., S. phylicifolia L. (14 sites) and S. myrsinifolia Salisb., Salix ×fragilis L. (13 sites). Voucher specimens from this study are deposited in the herbaria of the Natural History Museum (BM) and the Royal Botanic Gardens Kew (K). These samples provide a "snapshot" of willow diversity along a latitudinal gradient and an indication of the geographically changing taxonomic diversity that is presented to willow-feeding herbivores across Europe. It is anticipated that further papers will examine the insect fauna collected from these sites as part of this study.

Keywords

Biogeography, Bulgaria, ecospace, Estonia, Finland, Greece, Hungary, latitudinal gradient, Latvia, Lithuania, megatransect, Norway, Poland, Romania, Salicaceae, salicophagy, spatial analysis, willow-feeding insects

Introduction

The ecological significance of the genus Salix

Willows (the genus Salix L.) are "foundation species" (Ellison et al. 2005) in many wet habitats in the north temperate region. By providing an abundant food-source for many willow-feeding animals (generalists and specialists) they provide the basis for characteristic ecosystems (Brändle and Brandl 2001, Nyman et al. 2007). Willow leaves frequently show signs of leaf damage resulting from herbivore feeding. Herbivores include mammals: rodents (Tahvanainen et al. 1985b), deer, elk and, in the arctic, reindeer (den Herder and Niemelä 2003) and also phytophagous insects, notably Lepidoptera, Coleoptera and Symphyta Hymenoptera (sawflies) (Volf et al. 2015).

Phytophagous Coleoptera have, in addition to generalists that may potentially or sporadically feed on willow, several Salix specialists (Häggström and Larsson 1995, Volf et al. 2015) that will cue in on willow phytochemistry (Kolehmainen et al. 1995, Rowell-Rahier 1984a, Rowell-Rahier 1984b, Tahvanainen et al. 1985a). The suborder Symphyta contains a number of highly specialised willow-feeders (Leppänen et al. 2014, Roininen and Tahvanainen 1989) and are particularly abundant in Northern Europe, a fact which has been attributed to the greater number of willows in the north (Kouki et al. 1994). Willows are also host to numerous sap-sucking insects in the Hemiptera, especially aphids (Aphididae) (Blackman and Eastop 2014), psyllids (Psyllidae and Triozidae: Hill and Hodkinson 1995, Hill et al. 1998, Hodkinson et al. 2001, Serbina et al. 2015), leaf-hoppers (Cicadellidae) and spittle-bugs (Cercopidae).

The abundant herbivores further support a predator trophic level, from birds (Sipura 1999), ants and predatory beetles, as well as large numbers of parasitic wasps (Callan 1940). The diversity of willow-feeding herbivores suggests that willows can be considered a "superhost". The concept of superhost is usually applied to hosts of galling insects (de Araújo et al. 2013). Willows do indeed host many galling insects, but also act as a superhost more generally for many guilds of herbivorous insects. In a survey of 25 European tree species, willows had both the greatest number of phytophages and the greatest number of specialist herbivores (Brändle and Brandl 2001).

Taxonomy of willow

The genus Salix in Europe is usually considered to be a difficult and confusing group for classification and identification (Karrenberg et al. 2002, Meikle 1992, Rechinger 1992, Skvortsov 1999). The main reasons for this are: (1) genetically-based morphological polymorphism, (2) phenotypic plasticity (3) the prevalence of hybridization (4) differences in taxonomic opinion. Although some willows (such as S. pentandra L.) are rather uniform, other species are highly variable. Salix myrsinifolia is a good example of a species that shows extensive polymorphism: notably in leaf indumentum (hairy to glabrous) and leaf shape (narrowly to broadly elliptical). Although willow polymorphism is rarely formally tested in common garden experiments it is likely that much of this polymorphism is genetically based as different morphs can be found mixed in populations, having developed under the same environmental conditions.

Willows also exhibit phenotypic plasticity, such that even different plants of the same clone can look quite different, particularly if coppiced. Coppice shoots and their leaves can be very different from those of normal branches. However, probably the most remarkable and problematic aspect of willow taxonomy is the great ability for willows to hybridize. Crosses between quite unrelated species occur and many hybrids have a high degree of fertility. A recent study has shown that widespread hybridization is sufficient for chloroplast capture to occur even when species boundaries are maintained (Percy et al. 2014).

Coupled with this is the fact that many hybrids are of economic importance, due to their fast growth, and are widely planted. An example is the widespread hybrid Salix ×rubra Huds. (S. purpurea L. × S. viminalis L.). Another case is S. ×meyeriana Rostk. ex Willd. (S. euxina I.V.Belyaeva × S. pentandra) frequently planted as a more easily propagatable alternative to S. pentandra (Zinovjev 2011). Sometimes hybrids are so widespread they behave effectively as homoploid hybrid species. An example is the crack willow (S. ×fragilis) which is a hybrid between S. alba and S. euxina (Belyaeva 2009) but which constitutes a characteristic landscape feature over much of Europe and which authors have in the past considered a species (Meikle 1984). Another case where taxonomic treatment varies is S. bebbiana Sarg. Here, we follow Skvortsov (Skvortsov 1999) in recognizing S. bebbiana as an Eurasian as well as a North American species, despite considerable variation across the range. However, many European authors (e.g. Bennett et al. 1991, Rechinger and Akeroyd 1993) consider the European S. bebbiana to represent a separate species, S. xerophila Flod. Salix bebbiana (=S. xerophila) is closely related to the glabrescent Pale Willow (S. starkeana Willd.). However, S. starkeana is a comparatively rare willow.

Geography of willow and stand level sampling

Species of the genus Salix have a long history of being mapped in Europe starting with the monumental Atlas Florae Europaeae project (Jalas and Suominen 1976). In turn, these data have been used for detailed analyses of geographic distribution using numerical methods at a continent-wide (Myklestad and Birks 1993) and regional (Myklestad 1993) scale. A more recent resource at the country level (with more up-to-date taxonomy) is that of the Euro+Med Plantbase (Uotila 2011). However, stand composition cannot be easily predicted from occurrences in large grid squares or whole countries. Willows in natural stands across Europe provide a distinctive ecospace for the willow-feeding organisms and understanding the changing stand-level taxonomic composition of the Salix species is important for understanding the host choice and distribution of willow herbivores. It is the stand that provides the landscape unit and the ecospace within which host choice operates. Also large-scale mapping projects often exclude hybrids, which may be an important part of natural stands and particularly important as they may possibly form "hybrid bridges" (Floate and Whitham 1993) for herbivorous insects to move between hosts. Furthermore, direct observation of natural willow stands, as in this study, allows the co-collection of herbivores with the collection of voucher herbarium specimens.

The collection of data over a long geogrphical distance falls into the category of studies now dubbed 'megatransects'. The power and utility of this technique has been amply demonstrated by numerous recent studies. Some recent examples include: Anstett et al. 2014, Baltensperger and Huettmann 2015, Baltensperger et al. 2015, Barrios-Garcia et al. 2014, Hernández et al. 2007, Huber 2015, Senterre et al. 2004.

Material and methods

Stand selection

Willow stands were examined during two journeys by road across Europe: Greece to Poland in April 2015 and Poland to Norway in June 2015 (Fig. 1). Sites were selected by driving approximately 100 km north of the previous site and opportunistically locating a suitable habitat in which to find willows, generally a river or low-lying ground. The spacing of sites varied according to logistic and travel constraints. In southern Europe, willows are largely restricted to riparian habitats, but northwards they become commoner in many more habitats. The sampling unit was the willow stand (willow dominated local area). The requirement for a site was that it had a stand of willows that met certain minimum size requirements (at least 100 m in longest linear dimension). A stand of willows is defined as a contiguous area where willows are the dominant vegetation for at least 100 m in linear dimension (as for instance along a river bank). Some stands are very extensive, in which case our sampling was limited to approximately 200 m in largest linear dimension. Because willow stands differ so much in size, shape and density, it was not practical to impose equal area or grid sampling. Time constraints limited entomological and botanical sampling to 1-2 hours per site. A total of 42 sites were sampled across Europe from south to north (38.8 to 70.6°N) while minimising east-west deviation to between approximately 21.2 and 26.1°E. In addition to the 42 sites, a series of "Supplemental sites" are recorded at which additional insect collections were made but the full site recording process was not carried out.

Figure 1.

Map of sampling sites with sites 10, 20, 30 and 40 arrowed.

Data collection

At each site latitude, longitude and altitude data were collected using a hand held Garmin Etrex global positioning system, accurate to within 3 m. Basic notes on the immediate environment were taken to provide a habitat profile of the sites. At each site, the willow diversity was determined and voucher specimens were made in order to validate the species present and to capture variation in species that exhibited considerable phenotypic variation. If the willows were flowering, an attempt was made to collect both male and female individuals. Willow abundance relative to abundance of other woody plants was estimated on a four-point scale: 1) abundant - 30% of individuals or more; 2) common - 10-30%; 3) occasional <10%; 4) rare - one or two individuals only were detected. Samples were processed using standard herbarium techniques and specimens are deposited at the Natural History Museum, London (BM) or at the Royal Botanic Gardens Kew (K). Field identifications were made by QC and DMP. Confirmation, and critical determination of all vouchers, including hybrids, was done by IB. In addition to herbarium samples, samples of leaf tissue were dried using silica gel to permit future DNA-based studies and retained at NHM.

Climate data

As background information, climate data for three contrasting individual sites is given from publically available data sources (Table 2). These use a dataset of mean historical monthly temperature (°Celsius) and rainfall (mm), computed globally for the period 1990-2009 by the Climatic Research Unit (CRU) of University of East Anglia (UEA) and available through the World Bank Climate Portal (World Bank Group 2015).

Latitude, longitude and general details of collecting sites, Salix transect across Europe, April-June 2015.

SITE

Lat °N

Long °E

Alt (m asl)

date

country

river/site

habitat

1

38.80007

22.462900

37

21-iv-2015

Greece

River (R.) Asopos, west of Thermopylae

Bank of fast-flowing rocky river partly shaded by Platanus and with Arundo and Tamarix along the stream bank

2

38.902000

22.310150

33

21-iv-2015

Greece

R. Sperchios, near Leianokladi, east of Lamia

Bank of wide and rocky river bed in wide floodplain with Tamarix and Rubus etc.

3

39.306694

22.528323

177

22-iv-2015

Greece

R. Enipeas east of Farsala

By irrigation pumping station, bank of river flowing through agricultural area (fields of wheat). With Cercis, Populus, Rubus etc.

4

40.032685

22.175437

534

22-iv-2015

Greece

Stream near Kokkinogeia, Thrace

Damp drainage in foothills of the Olympus range with rapidly flowing stream meandering through. Rough grassy terrain with poplar and willow trees.

5

41.113317

23.273893

31

23-iv-2015

Greece

At R. Struma, near Lithotopos

In mud and shallow water at the edge of a broad and muddy irrigation canal through agricultural land with willows, poplar and Rubus etc.

6

41.412468

23.318609

90

23-iv-2015

Bulgaria

R. Struma, near Topolnitsa

Bank of river in narrow sandy grazed floodplain between the river and hills.

7

42.165622

22.998141

392

24-iv-2015

Bulgaria

R. Struma, north of Boboshevo

Sandy riverbank with poplar, ash, willow and elm between rocky side of gorge and sandy flat with small church/shrine.

8

42.923989

23.810563

339

24-iv-2015

Bulgaria

R. Kalnitza, near Botevgrad

Sandy banks of small polluted river in construction area with Rubus, Urtica, Prunus spinosa etc.

9

43.739343

23.966755

35

24-iv-2015

Bulgaria

R. Danube, at Oryahovo

River bank by light industrial area at ferry port on the Danube with poplars, tree willows, Phragmites etc.

10

44.260343

23.786781

81

25-iv-2015

Romania

R. Jiu, at Podari, near Craiova

Clayey/sandy bank of river with white mulberry, poplar etc

11

44.961981

23.190337

172

25-iv-2015

Romania

R. Jiu, north of Rovinari

Along ditches in middle of ploughed fields with poplars and Phragmites etc.

12

45.510676

22.737225

556

26-iv-2015

Romania

Meadow near Paucinesti, Carpathian region

Along ditches and in fields in grazed sedgy meadows in agricultural valley. Many plum trees in blossom.

13

46.518504

21.512839

102

26-iv-2015

Romania

R. Crişul Alb, at Chișineu-Criș

Embanked river through town, grassy slope with thick willow patches.

14

46.700744

21.312680

94

27-iv-2015

Hungary

R. Fekete-Körös, near Gyula

Bank of 20m wide river, grassy bank with willows and nettles etc

15

47.665648

21.261768

91

27-iv-2015

Hungary

Drainage ditches near R. Hortobagy, north-east of Balmazújváros

Broad drainage ditch between road and ploughed field with Phragmites etc.

16

48.374291

20.725264

148

28-iv-2015

Hungary

R. Bodva, south of Szendrő

Bank of small river running through landscape of forest and agricultural fields. With poplars, Euonymus europaeus and Prunus spinosa etc

17

49.463447

21.697255

385

28-iv-2015

Poland

R. Panna, at Tylawa

Bank of small river (7-8 m wide) with stony to muddy bottom and alders, birches and blackthorns etc

18

50.470234

22.238372

157

29-iv-2015

Poland

Fields north of Rudnik nad Sanem

Agricultural land by E77 highway with old and young Salix viminalis plantations. Birch, alder and blackthorn common.

19

50.673994

21.823391

141

29-iv-2015

Poland

R. Łęg, near Gorzyce

Wet meadow near embanked river with Phragmites etc.

20

51.775039

21.197100

101

30-iv-2015

Poland

R. Pilica, at Warka

Sandy banks of large river (30m wide), banks managed for angling.

20a

51.775039

21.197100

101

11-vi-2015

Poland

R. Pilica, at Warka

Sandy banks of large river (30m wide), banks managed for angling.

21

52.693980

21.852900

96

12-vi-2015

Poland

R. Bug, near Brok

Rough banks of wide muddy river used for angling with nettle, Rubus, Symphytum etc.

22

53.554830

22.302990

128

12-vi-2015

Poland

Meadow near R. Biebrza at Wasocz, near Szczuczyn

Wet meadow with Typha, Menyanthes, Comarum etc.

23

54.069430

23.117450

137

13-vi-2015

Poland

R. Czarna Hańcza, near Sejny on road from Suwalki

Sluggish 12m wide river with waterlilies and lined with Phragmites and Alnus, and adjacent meadow with Cirsium, Dactylorhiza etc.

24

54.925830

23.774200

28

13-vi-2015

Lithuania

Embankment of River at Kaunas

Dry sandy ridge overlooking wide muddy river

25

55.795570

24.566780

62

13-vi-2015

Lithuania

R. Levuo at Karsakiškis near to Panevėžys

Banks of river with birch and willow thicket

26

56.711410

24.251620

23

14-vi-2015

Latvia

Near R. Misa, between Iecava and Kekana

Scrubby meadow beside farm track

27

57.749630

24.402300

7

14-vi-2015

Latvia

R. Salaca short distance inland from Salacgriva

Rough meadow beside river with Alnus, Acer, Prunus etc

28

58.422570

24.440630

18

15-vi-2015

Estonia

Field near Parnu

Rough pasture beside road, invaded by willows

29

59.402890

24.935770

48

15-vi-2015

Estonia

R. Pirita at Lagedi near Tallinn

Banks of small shallow river with abundant aquatic macrophytes, by suspension footbridge

30

60.272990

24.658430

33

16-vi-2015

Finland

Near Lake Bodom, Espoo, Finland

Along ditches near lake, in agricultural landscape of cereal fields and meadows, and aspen/birch groves

31

61.099650

25.628200

84

16-vi-2015

Finland

Drainage flowing into lake Vesijärvi at Paimela near Lahti

Banks of small muddy river 6-7 m wide, in agricultural landscape with abundant aspen and birch

32

62.049620

26.123690

174

17-vi-2015

Finland

Lake near Toivakka

Forest and lake margin where road crosses end of lake in birch, aspen, pine and spruce forest

33

63.015890

25.804570

139

17-vi-2015

Finland

Near Viitasaari

Along ditches beside forest track at margin of Pinus, Betula forest

34

64.050740

25.526640

91

17-vi-2015

Finland

R. Pyhäjoki, at Joutenniva, south of Haapavesi

Banks of fast flowing rocky river through agricultural landscape with aspens, birches, alders and willows along banks

35

64.612870

25.538050

58

18-vi-2015

Finland

Tributary of the R. Siikajoki near Mankila

Banks of small river (6m wide) and ditches, in agricultural area, with aspen and birch common

36

65.328350

25.291750

1

18-vi-2015

Finland

R. Iijoki at mouth, near Kestilä, north of Oulu

Banks of very wide river

37

66.249470

23.89450

51

19-vi-2015

Finland

Small river between Kainuunkylä and Väystäjä

Wet scrub and woodland edge (birch and spruce) with abundant Trollius and other northern herbs

38

67.212530

24.126290

160

19-vi-2015

Finland

Near Vaattojärvi

Between two small rivers flowing into lake with wet areas and ditches around birch, pine forest

39

67.911830

23.634110

233

19-vi-2015

Finland

R. Muonion (Muonionjoki) just south of Muonio

Banks of wide (100-200m), rocky, fast-flowing river

40

68.813800

23.266580

374

20-vi-2015

Norway

South of Siebe

In birch scrub on heathy ridge above lakeshore in reindeer management area

41

69.724870

23.405810

289

20-vi-2015

Norway

Shores of Lake Trangdalsvatn

Rocky slope down to clear, gravel bottomed lake, surrounded by birch and willow scrub

42

70.652340

23.665830

67

21-vi-2015

Norway

Jansvannet Lake, Hammerfest

Wet areas and margins of birch wood around lake

SUPPLEMENTARY SITES

i-A

46.847908

8.631778

455

17-iv-2015

Switzer-land

R. Reuss, near Erstfeld

Gravel-bottomed river near motorway

i-C

39.235768

20.523075

-3

19-iv-2015

Greece

R. Acheron, at Mesopotamo

Drained cultivated fields surrounded by drainage ditches in river delta

i-J

45.447181

22.228965

236

26-iv-2015

Romania

Near Caransebes

Wet area near highway interchange [no willows collected]

i-K

52.302400

5.525235

11

1-v-2015

Nether-lands

Dyke in Flevoland

Near abundant drainage dykes by sea on reclaimed land.

ii-A

56.411000

24.167880

33

13-vi-2015

Lithuania

Near Bavska

Planted S. alba

ii-B

56.715700

24.249580

12

14-vi-2015

Latvia

R. Misa

Banks of river

ii-C

59.403880

24.932620

43

15-vi-2015

Estonia

Lagedi

Rough grassland invaded by shrubs near houses

ii-D

65.324430

25.315300

6

18-vi-2015

Finland

Kestilä

Margins of birch wood by road

ii-E

66.229570

23.785480

87

19-vi-2015

Finland

Near Kainuunkylä

Wet ditches by road at edge of birch wood

ii-F

67.934880

23.656410

238

20-vi-2015

Finland

Muonio

By river

ii-G

68.458680

23.425840

346

20-vi-2015

Finland

North of Hetta

Rocky area of birch and pine scrub

ii-H

69.343310

23.601290

317

20-vi-2015

Norway

South of Masi

Birch scrub with juniper

ii-I

69.881290

21.731950

7

22-vi-2015

Norway

West of Badderen

Rocky scrub by fjord

ii-J

69.512520

20.703190

13

22-vi-2015

Norway

Near Birtavarre

Rocky scrub by fjord

Summary climate variables taken from publically available resources (see Methods) for three contrasting sites on the transect: 1 and 42, the most southerly and most northery sites on map (Fig. 1) together with a middle site, 20, indicated by an arrow on map. Mean monthly temperature (°C) and mean monthly precipitation (mm) are given here. This table is provided as background information on the climatic gradient represented by the megatransect.

SITE 1 (nr. Thermopylae: 38.80, 22.46) SITE 20 (Warka: 51.78, 21.20) SITE 42 (Hammerfest: 70.65, 23.67)
Temperature,°C
Jan 5.31 -0.53 -12.51
Feb 7.01 1.02 -12.58
Mar 9.9 3.58 -10.92
Apr 12.41 8.39 -6.08
May 17.8 14.15 0.21
Jun 22.65 16.25 6.96
Jul 24.99 19.4 10.14
Aug 24.76 18.88 8.73
Sep 20.53 13.02 3.99
Oct 16.32 8.65 -3.07
Nov 10.76 2.82 -9.08
Dec 5.28 -3.33 -11.62
Precipitation, mm
Jan 49.06 25.28 79.46
Feb 28.5 29.7 76.35
Mar 43.45 29.48 73.6
Apr 52.78 34.13 54.42
May 44.53 40.14 33.99
Jun 15.56 65.88 42.68
Jul 24.76 79.65 53.09
Aug 15.41 55.42 55.79
Sep 34.78 53.66 51.47
Oct 42.38 42.62 82.58
Nov 66.34 42.03 75.12
Dec 91.11 27.17 87.93

Results

Sites

Table 1 shows the 42 sites recorded in this study as well as details of further "supplemental sites" where insect collections were made but without the level of sampling accorded to the main sites. The geographical distribution of sites is shown in Fig. 1. The supplemental sites will not be discussed here but their basic details are given, as subsequent papers on the insects sampled along the transect may refer to them. The latitudinal variation provides an enormous variation in climate. Table 2 shows summary climatic statistics for three sites: the most southerly, the most northerly and a central site (Poland).

As can be seen from Table 1, site elevations varied from 1 m to 556 m above mean sea level (amsl), with a median of 95 m. Because the sampling was predominantly in lowlands, the diversity of mountain or upland willows was not captured in this study, nor was it intended to be. Instead we capture the diversity of large stands of willow found in wet low-lying areas, which from an "insect eye view" or "insect chemosensory perspective" represent the largest areas of willow resource in the landscape, generally associated with landscape features such as rivers, lakes, drainage ditches or poorly drained meadows.

Willows

Table 3 lists the total of 20 willow species that were recorded, together with the 12 hybrids. For each taxon the total number of sites (out of 42) is given. In this transect, the most widespread willow is Salix alba, which occurs in 20 sites (out of 42). This species is followed by S. triandra (with 15 sites), S. capreaS. phylicifolia (with 14 sites each) and by S. myrsinifoliaS. ×fragilis (with 13 sites).

Salix taxa (species and hybrids) on transect.

Taxon

Number of sites on transect

Species or hybrid

Hybrid binomial (if available)

S. alba L.

20

sp

-

S. amplexicaulis Bory & Chaub.

4

sp

-

S. aurita L.

6

sp

-

S. bebbiana Sarg.

7

sp

-

S. caprea L.

14

sp

-

S. cinerea L.

9

sp

-

S. eleagnos Scop.

1

sp

-

S. euxina I.V.Belyaeva

4

sp

-

S. glauca L.

5

sp

-

S. gmelinii Pall.

1

sp

-

S. hastata L.

5

sp

-

S. lanata L.

1

sp

-

S. lapponum L.

4

sp

-

S. myrsinifolia Salisb.

13

sp

-

S. pentandra L.

7

sp

-

S. phylicifolia L.

14

sp

-

S. purpurea L.

8

sp

-

S. silesiaca Willd.

1

sp

-

S. triandra L.

15

sp

-

S. viminalis L.

9

sp

-

S. alba × S. pentandra

1

h

S. ×ehrhartiana G.Mey

S. aurita × S. myrsinifolia

1

h

S. ×coriacea J.Forbes

S. cinerea × S. aurita

1

h

S. ×multinervis Döll

S. cinerea × S. triandra

1

h

S. ×krausei Andersson

S. euxina × S. pentandra

1

h

S. ×meyeriana Rostk. ex Willd.

S. myrtilloides × S. glauca

1

h

-

S. phylicifolia × S. myrsinifolia

2

h

S. ×tetrapla Walk.

S. purpurea × S. viminalis

8

h

S. ×rubra Huds.

S. triandra × S. viminalis

3

h

S. ×mollissima Sm.

S. viminalis × S. cinerea

1

h

S. ×smithiana Willd.

S. alba × S. euxina

13

h

S. ×fragilis L.

S. ×fragilis × S. triandra

1

h

S. ×alopecuroides Tausch

Site diversity (Table 4) was modest with the overall median number of willow taxa (species and hybrids) per stand being four. However, the stands showed a progressive increase in diversity from south to north with the median number of willow taxa per stand in southern Europe (Greece, Bulgaria, Romania) being three; the median number in central Europe (Hungary, Poland) being five; and the median number in northern Europe (Lithuania, Latvia, Estonia, Finland and Norway) being six.

Salix collections per site, trans-Europe transect, April-June 2015. Frequency in stands is given in brackets as: A=abundant, C=common, O=occasional, R=rare (see Methods under Data Collection).

Site Country No. of taxa Willow species and hybrids
1 Greece 3 S. alba (O), S. eleagnos (O), S. purpurea (C)
2 Greece 3 S. alba (C), S. amplexicaulis (C), S. triandra (O)
3 Greece 1 S. alba (C)
4 Greece 3 S. alba (C), S. amplexicaulis (C), S. triandra (O)
5 Greece 2 S. alba (C), S. triandra (O)
6 Bulgaria 4 S. alba (C), S. amplexicaulis (C), S. purpurea × S. viminalis (O), S. ×fragilis (C)
7 Bulgaria 4 S. alba (C), S. amplexicaulis (C), S. euxina (O), S. triandra (C)
8 Bulgaria 2 S. alba (O), S. euxina (C)
9 Bulgaria 1 S. alba (A)
10 Romania 1 S. alba (A)
11 Romania 3 S. alba (C), S. purpurea × S. viminalis (C), S. triandra × S. viminalis (O)
12 Romania 3 S. cinerea (C), S. silesiaca (C), S. ×fragilis (O)
13 Romania 4 S. alba × S. pentandra (O), S. purpurea (O), S. triandra (O), S. ×fragilis (A)
14 Hungary 7 S. alba (O), S. euxina (O), S. purpurea × S. viminalis (O), S. triandra (C), S. triandra × S. viminalis (C), S. viminalis (O), S. ×fragilis (O)
15 Hungary 4 S. alba (O), S. cinerea (C), S. purpurea × S. viminalis (C), S. ×fragilis (C)
16 Hungary 5 S. alba (C), S. aurita (O), S. purpurea (C), S. triandra (C), S. viminalis (O)
17 Poland 3 S. caprea (R), S. euxina (C), S. purpurea (C)
18 Poland 5 S. aurita (O), S. cinerea (C), S. purpurea × S. viminalis (O), S. triandra (O), S. viminalis (C)
19 Poland 6 S. alba (O), S. cinerea (C), S. purpurea × S. viminalis (C), S. triandra × S. viminalis (C), S. viminalis (C), S. ×fragilis (C)
20 Poland 6 S. alba (R), S. gmelinii (O), S. purpurea (C), S. triandra (C), S. viminalis (O), S. ×fragilis (A)
21 Poland 7 S. alba (O), S. cinerea (R), S. cinerea × triandra (R), S. purpurea (C), S. triandra (A), S. viminalis (C), S. ×fragilis (C)
22 Poland 2 S. bebbiana (A), S. ×fragilis (C)
23 Poland 3 S. bebbiana (A), S. myrsinifolia (R), S. pentandra (O)
24 Lithuania 7 S. alba (R), S. caprea (A), S. purpurea (C), S. triandra (C), S. viminalis (O), S. viminalis × S. cinerea (O), S. ×fragilis (O)
25 Lithuania 7 S. alba (O), S. cinerea (O), S. myrsinifolia (A), S. pentandra (C), S. purpurea (C), S. triandra (O), S. ×fragilis (C)
26 Latvia 9 S. alba (O), S. bebbiana (O), S. caprea (O), S. cinerea (A), S. myrsinifolia (O), S. pentandra (C), S. purpurea × S. viminalis (O), S. triandra (O), S. viminalis (O)
27 Latvia 5 S. bebbiana (O), S. myrsinifolia (A), S. triandra (O), S. viminalis (C), S. ×fragilis (C)
28 Estonia 6 S. caprea (C), S. cinerea x aurita (R), S. myrsinifolia (A), S. phylicifolia (C), S. triandra (O), S. ×fragilis (O)
29 Estonia 4 S. myrsinifolia (A), S. phylicifolia × S. myrsinifolia (R), S. purpurea × S. viminalis (C), S. ×fragilis × S. triandra (R)
30 Finland 5 S. aurita (R), S. caprea (R), S. cinerea (R), S. pentandra (R), S. phylicifolia (A)
31 Finland 6 S. cinerea (O), S. euxina × S. pentandra (O), S. myrsinifolia (A), S. pentandra (O), S. phylicifolia (O), S. phylicifolia × S. myrsinifolia (C)
32 Finland 6 S. aurita (C), S. bebbiana (O), S. caprea (O), S. myrsinifolia (A), S. pentandra (C), S. phylicifolia (C)
33 Finland 4 S. aurita (O), S. caprea (C), S. myrsinifolia (C), S. phylicifolia (A)
34 Finland 3 S. caprea (O), S. pentandra (R), S. phylicifolia (A)
35 Finland 4 S. aurita (R), S. caprea (O), S. aurita × myrsinifolia (R), S. phylicifolia (A)
36 Finland 2 S. myrsinifolia (O), S. phylicifolia (A)
37 Finland 4 S. caprea (O), S. hastata (O), S. myrsinifolia (O), S. phylicifolia (A)
38 Finland 6 S. caprea (R), S. glauca (A), S. hastata (O), S. lapponum (R), S. myrtilloides × S. glauca (R), S. phylicifolia (A)
39 Finland 6 S. bebbiana (R), S. caprea (C), S. glauca (O), S. hastata (O), S. lapponum (A), S. phylicifolia (A)
40 Norway 2 S. glauca (O), S. phylicifolia (A)
41 Norway 7 S. bebbiana (O), S. caprea (C), S. glauca (C), S. hastata (O), S. lapponum (C), S. myrsinifolia (C), S. phylicifolia (C)
42 Norway 7 S. caprea (O), S. glauca (C), S. hastata (C), S. lanata (R), S. lapponum (C), S. myrsinifolia (C), S. phylicifolia (C)
SUPPLEMENTAL SITES
A-i Switzerland 2 S. eleagnos, S. purpurea × S. viminalis
C-i Greece 1 S. alba
J-i Romania 2 S. cinerea [not collected], S. ×fragilis [not collected]
K-i Netherlands 1 S. caprea
C-ii Estonia 7 S. bebbiana, S. cinerea, S. euxina × S. pentandra, S. myrsinifolia, S. phylicifolia, S. ×fragilis, S. ×fragilis × S. triandra
D-ii Finland 3 S. aurita × S. cinerea, S. caprea, S. myrsinifolia × S. phylicifolia
E-ii Finland 2 S. bebbiana, S. lapponum
H-ii Norway 5 S. glauca, S. hastata, S. lapponum, S. myrsinifolia, S. phylicifolia
I-ii Norway 3 S. caprea, S. hastata, S. myrsinifolia

Three stands (in Greece and Bulgaria) had just one willow taxon and in all cases this was S. alba. One stand (site 26 in Latvia) had the maximum recorded number of taxa, nine per stand, while six sites (in Hungary, Poland, Lithuania and Norway) had seven taxa. Finally, Table 5 lists the voucher specimens collected.

Salix collections (collectors: Quentin Cronk and Diana Percy), trans-Europe transect, April-June 2015. Accession number is collector-site-number (e.g. QCDP-A-1; QCDP-19-2). Sex is recorded as m=male, f=female, v=vegetative, b=in bud.

Site

No.

Country

Sex

Name

Notes

1

1

Greece

f

S. purpurea

To 6m

1

2

Greece

m

S. eleagnos

To 2m

1

3

Greece

m

S. cf. alba

River-side shrubs to 2m

1

4

Greece

f

S. alba

River-side shrubs to 2m

2

1

Greece

f

S. amplexicaulis

Shrub to 5m, opposite leaves

2

2

Greece

f

S. triandra

Pale bracts and stipules

2

3

Greece

f

S. alba

Large tree willow to 20m

3

1

Greece

m

S. cf. alba

Large tree to 20 m with fissured bark, 2 stamens per flower

3

2

Greece

f

S. cf. alba

Female flowers pedicillate, bracts relatively narrow, not very hairy, brown tipped

3

3

Greece

m

S. cf. alba

Small tree to 4m, 1 stamen per flower

3

4

Greece

f

S. cf. alba

Female flowers sessile, bracts relatively wide, very hairy

4

1a

Greece

m

S. cf. alba

Tall tree to 20m with fissured bark

4

1b

Greece

f

S. cf. alba

Tall tree to 20m with fissured bark

4

2a

Greece

m

S. amplexicaulis

Shrub to 4m

4

2b

Greece

f

S. amplexicaulis

Shrub to 4m

4

3

Greece

m

S. triandra (var.)

Shrub to 4m

5

1a

Greece

m

S. alba

Grey-barked tree to 10m

5

1b

Greece

f

S. alba

Grey-barked tree to 10m

5

2a

Greece

m

S. triandra (var.)

Small shrub to 4m

5

2b

Greece

f

S. triandra

Small shrub to 4m

6

1a

Bulgaria

m

S. alba

Tall grey barked tree to 15m

6

1b

Bulgaria

f

S. alba

Tall grey barked tree to 15m

6

2

Bulgaria

f

S. ×fragilis

Small tree to 6m

6

3

Bulgaria

m

S. purpurea × S. viminalis

Small shrub with reddish twigs, 2m

6

4

Bulgaria

f

S. amplexicaulis

Small shrub 2m

7

1a

Bulgaria

m

S. amplexicaulis

7

1b

Bulgaria

f

S. amplexicaulis

7

2a

Bulgaria

m

S. triandra

7

2b

Bulgaria

f

S. triandra

7

3a

Bulgaria

m

S. alba

7

3b

Bulgaria

f

S. triandra

7

4a

Bulgaria

m

S. euxina

7

4b

Bulgaria

f

S. euxina

8

1a

Bulgaria

m

S. euxina

8

1b

Bulgaria

f

S. euxina

8

2

Bulgaria

m

S. alba

9

1a

Bulgaria

m

S. alba

Tall trees to 30m and possibly planted. Similar trees are very common along the banks of the Danube.

9

1b

Bulgaria

f

S. alba

Tall trees to 30m and possibly planted. Similar trees are very common along the banks of the Danube.

10

1a

Romania

m

S. alba

Large tree to 20m

10

1b

Romania

f

S. alba

Large tree to 20m

11

1

Romania

m

S. alba

11

2

Romania

f

S. purpurea × S. viminalis

11

3

Romania

f

S. triandra × S. viminalis

12

1a

Romania

m

S. silesiaca

Shrub, twigs ridged under bark

12

1b

Romania

f

S. cinerea

Tree to 10m, twigs ridged under bark

12

2

Romania

m

S. ×fragilis (towards S. euxina?)

Glabrous tree to 10m

13

1a

Romania

m

S. ×fragilis

Small coppiced growth by river

13

1b

Romania

f

S. alba × S. pentandra

Small coppiced growth by river

13

2

Romania

f

S. triandra

13

3

Romania

f

S. purpurea

14

1a

Hungary

m

S. triandra × S. viminalis

To 5m

14

1b

Hungary

f

S. triandra

To 5m

14

2a

Hungary

m

S. euxina

To 8m

14

2b

Hungary

f

S. ×fragilis

To 8m

14

3

Hungary

f

S. viminalis

To 6m

14

4

Hungary

f

S. purpurea × S. viminalis

To 2m

14

5

Hungary

f

S. alba

To 10m

15

1a

Hungary

m

S. cinerea

Shrub to 4m with striae

15

1b

Hungary

f

S. cinerea

Shrub to 4m with striae

15

2

Hungary

m

S. purpurea × S. viminalis

15

3

Hungary

f

S. ×fragilis

15

4a

Hungary

m

S. alba

Small tree

15

4b

Hungary

f

S. alba

Large tree, branches weeping

16

1

Hungary

m

S. alba

16

2

Hungary

m

S. triandra

16

3

Hungary

f

S. purpurea

16

4

Hungary

v

S. viminalis

16

5

Hungary

f

S. aurita

16

6

Hungary

m

S. alba

17

1a

Poland

m

S. euxina

17

1b

Poland

f

S. euxina

17

2

Poland

f

S. purpurea

17

3

Poland

f

S. caprea

18

1

Poland

f

S. viminalis

Young coppice plantation

18

2a

Poland

m

S. aurita

Shrub to 4m weakly striate

18

2b

Poland

f

S. cinerea

Shrub to 4m

18

3

Poland

f

S. purpurea × S. viminalis

18

5

Poland

f

S. triandra

18

6

Poland

f

S. purpurea × S. viminalis

19

1

Poland

m

S. ×fragilis

Abundant at this site

19

2

Poland

f

S. purpurea × S. viminalis

19

3

Poland

f

S. viminalis

19

4

Poland

f

S. triandra × S. viminalis

19

5

Poland

m

S. alba

Occasional at this site

19

6

Poland

f

S. cinerea

20

1

Poland

m

S. ×fragilis

20

2

Poland

f

S. purpurea

20

3a

Poland

m

S. triandra

20

3b

Poland

f

S. triandra

20

4

Poland

f

S. viminalis

20

5

Poland

f

S. gmelinii

20a

1a

Poland

f

S. triandra

small tree/shrub to 4m

20a

1b

Poland

f

S. triandra

small tree/shrub to 4m

20a

2

Poland

f

S. ×fragilis

to 40m

20a

3

Poland

f

S. alba

to 30m

20a

4

Poland

f

S. ×fragilis

20a

5

Poland

v

S. gmelinii

large open sprawling shrubs to 5m

20a

6

Poland

v

S. viminalis

shrub to 5m

20a

7

Poland

v

S. purpurea

shrub to 4m

20a

8

Poland

v

S. triandra

20a

9

Poland

v

S. ×fragilis

very glossy green upper sides to leaves

21

1a

Poland

m

S. triandra

small trees or multi-stemmed shrubs to 10m high ×20m across

21

1b

Poland

f

S. triandra

21

2

Poland

f

S. ×fragilis

small bush to large tree to 20m

21

3

Poland

v

S. ×fragilis

21

4

Poland

f

S. purpurea

shrub to 4m

21

5

Poland

f

S. cinerea

21

5a

Poland

f

S. cinerea × S. triandra

shrub to 3m

21

5b

Poland

v

S. purpurea

21

6

Poland

f

S. viminalis

bush to 6m

21

7

Poland

f

S. alba

tree to 30m

21

8

Poland

f

S. ×fragilis

21

9

Poland

f

S. triandra

21

10

Poland

v

S. purpurea

22

1

Poland

f

S. ×fragilis

15m medium tree

22

2

Poland

v

S. bebbiana

low bushes 2-3m with abundant cercopid spittle bugs and willow feeding scaraboid beetle

22

3

Poland

v

S. ×fragilis

shrub to 3m

22

4

Poland

v

S. ×fragilis

sapling 1.5m

22

5

Poland

v

S. bebbiana

23

1

Poland

v

S. bebbiana

blue/grey-green foliage, 2-5m high

23

2

Poland

f

S. pentandra

trees all multistemmed (c. 4), to 15m, rugged bark, foliage with somewhat weeping habit

23

3

Poland

v

S. myrsinifolia

a few low bushes in the meadow with yellow-green foliage and some red pigmentation on stems, 1-1.5m

23

4

Poland

v

S. bebbiana

23

5

Poland

v

S. bebbiana

24

1

Lithuania

v

S. caprea

large shrub to 10m, planted?

24

2

Lithuania

v

S. purpurea

bushes to 2m

24

3

Lithuania

v

S. viminalis × S. cinerea

small sapling, no striae

24

4

Lithuania

f

S. ×fragilis

medium tree to 20m

24

5

Lithuania

v

S. viminalis

24

6

Lithuania

v

S. purpurea

24

7

Lithuania

m

S. triandra

24

8

Lithuania

f

S. triandra

24

9

Lithuania

f

S. ×fragilis

24

12

Lithuania

f

S. alba

25

1

Lithuania

v

S. purpurea

25

2

Lithuania

f

S. alba

25

3a

Lithuania

m

S. pentandra

25

3b

Lithuania

f

S. pentandra

25

4

Lithuania

f

S. triandra

25

5

Lithuania

v

S. myrsinifolia

25

6

Lithuania

v

S. myrsinifolia

25

7

Lithuania

v

S. myrsinifolia

25

8

Lithuania

v

S. myrsinifolia

25

9

Lithuania

f

S. cinerea

25

10

Lithuania

v

S. triandra

25

11

Lithuania

v

S. ×fragilis

26

1

Latvia

v

S. cinerea

5m h ×6m w

26

2

Latvia

m

S. triandra

26

3

Latvia

m

S. pentandra

small trees to 8m

26

4

Latvia

v

S. bebbiana

26

5

Latvia

v

S. purpurea × viminalis

26

6

Latvia

v

S. bebbiana

26

7

Latvia

v

S. viminalis

26

8

Latvia

v

S. myrsinifolia

26

9

Latvia

f

S. myrsinifolia

26

10

Latvia

v

S. caprea

26

11

Latvia

v

S. alba

26

12

Latvia

v

S. purpurea × S. viminalis

26

13

Latvia

f

S. triandra

26

14

Latvia

f

S. triandra

27

1

Latvia

f

S. myrsinifolia

subglabrous shrubs to 5m

27

2

Latvia

v

S. bebbiana

small bush with striae

27

3

Latvia

v

S. viminalis

27

4

Latvia

f

S. triandra

27

5

Latvia

v

S. ×fragilis

28

1a

Estonia

m

S. triandra

wide bush, 3m h ×4m w

28

1b

Estonia

f

S. triandra

28

2

Estonia

v

S. ×fragilis

young plants to 4m

28

3

Estonia

v

S. ×fragilis

28

4

Estonia

f

S. cinerea × S. aurita

bush 3 ×4m, with striae

28

5

Estonia

v

S. caprea

vigorous bush to 4m, no striae

28

6

Estonia

f

S. myrsinifolia

bush to 2m

28

7

Estonia

f

S. myrsinifolia

28

8

Estonia

v

S. phylicifolia

28

9

Estonia

v

S. myrsinifolia

29

1

Estonia

v

S. purpurea × S. viminalis

29

2

Estonia

v

S. ×fragilis × S. triandra

29

3

Estonia

f

S. myrsinifolia

29

4

Estonia

v

S. myrsinifolia

29

5

Estonia

v

S. phylicifolia × S. myrsinifolia

29

6

Estonia

f

S. myrsinifolia

30

1a

Finland

b

S. phylicifolia

shrub to 4m

30

1b

Finland

f

S. phylicifolia

30

2

Finland

f

S. pentandra

30

3

Finland

v

S. aurita

shrub to 4m, weakly striate

30

4

Finland

v

S. caprea

tree straight-trunked to 15m

30

5a

Finland

b

S. phylicifolia

30

5b

Finland

f

S. phylicifolia

30

6

Finland

v

S. cinerea

large shrub, no striae

30

7a

Finland

b

S. phylicifolia

30

7b

Finland

v

S. phylicifolia

31

1

Finland

f

S. myrsinifolia

small shrub to 2m

31

2

Finland

v

S. phylicifolia × S. myrsinifolia

31

3

Finland

f

S. myrsinifolia

31

4

Finland

v

S. phylicifolia

31

5

Finland

v

S. euxina × S. pentandra

very small plants 1-2m

31

6

Finland

v

S. myrsinifolia

31

7

Finland

f

S. cinerea

31

8

Finland

f

S. myrsinifolia

small plants 1-2m

31

9

Finland

f

S. myrsinifolia

31

10

Finland

f

S. myrsinifolia

31

11

Finland

m

S. pentandra

32

1

Finland

f

S. pentandra

to 4m

32

2

Finland

v

S. aurita

to 2m

32

3

Finland

f

S. myrsinifolia

to 2m

32

4

Finland

f

S. phylicifolia

to 2m

32

5

Finland

f

S. myrsinifolia

to 4m

32

6

Finland

f

S. phylicifolia

to 3m

32

7

Finland

f

S. myrsinifolia

32

8

Finland

f

S. myrsinifolia

32

9

Finland

v

S. bebbiana

32

10

Finland

v

S. caprea

33

1

Finland

v

S. caprea

small shrubs to 3m

33

2

Finland

f

S. aurita

to 2m

33

3

Finland

f

S. myrsinifolia

to 4m

33

4

Finland

f

S. phylicifolia

to 3m

34

1

Finland

f

S. phylicifolia

to 3m

34

2

Finland

v

S. caprea

to 3m

34

3

Finland

m

S. pentandra

c. 4m

34

4

Finland

f

S. phylicifolia

34

5

Finland

f

S. phylicifolia

35

1

Finland

v

S. caprea

small tree to 4m

35

2

Finland

v

S. aurita × S. myrsinifolia

shrub to 1.5m

35

3

Finland

f

S. aurita

old tree, 8m high

35

4

Finland

f

S. phylicifolia

bushes to 6m

35

5

Finland

f

S. phylicifolia

35

6

Finland

f

S. phylicifolia

35

7

Finland

f

S. phylicifolia

36

1

Finland

f

S. phylicifolia

to 4m

36

2

Finland

f

S. myrsinifolia

to 6m

36

3

Finland

f

S. phylicifolia

37

1

Finland

f

S. phylicifolia

shrub 1-3m

37

2

Finland

m

S. hastata

5m high spindly tree

37

3

Finland

v

S. caprea

3-4m high

37

4a

Finland

m

S. hastata

in wet heathy scrub, less than 75cm

37

4b

Finland

f

S. hastata

37

5

Finland

f

S. myrsinifolia

38

1

Finland

f

S. phylicifolia

to 3m

38

2

Finland

f

S. glauca

to 1.5m

38

3

Finland

f

S. glauca

38

4

Finland

f

S. hastata

1-1.5m

38

5

Finland

m

S. phylicifolia

38

6

Finland

v

S. caprea

38

7

Finland

f

S. hastata

38

8

Finland

f

S. myrtilloides × S. glauca

38

9

Finland

v

S. caprea

38

10

Finland

f

S. lapponum

39

1

Finland

f

S. phylicifolia

bushes to 2m

39

2

Finland

f

S. hastata

shrub to 1.5m

39

3

Finland

f

S. hastata

39

4

Finland

f

S. phylicifolia

39

5

Finland

f

S. bebbiana

39

6

Finland

f

S. hastata

39

7

Finland

m

S. phylicifolia

39

8

Finland

m

S. hastata

39

9

Finland

m

S. hastata

39

10

Finland

v

S. caprea

39

11

Finland

v

S. caprea

39

12

Finland

f

S. glauca

39

13a

Finland

m

S. glauca

39

13b

Finland

f

S. glauca

39

14

Finland

f

S. lapponum

39

15

Finland

f

S. lapponum

40

1a

Norway

m

S. phylicifolia

40

1b

Norway

f

S. phylicifolia

40

2

Norway

b

S. glauca

40

4

Norway

m/f

S. phylicifolia

catkins bisexual

40

5

Norway

f

S. phylicifolia

40

6

Norway

f

S. phylicifolia

41

1

Norway

f

S. lapponum

grey bush willow 1-1.5m

41

2

Norway

b

S. glauca

41

3

Norway

f

S. glauca

41

4

Norway

f

S. phylicifolia

green bush willow 1-2m

41

5

Norway

f

S. phylicifolia

41

6

Norway

v

S. hastata

dwarf willow

41

7

Norway

v

S. glauca

41

8

Norway

f

S. myrsinifolia

41

9

Norway

f

S. myrsinifolia

5m high with slender dark grey stems

41

10

Norway

m

S. myrsinifolia

41

11

Norway

f

S. bebbiana

41

12

Norway

v

S. caprea

5m high with slender pale grey stems

42

1a

Norway

v

S. lapponum

42

1b

Norway

f

S. lapponum

shrub 1-1.5m

42

2

Norway

m

S. glauca

shrub c. 1m

42

3a

Norway

m

S. glauca

dwarf shrub

42

3b

Norway

f

S. lanata

dwarf shrub

42

4

Norway

v

S. caprea

to 5m

42

5

Norway

f

S. lanata

shrub less than 75cm

42

6a

Norway

m

S. myrsinifolia

shrub to 4m

42

6b

Norway

f

S. phylicifolia

42

7

Norway

f

S. myrsinifolia

shrub 5m

42

9a

Norway

m

S. myrsinifolia

42

9b

Norway

f

S. myrsinifolia

42

10

Norway

f

S. hastata

42

11

Norway

f

S. hastata

42

13

Norway

f

S. lapponum

42

14

Norway

f

S. glauca

42

15

Norway

f

S. glauca

A-i

1

Switzerland

f

S. eleagnos

A-i

2

Switzerland

m

S. eleagnos

A-i

3

Switzerland

f

S. purpurea × S. viminalis

A-1

4

Switzerland

m

S. purpurea × S. viminalis

C-i

1

Greece

m

S. alba

J-i

1

Romania

-

S. cinerea [Not collected]

J-i

2

Romania

-

S. fragilis [Not collected]

K-i

1

Holland

f

S. caprea

C-ii

1

Estonia

f

S. euxina × S. pentandra

C-ii

2

Estonia

v

S. ×fragilis

C-ii

3

Estonia

v

S. ×fragilis × S. triandra

C-ii

4

Estonia

b

S. phylicifolia

C-ii

5

Estonia

v

S. myrsinifolia

C-ii

6

Estonia

f

S. cinerea

C-ii

7

Estonia

f

S. cinerea

C-ii

8

Estonia

v

S. bebbiana

C-ii

9

Estonia

f

S. bebbiana

C-ii

10

Estonia

f

S. phylicifolia

C-ii

11

Estonia

f

S. myrsinifolia

C-ii

12

Estonia

f

S. myrsinifolia

D

1

Finland

f

S. aurita × S. cinerea

D

2

Finland

v

S. caprea

D

3

Finland

f

S. myrsinifolia × S. phylicifolia

E

1a

Finland

m

S. lapponum

E

1b

Finland

f

S. lapponum

E

2a

Finland

m

S. bebbiana

E

2b

Finland

f

S. bebbiana

H

1

Norway

m

S. hastata

H

2

Norway

m

S. hastata

H

3

Norway

m

S. hastata

H

4

Norway

f

S. phylicifolia

H

5

Norway

f

S. phylicifolia

H

6

Norway

f

S. myrsinifolia

H

7

Norway

f

S. hastata

H

8

Norway

f

S. phylicifolia

H

9

Norway

m

S. glauca

H

10

Norway

f

S. phylicifolia

H

11

Norway

f

S. hastata

H

12

Norway

f

S. glauca

H

13

Norway

f

S. glauca

H

14

Norway

f

S. lapponum

H

15

Norway

f

S. glauca

H

16

Norway

f

S. phylicifolia

I

1a

Norway

m

S. myrsinifolia

I

1b

Norway

f

S. caprea

I

1c

Norway

v

S. caprea

I

2a

Norway

f

S. myrsinifolia

I

2b

Norway

f

S. myrsinifolia

I

2c

Norway

f

S. myrsinifolia

I

2d

Norway

f

S. myrsinifolia

I

2e

Norway

f

S. myrsinifolia

I

3a

Norway

f

S. hastat a

I

3b

Norway

f

S. hastata

I

3c

Norway

f

S. hastata

I

3d

Norway

f

S. hastata

I

3e

Norway

f

S. hastata

Discussion

These samples provide a "snapshot" of willow diversity along a latitudinal gradient and an indication of the geographically changing taxonomic diversity that is presented to willow-feeding herbivores across Europe. What is particularly noticeable is the role in taxic diversity of hybrids. One third (10 out of 30) of the total taxa recorded were hybrids. This highlights the importance of recording hybrids, which are often inadequately reported in surveys. Salix hybrids are notable for their frequency in nature but comparative rarity in the literature on willows.

Also worthy of comment is the general increase in willow diversity from south to north (Table 4). This is the opposite of a common biogeographical pattern that species diversity is higher in warmer regions nearer the tropics, and lower nearer the poles. The genus Salix has undergone a major radiation in boreal regions which may go some way towards explaining this inversion of the norm.

Finally, it should be noted that these willows formed the background for a major sampling of insects and it is anticipated that further papers forming part of this study will examine the insect fauna collected.

Acknowledgements

Funding for the fieldwork was partly provided by the Natural History Museum (London, UK) Life Sciences Departmental Investment Fund (SDF13010) to DMP. QCC acknowledges appointments by RBG Kew (as Honorary Research Associate) and by Queen Mary University of London (as Visiting Professor), which greatly facilitated the conduct of this study. Finally, we thank the two reviewers for their many helpful  suggestions.

Author contributions

QCC collected the willows, made preliminary identifications of the willow specimens and wrote the paper; ER assisted in the collection of willows and contributed to the writing; IB identified the willow specimens and contributed to the writing of the paper; DMP conceived and planned the study, jointly collected the willows and co-wrote the paper.

References