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Carex bigelowii Torrey ex Schweinitz ( C. rigida Good., non Schrank; C. hyperborea Drejer)

2001; Wiley; Volume: 89; Issue: 6 Linguagem: Inglês

10.1111/j.1365-2745.2001.00623.x

1365-2745

R. W. Brooker, Bengt Å. Carlsson, Terry V. Callaghan,

Botany and Plant Ecology Studies

A clonal perennial sedge with creeping rhizomatous growth. Mature clones with a below-ground moderately branched tiller system have up to 27 generations of connected tillers. Rhizomes stout; roots grey-brown, often as thick as rhizomes (Jermy et al. 1982). Culms rigid, up to 30 cm, sharply three-angled (occasionally four-angled; Sell & Murrell 1996) and usually rough towards the top, developing on tiller generations 3–5 (the youngest generation being generation 1). Leaves present on tillers 1–3 (possibly 4), 2–4(–7) mm wide and shorter than stems, up to 25 cm long, hairless and somewhat glaucous, recurved and keeled. The leaf margins roll outward on drying, and leaf tissue turns reddish-brown with senescence. Ligules 1–2 mm long, ± triangular, with an acute apex. Basal sheaths reddish-brown, shiny and persistent, with the lower sheaths leafless. Inflorescence one-eighth to one-fifth (if short, one-half) the length of the stem with 1 (rarely 2) male and generally 2–3 (possibly 1–6) female spikes. Lowest bract leaf-like, shorter than the inflorescence; all other bracts small, with all bracts having large, blackish-brown auricles. Male spike 5–20 × 3–5 mm, purplish-black, subsessile or with a peduncle up to 0.5 cm. Male glumes 3–4 mm long with a pale midrib and base, the apex being acute/rounded. Female spikes 5–15 × 3–5 mm, closely spaced, or with the lower spike more distant; peduncles absent or very short. Female glumes 2.5–3.5 mm long, purplish-black with pale (sometimes inconspicuous) midrib, mostly equalling utricle in length but not in width, ovate and obtuse. Utricle 2–3 mm, smooth and green below, purplish-black above with a short, slightly notched beak up to 0.2 mm, and 2 stigmas. Nut 2 mm long, ovate and biconvex. Air-dried nut mass at individual sites ranges between 0.2 and 0.5 mg (excluding utricle) and 0.1–1.7 mg. Carex bigelowii (stiff sedge) is similar to mountain varieties of C. nigra but distinguishable by reddish-brown (not blackish-brown) basal leaf sheaths (Stace 1997). In early botanical works from the UK or Ireland, C. bigelowii sensu stricto has commonly been listed under C. rigida Good. (e.g. Smith in Tansley 1911; Bot. Irl.; Tansley, Br. Isl.) and less commonly C. concolor auct. (Pearsall 1950) and C. bigelowii ssp. nardeticola Holub (Sell & Murrell 1996). The current classification for this species in the UK is sometimes given as C. bigelowii ssp. rigida Schultze-Motel (Fl. Eur. 5; Scott & Palmer 1987). This form (as described above) is the central European ssp. of C. bigelowii, being present within central and north-western Europe and western Fennoscandia (Fl. Eur. 5). In northern Europe it is replaced by C. bigelowii ssp. bigelowii (C. hyperborea Drejer) which has larger and relatively thinner female spikes and longer peduncles on the male spikes (Fl. Eur. 5). The circumpolar Carex bigelowii sensu lato complex has an intricate taxonomy, and a wide variety of synonyms and members across its circumpolar distribution (Table 1). The confusion over nomenclature has not been aided by the application of the species synonyms or subspecies names from one geographical race to other races (Falk 1940), and frequent hybridization with other members of the Carex section Acutae (Faulkner 1973; Standley 1985). Löve & Löve (1966) describe C. bigelowii ssp. arctisibirica from northern Siberia (Carex ensifolia ssp. arctisibirica – Tolmachev et al. 1996), which is similar to ssp. rigida but with shorter creeping rhizomes, more slender stems and thinner leaves, and with female glumes lacking the pale midrib. Other key Asiatic members of the circumpolar C. bigelowii complex are C. lugens and C. soczavaeana, having less creeping, more tufted growth-forms and thinner leaves than the western species (Tolmachev et al. 1996; Stenström 2000). Carex consimilis, common in eastern Siberia and western North America, has longer leaves and longer pedunculated spikes than C. bigelowii (Hultén 1964). Fl. Eur. (5) also lists C. bigelowii ssp. ensifolia from the southern Urals. Hultén (1964) provides a review of synonyms for the C. bigelowii complex (listed as C. rigida) throughout its geographical range, giving details of physiological variation between races. The original type for C. bigelowii was a specimen collected from the mountains of New Hampshire, North America, and is probably a member of the northern circumpolar ssp. Carex bigelowii ssp. bigelowii (Hultén 1964). In this account we will consider, unless otherwise stated, information on C. bigelowii sensu stricto, including ssp. rigida (UK and Ireland) and ssp. bigelowii (Northern Europe, North America and Greenland). Carex bigelowii is a common sedge species of open upland and arctic communities, often growing within carpets of moss. Its foraging clonal habit can produce high, dense coverage over large areas and it is often a target species for grazers. Carex bigelowii is a native species of the British Isles, with the stronghold of its UK range in Scotland (Fig. 1) both within the main Highland region (Fraser Darling & Morton Boyd 1964), and in the Western and Northern Isles (Jermy & Crabbe 1978; Scott & Palmer 1987). However, it is also found at upland sites within the Pennines and Lake District of northern England (Fig. 1). In Wales its distribution is extremely localized, occurring only in the mountainous parts of Merioneth and Caernarvonshire (Ellis 1983). It occurs at sites throughout Ireland, in 16 out of the 40 Irish botanical divisions (Bot. Irl.). The distribution of Carex bigelowii in the British Isles. (○) Pre-1950; (•) 1950 onwards. Each dot represents at least one record in a 10-km square of the National Grid. Mapped by Mrs J. M. Croft, Biological Records Centre, Centre for Ecology and Hydrology, mainly from records collected by members of the Botanical Society of the British Isles. Preston & Hill (1997) classify C. bigelowii as a Circumpolar Arctic-Montane species. It occurs throughout much of Europe where its distribution has a distinct northerly bias, being found in Austria, the Czech Republic, the Færoes, Germany, Norway, Poland, Romania, Sweden, Finland and Iceland (Fl. Eur. 5). Although Knuth (Poll. 3), Hultén (1960) and Tutin et al. (Fl. Eur. 5) list it as present on Spitsbergen, Elvebakk & Prestrud (1996) state that herbarium specimens from Svalbard are actually C. subspathacea. However, a small population of C. ensifolia ssp. arctisibirica has recently been found on the shore of Adventfjorden (S. Spjelkavik, personal communication). To the west of Europe, C. bigelowii occurs in the coastal areas of Greenland (Trapnell 1933; Bliss 2000) up to c. 78° N (Scoggan 1978). In Canada and North America it is found throughout the Alaskan Arctic (Hultén 1968) and the Canadian Arctic Archipelago, including Baffin, Devon, Ellesmere, Southampton and Banks Islands (Dallwitz 1980; Aiken et al. 1999 onwards; Dallwitz et al. 1993 onwards, 1995 onwards, 2000). Its range extends south to the alpine regions of New England and New York (Matthews 1992) being also found in the mountains of British Columbia, Alberta and New Hampshire (Hultén 1960), and north to 80° N on Ellesmere Island (Scoggan 1978). To the east of Europe, the C. bigelowii sensu lato complex extends across arctic Russia (Fl. Eur. 5; Fig. 2). For example, Carex ensifolia ssp. arctisibirica replaces C. bigelowii sensu stricto east of the Kola Peninsula, with C. lugens in Eastern Siberia and North America and Carex soczavaeana widespread in tundra near the Pacific Ocean (Tolmachev et al. 1996). Hultén (1964) provides reports of members of the C. bigelowii complex from Corsica, the Caucasus, the western Himalayas and the Szechwan area of China. It may also occur in Japan (Sell & Murrell 1996). The circumpolar distribution of Carex bigelowii sensu lato from Hultén & Fries (1986). • indicates isolated, fairly exactly indicated occurrences and solid lines indicate regions of incompletely or approximately stated occurrences. Hatched areas indicate regions of common or fairly common occurrence. Contours and shading define areas of 200, 1000, 2000 and 4000 m a.s.l. Reprinted with permission. In the UK, C. bigelowii is generally found over 600 m above sea level (a.s.l.) (Sell & Murrell 1996), although it extends from 1326 m on Ben Nevis (Fl. Br. Isl. 3) down to 30 m and even 15 m on Shetland (Alt. range Br. Pl.; Scott & Palmer 1987). In the Cairngorms it is found at 1050 m on Cairngorm itself (Bayfield 1979) and up to 1068 m on Glas Maol (Welch et al. 1992, R.W. Brooker, personal observation). In Scotland the lower altitudinal limits of C. bigelowii descend in a north-westerly direction, from approximately 457 m in the east-central Highlands (from Ben Lawers to the Cairngorms and Lochnagar) to 122 m in the north-west Highlands (Pl. Comm. Scot.). At Moor House, in the Westmorland area of northern England, it is reported in communities from 678 m to 830 m a.s.l. (Rawes & Welch 1969) and reaches its altitudinal limit at the summit of Cross Fell 893 m a.s.l. (D. Welch, personal communication). In Ireland, C. bigelowii is one of only eight species from Watson's (1868–1870) 'Highland' list to reach an altitude of over 914 m (3000 ft; Bot. Irl.). The highest site with C. bigelowii detailed by Praeger (Bot. Irl.) is the summit of Luqnaquilla in the Wicklow Mountains, at 926 m, and the lowest is the Knockalla Ridge, at 366 m. Throughout mainland Europe, C. bigelowii is a species of upland areas, although again there is a gradual lowering of altitudinal range and limits with increasing latitude or climatic continentality. On the Færoes it has been recorded between 100 m and 800 m on Blankeskoalefaeld (Ostenfeld 1901–08). Carex bigelowii occurs at a wide range of altitudes throughout the mountain ranges of Scandinavia, at 1950 m in Jotunheim at approximately 61°30′ N (Nilsson 1986), at 970 m at Ringebu, 61°42′ N (Heide 1992), and at 500–650 m in northern Norway, 25 km south-east of Alta, 69°46′ N (Moen et al. 1993). In northern Sweden it has been recorded at 1694 m in Lule Lappmark (Nilsson 1986), and at 1000 m at Latnjajaure (Stenström & Jónsdóttir 1997), 1150 m on Mt Slåttatjåkka (Carlsson & Callaghan 1990a,b) and at 600 m in Vassijaure (Wijk 1986), all of these last three sites being within the Torneträsk region at approximately 68°20′ N. Carex bigelowii is not uncommon in dry habitats below 100 m throughout Iceland. However the wetland habitats in which it occurs tend to be between 350 and 800 m (except in the extreme north of the country). There are 20 records of it above 1000 m, and three from above 1100 m (all in separate localities in the Tröllskagi mountains, west of Eyjafjörur in northern Iceland). The highest record is at 1220 m on Kirkjufjall near Hörgárdalur, west of Akureyri (H. Kristinsson, personal communication). In Godthaab Fjord, on the west coast of Greenland, Trapnell (1933) notes its descent towards sea-level, although it tends to inhabit the lower montane zone between 100 and 300 m. However, Trapnell points out that the limits of this zone may vary by as much as 300 m depending upon aspect and exposure, for example. North American Carex bigelowii populations are reported at high elevations in Montana, Idaho, Wyoming, Utah and Colorado, occurring at elevations ranging from 1818 to 3636 m in the Rocky Mountains (Matthews 1992). Further north, in interior Alaska for example, localities include 760 m at Eagle Creek (65°10′ N; Chapin & Chapin 1980), 730 m at Toolik Lake (68°38′ N; Shaver & Laundre 1997) and approximately 1400 m at Atigun Pass in the Brooks Range (R. W. Brooker, personal observation). Carex bigelowii has a wide ecological amplitude (Gjærevoll 1956), possibly as a result of its taxonomic complexity and frequent hybridization with other closely related species (Carlsson & Callaghan 1991a). It tends to occur in short, open vegetation dominated by creeping, procumbent vascular species and cryptogams (see Section III). Within the UK, the distribution of Carex bigelowii coincides with several environmental features, including land above 2500 ft (762 m), the January daily mean temperature isotherm of 40° F (c. 4.5 °C), and annual precipitation in excess of c. 1500 mm (Atl. Br. Fl.). The mean annual temperature maxima experienced by C. bigelowii are generally less than 21 °C (Conolly & Dahl 1970). However, the maximum can approach 23–24 °C within northern England and Wales (Rodwell 1992). The temperature maximum for C. bigelowii throughout its European distribution is 27 °C (Dahl 1998). Annual precipitation for Carex bigelowii-rich UK plant communities may vary from more than 3200 mm on the western coast of Scotland (with over 220 wet days) to 1600 mm in the central and eastern Highlands (with 180–200 wet days per year), with much occurring as snow, especially in the central and eastern Highlands (Rodwell 1991b, 1992). Carex bigelowii is commonly found in areas of late snow-lie (Jermy et al. 1982). In subarctic and arctic environments these snow patches may not thaw until June and possibly even July (Wijk 1986; R. W. Brooker, personal observation). With fresh snow-fall occurring at some localities by the end of October and even September, especially at more northerly latitudes, the growing season for C. bigelowii is sometimes only 3 months. Snow depth at some sites has been recorded at 250–300 cm (Wijk 1986). However, C. bigelowii is not restricted to communities with a deep snow cover, and may be found on ridges and summits where wind action prevents the build-up of a thicker snow-layer (Carlsson & Callaghan 1991b). The exposure of these ridges may lead to either high levels of winter-time moisture loss whilst soils are frozen, or summer-time drought during snow-free periods, and so throughout its annual cycle C. bigelowii can experience periods of drought even though annual precipitation levels may be high. The distribution of Carex bigelowii is not strongly restricted by soil type. McVean & Ratcliffe (Pl. Comm. Scot.) list it under 'plant indicators of soil-Indifferents', i.e. it is a species that occurs on soils with an extremely wide range of soil calcium-status and pH, ranging from markedly oligotrophic to eutrophic. Similarly, in a survey presented by Gjærevoll (1956) C. bigelowii grows in soils with pH values from 6.25 to 3.25, although it tends to occur on the more acidic soils. However, certain authors have described C. bigelowii as a species occurring only on base-poor substrates (e.g. Hegi Fl., edn 1). Its common presence on low pH soils is probably because of the development of such soils under environmental conditions that also favour its growth, rather than the presence of the sedge being dependent upon the occurrence of acidic soils. Analysis of soils from Scottish communities containing C. bigelowii (Pl. Comm. Scot.) indicate the fairly wide range of pH level, soil nutrient (K) availability and soil organic matter content that it will tolerate (Table 2), although these values may not encompass the full range of conditions under which it can grow. For example, the highest pH value recorded by McVean & Ratcliffe (Pl. Comm. Scot.) in a community containing C. bigelowii is 5.4, whereas at a high fellfield site with C. bigelowii in northern Sweden Michelsen et al. (1995) measured pH as 7.12. The fact that C. bigelowii can utilize both ammonium and nitrate N (Jónsdóttir & Callaghan 1990) may be related to its ability to grow on both acidic and basic substrates. The substrates on which C. bigelowii can grow range widely in organic matter content. Soils with high organic matter content include upland podsols (Bayfield 1979), peat hags and even exposed bare peat (Scott & Palmer 1987). Substrates with low organic matter content include dry, stony mountain-top detritus with low humus formation and a minimum of peat (Tansley, Br. Isl.). These soils may be well drained but periodically wet and are found, for example, on the more exposed plateaux regions of the Cairngorms (Rodwell 1992, R.W. Brooker, personal observation). Carex bigelowii can grow in cinder-based gravel and silt (Kershaw 1962), well-drained postglacial lava field (Jónsdóttir et al. 1995), aeolian-andic soils (Magnússon 1994), alluvial sand (Trapnell 1933) and glacifluvial sand (Callaghan 1976). The common factor of all these soils tends to be the low availability of soil nutrients, probably resulting from several factors including temperature and soil water content. In wetter areas anoxic soil conditions and low temperatures may limit microbial respiration and soil mineralization (Swift et al. 1979; Billings et al. 1982). As a result, despite the build-up of organic matter within the soil, the availability of soil nutrients can limit plant growth (Chapin & Shaver 1985; Walker 1995). In dry, exposed sites, for example on ridge tops, the erosive action of wind and high levels of frost-heave activity may prevent the build-up of an organic soil layer, although pockets of organic material may develop (Tansley, Br. Isl.). The National Vegetation Classification (NVC) records C. bigelowii as a component of many communities (Rodwell 1991a,b, 1992). NVC community codes are given in parentheses. Phytosociological synonyms (e.g. Pl. Comm. Scot.) are not given here but are provided by Rodwell (1991a,b, 1992). Although C. bigelowii can be described as a species of open upland and montane communities, these terms are sometimes confusing because such communities may approach sea level at more northerly or westerly latitudes within the UK and Ireland (Pl. Comm. Scot.; Fl. Br. Isl.; Smith in Tansley 1911; Bot. Irl.). Although the geographical range of C. bigelowii extends to northern England and Wales, the distribution of British communities has an overwhelming Scottish bias, with 19 out of the 33 communities confined to Scotland. The major exception to this rule is the Eriophorum vaginatum blanket and raised mire (M20) which is concentrated within the Pennine region of northern England, possibly owing to the prevailing vegetation management regimes within this area (Rodwell 1991b). The relatively wide tolerance of C. bigelowii for varying climatic conditions is reflected by its presence within both wet mire and spring communities and drier heath and fellfield communities of exposed ridges and summit plateaux. For example, it occurs within the moss-dominated Philonotis fontana–Saxifraga stellaris spring community (M32; Rodwell 1991b), and within the Juncus trifidus–Racomitrium lanuginosum rush-heath (U9) on exposed areas of shifting gravel (Rodwell 1992). Carex bigelowii reaches the status of constant within this latter community, and within four other communities. All five communities in which it is a constant tend to develop within areas with mean annual temperature maxima of less than 21 °C (Conolly & Dahl 1970) and at relatively high altitudes, being within the low-, low-mid or mid-alpine zones (Rodwell 1991b, 1992). The altitudes at which these communities occur are related to the continental–oceanic, east–west climatic gradient, the exposure and aspect of the terrain, and the length of snow-lie. For example, the C. bigelowii–Racomitrium lanuginosum moss-heath (U10) develops above 500 m along the oceanic western seaboard and inner Western Isles (Rodwell 1992), in comparison to C. bigelowii–Polytrichum alpinum sedge heath (U8) which approaches the upper limit of vegetation at 1200 m within the more continental Cairngorms. Carex bigelowii is classified as a member of the arctic-alpine or 'Highland' component of the UK flora (Watson 1868–1870; Tansley, Brit. Isl.). As such, its distribution often corresponds to those of other arctic-alpine species, with the nature of the association (e.g. frequency and distribution) being dependent more often on the requirements of the other species than on the relatively indifferent C. bigelowii. For example, it is found on heaths with Arctostaphylos alpinus (Rodwell 1991b), in wet mires with Juncus trifidus (Rodwell 1991b), in relatively species-rich meadows with Alchemilla alpina (Tansley, Brit. Isl.) and in snow-bed communities with Salix herbacea (Beerling 1998). At any given altitude, changes in exposure may cause large differences in community composition, although again the relative indifference of C. bigelowii may mean that it remains a constant feature of the communities across environmental gradients. For example, at high altitude (1000–1200 m) within the Cairngorms, late-lying snow beds within Juncus trifidus–Racomitrium lanuginosum rush-heath (U9) are fringed by a zone of Nardus stricta–C. bigelowii grassland (U7) which surround an inner area of C. bigelowii-Polytrichum alpinum sedge heath (U8). Carex bigelowii also occurs as a component of extremely late-lying snow-bed communities, e.g. Polytrichum sexangulare–Kiaeria starkei snow-beds (U11), although with reduced abundance and frequency. Carex bigelowii occurs in only one UK 'woodland and scrub' community (Rodwell 1991a), i.e. Salix lapponum–Luzula sylvatica scrub (W20). It may be that the distribution of the vast majority of woodland species does not coincide with that of C. bigelowii, or that it is unsuited to denser vegetation where it may be shaded. W20, a mountain dwarf-shrub heath community, is dominated by short Salix supporting the proposition that C. bigelowii is favoured by low-stature open canopies. This preference may be reflected by the fact that, although occurring in upland communities with relatively dense vegetation, e.g. Calluna vulgaris–Vaccinium myrtillus-Sphagnum capillifolium heath (H21), it achieves its highest abundance and constancy in very open communities such as Carex bigelowii–Polytrichum sedge heath (U8). The cover of these high-altitude communities may range from a carpet of continuous (often moss-dominated) vegetation to small islands of vascular and bryophyte species within open scree and boulder fields (Tansley, Brit. Isl.). In Shetland, C. bigelowii is frequent on the summits of the higher hills on stony plateaux (fellfield and ridges) often with Salix herbacea (Scott & Palmer 1987). In Mull it is found in high-altitude grass heaths and, on ledges, in Racomitrium heath (Jermy & Crabbe 1978). The overriding features of communities containing C. bigelowii are twofold. Firstly, C. bigelowii does not grow as an understorey species. Secondly, it is very commonly associated with the presence of well-developed lichen and moss populations. In Ireland, C. bigelowii is found within communities similar to those of upland Britain, including those of the high moors of the Comeraghs in Co. Waterford (up to 792 m), the relatively arctic-alpine species-rich communities in the Kerry-Cork highlands (above 914 m), and (more rarely) on the limestone escarpments of the Ben Bulben Plateaux (644 m). Praeger (Bot. Irl.) lists species associated with C. bigelowii at these, and a number of other sites, including Arctostaphylos uva-ursi, Listera cordata, Salix herbacea, Vaccinium vitis-idaea, Lycopodium alpinum and Racomitrium lanuginosum. Carex bigelowii is a characterizing species of 43 communities defined in the Nordic Vegetation classification (Vegetationstyper i Norden – Database v2.4, 31st January 2000). It achieves dominance in four of these communities, these being Carex bigelowii–Carex lachenalii-type heath vegetation on poor soils, Carex bigelowii–Sphagnum compactum-variant fen vegetation, Juncus balticus-type fen vegetation and Carex bigelowii-variant fen vegetation. Mosses are key components of all these community types. In mainland Fennoscandia, as in the Scottish Highlands, C. bigelowii occurs in a range of communities, from dry heaths to late-lying snow-beds, and at a variety of altitudes. On high-altitude dry heaths in the Torneträsk region of northern Sweden (Mt Slåttatjåkka, 1150 m a.s.l., 68°21′ N, 18°42′ E), C. bigelowii grows in association with creeping dwarf-shrubs such as Cassiope tetragona, Vaccinium vitis-idaea, Empetrum nigrum ssp. hermaphroditum and the dwarf willows Salix polaris and S. herbacea (Jónsdóttir & Callaghan 1988; Carlsson & Callaghan 1991a,b). In these relatively dry communities, lichens, e.g. Ochrolechia frigida, Alectoria ochroleuca and Cetraria islandica, are also numerous (Carlsson & Callaghan 1990a). Within the same region, but in more shallowly sloping areas, C. bigelowii grows within 2–4 cm thick Racomitrium lanuginosum mat communities along with the dwarf shrubs mentioned above (Jónsdóttir & Callaghan 1988). These communities are similar to Carex bigelowii-Racomitrium lanuginosum moss-heaths (U10) found within the Scottish Highlands, having creeping vascular species such as the dwarf shrubs and C. bigelowii within a moss-dominated matrix. At lower altitudes, C. bigelowii is found within grass heath communities (with Salix herbacea and Racomitrium lanuginosum), and in base-rich fen communities (within moss-mats mainly of Drepanocladus revolvens and Campylium stellatum), the dominant vascular species at these latter sites being C. bigelowii and Eriophorum angustifolium (Carlsson & Callaghan 1990a). A rich sedge heath community from a north-west-facing slope of Mt Latnjatjårro (a few kilometres to the west of Mt Slåttatjåkka) is described by Stenström & Jónsdóttir (1997). The vegetation is partly flushed during snow melt, the most abundant vascular plants along with C. bigelowii being C. vaginata, Salix reticulata, Polygonum viviparum, Dryas octopetala, and the mosses Hylocomium splendens, Tomenthypnum nitens and various Dicranaceae. In the Norwegian mountains, C. bigelowii occurs in similar lichen and moss dominated heaths to those of northern Sweden. The lichen heath site from Stigstuv, Hardengervidda (60°17′ N, 7°35′ E), is similar to those from 1150 m a.s.l. in northern Sweden, but also containing Arctostaphylos alpinus and Vaccinium uliginosum (Callaghan 1976), possibly as a result of warmer or wetter conditions in this more oceanic lower latitude region. The plant communities of Scandinavian alpine snow-beds have been comprehensively reviewed by Gjærevoll (1956), who subdivided his Caricetum bigelowii–Carex lachenalii association into five 'sociations': the C. bigelowii–C. lachenalii–Drepanocladus sociation, the C. bigelowii–Leontodon autumnalis sociation, the C. bigelowii–Agrostis borealis sociation, the C. bigelowii–Deschampsia atropurpurea sociation, and the C. bigelowii–Luzula wahlenbergii sociation. Gjærevoll (1956) provides detailed accounts of the communities and environmental conditions at sites from which the separate sociations were recorded. As in the Scottish Highlands, very steep ecotones within snow-beds produce rapid changes in plant community species composition over short distances. In the Scandinavian snow-bed communities C. bigelowii occupies something of a middle ground within these community series. Wijk (1986) found that the vegetation along an 18-m transect in northern Sweden changed from being dominated by Empetrum nigrum ssp. hermaphroditum in areas with extremely short periods of snow cover (0 m), to Betula nana shrubs at the extreme edge of the snow bed (0–1.5 m), to a Vaccinium myrtillus zone with some Juncus trifidus (1.5–7 m), to Deschampsia flexuosa within the snow-bed vegetation proper (7–15 m), and finally to Salix herbacea and C. bigelowii-dominated vegetation toward the very centre of the snow bed (15–18 m). Carex bigelowii is replaced in very late-lying snow-beds and in communities from the more oceanic parts of the Scandinavian mountains by Carex lachenalii (Gjærevoll 1956). Moen et al. (1993) described a snow-bed community from a highland plateaux region (500–650 m a.s.l.), northern Norway, with C. bigelowii and Salix herbacea as the dominant vascular species along with lichens of the Cladonia and Stereocaulon genera and crustose species. These species occurred in 100% of twenty-four 50 × 80 cm plots. Väre et al. (1997) present details of the composition of four C. bigelowii-dominated snow-bed communities, from between 600 m and 900 m a.s.l., on the west slope of Jeähkkas mountain in the Kilpisjärvi area, north-western Finland (69°03′ N, 20°50′ E). Species with 100% occurrence (in all five plots at each site) at all four sites (600, 700, 800 and 900 m a.s.l.) were C. bigelowii, Polygonum viviparum and Salix herbacea. Other important snow-bed species at this site (with 100% occurrence in plots at given altitudes) were: Gnaphalium supinum (600 m, 700 m, 800 m), Leontodon autumnalis (600 m), Vaccinium vitis-idaea (600 m), Deschampsia flexuosa (600 m), Vahlodea (Deschampsia) atropurpurea (700 m), Oxyria digyna (700 m), Sibbaldia procumbens (700 m, 800 m) and Veronica alpina (700 m). On Iceland, as in mainland Fennoscandian and Scottish U10 communities, C. bigelowii is a component of Racomitrium lanuginosum-dominated communities. At Heidmörk in south-west Iceland (64°04′ N, 21°44′ W, 170 m a.s.l.) Empetrum nigrum ssp. hermaphroditum, C. bigelowii, Deschampsia flexuosa, Salix herbacea, Galium normanii, Festuca rubra, F. vivipara, Thalictrum alpinum and Juncus trifidus, all grow in the 5–10 cm thick moss mat (Jónsdóttir & Callaghan 1988). At Hnausheide, in the south-western part of the Icelandic Highlands (botanically those areas above 300–400 m –Thórhallsdóttir 1997 from Steindórsson 1964), the main vascular species in the moss heath are C. bigelowii, Empetrum nigrum ssp. hermaphroditum, Betula nana and Festuca rubra (Jónsdóttir 1991). Moss heath is widespread in Iceland, both in lowland and large tracts of the southern highland areas, especially in areas with high rainfall (Thórhallsdóttir 1997). As well as Racomitrium heath vegetation, C. bigelowii is a characteristic species of dwarf-shrub heaths. Both heath communities occur on mesic to fairly dry substrates. Characteristic species of the dwarf-shrub heath are Betula nana, Dryas octopetala, Empetrum nigrum, Loiseleuria procumbens, and Vaccinium uliginosum. Willows are usually present on the dr