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116 H. PETER HALLBERG .A.ND REINHOLD IV .A.RSSONtrue heather heath (Hylocomieto-:-Ca1lunetum accordingto DAMMAN 1957, see also MALMER'spaper inthe present book), apart from Oalluna, also otherdwarf shrubs dominate, e.g., Arctostaphylos uva-ursi(on rocks and boulders), Empetrum nigrum, andV accinium vitis-idaea. On strongly exposed cliffsin the bare skerry region, Empetrum often growsnearer to the sea than Oalluna (GILLNER 1964b,p. 8, cf. DAMMAN 1957, p. 371). Exposure to strongwind causes Calluna to be replaced by Deschampsiaflexuosa. Other plants frequenting the heatherheath are some other grasses (see below), herbs likePotentilla erecta, mosses, and lichens.The heaths are nowadays seldom grazed, and thetree and bush vegetation on these heaths is graduallyincreasing, particularly Sorbus aucuparia havingbecome much more widespread. The heather heathcorresponds in the woody vegetation to Vaccinietummyrtilli (IVARSSON 1962, p. 98) under a tree andbush layer of varying composition.On dry ground the heather heath is rich in grasses,mosses, and lichens. Among these species,Agrostis canina ssp. montana, Anthoxanthum odoratum,Deschampsia flexuosa, Hylocomium splendens,and Oladonia silvatica sens. lat. may be mentioned.On thin soil on rock the heather heath is replacedby grass heath, rock crevice vegetation, or lichencommunities (see above).Carex nigra, C. panicea, Oornus suecica, Ericatetralix, Eriophorum angustifolium, E. vaginatum,and Molinia coerulea belong to heath (moor) communitieson wet ground. They are, as a rule, countedin Sweden among the so-called wet heaths, whichconsist of a whole series of plant communities,transitional between heather heath and peat-formingvegetation, meadows, and pastures. In wetheaths, which resemble the fens closely, Nartheciumossifragum grows together with Drosera rotundifolia,Erica tetralix, and M olinia coerulea, etc. in a matof different Sphagnum species. However, N artheciumis more common inland where the precipitationis higher and the peaty areas niore extensive(cf. H. FRIES 1945, p. 42, Fig. 6).Lakes and fensReal lakes are few (cf. LYSEN 1960) but smallbodies of water and wet fen areas are common. Inthe littoral zone, particularly in the bare skerryregion, rock pools are numerous, whose brackishwater and high pH values reduce the vegetation toa green soup of Enteromorpha species. Inland ofthe littoral, the vegetation of the ponds has agreater variety of species. Limosella aquatica andTillaea aquatica are characteristic of a type ofshallow, periodically desiccated pools. The deeperponds near tlie sea shore are the habitats of Sparganiumangustifolium, Typha latifolia, and severalScirpus species, e.g., S. maritimus. Not infrequently,Scorpidium scorpioides may appear in these localities,sometimes indicative of the presence of shelldeposits. Typical Lobelia lakes are very rare in thebare skerry and deciduous woodland regions (cf.H. FRIES 1945, map 178).Owing to the peat cutting, which was formerlyvery frequent, a number of minor bodies of water,originally peat-hags, are to be seen in the small peatareas formed in the rocky depressions. These peatdeposits were often dug out right down to the rock.Typical plants of the peat-hags are Equisetumfluviatile, Glyceria fluitans ad M enyanthes trifoliata,sometimes also . Iris pseudacorus. In thedeeper trenches, colonies of Hippuris vulgaris,Nymphaea alba and Sparganium angustifolium maybe seen, among others. Remains of bog vegetationmay be observed here and there in the peat-hags(IVARSSON 1962, p. 12). The hummocks are cappedwith, for instance, Calluna, Empetrum nigrum,Erica tetralix, Sphagnum spp. and, occasionally,Rubus chamaemorus, which is rare in the coastalregion. The margins of ponds and peat-hags aremostly overgrown, forming transitional stages betweenthe communities of the open waters andthose of the moist meadow, wet heath, and firmerpeat-forming vegetation.In wet and poorly nutrified fens, Carex rostrata,Equisetum fluviatile and Drepanocladus fluitans,grow among others, while Carex vesicaria, Eriophorumlatifolium, Phragmites communis, Oalliergongiganteum, and Scorpidium scorpioides occur in thewell nutrified fens. On slightly drier ground, Oarexnigra, Lythrum salicaria and Potentilla palustris arecharacteristic, while Oarex flava and 0. hostianainhabit firm fens richer in nutrients..Acta Phytogeog1·. Suec. 50