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122 H. PETER H.ALLBERG AND REINHOLD IV ARSSONAgrostidetum t€nuis (IVARSSON 1962, p. 100),containing Achillea millefolium, Agrostis tenuis,Fragaria vesca, Galium verum, Pimpinella saxifraga,etc., species that belong to the pastures or mownmeadows, has a wide distribution in the brush woodsof southern Bohuslan. The substratum is relativelydry, and the habitat as a rule exposed more or lessto the south, bordering arable land or pastures.Outside the brushwood, Agrostidetum splits into anumber of different communities (see above). Thiscommunity is not found_in closed forest.FoRESTS.-The trees that grow farthest to thewest are Betula verrucosa, Populus tremula, Quer,cusrobur and Sorbus aucuparia. A kind of wood,comprising only Betula verrucosa or Quercus robur,seldom B. pubescens or Q. petraea, is found even insites that are rather strongly exposed to the wind.The lower branches are extremely elongated in thewind-exposed localities and often pressed towardsthe ground, while the upper part of the crown isbent in the leeward direction. The annual shoots onthe windward side are generally greatly hindered ingrowth and much shorter than those directed leeward.The uppermost shoots are largely killed offand protrude dead above the shelter of adjacenthills. Thus the wood grows densely but appearsclipped at a distinct, usualy inclined plane. TheQuercus species mostly grow on bare boulder groundexposed to the south, as does also, further eastwards,Tilia cordata. The Betula species, in contrast,belong to horizontal, moist sites. Also Populustremula alone can develop exposed woodland vegetation.It is much more sensitive to the effect ofthe wind than Betula and Quercus, aspenwoodbeing probably only a pioneer stage. It sometimesgrows on bouldery ground, but the substratum isoften moist, never completely dry. As a consequencePopulus prefers more or less the northfacingslopes which are moister than those facingthe south.Further eastwards, Alnus glutinosa, Corylus avellana,Fraxinus excelsior and Ulmus glabra, togetherwith Quercus robur and petraea form a variant ofthe deciduous forests of south Sweden. Alnusglutinosa grows on a wet substratum rich inActa Phytogeogr. Suec. 50nutrients. It may also grow on the shore, butseldom forms dense littoral scrub, as it often doesin the upper geolittoral on the East Coast. In theAlnus forest, Salicetum species may develop asparse shrub layer.Fraxinus excelsior may, apart from appearingtogether with Alnus, stand alone, often then as apioneer on abandoned cultivated land with a highsubsoil water level, when hay-cutting and grazinghave ceased.Ulmus glabra grows on nutrient-rich ground witha good supply of water. Together with the Quercusspecies it can develop into groves of forest, oftenwith a shrub layer of Corylus avellana. Ulmusglabra ssp. scabra is the most frequent, but in thenorth also ssp. montana is seen. Marginal thicketsalong the edges of the clumps of forest very oftenconsist of Prunetum, which may also form a sparseshrub layer inside the forest, but only when this isnot too dense. The Geranietum sanguinei forms afringe in front of the marginal thickets of Prunetumspinosae.On ground deficient in nutrients Vaccinietummyrtilli forms a field laye;r vegetation also in thewoods. In the Quercus forest Vaccinietum deschampsietosumgrows on dry ground. When thesubstratum is moister, Quercus is replaced mainly byBetula verrucosa as a dominant tree. The field layerthen consists of Vaccinietum molinietosum, and amarginal vegetation of Salix repens and V acciniumuliginosum is often developed.The field layer under oak groves, growing onbetter nutrified ground, is as a rule formed byAnemonetum hepa ticae with Anemone hepatica,Carex digitata, Ranunculus auricomus sens. lat.,Viola riviniana, and others. These species are sensitiveto the effect of the westerly winds. Anemonetumhepaticae therefore occurs only in well protectedsites. The composition by species of this communityhas a south-eastern and continental trend.In the alder forest, where the subsoil water levelis high and the nutrient content of the ground isgood, Filipendula ulmaria dominates a conspicuouscommunity, Filipenduletum ulmariae, also containingfor example Deschampsia caespitosa, Geumrivale, Lycopus europaeus, Lysimachia vulgaris andMentha arvensis.