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Glimpses of the Bothnian coast 177destroyed by the fire, are conditions given for thesuccessful germination of spruce and pine seeds insufficient shelter (TIREN 1934, pp. 226-267). Thebirch leaf litter also helps to check the expansionof other vegetation.Another factor influencing the genesis anddevelopment of the coastal forest is land upheaval.The additional ground thus laid bare usually firstattracts the alders, which grow rapidly to beginwith. The damp substrate and often rich supplyof diaspores give the alder an initial · great advantagein the competition. In the southern partof the region, i.e. Gastrikland and southern Halsingland,the alder border consists mostly of Alnusglutinosa. Northern Halsingland, Medelpad andsouthernmost Angermanland form an intermediatezone with both A. glutinosa and A. incana and occasionallytransitional forms. Further north A.incana takes over almost entirely.The most common type of soil is coarse seawashedglacial till, rich in boulders and locallytransformed into boulder fields or shingle. At theriver mouths there are often deposits of fine-grainedriver sediment. Apart from this there are drumlinsand_ occasional fields of sea-washed glacifluvialgravel and sand, but dunes are exceptional. Thanksto submarine bedrock of Palaeozoic limestoneunder part of the Bothnian Sea and numerous butlocal Post-glacial deposits of shell debris along andabove the present coastline, the soil is rather calcareousin certain areas. The bedrock, on the otherhand, is nearly always Archaean and non-calcareous.TYPICAL ZONATION.-On morainic shores thevegetation nearest the sea consists of:Triglochin maritimumSonchus arvensisParnassia palustrisPlantago maritimaOaltha palustrisJ uncus GerardiRumex crispusEuphrasia frigidaEleocharis acicularisE. palustrisA common grass 1s the endemic Deschampsiabottnica. At times the more northerly endemicEuphrasia bottnica occurs. Species such as Centauriumvulgare are confined to the southern partof the area.Normally the alder border is preceded by a zonewith an abundant growth of alder seedlings (androot-shoots in A. incana). Here grow species such asFilipendula ulmariaLythrum salicariaAngelica silvestrisPotentilla palustrisa ornus suecicaV aleriana salinaParnassia palustrisJ uncus balticusCarex glareosaCarex MackenzieiThe part of the alder border itself nearest theshore is often very dense. The undergrowth containsthe same tall herbs as the adjoining outer zone,as well as some high grasses. The alder's abilityto bind ·nitrogen and the deep litter give rise to amull-like soil. The older parts of this alderwoodare invaded by the spruce, but its advance seemsto be checked by the severe storms that recurevery ten years or so, when inundation by brackishwater kills the young trees or seedlings nearest theshore (SKYE 1963). Even during the normal, lesserstorms the sea advances rather far into the alderborder and deposits quite large amounts of litterand also a lot of rubbish, driftwood and logs. Inthis way the ecosystem s provided with a good dealof nutrition.The spruce makes use of the rich substrate, andstands of fairly high trees develop rapidly. However,the soil is soon impoverished and at the same time.the alder is checked, the alderwood rich in herbsbeing gradually replaced by spruce forest carpetedwith mosses and a field layer of bilberry ( Vacciniummyrtillus). Where the ground is moist enough theseforests can even reach impressive dimensions.Normally, however, the crustal upheaval causesthe land to become increasingly drier, causing thespruces to lose their vigour and finally, after rathermore than a century, to stop growing nearly altogether.BOULDER SHORES.-On parts of the coaststrongly battered by the sea the ground often consistsof coarse mineral soil. Here conditions favourthe pine rather than the spruce. The seaward fringe .. (nearest the alder border) consists of a mixture ofspruce and pine but this is later succeeded by a purepine forest rich in lichens, with V accinium vitisidaea,Calluna vulgaris or Arctostaphylos uva-ursiActa Phytogeogr. Suec. 5()