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286 ERIK SKYEof old trees, substitution of unfavourable exotictrees for indigenous trees, general desiccation ofthe .. atmosphere through draining of lakes andmarshy and and through felling and thinning offorests. The dominant influence however, seems tobe the pollution of the air by the smoke of housesand factories and the exhaust gases of motorisedtraffic."In principle the air is polluted from three differentsources: 1) the heating systems of dwellinghouses, including all types from the coal fire to theoil furnace, 2) factories, large heating · plants andsimilar installations and 3) motor traffic. Of thesethe last two have received most attention, whichdoes not mean the first can be neglected. Especiallyin regions with a protracted cold season it probablyplays a significant part. Concerning the main causeof the reduction of the lichen flora in cities opinionsdiffer. Some authors, such as RYDZAK (1953, 1956aand b, 1957 and 1959), ascribe it to the "desertclimate'-'- of the urban environment. As RYDZAKpoints out, even in towns without industries alichen-free zone is formed in the centre, i.e. thedriest part.Of great interest is a comparison between BARK­MAN's (1963) map and the results so far obtained inStockholm (see below). No essential divergencescan be noted, and it seems difficult to explain thereduction of the lichen flora by means of the "desertclimate" theory. A similar impoverishment hasbeen reported from Great Britain and Germany aswell, and it looks as though the lichen flora isdecreasing all over Western Europe. The fact thatthere is good reason to ascribe this to air pollutionmakes the _picture no less frightening.In Sweden the foremost event drawing generaland official attention to the problem of air pollutionwas the severe damage to the vegetation surroundingthe shale oil wcrks at Narkes K varntorpin the years around 1950. The factory was erectedfar out in the country in connection with fields ofalum shales and around it grew a small settlement,mostly situated in the direction of the prevailingwind. It did not take long before damage appearedin forest and fields, and the management wasbesieged by claims for damage. At this junctureexperts were called in to analyse the situation fromdifferent points of view. The main source of damageat K varntorp was found to be S02, and for thefirst time it was possible to study the occurrenceof lichens in relation to the proportion of this gasin the atmosphere (SKYE 1958). In spite of theabsence of a "desert climate" a fairly large lichenfreezone had formed around the factory. Betweenthis and areas where the vegetation was not affecteda transitional belt could easily be discerned.Around the time this matter was being investigatedanother botanist, A. F. FENTON (1960, 1962 and1964), was engaged on similar research. Quite unawareof each other we both reached much the sameconclusions.Field work at present proceeding in Sweden seems.to support the suggestion that air pollution is.mainly responsible for the disappearance of lichens.from the 'cities as well. For the last year or sostudies of the lichen flora in Stockholm and Goteborg(SKYE 1964), the Ornskoldsvik district inAngermanland and Koping in Vastmanland havebeen in progress. A number of other areas are tobe covered, the aim being to get as comprehensivea picture as possible of how lichens react to differenttypes of air pollution. In this way it may bepossible to find suitable indicator plants for differentpollutions.The results of the Stockholm and Goteborg investigationshave not yet been fully analysed; infact some of the fieldwork has only just begun.However, enough material is available for certaintendencies to be detected, e.g., as might have beenexpected, the lichen-free zone in Stockholm has.expanded considerably since SERNANDER's andH0EG's investigations. Indeed, this zone keeps increasingas the city grows, and some of the membersof the lichen flora are so sensitive that thebotanist must thus keep a watchful eye on townplanningactivities and get to the site well beforethe bulldozers.Of the species found Lecanora conizaeoides andLecidea scalaris are least sensitive to a pollutedenvironment. They occur abundantly on various.kinds of trees in the periphery of cities such asStockholm. In the less disturbed lichen vegetationfarther from the centre they are a very minor constituentwhere they occur at all. Though competi-Acta Phytogeogr. S1tec. 50