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Botanical Indications of Air PollutionBy ERIK SKYEUntil recently the subject of air pollution hasaroused little attention in Sweden, thanks to the factthat we have been entirely spared such catastrophesas those of the Meuse valley near Liege, Belgium1930 and London 1952 and 1956. However,in the last few years the marked deterioration inour atmosphere has made research a necessity hereas well. We now also have a large foreign materialto draw upon.The term air pollution includes the release ofgases and solid particles of various kinds into theatmosphere as well as such natural phenomena asthe escape of dust from sandy areas and fields.Signs of the latter type of air pollution wereamong the earliest observations made in this fieldby a number of authors. Thus it is known that inthe mountains the vegetation at the edge of a spotexposed to wind often receives a considerablenutrient supplement in the form of airborne humusand mineral soil deposited onto an unbroken blanketof vegetation, such as a border of Betula nanaor various Salix species. The epiphytic vegetationon trees lining roads largely consists of speciesbenefiting from the escape of dust from the surroundingfields. As pointed out by SERNANDER(1912d) and later, amongst others, ALMBORN (1948)such trees have a lichen vegetation only partlyresembling that infesting the same kind in more orless virgin forest. Some species found in naturalsurroundings do not occur; · others-which SER­NANDER considered nitrophilous-are new additions.The latter are thus positive indicators of thistype of air pollution. In certain parts of Sweden,e.g. southern and eastern Skane and bland, theescape of soil often causes problems. However,these are outside the scope of the present article.19* - 652151Generally speaking the term "air pollution" isused to connote quite a different type of phenomena,such as dispersal of industrial waste, exhaustgases from cars and smoke and soot from chimneys.Of this type of pollution, too, the lichens are goodindicators. Already in 1866 NYLANDER commentedon the sparse, weak lichen vegetation on the treesin Paris. ARNOLD (1891-1901) made the same observationin Munich. SERNANDER took up the questionin his essay of 1912 d and later in "StockholmsNatur" 1926. In the latter work he notes that largeparts of central Stockholm are completely devoidof lichens (a "lichen desert"). This is surroundedby a transitional or "struggle zone" which is surroundedby a normal zone of lichen vegetationin the periphery of the city. SERNANDER furthernotes that the conifers in the city are apt to beinjured by air pollution. Especially the sprue tree,Picea abies, is sensitive in this respect. SERNANDER'sworks stimulated several other Fennoscandian botanists,e.g. HAUGSJA (1930), H0EG (1934) andV AARNA (1934). The lichen flora of Stockholm andits surroundings has been studied by a number ofpeople over the years, but no detailed investigationshave been published.Among the many non-Scandinavian works onthe occurrence of lichens in urban environments isBARKMAN (1961), describing the impoverishment ofthe lichen flora in the Netherlands. This, the authormaintains, is the work of man, and he claims thefollowing concerning epiphytes, "The cryptogamicepiphytes (bryophytes as well as lichens) have beenreduced in both number and species by the followingprocesses: the use of artificial manure, cleaning,white-washing and spraying of poison on fruit-trees,removal of dead boles and stumps, cutting downActa Phytogeog1·. Suec. 50