IMMIGRATION AND DISPERSAL OF THE SCANDINAVIAN FLORA 227 



expense of the broadleaf forests. That might have happened even under a 

 uniform cHmate. On the other hand, the broadleaf forest also disappears 

 where no Picea comes in, and one may also ask why the spruce has started its 

 immigration at all. But conditions are by no means as simple as they are 

 frequently described. In other cases, e.g. that of Cladium, the disappearance 

 may be due to a change of environment. Again, one may raise the question 

 whether menotrophication came as a result of a climatic change. The appear- 

 ance of similar changes during the climatic decline at the end of the Last 

 Interglacial is indicative. 



The Post-Arctic climatic amelioration has a double aspect: the positive, 

 viz. the immigration and settling of new plants and communities, and also 

 the negative, viz. the crowding out of earlier types. The Late-Glacial cHmate is 

 still enigmatic, and again we may have been deluded by overlooking ecological 

 problems. The open, fresh soil left by the Pleistocene glaciers had a selective 

 influence on the germination of the few diaspores that initially landed on it. 

 To be able to establish themselves, the species have to be such plants that today 

 also occur on open, humus-free soil, a soil type that is primarily found in the 

 semi-deserts. It maybe that we have been too much inclined to interpret the 

 finding of steppe plants (and animals) as witnesses of a steppe climate instead 

 of a steppe soil. Even from a very early period pollen grains have been found 

 of plants which are decidedly not Arctic in their requirements. 



This has certain implications for our understanding of living conditions 

 during the Late-Glacial, but even more so for our understanding of the 

 disappearance of the Late-Glacial biota. Did animals and plants vanish 

 because of a change from a steppe climate to a more humid type, or did they 

 do so because the soil evolved towards a more humic type without change 

 of climatic humidity? Or both? Or because of the onset of competition? 

 The role played by mycorhiza should not be left out, but mycorhizal plants 

 do appear rather early, at least in humid regions. 



The absence of competition is another important factor not to be discounted. 

 Where "Late-Glacial plants", e.g. Ephedra, can maintain themselves well 

 into the Atlantic (at least) in Norway (Hafsten, 1953), this is certainly due not 

 only to the fact that small crags and shelves in steep rock faces were not 

 shaded by forest trees, but also to the fact that in such habitats competition 

 never became very severe. Just as such localities have been proposed as 

 possible stations for Digitalis and other species of the same ecology, they 

 most probably have also furnished havens for the Late-Glacial flora long 

 after it had been crowded out in soil progressing towards a climax condition. 



We know that the Late-Glacial flora of southern Scandinavia contained a 

 number of Alpine plants, i.e. plants that are today chiefly found in the Alpine 

 zone of our mountains. Dryas octopetala, Salix herbacea, Saxifraga oppositi- 

 folia, and many others have been identified among pollen and macro-fossils. 

 The fate of this flora has for a very long time been controversial. That these 



