IMMIGRATION AND DISPERSAL OF THE SCANDINAVIAN FLORA 225 



three groups. We know of very few plants belonging to Category 1 being 

 utilized to such an extent as to be exterminated locally (Archangelica officinalis, 

 Gentiana purpurea) or very much reduced in number {Ulmus scabra). We 

 must assume that under Category 2 we have completely lost important plant 

 communities of easily arable soil and that vegetation types known today are a 

 one-sided selection of those that would have existed in the absence of agricul- 

 ture. How many species are involved in the third principle will forever 

 remain conjectural, but I should personally be inchned to assign to this group 

 (3c) a rather high percentage of our flora — some 20 per cent perhaps. 



The development of climate and vegetation since the advent of agriculture 

 must be taken into account. For those parts of the country where land 

 occupation took place during the Iron Age, the remaining spontaneous 

 vegetation types are probably not too different from those existing before the 

 land was cleared. For the old agricultural regions we may presume that 

 changes due to climate — in addition to those due to agriculture — have 

 completely altered the face of the land, and that nothing remains of what was 

 the original vegetation. When discussing these problems, we should keep in 

 mind that much good soil, i.e. soil that is considered good today, was unsuit- 

 able for farming with the primitive tools of the pre-Iron Age man. 



Wendelbo (1957) has suggested a very interesting recent replacement of 

 species, viz. the disappearance of Centaurea pseudophrygia and its gradual 

 replacement by C. nigra. The former is a species of open deciduous forests, 

 such as are now gradually disappearing with the abandonment of grazing in 

 forested areas. C. nigra, however, grows in meadows, on roadsides, etc., and 

 is better adapted to conditions of modern agricultural practices. Interbreeding 

 and introgressive hybridization may contribute to speeding up the replace- 

 ment. One must assume that similar processes took place on a grand scale 

 during the more radical transformation of the landscape concomitant with 

 the primary introduction of agriculture. 



During the very long time between the disappearance of the Pleistocene 

 ice and the advent of agriculture, vegetation could develop under the influence 

 of two major factors: the changing climate, and the immigration of species. 

 The latter was again a function of the changing climate, but also of two other 

 factors: the location of glacial refugia, and the speed and possibilities of 

 dispersal. There is no doubt that in the discussion of the immigration of the 

 Scandinavian flora, these conditions have not been adequately treated. 



It has become increasingly clear that the sequence of forest types met with 

 in pollen-analytical investigations in Scandinavia cannot be interpreted in 

 such a one-sided climatological manner as has been the case. Registered 

 regional reversals must have a background in climatic shifts : thus the vegeta- 

 tional changes indicating the onset of the Younger Dryas and the Sub- 

 Atlantic periods are generally due to chmatic deterioration. But we have no 

 guarantee that the immigration of, for example, Corylus came just at the 



