226 KNUT FAiGRl 



moment when the "Corylus cUmate" first estabhshed itself. The sequence 

 {PopuIus~)Betula-PhiuslCory/us~broadlesif forest is very reminiscent of what 

 one would expect from an ordinary succession under relatively uniform 

 climatic conditions. We possess at least one positive indication that differen- 

 tials of immigration speed are of importance: the gradual delay of Pinus in 

 relation to Corylus along a line from Denmark to western Norway. One 

 might conceive of climate first running through a Pinus-non-Corylus type in 

 Denmark and through a Corylus-non-Pinus type in west Norway, but the 

 assumption is not very probable. Different rates of dispersal give a better 

 explanation. It should be noted that, of the two, Piinis diaspores are better 

 adapted for long-distance dispersal, but Corylus nevertheless advances faster. 



Given the disadvantageous climate of the Younger Dryas period, we 

 must conceive of a comparatively rapid amelioration most probably bringing 

 climate up to at least present-day level in a rather short time. The omni- 

 presence of a (sub-)pioneer Betula phase at the beginning of the Post-Arctic 

 forest period indicates that the present laws of plant succession were valid 

 then also. 



The succession of forest types in northern Europe on the whole matches the 

 one to be expected under a static climate (which does, of course, not prove 

 anything). But there are some remarkable exceptions, the main one being the 

 history of Alnus. There is hardly any doubt that the species generally registered 

 in the lowlands is A. glutinosa. Its appearance has been seen in connection 

 with the old conception of a change from a drier to a more humid climate at 

 the Boreal-Atlantic transition, and it is difficult to suggest a better alternative 

 explanation. There has been a tendency on the part of Quaternary geologists 

 to operate with precipitation and temperature as if they were independent 

 entities instead of being tied together in the same circulation system. However, 

 it must be admitted that till today meteorologists have not been able to give 

 much help in explaining the changes inferred from geologic-biologic evidence, 

 and even less the astonishing parallelism in alternations between humid and 

 dry periods according to the Blytt-Sernander concept in the cyclonic belts of 

 north Europe, and of the Sahara desert as shown by recent finds (Meriel, 

 1962). 



The occurrence during the Hypsithermal of exigent species which are today 

 much rarer (Cladium) in or even absent (Trapa) from Scandinavia undoubtedly 

 indicates that climate deteriorated, and so does the general recession of 

 vegetation boundaries both from the mountains and from the north. It is 

 very doubtful if new competition can completely exterminate a species within 

 its proper climatic area, but it is obvious that every new immigrant that 

 reaches domination must do so at the expense of the plants previously extant. 

 The changes from, for example, Qiiercetum mixtum to Picea in Scandinavian 

 pollen diagrams at the last zone transition (Sub-Boreal-Sub-Atlantic) first 

 and foremost indicates that the newcomer has conquered its area at the 



