IMMIGRATION AND DISPERSAL OF THE SCANDINAVIAN FLORA 223 



The importance of the opening up of new routes is vividly realized by any 

 European botanist visiting the U.S.A. for the first time. Along all the roads, 

 in all waste places in the northeast, the ruderal flora is completely "European" 

 in appearance, and a botanist could not tell from the flora on which side of 

 the ocean he was. The few non-European plants are such ones that also 

 occur in similar places in Europe: Erigeron canadense, Oenothera biennis, etc. 



Obviously, the reason for the dominance of European weeds is the fact 

 that these, coming from an area of more intensive agricultural practice and 

 human activity as a whole, had been more rigidly selected and adapted to 

 ruderal conditions. European agricultural practice and settlement density 

 created conditions under which the local flora could not compete — except for 

 a few species, which, on the contrary, managed to establish themselves in 

 Europe as well. This example is important because it shows the significance of 

 ecological changes taking place within the area concomitant with or at least 

 more or less contemporaneous with the introduction of alien plants. Due to 

 the changes, new ecologic niches are created which may open up room for 

 immigrants. What has happened in North America during the last few 

 hundred years is important as a picture of what must have happened when 

 Neolithic man first came to northwestern Europe with his husbandry and 

 agriculture. 



Whereas ecologic conditions in America have changed radically during 

 the "modern" period, due to the introduction of European agriculture, those 

 in Europe have not changed similarly during the same period. The changes in 

 agricultural methods have altered the aspects of the weed flora, but not 

 fundamentally. We may therefore, in the discussion of this group, neglect 

 point 4. Coming now to the next group we shall see how the historical develop- 

 ment is of much greater importance. 



The "middle" period is introduced by two overlapping events, the advent 

 of agriculture and the deterioration of climate. In some places the first, in other 

 places the latter, is the older of the two. In southern coastal districts agriculture 

 arrived during the Post-Glacial Hypsithermal period; in central and northern 

 districts it came (much) later. The effects of agriculture thus intergrade with 

 those of a major cHmatic change. 



Before the introduction of agriculture, vegetation must have proliforated 

 itself to an extent inconceivable today. The picture drawn by Iversen (1949) 

 is based upon the flat Danish landscape; in Norway it must have been more 

 varied, but, even so, there is no doubt that the forested pre-agricultural 

 landscape was a relatively monotonous one. Within the conifer region we 

 still have relatively untouched areas which in their floristic composition are 

 even more monotonous than the oceanic heaths in their terrible poverty. Even 

 the smallest clearing around a dairy chalet in the forest looks like an oasis in 

 these green deserts. Just as our ruderal and weed flora had been subjected to a 

 very strong selection before being spread to America, so our pre-agricultural 



