IV. THE ALTITUDINAL DISTRIBUTION 



OF THE SPECIES. 



THERE are only a few species which are equally common in the 

 low land and on mountain heights. The majority of them are 

 either Lowland forms, which rapidly decrease in frequency above a 

 certain altitude and at last disappear completely, or Alpine forms 

 which have their main distribution at greater heights above sea- 

 level and decrease in frequency downwards. In countries with a 

 continental climate it is, as a rule, not difficult to draw a some- 

 what sharp limit for the altitudinal distribution of plants, but the 

 conditions for survey are far less favourable in a country like Ice- 

 land, where a cold and damp insular climate prevails in the coastal 

 districts. On account of the low summer temperature, the majority 

 of the Alpine species can thrive as far down as the low land, and 

 conversely many of the lowland mosses will be able to establish 

 colonies in favourable localities, which are situated essentially higher 

 than their usual limit of growth. In South Iceland, for instance, 

 I have seen a southern slope, at an altitude of above 500 metres, 

 which was covered with species of Hylocomium together with other 

 species belonging to the same formation. 



As regards Iceland a fairly distinct boundary line may be drawn 

 between the Lowland and Highland formations by taking the upper 

 limit of the heather moor (and of the birch) as a basis for the 

 classification (Helgi Jonsson, 189518991900); it is then seen 

 that the upper limit of growth of a very great number of Bryophyta 

 nearly coincides with the upper limit of the heather moor. The 

 upper limit of the heather moor varies somewhat in the different 

 parts of Iceland, and is also in a high degree dependent on local 

 conditions such as shelter, direction of exposure, and conditions per- 

 taining to moisture. In East Iceland the boundary line lies, as a 

 rule, at about 300 metres, and the same is the case in the greater 

 part of South-west, West and North Iceland. Around Myvatn, ac- 



41* 



