344 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



like — indeed Birches actually form the northernmost (albeit scrubby 

 and open, see Fig. 96) ' forests ' of much of Europe, and supply 

 the only native arborescent growth in Iceland and Greenland. The 

 groves of Aspen and Balsam Poplars (all Populus spp.) are usually 

 (but apparently not always — see p. 343) secondary, replacing conifer- 

 ous stands after felling or fires, while those of Alders {Alnus spp.) 

 are normally merely serai. 



The undergrowth and ground-flora in well-developed examples 

 of these coniferous, etc., forests tend to be less dense and diverse 



Fig. 96. — Open scrubby Birch ' forest * in Finmark, northern Norway (' Nor- 

 wegian Lapland '). Dark ground-shrubs and Lichens cover the ground, the 

 former being here locally predominant. 



than in most broad-leafed deciduous ones. The reasons are 

 apparently (a) that there is no season during which the lower layers 

 are not shaded by the leaves of the dominants, {b) that the thick 

 and dry carpet of slowly-decaying resinous leaves hinders the 

 establishment of seedlings, and (c) that the generally less favourable 

 regions offer fewer potentialities for growth. Nevertheless there 

 may be a fair shrub layer, especially in the damper situations where 

 the ground is commonly moss-covered ; on the other hand, in 

 drier areas and in the northernmost sparse forests, luxuriant Lichens 

 often form a continuous ground-investment over vast areas. 



One or another type of northern coniferous forest (or, occasionally, 



