142 Plant-formation of Siliceous Soils 



occur in connexion with the oak-birch heath association 

 in southern England, and are to be regarded merely as 

 stages in the multiform succession exhibited by the heath 

 formation. There is little or no distinction of habitat 

 between such birchwoods and the associations which 

 precede or follow them in succession. 



The third kind of birchwood, however, has a different 

 status, for it occupies a distinct climatic zone, above that 

 of the oakwood, but the edaphio factors of its habitat 

 appear to be the same. . It might perhaps be regarded as 

 a separate climatic formation, but its characters are too 

 negative, and it is better considered simply as a separate, 

 climatically differentiated, association of the formation of 

 siliceous soils. 



Robert Smith, who first described the Highland birch- 

 woods from the ecological standpoint, treated them along 

 with the pinewoods of the same region, and wrote^: " The 

 subordinate vegetation in the sub-alpine woods is of a less 

 marked character than that in the temperate woods 

 [e.g. the oakwoods], hardly differing, except in the pro- 

 portion of the individuals present, from the adjoining 

 moorland or hill-pasture. ... In the pinewoods heather and 

 its associates usually constitute the plant carpet, whilst 

 in... birchwoods the ground is more often covered by 

 grasses and bracken, although this rule is by no means 

 invariable." 



The sub-alpine Scottish birchwoods possess, in fact, 

 a ground vegetation made up of the oakwood species 

 which can endure the higher altitude together with any 

 species of siliceous grassland. They appear to have no 

 species peculiar to them. Thus these birchwoods take 

 their place just like those of the Pennines as a climatic 

 zonal association of the formation of siliceous soils ; while 

 the pinewoods, as we have already seen (p. 117), appear 

 to be naturally included in the heath formation. The 

 1 B. Smith, 1900, p. 451. 



