II] WOODLAND ASSOCIATIONS 61 



reduced numbers. Since no Alpine and very few sub-Alpine 

 species take the place of the absent lowland species, the floristic 

 features separating the birch woods from the oak woods are, in 

 this district, largely of a negative character. From the stand- 

 point of vegetation, however, there are positive differences, as 

 there is a rearrangement of the common members of the two 

 associations. 



There is little doubt that the birch woods of Betula pube- 

 scens must be placed in the same plant formation as the oak 

 woods of Quercus sessiliftora, not merely because the two 

 associations are connected by all possible gradations and 

 because one may easily replace the other, but because the 

 general habitats have so much in common and the floristic 

 elements are so very much alike. A birch wood, in this district, 

 is simply a wood in which the oaks, on account of the increased 

 exposure consequent on the increased altitude, have largely or 

 entirely disappeared, and in which the birches have profited 

 by the absence of the competition of the oaks. However, the 

 difference in altitude and the consequent differences of the 

 flora and of the vegetation are sufficient to justify the placing 

 of the two communities in separate associations, in spite of 

 the existence of numerous connecting links. 



Following a system of universal nomenclature (see Moss, 

 1910 6 : 41, et seq.), the two most important woodland associa- 

 tions of the non-calcareous soils may be designated as follows : 



(i) Quercetum sessiliflorae or association of Quercus sessiliflora. 

 (ii) Betuletum pubescentis or association of Betula pubescens. 



Regarding these as belonging to the formation Silicion, the 

 above names may be combined as follows (Moss, loc. cit.} : 



(i) Silicion Querceti- sessiliflorae. 

 (ii) Silicion Betuleti-pubescentis. 



Not only do the meagre birch woods of the Pennines 

 closely resemble the oak woods, but the more typical birch 

 woods of Perthshire would appear to be very closely allied to 

 the Scottish oak woods. R. Smith (1900 b: 45) in describing 

 the birch woods of Perthshire says: "The birch woods in 

 the shelter of the river valleys may, however, have as rich a 

 vegetation as the oak coppice. Thus, for example, a list taken 

 in the birch wood between Loch Tummel and the Falls of 



