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 sessiliflora, 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 b: 41, et seg.), 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 
