II] WOODLAND ASSOCIATIONS 59 



primaeval oak (Quercus sessiliflora} forest. On the accompanying 

 vegetation maps, they are given the same colour as the oak 

 woods of which they originally formed a part. 



Most of the alder-willow thickets occur on the Pendleside 

 shales; and on these soils at the lower altitudes, woods are 

 now rare. It seems highly probable that this is due to the 

 fact that the shales make excellent agricultural land ; and the 

 original woods on the shales, therefore, have nearly all been 

 felled, and the land put down to cultivation, chiefly as permanent 

 pasture (see Chapter VIII). 



The flora of the alder-willow thickets does not differ 

 materially from that of the damper parts of the oak woods. 

 Occurring in the cultivated area, it is natural to find in them 

 some alien trees, and some invading pasture species among the 

 ground flora. The following trees and shrubs were noted in 

 the ash-alder thickets near Edale and Castleton : 



Subdominant species 



Salix fragilis Alnus glutinosa 



S. cinerea Fraxinus excelsior 



Locally abundant species 



Salix caprea Rubus spp. 



Corylus Avellana Rosa tomentosa (rare) 



Betula pubescens R. canina 



Ulmus glabra Pyrus Aucuparia 



( = TJ. montana) Hedera Helix 



Prunus spinosa Lonicera Periclymenum 



Occasional and rare species 



*Larix decidua Q. sessiliflora 



*Populus canadensis *Prunus insititia 



*Castanea sativa Rosa arvensis 



*Quercus Robur *Acer Pseudoplatanus 



BIRCH WOODS OF BETULA PUBESCENS 



Birch woods were recognized in Scotland by Robert Smith 

 (1900, a and 6) who stated that they were quite natural and 

 self-sown. Smith did not state which of the two British species 

 form the dominant element; but both are actually abundant 



