58 VEGETATION OF THE PEAK DISTRICT [CH. 



Influence of Shade on the Ground Vegetation 



There are practically no places within the oak woods where 

 the shade is too great for the growth of a carpet of vegetation ; 

 but where the beech {*Fagus sylvatica) has been introduced 

 and planted in mass or even only in patches, such places occur. 

 Where the shade is most dense under the beeches, practically 

 no plants are to be found except a few mosses, and in autumn 

 some of the larger fungi. Where the shade is less dense, a few 

 grasses, such as Holcus mollis and Deschampsia fiexuosa, may be 

 scattered about ; and mixed with them are a few shade-loving 

 species, such as dog's mercury (Mercurialis pe7'e7inis) and wood 

 sanicle (Sanicula europaea). 



The sycamore {*Acer Pseudoplatanus) also casts much shade ; 

 but, as this is usually introduced singly among the indigenous 

 oaks, the tree does not usually bring about a great change in 

 the ground vegetation. 



The pine {* Pinus sylvestris) is frequently planted in mass ; 

 and, when planted closely, much shade is thrown, and the 

 original ground species tend to disappear. 



Of indigenous trees, the wych elm ( U. glabra = U. montana) 

 casts the greatest shade ; but on the siliceous soils this species 

 does not usually occur as a social tree except where planted. 



Alder-willow thickets 



In some of the valleys, there is, at the present time, no 

 extensive tract of woodland at all. This is the case, for 

 example, in the Edale Valley and around Chapel-en-le-Frith. 

 There can be little doubt that, in all such cases, the primaeval 

 woodland has been destroyed and the land put under cultivation. 

 Trees, however, often line the banks of the streams which flow 

 through such localities, and form narrow fringing thickets which 

 may be a mile or so in length. The most abundant trees are 

 the alder {Alnus glutinosa), willows (e.g. Salix fragilis and 

 S. cinerea), and the ash {Fraxinus excelsior). These alder- willow 

 thickets are to be regarded as the persisting and lingering 

 remains of the alder-willow association which doubtless fringed 

 practically all the streams when the latter flowed through the 



