70 VEGETATION OF THE PEAK DISTRICT [CH. 



as they are very much more abundant further south, as on the 

 Chalk of south-eastern England, and on limestone further north, 

 as in north-west Lancashire. 



Of introduced conifers, the larch {*Larix decidua) and the 

 Scots pine {*Pinus sylvestris) are locally very abundant; but 

 no evidence has been obtained that either of these trees 

 rejuvenates itself from self-sown seed. 



The aspen {Populus tremula) is the only indigenous poplar 

 of the Peak District. It is decidedly uncommon on the whole; 

 but occasionally, as in Cressbrookdale, aspen societies occur. 



Of willows which are certainly indigenous, there are the 

 crack willow (Salix fragilis), the osier willow (S. viminalis), 

 and the sallows {S. caprea and S. cinerea); but S. alba, 

 S. triandra, S. pentandra, S. purpurea, x S. Smithiana also 

 occur by some of the stream sides. S. aurita and S. repens 

 appear to be absent from the limestones. 



The hazel {Gorylus Avellana) is a very abundant and 

 characteristic shrub, more so even than in the oak woods. 

 Dense thickets of hazel frequently occur, especially in the 

 subordinate scrub associations (see next chapter). 



The alder (Alnus glutinosa) is even less abundant and less 

 characteristic in the ash than in the oak woods ; but locally it 

 forms societies at the bottom of some of the damper dales, as in 

 Cressbrookdale. 



Birches are as rare as oaks in the ash woods of the Peak 

 District, and are perhaps not indigenous. *Betula alba and 

 *B. pubescens have both been planted in Haydale (Monsaldale), 

 along with beeches and conifers, on the site of a former ash 

 wood. The absence of oaks and birches from the ash woods 

 of this district is interesting; as, in other parts of England, 

 both trees occur more or less abundantly in ash woods. 



Oaks are very rare and perhaps not indigenous on the 

 Carboniferous Limestone of the Peak District. In the Wye 

 valley, which is locally well wooded, only about half a dozen 

 oaks were noted; and these did not occur in the more primitive 

 of the ash woods, but only among trees which were obviously 

 introduced, as in parklands and plantations. 



The beech {*Fagus sylvatica) is planted abundantly, but 

 does not appear to be indigenous on the Pennines. 



Several species of Ribes {R. Orossularia, R. alpinum, 



