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 (Salia fragilis), the osier willow (S. viminalis), 
and the sallows (S. caprea and S. cinerea); but S. alba, 
S. triandra, 8. pentandra, S. purpurea, x S. Smithiana also 
occur by some of the stream sides. S, aurita and 8. repens 
appear to be absent from the limestones. 
The hazel (Corylus 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. Grossularia, R. alpinum, 
