60 VEGETATION OF THE PEAK DISTRICT [CH, 



in Perthshire, though Betula pubescens ascends to higher 

 altitudes than B. alba. In the Leeds and Halifax district, which 

 lies immediately to the north of the Peak District, " the birch 

 wood or uppermost tree zone of the Scottish Highlands is 

 represented by a modification of the oak wood.... The oak 

 [Quercus sessiliflora] is usually dominant ; but when the best 

 of these are removed and no others planted, the birch [Betula 

 pubescens] becomes dominant, either alone or with stunted 

 oaks" (Smith and Moss, 1903: 388). In the Harrogate and 

 Skipton district of the mid-Pennines, an uppermost woodland 

 zone was also recognized where the dominant plants form a 

 loose scrub of birch [Betula pubescens], mountain ash [Pyrus 

 Aucuparia], holly [Hew Aquifolium], hawthorn [Crataegus Oxy- 

 acantha], blackthorn [Prunus spinosa], and willows [S. cinei^ea, 

 S. caprea, and S. aimta] (Smith and Rankin, 1903 : 159). 



Birch woods (see figure 8) are only feebly developed in 

 the Peak District : none is of great extent ; and none shows 

 a sharp line of demarcation from the upper oak woods. As 

 one ascends a wooded hill-slope composed of non-calcareous 

 rocks, the oak (Quercus sessiliflora) becomes rare at altitudes 

 above 1000 feet (305 m.), and usually ceases at 1100 feet 

 (335 m.) or 1200 feet (366 m.). The diminution in number 

 of the oaks is attended by an increase in number of the birches ; 

 so that there is a gradual transition from oak woods to oak- 

 birch woods and to pure birch woods. Woods of the intermediate 

 oak-birch type are of frequent occurrence in Longdendale and 

 upper Derwentdale. Pure birch woods, however, are rare in 

 the Peak District, though two or three rather small examples 

 occur at altitudes above 1000 feet in the two valleys just 

 mentioned. Along with the oak, most of the other trees, such, 

 for example, as the alder (Alnus glutinosa), the wych elm 

 ( Ulmus glabra = U. niontana), and the ash (Fraodnus excelsior), 

 are left behind when the higher altitudes are reached. The 

 mountain ash {Py7'us Aucuparia), on the other hand, becomes 

 more abundant. Similarly, most of the shrubs of the oak 

 woods become rare at the higher altitudes, the hawthorn 

 (Crataegus Oxyacantha) and Salix cinerea perhaps ascending 

 higher than most of the others. The number of shade-loving 

 gi-ound species also becomes greatly reduced ; and such species 

 (see page 55) are either totally absent or present in greatly 



