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 [Ilex Aquifoliwm], hawthorn [Crataegus Oxy- 
acantha], blackthorn [Prunus spinosa], and willows [S. cinerea, 
S. caprea, and S. aurita] (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. montana), and the ash (F'raainus eacelsior), 
are left behind when the higher altitudes are reached. The 
mountain ash (Pyrus 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 
ground species also becomes greatly reduced; and such species 
(see page 55) are either totally absent or present in greatly 
