Scottish Birchwoods 141 



found in the west and south, Ewphorhia hiberna is mainly 

 though not quite exclusively Irish. All these belong, 

 outside the British Isles, mainly or exclusively to the 

 Iberian peninsula. 



Birchwood association (Betuletum tomentosm). 

 (Plate XII.) 



It has already been shown that on the siliceous rocks 

 of the Pennines a fringe of birchwood frequently occurs 

 immediately above the oakwoods at an altitude of 1000 — 

 1500 feet (c. 300 — 450 m.), but the association is, on the 

 whole, poorly developed. In Scotland, however, birch- 

 woods are the characteristic woods of the sides of the 

 Highland valleys, where they are often extensively de- 

 veloped above the upper limit of the oakwoods (c. 1000 

 feet = 300 m.) and sometimes, though rarely, attain a 

 height of about 2000 feet (c. 600 m.) on the slopes of the 

 higher mountains (Plate XII a) . This increase of altitude 

 is of course in accord with the general rule that the 

 greater the height of a hill-mass the higher any given 

 altitudinal zone of vegetation will ascend. 



W. Gr. Smith writes of the Scottish birchwoods' that 

 their occurrence may indicate : — 



(1) " An oakwood from which the oak, originally 

 dominant, has been removed, while the less valuable 

 birches have been left, and along with naturally sown 

 seedlings have in time formed a wood. 



(2) A wood-clearing or a piece of moorland preserved 

 from excessive grazing by sheep, deer, and rabbits. Under 

 these conditions, birch seedlings and mountain ash fre- 

 quently develop in such numbers that a thicket or wood 

 results.... 



(3) The sub-alpine deciduous wood, the highest zone 

 of woodland in the Highland valleys." 



The first two cases we have already seen constantly 

 1 "W, G. Smith, 1905, p. 5. 



