CHAPTER III 



SCRUB ASSOCIATIONS 



Past and present upper altitudinal limit of trees. Buried timber in the 

 peat. Degeneration of woodland. Distribution and character of the 

 existing scrub. Scrub in other districts. Relation of the ground 

 vegetation of woodland to retrogressive scrub. Progressive and 

 retrogressive scrub. Comparison of the types of retrogressive scrub. 



Past and present upper altitudinal limit of trees 



It was pointed out in the last chapter that the upper 

 altitudinal limit of oak and ash woods at the present time is 

 in this district about 1000 feet (305 m.) and the upper limit of 

 birch woods is about 1250 feet (381 m.). Isolated trees and 

 patches of scrub, however, ascend to about 1550 feet (472 m.); 

 and there can be no doubt that formerly trees ascended, in the 

 Peak District, to about 1750 feet (533 m.) or 1800 feet (549 m.). 

 These figures represent the upper limits of trees and woods on 

 the highest hills alone : on the lower hills, the upper limits of 

 trees and woods are not so high. For example, in the latitude 

 of Halifax (a few miles to the north of the Peak District), the 

 Pennines only rise to about 1550 feet (472 m.) ; and the present 

 tree limit there occurs at about 1250 feet (381 m.) and the 

 woodland limit at about 1000 feet (305 m.). According to 

 W. G. Smith (1911 : 20), in the Highland glens of Scotland, 

 birch woods sometimes ascend to an altitude of 2000 feet 

 (610 m.), in spite of the more northern latitude ; but here moun- 

 tains are massed together and rise to more than 3000 feet (915 m.). 

 On Ben Nevis, the highest mountain (4400 feet = 1313 m.) in 

 the British Isles, a tree is said to occur at 2700 feet (823 m.). 

 Still further north, in Scandinavia, where the mountains are 

 still higher, the trees commonly ascend to 3000 feet (915 m.). 



