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 (805 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 1759 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 (881 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.). 
