Ill] SCRUB ASSOCIATIONS 97 



In districts where the rainfall is low, as on the borders 

 of steppes and at very high altitudes, where the amount of 

 precipitation is insufficient to permit of the growth of large 

 trees, there can be no doubt that static, climatic scrub occurs ; 

 and, on certain very dry soils in moderately rainy localities, 

 it is also certain that static, edaphic scrub occurs. 



The relations of the most important types of scrub are 

 shown in the following scheme : 



Scrub 



Kinetic Static 



(1) Retrogressive (2) Progressive (3) Edaphic (4) Climatic 



Of these, the examples of scrub met with in the Peak 

 District belong mostly to the first type; and no doubt the 

 great majority of the British examples of scrub should be 

 placed in the same class. 



RELATION OF THE GROUND VEGETATION OF WOODLAND 

 TO RETROGRESSIVE SCRUB 



Although nearly all the examples of scrub on the Pennines 

 belong to the retrogressive type, they are important, and no 

 account of the vegetation of a district which failed to account 

 for them, could be regarded as complete. At the same time, it 

 does not appear to be reasonable to regard subordinate asso- 

 ciations such as retrogressive or progressive scrub as of the 

 same ecological rank as chief associations like flourishing 

 tracts of woodland. 



From the standpoint of succession, the study of the ground 

 vegetation of woodland is a matter of prime importance both 

 to the phytogeographer and to the forester. By such a study, 

 an inkling may be gained of the possible fate of particular 

 tracts of degenerating forest. For example, where the ground 

 vegetation consists of heathy undershrubs, such as bilberry 

 (Vacdnium Myrtillus) and heather (Galluna vulgaris), and of 

 humus-loving grasses, such as purple moor-grass (Molinia 

 caerulea) and silver hair-grass (Deschampsia flexuosa), it may 



M. 7 



