Ill] SCRUB ASSOCIATIONS 93 



this district are, on the whole, characterized by such closed 

 ground societies, it would seem that here is an additional reason 

 which helps to explain the gradual degeneration of the forests 

 of the Pennines. It is difficult, for example, to see how a close 

 turf of silver hair grass (Deschampsia flexuosa) can be colonized 

 by oak or beech seedlings; and, in fact, such seedlings are rarely 

 seen in these situations. This fact is known to some foresters 

 of the country ; and use is made of their knowledge in that 

 many of the woods and plantations of which they have charge 

 have the ground kept more or less free of woodland " weeds." 



The difficulty which larch seeds experience in germinating 

 in closed herbage in the larch forests of the Altai Mountains has 

 been pointed out by Krassnoff (1886) and quoted by Warming 

 (1909: 316): "the herbaceous vegetation consists of species 

 of Aconitum, Delphinium, Paeonia, Clematis, and others. Each 

 year millions of larch seeds fall into this sea of herbage ; yet 

 only a few find places where they can germinate : the forest is 

 apparently doomed to extinction." 



The remarkable series of climatic changes within the 

 historical period, which are invoked by certain writers to 

 account for plant-successions, are always open to a certain 

 amount of suspicion. In general, plant-successions, which have 

 taken place since early post-glacial times and in a region of 

 fairly uniform present-day climate, would seem to be explicable 

 by changes in the physiographical and edaphic conditions of 

 plant habitats. 



DISTRIBUTION AND CHARACTER OF THE EXISTING SCRUB 



The existing woodlands, at their upper altitudinal limits, 

 often pass imperceptibly into open scrub. On many of the 

 hill-slopes of the remoter valleys, trees are more or less thinly 

 scattered about; and it is, in fact, not always easy to decide 

 whether or not a particular tract of vegetation should be con- 

 sidered scrub or poor woodland. Longdendale, Upper Derwent 

 Dale, and Upper Cressbrook Dale furnish excellent examples of 

 scrub. In some cases, the ground vegetation is grassy, in 

 others heathy undershrubs are abundant. In some cases, the 

 tallest plants are shrubs; and these sometimes form dense 

 thickets : in others, shrubs are absent, and the uppermost layer 



