IIT] 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 
