130 Plant-formation of Siliceous Soils 



(3) Sub-association of more open situations with much acid humus. 



SUBOEDINATE (EeTROGEBSSIVE) ASSOCIATIONS 



Scrub associations. Every stage can be traced from 

 typical woodland to open scrub ; and there can be little 

 or no doubt tbat, so far as tbe Pennines are concerned, 

 the great majority of examples of open scrub are simply 

 retrogressive woodland. In ancient times, when the 

 woods ascended to their climatic limits, there was doubt- 

 less a natural region of climatic scrub at a higher level 

 than the woods; but the woodland limit has been 

 depressed some hundreds of feet since that particular 

 period. 



There can be no doubt that a great deal of the 

 degeneration of the woodland has been brought about 

 by the indiscriminate felling of trees, the absence of any 

 definite system of re-planting, and the grazing of quad- 

 rupeds. It is doubtful, however, if these causes are quite 

 sufficient to account for all the facts. It has to be 

 remembered that the population of the remoter valleys, 

 many of which are now almost or wholly treeless, is very 

 small. A matter which is perhaps not sufficiently em- 

 phasised is that, in a closed plant association, seedlings 

 especially of plants with large seeds like the oak, are 



