CH. Ill] SCRUB ASSOCIATIONS 89 



It is clear, therefore, that the upper altitudinal limit of woods 

 or of trees in any particular district cannot be accounted for 

 merely by the factors connected with the altitude of that 

 district. This point of view, although quite an old one, is 

 frequently ignored. 



Smith (1911: 13) says that "tree growth ceases wherever 

 the wind attains such a force the formation of young shoots is 

 prevented. The determination of this wind zone must always 

 be considered in forestry. It cannot be defined as so many 

 feet above sea-level. The action of the wind may be very 

 marked on the coast itself.... It is also the case in a hill-mass, 

 that all the zonal limits of plants are lifted up." This is indeed 

 quite true, and helps to explain the occurrence of woodland plants 

 under the peat of Cross Fell (which rises to 2930 feet = 893 m.) 

 at an altitude of 2400 feet (731 m.), whilst under the peat of 

 the Peak of Derbyshire (which only rises to 2088 feet = 636 m.) 

 woodland plants have not, during the course of the present 

 investigation, been observed higher than 1800 feet (549 m.). 

 It is not necessary to invoke post-glacial climatic changes to 

 account for apparent discrepancies of distribution of this 

 nature, for they can easily be paralleled by similarly apparent 

 discrepancies in existing vegetation. 



Warming (1909 : 39) has stated that trees cease on 

 mountains at the altitude where they break up into separate 

 peaks. It seems highly probable that at the conclusion of 

 the glacial period, this country was invaded by Arctic-Alpine 

 species, and afterwards by forest or woodland species. Lewis 

 (1905, etc.) finds two forest layers in some of the Scottish peats, 

 so that perhaps there were two separate invasions of forest 

 plants. However, only one such layer appears to be represented 

 as a rule in the peat of the Peak District, though in other parts 

 of England two or more layers of trees are found buried by peat. 

 The primitive woods probably ascended the mountains up to or 

 nearly up to the limit indicated by Warming. Perhaps there 

 was above this primitive forest limit, a narrow girdle of climatic 

 scrub and a still higher girdle of climatic grassland ; but, as 

 regards the present district, this is not certain. In any case, 

 from that time to this, there has been a gradual lowering of the 

 forest limit; and the scrub and grassland, which now characterise 

 the higher slopes of the district (and indeed those of Europe 



