QliMlt Ashwoods 167 



Ashwoods characterise the south-western extremities 

 . of the chalk outcrop, lying in the Isle of 



Wight, Dorset and East Devon. The line of 

 contact between the ashwood and the beechwood regions 

 occurs near Butser Hill north of Portsmouth, where the 

 South Downs attain their greatest elevation (889 feet = 

 c. 270 m.). Ashwoods are also developed on those portions 

 of the Upper Greensand (which often forms a low terrace 

 at the foot of the chalk scarp) that are highly calcareous, 

 as in parts of the western Weald, in the Isle of Wight, 

 Dorset and Bast Devon. Within the beechwood regions, 

 ashwoods scarcely occur on the chalk. 



The factors determining the distribution of beechwood 

 and ashwood on the chalk are by no means clear. One 

 factor may be atmospheric and soil moisture. The escarp- 

 ments of the south-western chalk, such as the Isle of 

 Wight and Purbeok ridges, as well as the heights of 

 Butser Hill, receive a higher rainfall than any other part 

 of the English chalk, while the more clayey and therefore 

 moister nature of the Lower Chalk and somewhat similar 

 Upper (xreensand, which carry most of the ashwoods of the 

 Isle of Wight, is another difference which may favour ash- 

 woods rather than beechwoods. 



The competition between beech and ash is an important 



factor in this question. Where the two 



Competition trees are placed in direct competition on 



... equally suitable soils the beech must even- 



beech and m 



ash. tually suppress the ash because of the deeper 



shade which it casts and its correlated lower 

 demand for light. It may be that the beech, advancing 

 westwards from the continent, across what are now the 

 Straits of Dover, drove the ash, so to speak, before it and 

 came to occupy those soils of south-eastern England pre- 

 viously in possession of the ash ; and that this tide of 

 invasion was checked, owing to climatic or other factors 



