Il] WOODLAND ASSOCIATIONS 43 



soil, (2) the depth of the soil, and (3) the altitude of the 

 woodland site. Other factors are of importance with regard 

 to the distribution of the various types of ground vegetation 

 (see pp. 53 and 71). 



1. The chemical nature of the soil. The soils of the 

 slopes of the limestone hills, to which the ash woods are 

 limited, are rich in calcium carbonate: on newly exposed 

 soils in such situations, the soil, in fact, may consist almost 

 wholly of this substance ; but commonly the percentage of lime 

 varies from about five to about thirty per cent. The percentage 

 of lime must be very high in the rock-strata in which the tap 

 roots of the ash are fixed. On the chalk of the south-east of 

 Elngland, the woods are usually dominated by the beech (Fagus 

 sylvatica), though the ash association does occur to some extent 

 on the chalk. 



The soils of the sandstones and shales have a very low 

 lime-content. In several analyses, the percentage of lime 

 (calculated as calcium oxide) was so low as 0*02 per cent. ; and 

 in no case was it higher than 0*05 per cent. On these soils, 

 ash woods do not occur, whereas oak and birch associations are 

 numerous and typical. It is not suggested here that the presence 

 of lime is the direct cause of the occurrence of ash woods on the 

 limestone slopes, or that the absence of it is the direct cause of 

 the occurrence of oak and birch woods on the siliceous slopes, as 

 much more experimental work must be done by plant physio- 

 logists before this classical problem can be approached from a 

 point of view which is likely to afford a satisfactory outlook on 

 the problems involved : all that is stated is that, so far as this 

 district is concerned, the ash woods invariably occur on the 

 calcareous strata and never elsewhere, and that the oak and 

 birch woods invariably occur on the siliceous rocks and never 

 on the calcareous ones. It seems, however, reasonable to suggest 

 that the presence or absence of a high lime-content of the soil 

 is concerned either directly or indirectly with the present distri- 

 bution of the principal types of woodland of this district. 



2. The depth of the soil. The fluvio-glacial sands in 

 the west of the district are deep, and hence offer no physical 

 obstacle to the growth of the long tap roots of Quercus Rohur. 

 Woods of Quercus Rohur occur on these soils. Further, so far as 

 this district is concerned, such woods are restricted to these soils. 



