42 VEGETATION OF THE PEAK DISTRICT [CH. 



equal strictness, to calcareous soils. The associations of Quercus 

 sessiliflora and of Betula pubescens, as contrasted with the 

 association of Quercus Robur, are also edaphic associations ; for 

 the former occur solely on the shallow siliceous soils of the 

 sandstones and shales whilst the latter is limited to the deep 

 fluvio-glacial sands. 



On the other hand, the association of Quercus sessiliflora, as 

 contrasted with that of Betula pubescens, is related to those 

 climatic factors connected with altitude; for both associations 

 occur on shallow siliceous soils, but the former occurs at 

 altitudes below 1000 feet (305 m.) and the latter at altitudes 

 between 1000 feet and 1250 feet (381 m.). 



The mean annual rainfall of the sites of the various woodland 

 associations varies locally from about 35 inches (89 cm.) to about 

 50 inches (127 cm.); but, so far as one is able to judge, there 

 is, in this district, no definite relationship between the different 

 woodland associations and local differences in rainfall. On the 

 whole, it may perhaps be said that the woods of Quercus Robur 

 have the lowest rainfall and those of Betula pubescens the highest 

 rainfall ; whilst the rainfall of the woods of Quercus sessiliflora 

 and of JFraxinuB excelsior is intermediate and approximately 

 equal in amount. 



As regards the water-content of the soils of the various 

 woods, that of the woods of Quercus Robur is the lowest, that 

 of the woods of Q. sessiliflora and Fraxinus excelsior is rather 

 higher, varying from moderately dry to very wet, and that of 

 the woods of Betula pubescens is, on the whole, the highest. 

 The surface soils of the birch association and of the two oak 

 associations show a marked tendency to form acidic humus, a 

 tendency which is scarcely discernible in the soils of the ash 

 woods. The nature of the surface soil is doubtless important 

 in relation to the germination of the seeds of the dominant 

 species, and therefore in relation to the rejuvenation of the 

 woods; but few or no experiments appear to have been con- 

 ducted with the view of testing such a hypothesis. In any 

 case, the surface soil cannot have much to do with the biology 

 of mature trees with deep tap roots, such as the ash. 



Factors, then, which appear to be of importance in accounting 

 for the differential distribution of the various woodland associa- 

 tions of the Peak District are (1) the chemical nature of the 



