22 



BRITISH FOREST TRKI-:> 



attain this normal degree of density varies with the age of 

 the crop and the nature of the species of trees ; l thus on the 

 Harz mountains Th. Hartig found the following results with 

 regard to spruce : 



The natural tendency of forests to thin themselves depends 

 on the fertility of the soil ; on good soils density is main- 

 tained for a longer time than on inferior soils, and the process 

 of elimination of suppressed and dominated individual 

 poles does not take place to any great extent until the develop- 

 ment of the total length of bole is nearly completed. 



The degree to which different species of trees are able to 

 form close canopy varies greatly ; thus a spruce forest can 

 overshadow a soil to more than double the extent that 

 Scots pine does. 



In general, trees growing in the full enjoyment of light 

 and air have a greater increment than such as are grown 

 with only a restricted measure of these, for they are enabled 

 to draw more nutriment from the soil, and have greater 

 opportunities of assimilating these supplies. But in closed 

 forest on soils of similar quality the total production per 

 unit of area will be greater than in the more open forest with 



1 Vide comparative table on p. 43. Hartig's data refer however to 

 sowings, which are very much denser than plantations during the thicket 

 and pole-forest stages of growth. 



