210 A MANUAL OF FORESTRY. 



291 per cent, of that in the 20th year, that it sinks to 

 3,0 per cent, between the 60th and 80th year, and that 

 it is stationary between the 80th and 100th year, after 

 which it gradually becomes very small until it falls to 

 nil. Hence, many trees must be removed during the 

 earlier part of a wood's life, and comparatively few 

 afterwards, always assuming that the area is to remain 

 fully stocked. 



c. Species. 



Light-demanding species require more space than 

 shade bearers. Schuberg's statistics for the Black 

 Forest give the following proportion, for woods ranging 

 from 40 to 80 years of age, placing Scotch Pine equal 

 to 100 :- 



Scotch Pine .... 100 

 Spruce .... 87 



Beech 79 



Silver Fir .... 63 



Comparing Scotch Pine, Larch, Oak, and Birch with 

 Spruce, Beech, and Silver Fir, the proportion is about 

 100 to 65. 



d. Soil. 



Until some time past middle-age, the number of trees 

 is larger on poor than on rich soil; afterwards the 

 difference disappears. The reason is, that the struggle 

 for existence commences earlier and is more energetic 

 on good than on poor soil. In this respect considerable 

 differences exist between the various species. 



e. Altitude. 



Under otherwise equal conditions, the number of trees 

 per acre increases with altitude, at any rate up to a 

 certain elevation. The statistics of the Black Forest for 



