THINNING 81 



air and also sufficient space in the soil for the 

 development of their roots. 



From this consideration of the issue of a 

 struggle for existence, uninterfered with by arti- 

 ficial thinning, it would appear that the advan- 

 tage of density of a crop is that it encourages 

 upward growth and freedom from knots, and 

 that it has the disadvantage that it is a strain 

 on the fertility of the ground and on the vitality 

 of the trees. 



Throughout the life of the plantation, density 

 should not be maintained beyond what is abso- 

 lutely necessary to maintain upward growth and 

 freedom from knots. If this limitation of density 

 is exceeded there is, in addition to the risk to 

 the fertility of the soil and the vitality of the 

 trees, also a risk of completely stopping diameter 

 growth. A check on diameter growth is inevit- 

 able in the period of vigorous upward growth, 

 but the diameter growth must not be killed. 



Experience shows that if growth, either in 

 height or diameter, is once seriously stopped by 

 unfavourable conditions, it cannot be restarted 

 when the conditions are made favourable. The 

 density of a wood which is ultimately to be 

 sold at a good price to a timber merchant should 

 be such as to hasten vigorous upward growth 



