158 ECONOMICS OF FORESTRY. 



According to climatic and soil conditions, which, 

 in combination, are technically called "site," the 

 annual production of available dry wood substance 

 above ground, when the site is fully utilized, varies 

 from at least 3500 pounds on the best sites to 1200 

 pounds on the poorest. This production remains 

 the same, regardless of the number of trees partici- 

 pating in it, provided that the entire available light 

 space be filled with active foliage, or, that, techni- 

 cally speaking, there is a full crown cover. 



From this observation it appears that not the 

 number of trees, but the density of crown cover, 

 i.e. the intensity of utilization of the light, is the 

 important factor in weight production, and, ceteris 

 fiaribus, in volume production. In other words, 

 there may be two and three times as many trees 

 on the same area, and yet no difference in total 

 volume. The difference due to numbers will ap- 

 pear in difference of the distribution of volume in 

 more or less useful form ; hence the proper gauging 

 of numbers is one of the most important operations 

 of the forester. 



As we have seen before, in a dense young growth 

 of nature's sowing, there may be 50,000 or more 

 trees per acre, which, by natural thinning after the 

 twentieth year, are reduced to 2000 or 2500, and then 

 diminishing steadily in number at a slower rate ; at 

 the end of the hundredth year only 200 to 250 occupy 

 the upper crown level, or only 10 per cent are left, 

 90 per cent having succumbed to the shading, or 



