l6o ECONOMICS OF FORESTRY. 



While in our natural unmanaged woods the final 

 useful crop, which usually has accumulated over 200 

 years before it is considered fit for harvest, rarely 

 * exceeds 8000 cubic feet, in the managed German 

 spruce forest, fully covering the ground, from which 

 all useless species are eradicated, we may find at 

 30 years over 3000 cubic feet of wood, more than 

 three times that amount at 60 years, and at 100 

 years 14,000 cubic feet of timber wood, having pro- 

 duced at the rate of 70 cubic feet during the first 

 two decades, at the rate of 240 cubic feet in the 

 third decade, reaching its maximum with 267 cubic 

 feet in the fourth decade, declining after this dec- 

 ade so that in the ninth decade the rate may be 

 only 100 cubic feet per year, and at 100 years the 

 average rate for the whole period has become only 

 140 cubic feet. On poorer soils much less, down 

 to one-half, of this production may be expected, and 

 with other species, of course, the general progress 

 of accretion and final result must differ ; yet there 

 is a remarkable regularity, a law of accretion ob- 

 servable in all conditions, upon which an analysis of 

 the assiduously gathered data lets in a flood of Hght. 

 While the natural forest, if not interfered with 

 by man or by accident such as fire, would follow, 

 of course, the same laws, yet practically the result 

 is a different one, because the economic point of 

 view is left out, and tree weeds are mixed with 

 the valuable species, thus naturally reducing the 

 amount of tisefid production. 



