196 FOREST VALUATION 



in age. Yield tables attempt to state the actual results in volume 

 per acre, produced by stands at different ages. When applied 

 to young timber growing upon sites of the same quality, and of 

 approximately equal density or degree of stocking, the growth 

 and future volume of such stands is predicted with sufficient 

 accuracy. For scattered timber or poorly stocked stands, a 

 reducing factor is required to prevent over-estimates. Methods 

 of studying and predicting yields per acre form part of the 

 subject of forest mensuration. Tables of growth and yield 

 are published by the U. S. Forest Service and in various state 

 bulletins, but are still very deficient in number and reliability. 



184. The Stand; Improvement in Quality of Products. 

 Volume alone does not measure the increase in value of a stand 

 of timber. As the trees become larger, the quality of the prod- 

 ucts which may be cut increases, for two reasons, The lower 

 branches on young trees form knots as long as they remain 

 attached, either alive or dead. After the branch falls and the 

 scar heals, the wood laid on is free from knots, or clear, and 

 produces the most valuable grades. The slabs contain the 

 greatest per cent of clear lumber of any portion of the tree, and 

 old trees contain enormously greater per cents of high-grade 

 products than young timber. The second factor of increase 

 in quality is due to the larger sizes which can be cut from older 

 trees. Both in length and breadth, higher prices are paid -for 

 increased dimensions. 



Both volume and quality increase the value per acre entirely 

 independent of changes in prices due to economic conditions 

 outside the stand. 



185. Deterioration of Standing Timber. After timber 

 reaches a certain age and condition, the stand as a whole ceases 

 to increase in volume, and at a much later period it begins to 

 diminish in quality as well. The trees continue to grow as 

 long as they live, but the total growth of the stand is offset by 

 the loss of trees through windfall, insects and disease. 



The remaining trees are putting on clear lumber, but in old 

 stands, decay caused by fungi becomes increasingly prevalent, 

 and finally offsets this increase in quality. 



