198 A MANUAL OF FORESTRY 



readily seen that yield tables, for even-aged stands, not only are 

 more easily made, but are of wider application than those for 

 uneven-aged stands. Since the chief use of yield tables is to 

 predict what forests will produce under favorable circum- 

 stances, they are based usually on the measurements of fully 

 stocked stands, and then are called ''normal." Often it is 

 impossible in our irregular forests to find acres that are fully 

 stocked, and consequently fractions of acres are measured rather 

 than entire acres which include open areas. Three classes of 

 forests need to be studied for the construction of such yield 

 tables: (a) un thinned pure forests; (b) un thinned mixed forests; 

 (c) thinned stands. Thus far all of the yield tables made in 

 this country have been for the first and second classes, especially 

 for pure forests, and do not indicate what these forests can 

 produce when properly treated. In the future this last (c) class 

 of yield tables will be chiefly needed, especially in forest regions 

 with the best market conditions. 



Normal yield tables cannot be made for uneven-aged forests, 

 since cuttings in such forests are made at regular intervals 

 when only a part of the stand is removed. For rough yield 

 tables with which to estimate the growth of such stands, it is 

 customary in this country to designate in tabular form the 

 number of years required for each diameter inch class to grow 

 one inch, and from this to estimate the amount of timber that 

 can be produced in a given period. Since the rate of growth 

 is based almost entirely on conditions different from those which 

 will prevail in the future, these tables should not be taken too 

 seriously. 



