424 THE USE OF YIELD TABLES 



of one acre, 43,560 square feet, by this area. This method is employed 

 in finding the number of trees per acre required to plant an acre, if 

 spacing is 4, 6, 8 or 10 feet apart in both directions. 



Density of Crown Cover. In actual stocking, the absolute number 

 of trees cannot be so simply determined. As crowns tend to adjust 

 themselves to light, they depart from a circular form, and the circular 

 spacing itself may permit of more trees per acre than the square. The 

 relation of the area of an inscribed circle to a square is .7854. That 

 of an inscribed circle to a hexagon is .9018. 



If either of these relations is consistently maintained, the total 

 number of trees per acre for full crown cover may differ, but the relative 

 number, for trees of different diameters will remain constant. From 

 the number so found, a curve of number of trees per acre based on diam- 

 eter can be plotted. This is a standard, intended to show relative, 

 not absolute, numbers. For instance, if the number per acre from 

 such a table for a given diameter is 400 trees, a stand of 200 trees per 

 acre of this average diameter would be 50 per cent of the standard. 



Two factors interfere to prevent the satisfactory application of such 

 a table in predicting yields. First, the number of trees in fully stocked 

 stands does not always decrease in direct proportion to their increase 

 in crown space. In tolerant species, a great over-lapping and suppres- 

 sion of crowns occurs, doubling the number of trees per acre over the 

 theoretical number indicated by the spread of crown, while in over- 

 mature stands, the increasing demand for light and moisture reduces 

 the stand per acre below that indicated by the crowns. The relation 

 is therefore not consistent except within rather narrow limits of age 

 and species; and yields based on this assumption will be excessively 

 large for over-mature age classes. 



The second factor tends to offset the first in stands not fully stocked — 

 this is the tendency (§ 301 and § 316) to improve the degree of stocking 

 with age. When a stand of a given age has only the number of trees 

 required for one twice this age, its rate of mortality will be very much 

 less since each tree has more than enough room to survive. Hence 

 the assumption, in stands not fully stocked, that the growth of a stand 

 can be predicted by determining the per cent which the number of 

 trees now in the age class bears to the normal number, will not be 

 borne out, but better results will be obtained. 



Method of Construction of the Yield Table. In stands which 

 possess a full crown cover, but whose age classes are distributed in 

 many-aged form, the rate of mortality may be assumed to hold for 

 all classes. An illustration of the above method of constructing a 

 yield table for yellow poplar in Tennessee is given below. 1 

 1 Based on data collected by W. W. Ashe. 



