TABLE WITH SITE CLASSES BASED ON HEIGHT GROWTH 401 



310. Construction of Yield Table with Site Classes Based on Height 

 Growth. There are two possible bases on which to sepai-ate site quality, 

 namely yields or rate of growth, and total height or height growth. 

 In choosing between these as the basis of site quality, not only must 

 the construction of the table be considered but also its later application 

 in the field. Whichever basis is used, the range of growth for a species 

 or region must be divided arbitrarily into site classes, once its maximum 

 and minimum limits are determined. When volume or yield is chosen 

 as the direct basis of site classes, regular and consistent results may be 

 obtained by eliminating most of the variables in the choice of plots. 

 But when these results are later used as a means of determining site 

 qualities in the field on the basis of mean annual rate of growth per year 

 or total yield based on age, the system breaks down. 



On the other hand, if the division of plots into site qualities is based 

 on height growth as indicated in § 296 not only are the original plots 

 apt to be separated more accurately into their true site classes since 

 variations in volume due to over- or under-stocking as reflected in the 

 board foot or other unit are minimized, but the division of a large 

 area in the field into site classes for the application of the growth data 

 in predicting yields is made possible in strict conformity with the 

 standard used in the table itself (§ 345). 



While volume has been made the direct basis of many European 

 yield tables, yet in these regulated and fully stocked stands most of 

 the variables are reduced to reasonable proportions. Under our con- 

 ditions of abnormal and accidental stocking, with the maximum of 

 damage to the stands during growth, the variations from the factor 

 of density of stocking due to variable number of trees per acre, even 

 in stands of full crown cover, is so great as to. discourage most investi- 

 gators on first attempt. 



The steps in the construction of a yield table based on height are 

 as follows: 



1. On cross-section paper on which age is plotted on the horizontal 

 scale, and height on the vertical scale, place the average height for each 

 plot above the age of the stand. These heights may be the heights of 

 the dominant trees (§ 296). These points will fall in a comet-shaped 

 band increasing with age. 



2. Draw a curve indicating the maximum height growth, and one 

 for minimum height growth as in Fig, 84. 



3. Decide upon the number of site classes to use. These will depend 

 largely on the total range of heights found for trees of a given age, and 

 the possibility of convenient subdivisions not too small to be serviceable, 

 i.e., large enough to overcome the slight variations in height based on 

 age which may be due to density of stand instead of site. 



