MEASUREMENTS REQUIRED ON EACH PLOT 399 



of measuring yields they are selected in stands which have reached 

 merchantable sizes. Plots on which a portion only of the trees are 

 merchantable may require the counting of the remaining stand and its 

 classification as to size. Dead trees are recorded by diameter. Species 

 are separately tallied. 



The height of trees for a yield table should be taken separately 

 on each plot. Several tr,;es of different diameters, whose heights are 

 average for the stand should be measured and recorded together with 

 their diameters, the number varying with the stand, from 5 to 15. 

 Where merchantable and not total height is desired, the satisfactory 

 determination of heights for the plot is made much more difficult by 

 the variation in top diameters and the danger of error in judging heights. 

 Such a yield table, while practical, is less reliable than one based on 

 total heights. Total height should always be recorded regardless of 

 whether merchantable height is used, since it is required for a permanent 

 standard of site quality. 



Where the merchantable height unit is used it may be better to tally 

 the merchantable length of every tree on the plot than to rely on a few 

 trees measured by the hypsometer. This introduces the element of 

 ocular guess. 



Age and Volume of Stand. The age of each plot is separately 

 determined by methods discussed in Chapter XXIII. The common 

 method of determining the volume on the plot is by standard volume 

 tables, based on diameter and height. This assumes that the variation 

 of the trees on each plot as to shape or form quotients from the average 

 form for this species or region, is not sufficient to require separate 

 determination. Since trees must either be felled or cut into, to deter- 

 mine age, except when the increment borer will suffice, and since the 

 trees selected for this purpose would be average in volume for the stand 

 or for diameter groups within it, these sample trees are sometimes used 

 to determine the volume of the stand. This method is useful when no 

 reliable volume table exists, and when cubic volume is sought. The 

 additional accuracy attained in measuring the volume of the sample 

 trees for the plot itself is offset by the possibility that the trees cut 

 may vary from the true average of the stand. The methods of deter- 

 mining the size of such sample trees for felling are described in § 241. 



Crown Classes. Each tree on the plot is usually tallied in the crown 

 class in which it falls, as classified in § 274. 



Description of Plot or Site. Since in the preparation of a yield no 

 effort is made to classify the plots into site qualities by inspection of 

 the site factors in the field, the description of the plot should be brief, 

 and serve merely to explain the results obtained and check their value. 

 The points to be covered are the following: 



