312 IMPROVING THE ACCURACY OF TIMBER ESTIMATES 



When cubic volume is used the average tree will not be the same 

 in diameter as when the board-foot unit is employed. The explanation 

 for this difference is that the volume sought is a weighted average of 

 the merchantable contents of all of the trees on the plot. Trees of 

 different diameters do not have the same weight in this average when 

 measured for board feet as when measured for cubic contents. The 

 tree containing the average board-foot volume will be larger than the 

 other. The smaller trees in the stand when measured in board feet 

 are more immature than they are for cubic feet and the merchantable 

 portion of the stand actually includes a lesser proportion of the whole. 

 In stands which are not of even age, this merchantable portion would 

 exclude many of the younger trees as being unmerchantable although 

 they would be included in the cubic volume, and the average age as 

 well as size of the portion merchantable for board feet is greater than 

 that included in the cubic volume. (The increase in average age of 

 stands due solely to the exclusion of a portion of the stand is a recog- 

 nized fact in European practice.) 



To determine the size as well as volume of the average tree of a 

 stand, we have two variables, height and diameter, one of which must 

 be fixed or eliminated before the other can be determined. The first 

 step is, therefore, to determine the average height of trees of each diam- 

 eter by a height curve (§ 209). The average tree can then have but a 

 single height and diameter and these dimensions may be found from 

 a curve of volume based on diameter for the plot. 



This curve may be taken from a standard volume based on diam- 

 eter and height (§ 143) by selecting the volumes corresponding to the 

 average heights for each diameter interpolated if necessary to the 

 nearest foot. At only one point on this curve will the average volume 

 coincide with the diameter. 



243. The Measurement of Permanent Sample Plots. The purpose 

 of locating and measuring permanent sample plots is to determine the 

 growth of stands. Their original measurement therefore must be made 

 by methods which will permit of an exact scientific comparison of these 

 with subsequent measurements. In this way, not only can the growth 

 of individual trees be determined, but all changes which take place in 

 the forest by decadence and by the operation of natural forces, insects, 

 fungi and cutting and thinning, or other silvicultural measures may be 

 noted. 



Permanent sample plots should be located on land under perma- 

 nent and stable ownership and should be accessible and easily found for 

 subsequent inspection and for a maximum of protection. The plot 

 should be square or rectangular and marked by permanent corners, 

 plainly labeled. Sample plots should be located in stands having 



