398 NORMAL YIELD TABLES FOR EVEN-AGED STANDS 



the 3'ield with the area measured. This error is best illustrated by the 



measurement of an isolated clump of trees with w^ide crown and root 



spread. A plot laid out to include their boles will have too small 



an area, and an excessive yield (Fig. 83). 



In dry regions especially, root spread exceeds that of crowns and 



cannot be determined accurately. The effect of these errors is especially 



noticeable when the size of the plots is small, the yield per acre varying 



inversely with area of plots. By increasing the size of the plot, the 



proportional influence of a faulty location of its boundaries is lessened, 



and when coupled with care in making these boundaries inclusive of 



crown space and probable root space of the trees measured, the error 



is negligible. Just as for other sample plots ('§ 243), it is better to 



have a smaller plot surrounded by a control strip of similar timber than 



to extend the 



r^ — ^^"^ — -^^^ /"^"^-'---s..,^ boundaries to in~ 



/ \^ I 'I xV/i / / \ elude the whole 



Yj/j-' [ \( v 1 1/ J ^^ ^ stand to be 



/^ ^"\ I 1^ measured, and it 



I \| I is usually possi- 



iL^ 1 1 Wll H \\ ble, in regions of 



|_ ^7 c~^ ' ] average rainfall, 



„ „„ T, , ,. , , . . , , to have such a 



)i\G. 83. — Keiation between growing space occupied by crowns 



or roots of trees and size of plot measured to secure control Strip. 



yield per acre. The size of plots 



^— Too small an area. under the above 



B — Correct for humid region or site. principles will 



C — Approximately correct for arid region. vary from y?- 



acre, for dense 

 young stands, to 5 acres for veteran scattered timber in dry regions. 

 Ordinary sizes run from ^ to 2 acres. Since these boundaries 

 should be accurately run, plots should be square or rectangular, 

 and since the area contributing to the growth of single trees is 

 in theory a circle, rectangular plots should not be too narrow; their 

 short dimension should be at least four times the average width of 

 cro\/ns of the trees measured. For the same reason plots should never 

 be triangular or have sharp angles. Unless intended for permanent 

 location and re-measurement, the corners of plots are marked tempora- 

 rily by any convenient means, and their side lines blazed or marked 

 so as to exclude all trees falling outside of the boundary. 



309. Measurements Required on Each Plot. Dimensions of Trees. 

 A diameter limit is determined, dependent on minimum merchantable 

 sizes. All trees above this are measured at B.H. and recorded in diam- 

 eter classes of 1 inch or 2 inches. Since these plots are for the purpose 



