398 



NORMAL YIELD TABLES FOR EVEN-AGED STANDS 



the yield with the area measured. This error is best illustrated by the 

 measurement of an isolated clump of trees with wide 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 

 boundaries to in- 

 clude the whole 

 of a stand to be 

 measured, and it 

 is usually possi- 

 ble, in regions of 

 average rainfall, 

 to have such a 

 control strip. 

 The size of plots 

 under the above 

 principles will 

 vary from yg- 

 acre, for dense 

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

 Ordinary sizes run from J 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 

 crowns 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 



Fig. 83. — Relation between growing space occupied by crowns 

 or roots of trees and size of plot measured to secure 

 yield per acre. 



A — Too small an area. 



B — Correct for humid region or site. 



C — Approximately correct for arid region. 



