28 THE THEORY AND PRACTICE OF WORKING PLANS 



the cruisers and used by them as a check on the width of the 

 estimating strips. 



Use of Yield Tables. — ^The estimating of timber by means 

 of yield tables unfortunately finds Httle or no apphcation in 

 America because of the lack of suitable tables. Yield tables 

 are constructed for even-aged fully stocked stands of a single 

 species for various site qualities. The age is usually given in 

 five- or ten-year intervals. European yield tables are separated 

 for final and intermediate yield (thinnings) and total. Normal 

 yield tables preponderate, but local (empirical) yield tables are 

 used as makeshifts. The methods of making yield tables is the 

 province of forest mensuration, but for purposes of forest organ- 

 ization the data should comprise : Age, number of trees per acre, 

 basal area, d.b.h. of average tree, height of average tree, yield per 

 acre, current and mean annual increment, for each site quality. 



The use of yield tables requires the determination in the 

 field of the following data (presupposing nearly even-aged 

 stands): Age, site quality,* density of stocking. f The corre- 

 sponding value for the age and site quality is read directly from 

 the yield table and this multiplied by the factor of density 

 (i.o to o.o). Where there are several species in the stand, 

 the percentage of each is determined and the corresponding 

 value in the various yield tables multiplied thereby; these 

 values are then added and their sum multipHed by the factor 

 of density (i.o to o.o). 



Cost. — The cost of estimating (field work only) averages 

 between the following figures: 



Ocular estimates i to 2 cents per acre 



2|-per cent strip estimates 2| to 5 



5-per cent strip estimates 5 to 10 



lo-per cent strip estimates 10 to 15 



* Site quality is most accurately gauged by the height of the trees. See 

 Roth: " Concerning Site," Forestry Quarterly, Vol. XIV, No. i, pp. 3-13. 



t The density or degree of stocking is most accurately gauged by considering 

 it as = area of cross-section of trees of the stand divided by normal area of cross- 

 section (basal area). See Roth: " Forest Regulation," Ann Arbor, Mich., 1914, 

 pp. 54-56. 



