THE THEORY AND PRACTICE OF WORKING PLANS 95 



errors may accumulate. A period of about twenty years 

 seems reasonable. Thus if the rotation is i8o years, the whole 

 working unit will be cut over in sixty years. If the period 

 during which the plan is to run be twenty years, the area is 

 divided on the basis of topography into three parts, each con- 

 taining about an equal volume, and each to be cut over in 

 twenty years. In some cases, where the working unit does not 

 lend itself to a division into parts containing equal volumes, 

 it may be divided into unequal parts, each part to be cut over 

 in a period bearing the same relation to the one-third of the 

 rotation as ^the part bears the whole working unit. The 

 part containing the largest proportion of overmature and 

 deteriorating timber should be cut during the first period. 

 This part may be further subdi\dded for convenience into 

 watersheds forming natural logging units or groups of units 

 (blocks) . 



(b) Example. — Total area of working unit = 200,000 acres. 



Minimum merchantable d.b.h. = 10 inches. 



Size of material desired: Sugar pine and yellow pine = 30 

 inches. Incense cedar = 24 inches. 



The group of old trees will include those 20 inches and over 

 d.b.h. The medium trees will include those between 10 inches 

 and 20 inches d.b.h. 



The average length of time required to produce a tree 30 

 inches d.b.h., considering the important species, is 160 years. 

 The period of reproduction is approximately twenty years. 

 Hence the rotation will be 160+20, or 180 years. Incense 

 cedar is shorter hved and more rapid growing, hence will be con- 

 sidered exploitable at 24 inches.* 



*This exploitable diameter for incense cedar will cause a slight inac- 

 curacy in that the medium trees should be taken to 8" instead of 10" to 

 correspond with the 24". On the oth;r hand, the volume between 8" and 10" 

 will be small, and if desired can be allowed for by sample tallies over a small 

 percentage of the strips. The cutting of a short-lived species to a lower 

 diameter limit is desirable in this case because the area is gone over only once 

 in 60 years. 



