432 Forestry Quarterly. 



ing to the size of the clearing, whereas the selection system by 

 single trees disturbs the forest conditions the least of any system; 

 hence for protection, the group method is not as desirable as the 

 single tree method. 



2. Since the group method aims at disposing of the old stand 

 within a comparatively limited time, many years must elapse after 

 the young crop is formed before a supply of saw timber is again 

 available. This would be a disadvantage if the stand is expected 

 to supply a steady demand always present, or is not of an area 

 large enough to be divided into compartments which are treated 

 successively, and enough compartments to round out the cycle 

 of rotation. 



Under present economic conditions, it seems that the selection 

 system by single trees is generally preferable to the group sys- 

 tem. 



Since it is not profitable to cut over a selection forest annually, 

 a period of years must elapse between cuttings. This should be 

 obtained by the Van Mantel formula expressed as follows : 



S . 



b=- in which "b" represents the amount of timber in board 



ia 



feet or cord which should be cut annually; "S" represents the 



actual amount of timber per acre in board feet or cords in the 



stand ; "a" represents the most profitable age at which the timber 



is ready for the ax. To illustrate : Suppose an Engelmann spruce 



stand averages 10,000 feet per acre, the total stand per acre. 



Suppose that we assume the trees average 200 years when it 



appears most profitable to cut them. 



10,000 



then b = = 100 ft. B. M. 



I of 200 



Since no purchaser would be willing to cut over the area annually 



for only 100 ft. B. M., we must determine the smallest amount 



which would justify a cutting. Suppose this amount is 2,500 



feet B. M. Then 2,500 divided by 100 equals 25, the number 



of years which should intervene between cuttings. These figures 



only hold good for a normal forest, but in the above illustration, 



if 1,000 feet is a greater wood capital than is normal, 100 feet is 



more than grows annually, and we would reduce the wood capital 



or if less, we would add to the wood capital. The advantage of 



