METHODS OF MAKING THINNINGS 161 



the principal basis for cutting or leaving a tree and the rela- 

 tive position of the crown of each tree must be observed with 

 respect to its associates. The distribution over the area of the 

 trunks of the trees is not so important because this may not 

 coincide with position of the crowns. 



The highest production as a result of thinnings can only 

 be secured by leaving the largest number of trees per acre 

 consistent with rapid growth of the individual. The indi- 

 vidual tree may grow fastest out in the open, entirely iso- 

 lated, but this would result in a low production per acre 

 because the area would be only partially stocked. It is 

 necessary to determine the minimum amount of growing 

 space which the tree demands for fast growth and then keep 

 the largest number of trees properly distributed over each 

 acre which this growing space admits. As the stand grows 

 older, the number of trees per acre must decrease because the 

 crown. space required by each tree is steadily enlarging; but 

 the total area occupied by the crown canopy should remain 

 approximately the same. Quick and accurate measurement 

 of the crown spread of the trees on a given area is difficult but 

 the basal (sectional) area at breast-high is easily measured 

 and can be safely substituted as an expression for the total 

 crown spread.* 



Using this idea the proper basal area (used as an indica- 

 tion of total crown spread and of the number of trees per 

 acre at each age) for a given species and site should be de- 

 termined and the stand kept close to this basal area by thin- 

 nings. "The total sectional area gives the best (single) indi- 

 cation of density, so much so that the extent of the thinnings 



* The growth in volume of trees is dependent upon the activity of the 

 crowns and may be considered proportional to the crown spread. Basal area 

 may, for comparative purposes, be used as an abbreviated expression of the 

 volume of the 'stand and for the purpose stated above be substituted for crown 

 spread. 



