392 FACTORS AFFECTING THE GROWTH OF STANDS 



eter growth of the remaining stand, which in stands continually under 

 management may be maintained at an almost constant rate until 

 the maturity of the stand. 



It has been found that in stands originally stocked with only part 

 of the normal number of trees for smaller ages, as the age of such stands 

 advances and the number of trees required in a stand of maximum or 

 normal density decreases, the poorly stocked stand tends to approach 

 and to equal the yield per acre of the stand which has been normally 

 stocked throughout its life. There is therefore a universal tendency 

 under natural conditions for stands to approach a full crown cover as 

 well as for the more densely stocked stands to become over-stocked. 

 This tendency must be recognized in dealing with density factors or 

 per cents in prediction of yield and forms a conservative factor in the 

 prediction of growth for partly stocked empirical or average stands. 

 Ideal conditions for growth are found in stands which have been main- 

 tained at a normal number of trees per acre as well as a normal crown 

 density through repeated thinnings. Not only is the total volume 

 produced per acre and the rate of growth greatly increased by a proper 

 balance between thinnings and the remaining stand, but the maturity 

 of the stand is hastened and its rotation may be reduced if desired. 



301. Density of Stocking as Affecting Growth and Yields. In 

 spite of the tendency of natural stands to approach normal density of 

 stocking through the expansion of their crowns, the attainment of 

 normality or full stocking under natural conditions of growth is seriously 

 interfered with by many agencies. Natural spacing or stocking is 

 largely a matter of chance and fails over extensive areas. Much of 

 the reproduction may be destroyed during these early years by grazing, 

 fires, frost or drought. Saplings and poles may be further destroyed 

 by fire, insects and disease. Later on, insects, disease, fire and wind 

 continue to make gaps in the age class and crown density. Most of 

 these detrimental factors are reduced under protection and the average 

 density greatly improved, yet forests covering .wide areas ordinarily 

 can not be brought to a perfect or full condition of crown cover or stock- 

 ing, no matter how intensive the care which is bestowed upon them. 



The yields of forests are desired on the basis of their actual average 

 production and not upon the small per cent of stands showing maximum 

 or perfect conditions of density and numbers per acre. This gives 

 rise to the problem of applying tables of yield to these conditions, first 

 as to the selection of areas or plots for the measurement of yields, and 

 second, as to whether the area so selected shall be an average of all 

 conditions of stocking within the site class or shall make no attempt 

 to attain this empirical average. 



It has been generally accepted that the best method of obtaining 



