388 FACTORS AFFECTING THE GROWTH OF STANDS 



A still more impracticable plan is to base site factors on the current annual 

 growth of a stand. ^ 



298. The Form of Stands. Even-aged versus Many-aged. There 

 is an essential ditfercnce in the character of even-aged stands and those 

 composed of all ages on the same area, and this difference constitutes 

 one of the greatest difficulties in determining the rate of growth or yields. 

 It has been shown (§ 274) that the competition between individual trees 

 made necessary by the expansion of their crowns and growing space 

 occurs in an even-aged stand between trees of the same age class. Except 

 around the borders of this age class there can be no expansion of the 

 areas occupied by the total stand belonging to this age class. The 

 factor of area can therefore be standardized in yield tables. Since 

 the yield of even-aged stands is composed of the volumes of trees which 

 have remained dominant throughout the life of the stand, the rate of 

 growth of the individual trees is a maximum both in height and diameter 

 and the mean annual growth resulting on an acre is the maximum for 

 the site when measured for the period required for the growth of the 

 average tree from seedling to maturity. 



The conditions are entirely different in many-aged stands, the dif- 

 ference being greatest for species which may be subjected to a long 

 period of suppression and yet retain the power to survive and recover. 

 In these stands several different age classes are brought into competi- 

 tion not merely with trees of their own age, but with older and younger 

 trees. The older trees have the advantage of the younger in appropriat- 

 ing space vacated by the death of veterans or by the removal of trees 

 for any cause. The young trees growing under partial shade are held 

 back in height growth, diameter growth and consequent volume growth. 

 The economic space occupied by the younger age classes growing under 

 partial shade may be defined as the actual percentage of the total grow- 

 ing space as represented by the available light, moisture and soil fer- 

 tility which is appropriated by these young trees to the exclusion of 

 its use by other age classes. This proportion of space so used is exceed- 

 ingly small and may be negligible, yet the reproduction may survive 

 as scattered individuals for many years. When old trees die, the space 

 released is not, as in the case of even-aged stands, occupied entirely 

 by reproduction, but is distributed among all of the trees so placed 

 that they may avail themselves of it by expanding their crowns. A 

 portion only of released space is taken by additional reproduction. 



1 " Concerning Site," Carlos G. Bates, Journal of Forestry, XVI, 1918, p. 383. 

 Not only is this basis impractical of measurement and classification in the field, but 

 it varies with age of the stand to a much greater degree than does mean annual 

 growth, hence is not trustworthy as a means of separating sites, though the postulate 

 that the best sites are capable of yielding the largest current annual growth is per- 

 fectly true. 



