EFFECT OF CROWN CLASS 353 



274. Effect of Crown Class. The individual rate of diameter 

 growth varies over a wide range with the same species, site and stand. 

 The rate of growth is coordinated directly with the crown spread of 

 the tree. There exists a relation between width of crown and diameter 

 which is found to hold good under almost every condition and for every 

 species, although varying with the species and its habit of growth. 

 This law, which might be of great use in determining the number of 

 trees which should exist per acre for a given species in mixed stands, 

 is somewhat interfered with by the fact that the volume of the crown, 

 rather than its mere diameter, is the factor affecting diameter growth, 

 and with western conifers, with very tall and slender crowns, width 

 alone does not properly express this value. As crowns receive more 

 growing space and expand, diameter growth correspondingly increases. 

 This elasticity of diameter growth correlated with crown spread is the 

 principal means of adjustment which a stand of trees possesses, by 

 which it constantly tends to fill in blanks and form a complete crown 

 canopy provided only that the distribution of the trees is such as to bring 

 these blanks within the possible maximum spread of individual crowns. 



Effect of Shade. Diameter growth during the life of a tree de- 

 pends upon its history with respect to the remaining trees in the stand. 

 A tree which has remained dominant since germination maintains a 

 maximum rate of diameter growth. The crown spread at successive 

 decades is a maximum. Trees which are at first dominant and later 

 suppressed, cease to grow in diameter because their crowns cease to 

 expand. The relation between -diameter and crown is maintained, 

 but neither continues to increase. Trees which were originally sup- 

 pressed and later freed may show a marked increase in diameter growth 

 coinciding with an increased spread of crown, thus maintaining the 

 proportion under the changed conditions. But if their crowns have 

 lost the power to recuperate, which depends upon both the specific 

 character and the age of the tree, no increase is made in diameter 

 growth by reason of this liberation. 



Effect of Treatment. The growth in diameter of trees can be pro- 

 foundly influenced by the artificial treatment of a stand. Since for 

 the individual tree it is a function of crown spread and its rate is governed 

 by the ability of the crown to expand, diameter growth is the most 

 easily governed and most adaptable function of tree growth. The 

 stand per acre or rate of growth for a period measured in cubic contents 

 may not be subject to great modification, but the sizes of the stock 

 produced and consequently the value per acre can be greatly influ- 

 enced by management. The behavior of trees in thinned stands and 

 on cutover lands must be studied separately from those subjected to 

 the natural laws of survival in original unthinned forests. 



