CHAPTER XXV 

 GROWTH OF TREES IN HEIGHT 



281. Purposes of Study of Height Growth. The rate of height 

 growth in trees is desired in order to determine the relative ability of 

 different species in a mixed stand to survive and dominate their com- 

 petitors. Height growth is the factor which largely determines the 

 future composition of mixed even-aged stands. A condition of sup- 

 pression is indicated by the diminution of height growth. Trees capable 

 of living under suppression have the power of maintaining a much 

 reduced height growth for a long period and of afterwards recovering 

 and increasing this rate. In the second place, data on height growth 

 are desired to determine the quality of site as a basis for classifying plots 

 in the study of yields per acre for yield tables. The relative heights 

 based on age which are attained by trees and stands are a close indica- 

 tion of the site quality, even superior to volume production as a reliable 

 index of site. Finally, height growth is desired as a step in the deter- 

 mination of the growth of trees in volume whenever the latter data are 

 required. 



282. Influences Affecting Height Growth. Species. The juvenile 

 period following germination (§ 257) is followed by a period of rapid 

 height growth which is maintained until the tree has reached from 

 two-thirds to three-fourths of its total maximum height. This period 

 is coincident with the rapid reduction of numbers in an age class and 

 with the expansion of the crowns and the elimination by suppression 

 of those trees which are unable to maintain their position and crown 

 spread in the stand through being overtopped. 



The third period is marked by increasing slowness and finally by 

 practical cessation of height growth and a marked change in form of 

 crown. In some hardwoods this is the result of division of the main 

 stem into several branches, and in conifers it is characterized by the loss 

 of the habit of producing annual whorls of branches. This habit, 

 however, is retained by many species such as spruce and fir. When 

 the power to produce annual whorls is lost, the growth in height becomes 

 similar to that of branches. The power of recovery of height growth, 

 which has been retarded or suppressed, is lost at an early age in intoler- 

 ant species, but with tolerant species may be retained for a long period. 



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