DIAMETER GROWTH OF TREES GROWING IN STANDS 351 



Since there can be no D.I.B. at 16 feet until the tree has reached 

 this point in height, the curve of these points would terminate at zero 

 diameter at an age equal to that required for the tree to grow 16 feet 

 in height, above the stump, which is 8 years in Fig. 70. In the same 

 manner the D.B.H. curve would terminate at a point representing the 

 year in which the tree reached 4^ feet in height, which is 4 years. The 

 stump curve has already been shown to terminate at an age repre- 

 senting the growth of the seedling to stump height at 3 years. This 

 principle is later explained more fully in connection with a method 

 of plotting the volume growth of different trees (§ 291). 



270. Factors Influencing the Diameter Growth of Trees Growing 

 in Stands. Diameter is the most variable factor of tree growth, dif- 

 fering with a wider range of conditions and showing greater diversity 

 between trees in the same stand than height growth. Growth in diam- 

 eter influences growth in volume of the tree to a much greater extent 

 than does height growth, the relation being that of dr or area. Since 



the growth in area bears this fixed relation — -, the area growth of indi- 

 vidual trees is never studied, as all problems for which it is desired 

 are solved by the study of diameter growth. The rate of diameter 

 growth is determined by four factors: species, quality of site, density 

 of stand, and crown class. 



Secondary factors modifying diameter growth are the amount of 

 shade endured by the specific trees studied, and the treatment of 

 the stand. 



271. Effect of Species on Diameter Growth. Different species 

 have developed specific differences in average rate of diameter growth. 

 Those accustomed to growing on soil of good quality as dominant 

 species have acquired the fastest growth rate. Intolerant trees usually 

 grow faster than tolerant since they must maintain their dominance. 

 Of this, the cottonwood is an example. Trees which have the power 

 of enduring shade usually grow, even in the open, at a somewhat slower 

 rate than intolerant trees. 



Trees do not indefinitely maintain a given rate of diameter grov/th. 

 Until a tree actually dies, it continues to increase in diameter, but there 

 comes a period when, in spite of the dominant position of the tree, 

 its rate of diameter growth diminishes. The period at which this 

 diminution sets in marks the maturity and the beginning of decadence 

 of the tree. The life cycle of different species of trees is as distinct 

 as that of different animals. Short-lived trees, like jack pine and 

 tamarack, show this falling off at 70 or 80 years or sooner, and disappear 

 within 30 or 40 years thereafter. The same is true of aspen. The life 

 cycle of conifers is apparently affected by general climatic conditions. 



