366 GROWTH OF TREES IX HEIGHT 



Unless trees can maintain a satisfactory continuous rate of height 

 growth individuals so stunted never attain the full height and form 

 of an average mature tree. 



The rapidity of height growth and the total heights ultimately 

 attained are a specific characteristic which is retained whether the 

 species is growing in mixture with other species having different rates 

 of height growth, or in pure stands. Competition of faster growing 

 species does not serve to stimulate the rate of height growth of a species 

 to an appreciable extent. Height growth plays an important role in 

 the survival, dominance and suppression of competing species. 



Quality of Site. The height growth of trees and stands is directly 

 affected by the quality of the site, to such an extent that the rate of 

 growth of trees in height, and the total heights attained serve as the 

 most reliable index for determining differences in site qualities and 

 formulating a basis of classification for sites. This relation between 

 height growth and site quality is largely independent of one of the factors 

 which influence diameter growth of trees (§ 270) namely, density of 

 stand. Although in some species, especially hardwoods with deliques- 

 cent stems, total height attained is less for open-grown trees than for 

 crowded trees, this is not always the case and the rate of height growth 

 is usually retained. On the other hand, stands, especially of conifers, 

 which are so densely stocked as to lead to stunting and starvation, 

 wiU show a decided loss of height growth. One instance is recorded 

 in which a stand of lodgepole pine 70 years old containing 70,000 trees 

 per acre, had attained a height of but 10 feet. 



The law of height growth of trees in a stand is to maintain as far 

 as possible an even rate of growth for all the trees in an age class or crown 

 canopy. There is considerable differentiation between trees with 

 dominant, intermediate and overtopped crowns, the individual rate 

 of height growth decreasing progressively with the loss of vigor and 

 dominance of the crown; but this differentiation is constantly dimin- 

 ished for the surviving trees in an age class by the death of the over- 

 topped trees whose rate of height growth has slowed down. 



When the growth in height for stands is measured, it is gaged by 

 the growth of dominant or sub-dominant trees, which gives very con- 

 sistent results. By thus eliminating the effect of crown class, height 

 growth of stands becomes almost directly an expression of species and 

 of site quality. 



Crown Class ana Suppression. The influence of shading, which 

 kills overtopped trees in an even-aged stand, also has a very marked 

 influence on height growth of trees of an age class growing under sup- 

 pression or in the shade of older trees. The normal rate of height 

 growth is checked by shade, and if it does not result in death the tree 



