RATE OF GROWTH. 



275 



'4,«;aiaSg^i 



Fig 37 —Oak tree grown in the open 



sometimes as many as 10 to 13, years before they appear to grow at all, their energy all going into 

 root growth. Then comes a period of more and more accelerated growth, which reaches its 

 maximum rate at 25 or 30 years; and when the cotton wood or aspen has reached the end of its 

 growth in height the spruce or pine is still at its best rate, and continues to grow for a long time 

 at that rate. In later life the rate decreases, yet height growth sometimes does not cease altogether 

 for centuries. As a rule, the light needing species are the ones which show the rapid height 

 growth at the start, while the shade enduring are slow at the start, but persistent growers. 



This fact is important in explaining the alternations of forest growth in nature 5 the persistent 

 shade-enduring species crowd out the light-needing, and the 

 latter rapidly take possession of any openings that lire or storm 

 has made. It is also important with refeience to the manage- 

 ment of wood crops and starting of mixed plantations ; the light- 

 needing species must be mixed only with such shade enduring 

 species as are slower growers than themselves. 



The diameter growth shows also periodic changes in its rate, 

 and is, of course, influenced in the same way by soil, climate, 

 and light conditions as the height growth. 



In the juvenile or brush stage, lasting G to 10 years in light- 

 needing and 20 to 40 years in shade enduring species, the cliame 

 ter grows comparatively little, all energy being directed to 

 height growth and root growth. When the crown has been definitely formed more food material 

 is available for wood formation, and the increase in foliage is accompanied by a more rax>id increase 

 of trunk diameter; in favorable situations the highest rate occurs between the fortieth and sixtieth 

 years; in the poorer situations, between the fiftieth and eightieth years, which rate continues for 

 some time. Then comes a period of slower rate, which finally m old age dwindles down almost 

 to zero. 



But neither the diameter growth nor the width of the annual rings alone tells us directly 

 what amount of wood is forming. The outer rings, being laid over a larger circumference, although 



narrower than the preceding rings, may yet have greater cubic 

 contents. The statements of diameter growth are, therefore, 

 misleading if we are interested in knowing how much wood is 

 forming. 



Accordingly the growth in volume must be considered 

 separately, as determined by the enlargement of the cross- 

 section area and the height. The growth in volume or mass 

 accretion is quite small in young trees, so that when wood is cut 

 young the smallest amount of crop per year is harvested, while, 

 if it is allowed to grow, an increase more than proportionate 

 to the number of years may be obtained. 



Only when the tree has a fully developed crown does it 

 begin to make much wood. Its volume growth progresses then 

 at compound interest, and continues to do so for decades, and 

 sometimes for a century or more. 



On poorer sites the rate is slower, but remains longer on the 

 increase, while on good sites the maximum rate is soon reached. 

 Of course in a forest, where light conditions are not most 

 favorable, because form development and soil conditions require 

 shade, the total wood formation is less than in an isolated tree favorably placed. Just so the domi- 

 nant trees in a forest— i. e., those which have their crowns above all others—show, of course, the 

 advantage they have over the inferior trees which are suffering from the shade of their neighbors. 

 Finally, if we would take into consideration an entire forest growth, and determine, for 

 instance, how much wood an acre of such forest produces at different periods, we must not over- 

 look the fact that the number of trees per acre changes as the trees grow older. Some of them 

 are overshaded and crowded out by the others, so that a young growth of spruce might start 

 with 100,000 little seedlings to the acre, of which in the twentieth year only 10,000 would be alive, 



Tig 38 —Maple tree grown m the forest. 



