342 HORTICULTURE, FORESTRY, FLORICULTURE 



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 cottonwood 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; tne persistent shade-enduring species crowd out 

 the light-needing, and the latter rapidly take possession of any 

 openings that fire or storm has made. It is also important with ref- 

 erence to the management 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 6 to 10 years in light- 

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

 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 rapid increase of 

 trunk diameter; in favorable situations^ the highest rate occurs be- 

 tween the fortieth and sixtieth years; in the poorer situations, be- 

 tween the fiftieth and eightieth years, which rate continues for 

 some time. Then comes a period of slower rate, which finally in 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 thin- 

 ner 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 separ- 

 ately, 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 tho 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 some- 

 times 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 



