GO 



lities as in unfavourable ones. It has been recently established that 

 a close leaf-canopy not only checks growth in girth but also 

 growth in height, although the latter is not influenced by the 

 density of the crop to the same extent as the former, and the un- 

 favourable influence begins to sho'w itself only in very dense 

 crops. 



B. Growth of girth and basal area. 



The course of growth of the girth at ground level is similar 

 to that of the height of the tree. The girth is, however, 

 usually measured at breast-height (4j feet). Hence a curve de- 

 lineating its growth cannot start from zero, but from the point 

 of time at which that height was attained. By the time a tree 

 reaches this stage, the rate of growth of the girth has either 

 entered upon its maximum or is on the point of doing so. There- 

 fore, as we are accustomed to measure it, the girth starts at 

 or near its maximum, rate of increase. When the rate of growth 

 begins to decline, it does so, at first more or less rapidly according 

 to species, soil and locality, slowly afterwards, and remains more 

 or less constant for some time. 



The rate of increase of the basal area is very slight at first, 

 then augments more or less rapidly up to a certain figure, after 

 which it remains constant or diminishes slowly. The continuance 

 of a dense leaf-canopy up to an advanced age results in an early 

 and rapid decrease of the rate of growth of the basal area of the 

 individual component trees. On the other hand, in the case of trees 

 standing isolated, the rate of growth increases, or at least remains 

 constant, beyond even the ordinary age of exploitability. In the 

 most favourable soils and localities the rate of .increase of basal area 

 O f the individual tree attains its maximum. rapidly (about the 40th V 

 or 50th year for European trees) and then declines; whereas under 

 opposite conditions it augments slowly, but the augmentation con- 

 tinues up to a great age. 



C. Developme'ht of the form -factor. 



The development of the form-factor of a tree is dependent on 

 the rate of increase of the girtfy at different heights, which rate itself 

 depends on the amount of standing room and the consequent expan- 

 sion of the crown. In the case of trees forming a leaf -canopy the 

 width of the annual rings of growth is generally greatest at the 



