44 Forest Mensuration 



As long as the tree lives, it must annually form a ring of growth (or 

 rather an additional coat, the sleeves of which cover the branches), the 

 outside of which becomes a layer of bark, the inside of which is a layer 

 of wood. In tropical countries this rule does not hold good provided 

 that there is no change of season. 



The formation of rings in the branches is regular. Branch-rings are, 

 however, eccentric and elliptical. The formation of rings in the roots 

 is said to be irregular, not representing the age of the root, possibly be- 

 cause there is no or little change of seasons in the soil. 



PARAGRAPH LXX. 



AGE OF STANDING TREES. 



The age of standing trees can be estimated only when regular annual 

 whorls of branches can be counted. 



The records of seed years and the history of the forest kept by many 

 forest administrations usually give an idea of the age of the trees. 



PARAGRAPH LXXI. 



AGE OF A FOREST. 



The age of a forest is the average age of the trees composing it. 



In the case of a thicket suppressed for a long time by the superstructure 

 of a leaf canopy overhead, a so-called "economic age" is frequently sub- 

 stituted for the actual age. In the case of Adirondack spruce, for ex- 

 ample, a diameter of i inch in the center of the trunk had better be 

 counted, as, say, 15 years, although it may contain as many as 60 rings. 



The mean age of an uneven-aged wood is defined as follows : 



1. That number of years which an even-aged wood would require on 

 the same soil, in order to produce the same volume as is now at hand. 



2. That number of years which an even-aged wood would require in 

 order to produce at the time of maturity the same volume which the 

 uneven-aged wood is likely to produce. 



The latter definition is scientifically more correct. Unless it is adopted, 

 an uneven-aged wood may get over 20 years older in 20 years, owing 

 to the fact that the trees dying in the meantime are mostly minors in age. 



