24 LIGHT IN RELATION TO TREE GROWTH. 
PRACTICAL VALUE OF THE SCALES. 
These scales have their practical use in silviculture, particularly in ~ 
giving a broad comparison of the different species. If one is seek- 
ing a species for underplanting, he naturally concerns himself with 
the most tolerant. If he is developing a plan for reproduction cut- 
tings, he knows that the least tolerant will require a clear cutting 
method, and that he can not reestablish new growth under the old 
cover. Thus, empirical knowledge of hght requirements establishes 
the fact that young growth of a tolerant species under the shade of 
a fully stocked stand of a tolerant species begins to die off, as a rule, 
after from 10 to 20 years of suppression; intolerant trees under a 
tolerant species perish after about 5 years; a tolerant species under 
an intolerant species may continue to live indefinitely, because the 
amount of light that penetrates through the thin crowns of the light- 
demanding species is sufficient for the growth and development of 
the tolerant species. Reproduction of an intolerant species under the 
shade of an intolerant species is capable of enduring for from 10 to 
20 years, or almost as long as reproduction of tolerant trees under the 
shade of tolerant species. Since the least amount of hght is ad- 
mitted to the ground in a fully stocked stand in its pole stage, only a 
tolerant species under the shade of an intolerant species can live 
through this period without injury. 
Tolerance scales based on experience are of distinct value to the 
practitioner and to the student of forestry first learning the char- 
acteristics of the different species. For accurate and scientific in- 
vestigation, however, they are defective, for they show only that one 
species is more or less tolerant than another. They do not show how 
much. What is needed is a mathematical expression for each species 
based on some definite scale. Thus, for example, that the tolerance 
of hemlock is 89, of yellow birch 61, of white oak 32, or whatever the 
actual values might be proved to be. 
METHODS OF DETERMINING TOLERANCE. 
The tolerance of trees may be determined by observation of their 
behavior in the forest in light of different intensities; by studying 
the anatomical structure and the functions of the different organs, 
especially the assimilative organs of trees grown in light and shade; 
and finally by measuring instrumentally the hght intensities them- 
selves. These methods may be classed respectively as empirical, 
anatomical or physiological, and physical. 
EMPIRICAL METHODS. 
The density of crowns of individual trees; the rate of natural 
pruning of different species in stands of the same density, or the 
same species in stands of different densities; the number of succes- 
sive branch orders found on trees of different species; the rate of 
