RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 45 
to be concerned with questions affecting the survival of the individual 
plant or tree. The individual tree of any age must be thought of as 
a spherical, conical, or cylindrical mass of irregular surface, such that 
a pencil of rays will affect equal absorbing surfaces, almost regard- 
less of its angle of approach. It is therefore logical that, in the study 
of individual trees, all determinations of light intensity and quality 
should take as the unit a pencil of rays of given cross section. This 
full cross section is to be obtained by exposing the absorbing surface 
of given area normal to the axis of the pencil. 
In the following discussion exposures for this purpose and of this 
nature will be implied, unless light quantities affecting stand growth 
are specifically mentioned. 
Total Radiation on the Site. 
To determine the quantity of radiant energy which is available 
for plants or trees on any particular site in relation to the growth 
of the whole stand, obviously the quantity should be measured at a 
point where it has not been intercepted or diminished in intensity. 
As previously pointed out, this will be above the crowns or in an 
opening of exposure similar to the plane of the forest canopy. After 
having determined the total amount of energy available, the amount 
actually utilized, if desired, might be measured as the difference be- 
tween the total and that which is available below the canopy. In 
any event, the intensity of solar radiation may be expressed in heat 
units, or calories per square centimeter of horizontal area. 
Insolation Under Canopies. 
The measurement of insolation or sunlight intensity under canopies 
may be for two distinct purposes: To determine the amount of 
energy which has escaped the tree crowns above and therefrom to 
deduce the amount utilized by them; and to determine the amount 
which is available for undergrowth, either in the form of subordi- 
nate species or reproduction. The first measurement, which is not 
concerned with the tolerance of the species, but rather with the com- 
pleteness of the canopy, the completeness of utilization, and the 
rate of growth of the stand should obviously be closely connected 
with measurements of total radiation on the site. Since the plan of 
such measurements has been explained, this subject may be dismissed 
and the attention turned to those problems which are concerned 
wholly with the question of tolerance, or the question of the relative 
requirements of the various species for light, especially in connec- 
tion with survival in their earliest stages. 
