METHODS OF DETERMINING TOLERANCE. 4] 
METHOD OF MAKING MEASUREMENTS. 
Measurements of light should be made on perfectly clear days, 
since cloudy, smoky, and hazy skies reduce the light intensity and 
are apt to lead to errors. For the same reason no measurements 
should be made when the sun is at the horizon or low altitude, for 
the composition of the solar spectrum at that time differs consider- 
ably from that when the sun is at a high altitude. Absolute light 
intensity is also influenced by the altitude above sea level. Its in- 
fiuence, however, is noticeably felt only when the difference in eleva- 
tion is 3,000 feet or more. 
The measurement of the relative light intensity enjoyed by forest 
trees presents considerable difficulties because the different parts of 
the crown are differently illumined. It is not enough, therefore, in 
measuring the relative light intensity to measure only the light on 
the periphery of the crown, but rather the minimum light intensity 
at which green leaves still continue to live or assimilate carbon from 
the air. This minimum light intensity is of the greatest importance 
in forestry, since it indicates the limit of light beyond which a given 
species can not exist, and consequently serves as a basis for classify- 
ing species as tolerant and intolerant. The maximum light intensity 
under which forest trees grow best does not differ as much with dif- 
ferent species as does the minimum light intensity which 1s charac- 
teristic of each species and which must be taken into account in car- 
rying on cultural operations in the forest. 
It is evident that in attempting to measure the maximum and 
minimum light intensities which are enjoyed by different species we 
are necessarily dealing with a variable quantity. Not only does the 
Neht vary with the altitude of the sun, but the light intensity within 
the crown varies with the density of the crown and the arrangement 
of the foliage. 
There are two possible ways of measuring the amount of light 
which is received by a given tree. One way is to obtain a summa- 
tion of the light intensities for the vegetative period. Another way, 
much simpler and yet probably as serviceable for the comparison of 
different species, is to make a series of measurements during the day 
at points within the crown where the foliage just continues to exist. 
An average of these measurements gives the average minimum light 
intensity for that species. ‘The measurements should be taken every 
hour, so as to include the periods of greatest and least illumination 
of the interior of the crown. 
These periods vary considerably with different species. Thus, 
thinly foliaged trees have a light intensity in the interior of the 
crown in direct proportion to the intensity of the daylight, and the 
relative minimum light intensity remains about constant. through- 
