FACTORS INFLUENCING TREE GROWTH. 15. 
minimum light intensity of 1/6 of the total daylight; at an elevation | 
of 8,500 feet it sinks to 1/6.4 and even to 1/6.9°. 
Thus the behavior of trees in their extension toward higher lati- 
tudes and higher altitudes is not the same. This is due to the fact 
that the intensity of both direct and diffused light decreases with the 
increase in latitude, and the hght limit of a species is reached when 
the intensity of the total daylight becomes equal to the tree’s mini- 
mum lght requirement. With increase in altitude, however, diffused 
light decreases, but direct sunlight increases. With an increase in 
the intensity of direct sunlight, even though there be a decrease in 
the diffused light and a lower temperature, the light requirements of 
a species remain constant or even become less at higher altitudes. 
A A 
4 S ae 
s 8 ; 
‘ N 
Ni 
W 8 
9 N a 
: | 
\ OF 2) 
A) >= 
S Ny 
; Rb 
N S| 
N Nf 
8 ; - . 
50° 60° 70° Boe B— = = Sere : 
7 JIOO0O -6000 3000" /2000 
POZE: ALTITUDE 
AB, total light intensity; BO, lati- AB, light intensity ; BC, altitude; ay, curve 
tude; az, total light intensity; ba, of the total light intensity; ab, curve of 
minimum light requirement of a the light requirements of trees. (After 
species; #, point at which total Wiesner.) . 
light intensity equals minimum 
light needed by the species, also 
limit of latitudinal distribution of 
the species. (After Wiesner.) 
Fie. 2.—Variation of light requirements with increase in latitude and altitude. 
This difference in the light requirements of trees in the horizontal 
and vertical distribution is well illustrated by the two diagrams 
shown in figure 2. 
SOIL MOISTURE. 
Tolerance of trees is emphatically influenced by moisture condi- 
tions in the soil, as well as by its quality. Fricke (1904: 315-825) 
clearly demonstrated by a series of convincing experiments that defi- 
cient moisture in the soil, brought about by competition of the roots 
of older trees, may cause the death of young growth under the shelter 
of mother trees. His experiments were made in a Scotch pine stand 
on poor, dry, sandy soil, on which, according to all authorities, the 
light requirements are greatest. In a stand from 70 to 100 years old,’ 
90431°—Bull, 92—11——3 
