18 LIGHT IN RELATION TO TREE GROWTH. 
seedlings is impossible in the dry soil produced by the roots of the 
older seedlings, which in the porous soils of the planting holes attain 
extra good development. In dry situations, therefore, fail places 
must be filled not later than three years after the first planting; 
otherwise it will be necessary to give up entirely the filling of the 
blank places, or the older competitors must be removed from small 
areas and these then replanted. 
These facts show that the so-called “ light increment,” or increase 
of growth after logging or thinning, is not due alone to the greater 
access of light to the remaining trees. By thinning a stand not only 
are the hght conditions changed, but the competition of the roots is 
diminished, which leads to an increase of moisture in the soil. The 
leaf litter is also more readily decomposed and the soil in this way 
becomes enriched with nutritive substances, all of which results, of 
course, in an acceleration of growth after thinning. 
SOIL FERTILITY. 
Plenty of chemical nourishment in and favorable physical condi- 
tions of the soil increases tolerance. At the same hght intensity the 
assimilative energy of the green leaf increases with increase of nour- 
ishment in the soil. Thus, the assimilative energy of trees grown in 
a deficient light but on good soil may be the same as that of trees 
grown in full light but on poor soil; or, in short, trees on good soils 
can stand more shade than trees on poor soils. This has been clearly 
demonstrated by Hartig (1897: 142-143), who thought that the 
weight of the young leaf-bearing shoots may, to some degree, serve 
as an indication of the amount of foliage in a tree. At the same 
time, the amount of wood produced in a tree for each pound of small 
leaf-bearing twigs serves as a criterion of the work of the leaves. 
Thus, measurements made by him on oaks in the Bavarian Spessart 
gave the average results shown in Table 3. 
TABLE 3.—Amouwint of wood produced annually fer each pound of small twigs. 
Pasiaus 
Age (years). | Cubic inches 
of wood. 
33 45.9 
90 16.8 
246 16. 0 
400 14.0 
These figures show that the leaves of young trees function with 
greater energy than the leaves of older trees. The same method may 
be applied to determine the effect of nourishment upon the work 
of the leaves. It was found that on different soils the annual produc- 
tion of wood in trees of the five different classes (dominant, codomi- 
