22 ELEMENTARY FORESTRY. 
other shade enduring. ‘The words tolerant and _ intol- 
erant are also used as distinguishing the peculiarity of trees in 
this respect, and they are perhaps better terms. Trees that are 
known as tolerant generally have a thicker mass of foliage 
than those that are intolerant of shade. ‘This simply means 
that their lower leaves can withstand the shade of their upper 
leaves. While it is not an absolute rule that tolerant species 
have a thick mass of foliage and intolerant trees open foliage, 
yet it is so generally true that where the characteristics of a 
tree are known it serves as a very reliable indication. Most 
trees are much more tolerant of shade when young than when 
old. Among our tolerant trees may be mentioned the Spruce, 
Oak, Balsam, White Cedar, Red Cedar, Hornbeam and Hard 
Maple. Among our intolerant species are Poplars, Cotton- 
wood, Willows, Soft Maple and Birch. Of our native pines, 
the White Pine is much more tolerant than either the Jack or 
the Norway Pine, which are quite intolerant of shade and soon 
succumb if they are protected from the full sunlight. 
Trees Protect One Another and are Mutually Helpful, 
and many trees that are quite hardy on the limits of their 
growth when grown in groups, will fail if grown singly, as for 
instance the Hard Maple in exposed parts of Minnesota. Trees 
also interfere with one another and struggle for light and soil 
conditions, and the weaker trees are destroyed. So true is this 
that where the land is thickly seeded with even-aged trees, they 
may all become weak and sickly because of hindering one 
another. On the other hand, this crowding of trees forces them 
to take on an upward growth and kills out the lower branches. 
Trees growing under such conditions make long timber free 
from knots, which is therefore most valuable. 
SOIL CONDITIONS. 
Water Supply. Water is the most important element in 
soils for tree growth, and the greatest attention must be given 
to its conservation and distribution through the soil. Trees 
do not grow to best advantage in very wet or in very dry soil, 
although some can live and almost thrive under Ritch euntaverds 
ble conditions. There is very little land, except in the arid 
