THE LIFE PEOCESSES OF TREES 27 



Moisture. — Soil moisture is essential for tree growth. We 

 have seen that a very large percentage of the tree by weight 

 is water either chemically combined in the wood substance or 

 present as free water or sap. A tree transpires moisture con- 

 stantly through the leaves and through the bark in both sum- 

 mer and winter. For tree growth there must be present in the 

 soil a supply of moisture adequate to supply the tree with 

 water needed to carry on the life processes. Should the supply 

 fall below the minimum necessary the tree will die. If trans- 

 piration from a tree takes place faster than the roots can absorb 

 the moisture from the soil, the tree will dry out and die. 

 This often takes place in winter when the ground freezes be- 

 low the roots of the tree and excessive transpiration takes 

 place through the bark induced by very bright sun. The 

 death of the trees in this way is known as winter killing. 

 In transplanting large trees it is customary to cut back the 

 crowns so as to reduce the transpiration of moisture, until 

 the roots have a firm hold on the ground. 



The presence of moisture in varying amounts in the soil 

 is" probably the principal factor in the local distribution of 

 trees. Soil moisture more than any other factor determines 

 what trees will grow in a certain situation, as on a ridge, 

 on a slope or on bottomland. Many trees demand a large 

 amount of water in the soil and are found growing naturally 

 in or near swamps and on damp soil, while others seek the 

 driest situations. Some trees are very particular as to the 

 amount of moisture in the soil, they demand a certain amount 

 and will not grow if the amount is increased or diminished, 

 while others will grow under diflferent moisture conditions. 

 The same tree is often found growing in the bottom-land 

 and on upper slopes and ridges, but it grows best where 

 the moisture conditions are most favorable to it. If a tree 

 that demands dry conditions is planted on a damp soil it will 

 soon die, and a tree that needs a moist soil will not live if 

 deprived of that moisture or planted on a dry soil. Because 



