116 STUDIES OF TREES 



light. It also explains why, in the forest, the lower branches 

 die and fall off— a process known in Forestry as " natural 

 pruning." The influence of light on the form of trees 

 should be well understood by all those who plant trees 

 and by those designing landscape effects. 



(4) Influence of heat: Trees require a certain amount 

 of heat. They receive it partly from the sun and partly 

 from the soil. Evaporation prevents the overheating of 

 the crown. The main stem of the tree is heated by water 

 from the soil; therefore trees in the open begin growth 

 in the spring earlier than trees in the forest because the 

 soil in the open is warmer. Shrubs begin their growth 

 earlier than trees because of the nearness of their crowns 

 to their root systems. This also explains why a warm 

 rain will start vegetation quickly. Too much heat will 

 naturally cause excessive drying of the roots or excessive 

 evaporation from the leaves and therefore more water 

 is needed by the tree in summer than in winter. 



(5) Influence of season and frost: The life processes of 

 a tree are checked when the temperature sinks below a 

 certain point. The tree is thus, during the winter, in a 

 period of rest and only a few chemical changes take place 

 which lead up to the starting of vegetation. In eastern 

 United States, growth starts in April and ceases during the 

 latter part of August or in early September. The different 

 parts of a tree may freeze solid during the winter without 

 injury, provided the tree is a native one. Exotic trees 

 may suffer greatly from extreme cold. This is one of the 

 main reasons why it is always advisable to plant native 

 trees rather than those that are imported and have not 

 yet been acclimatized. Frosts during mid-winter are not 

 quite as injurious as early and late frosts and, therefore, 

 if one is going to protect plants from the winter's cold. 



