THE PRINCIPLES OF FORESTRY 161 



lumber. (2) Forest planting, which is primarily for the 

 revenue to be derived from the sale of wood and lumber. 



It is a special problem in the management of each indi- 

 vidual farm to decide just what planting should be done. 

 The Forestry Service of the United States Department of 

 Agriculture offers to give practical assistance to tree planters 

 in deciding these questions. Circular 22 of that service 

 should be read in this connection. Careful study should be 

 given to the planting of trees under some good system, as 

 a forest is the most permanent thing that can be planted 

 on a farm. 



Need of Forest Planting. — Forests are necessary to the 

 highest material development of any country. The climatic 

 influences are very beneficial. It is desirable in behalf of 

 the public welfare to plant trees in great number. This is 

 true both on the treeless plains and also on the sites of de- 

 stroyed forests. To be of most public benefit the planting 

 should be well distributed over the region. All will share 

 the benefits and all should join in planting the trees. 



Forest Influences. — Growing trees conserve moisture, 

 modify climatic extremes, and purify the air. Careful 

 observation shows that large trees growing in a grove affect 

 climatic and soil conditions in several ways: 



1. During the day the ground under the trees is protected 

 from the sun's rays and is therefore cooler than soil not 

 protected. The air circulating over this cool soil tends to 

 cool the air in the immediate vicinity on sunny days. 



2. At night the trees retard the loss of heat from the 

 ground under them. This tends to equalize the temperature 

 of not only the soil and air under the tree but that in the 

 near vicinity. The soil and air are kept cooler by day and 

 warmer by night. This equalizing of temperature is notice- 

 able during short periods of very hot or very cold weather. 

 Gardens growing near trees are sometimes uninjured by 

 fall frosts which kill tender plants in other gardens. 



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