FORESTRY 421 



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The forest floor is not merely the surface of the soil, but 

 also the whole soil region in which the trees are rooted. This 

 is usually deep and rich, but, more than all, the humus gives 

 it the physical properties of a sponge in receiving and retain- 

 ing water. 



The character of the tree trunks is studied, not only to 

 insure freedom from lower limbs by means of a suitable 

 canopy, but also for the development of wood. Each tree 

 has a period of development during which it adds annually 

 to its wood mass enough to pay for the room and care it 

 requires ; but eventually it reaches a stage when it is not mak- 

 ing enough wood " to pay for its keep." The time to use a 

 tree, therefore, is when it has reached its maximum wood- 

 production and has not yet begun to decline. 



127. Forests and floods. Forests not only build up and 

 enrich the soil, but they also fix the soil, a fact of great im- 

 portance especially in a hilly country. The interlacing roots 

 grasp the soil, so that roots and soil are knit together in a 

 mass that resists erosion. It can be observed that hillsides 

 from which the forest has been removed soon become gullied 

 and stripped of soil. This protection against erosion serves 

 not only for the soil in which the forest grows, but also for 

 the soil of the fields at the lower levels. 



Forest soil holds water so persistently that heavy rains 

 do not run off quickly and produce floods, as they do in bare 

 regions. It is often remarked that streams that had a 

 steady flow when a region was first settled have become 

 alternately flooded and dry since the forests were removed. 

 Therefore, the forest-covered soil not only prevents erosion 

 of soil and flooded streams, but provides also a steady 

 supply of water to the streams. 



128. Formation of forests. It would not be useful to 

 give the details involved in the establishment of a forest 

 where one does not exist, or in the making over of an in- 

 ferior forest, but some idea of the things involved will add 



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