TOLERA^-CE OF FOREST TREES 015 



Nearby a second grove was found which grew in a wetter soil and 

 the light intensity was only about one-half as strong where the same 

 phenomenon was taking place. We r'aised a group pine in pots and 

 when they were about 18 inches high transplanted them to the green- 

 house, placing them 4 inches apart. As they were pot-grown this 

 damaged the roots systems very little. Arrangements were made to 

 water by pipes on the surface to avoid wetting the leaves. These trees 

 held their leaves and branches at a light intensity many times lower 

 than any that I have ever found in the woods. That is, when abundant 

 water was present the leaves remained and the trees formed very dense 

 crowns. The results of shading white pine with cheese cloth in beds 

 which were abundantly watered, and in beds which received only the 

 rain water which went through the cheese cloth, show conclusively 

 that light is not the chief factor in determining the length of the 

 needles. In the first set there was little difference in the length of the 

 needles grown under one or six layers of cheese cloth. In the second 

 set there was a marked difference, but these plants suffered from lack 

 of water because much of the rainfall was caught by the layers of 

 cheesecloth and evaporated without reaching the soil. 



I have not attempted to give an exhaustive review of the literature 

 but simply tried to show that the word tolerance does not mean a 

 light relationship only to all foresters; that it is a confusing term and 

 that its use has made concise thinking impossible. I believe that in 

 the interest of scientific silviculture it should be dropped from the 

 literature. If this is impossible it should be given an exact definition. 



The factors which influence tree growth — light, heat, humidity, soil 

 solution, etc. — must all be studied experimentally as they affect forest 

 development, before our silvicultural methods can be put on a scientific 

 rather than a philosophical basis. Until this work has been done, and 

 until we understand the relationship of individual trees and forest 

 types to these factors, we will not be able to understand the natural 

 succession of forests or to form a national policy of silvicultural man- 

 agement. To this end it is liighly desirable to establish silvicultural 

 research laI)oratories in the various forest regions in the country, and 

 the Forest Service should have the hearty support of everybody inter- 

 ested in forestry in its endeavor to advance fundamenta] research. 



