32 FORESTRY 



process is repeated. But should the soil be rich, and noth- 

 ing happen to the stand, the hemlock, and probably cer- 

 tain species of hardwood, would eventually survive the 

 pine, which could not come in as seedlings in their shade. 

 There are other tolerant species which do not do as well 

 as the hemlock in holding their own, but exist usually 

 as a secondary tree under the crowns of more rapidly 

 growing and less tolerant species. Such trees are the 

 dogwood, and hornbeam, and in the Lake States, the bal- 

 sam fir under white pine. 



Duration of Life of Trees. Once established, a spe- 

 cies whether tolerant or intolerant, has a great advantage 

 if it is long lived. Short lived trees will die out of a 

 stand, leaving the more enduring kinds in control. The 

 duration of life of different species varies from 70 to 80 

 years in the case of Jack pine and aspen, or even less, 

 up to the almost miraculous ages attained by the sequoias, 

 or big trees of California, which live to be over 2,000 years 

 old. It is hard to explain these differences, but they are 

 specific, and as much a part of the tree's characteristics 

 as its growth and tolerance. With old age, a tree simply 

 loses vitality. The power to heal up wounds in the bark 

 is lost. The roots seem to lose their grasp of the soil 

 in some species and are no longer able to resist the pres- 

 sure of strong winds on the large and heavy body of the 

 tree. Sometimes insect enemies in the form of bark beetles 

 attack these old trees and kill them. But more often the 

 spores of fungi enter through open knot holes or broken 

 limbs, or upwards through the roots, and the body of the 

 tree rots out until the strength is gone. Ultimately the 

 tree is blown over. Old woodsmen have often observed 

 the peculiar phenomenon of a tree tottering to its fall on a 

 still day, with apparently no reason for causing the fall 

 at that particular moment. The longest lived trees are 

 apt to be rather slow in growth. White oak which reaches 

 greater age than other oaks, does not compete with them 



