HERMAN H. CHAPMAN 31 



wider areas, our forests are either periodically burned 

 up or blown down. This may not occur in the same place 

 oftener than once every 100 years, but since the life of 

 most trees is much longer than this, it gives many species 

 a chance to perpetuate themselves indefinitely which would 

 otherwise be crowded out by more tolerant trees. So 

 widespread is this condition, that many of our most val- 

 uable timber trees, especially the pines, are found over 

 wide areas in stands which are practically even-aged, and 

 must therefore have come up after such natural clear- 

 ings. 



Two things, then, favor intolerant species: their ra- 

 pidity of height growth, and their ability to distribute seed 

 to great distances. Some tolerant trees can also distrib- 

 ute their seed in the wind, as hemlock. But this tree 

 grows very slowly in height. Take the case of white pine 

 and hemlock forests. In a windfall, the seeds of the white 

 pine and hemlock get a start, probably in company with 

 aspen and white birch — if any seed trees of these species 

 are near, The aspen and birch grow rapidly and the pine, 

 which is somewhat tolerant, holds its own in the light 

 shade cast by these species, or fills the openings in the 

 stand. The hemlock seed is also scattered but on the 

 bare soil these seedlings are apt to dry out and die, or 

 fail to take root at all. But as soon as there is some shade 

 cast, the moisture in the soil increases and hemlock seed- 

 lings take root. Long before they reach any size, the 

 poplars are almost mature, and the pine is thrifty and mak- 

 ing rapid growth. At 80 years the aspen begins to die 

 out. It has reached its old age and the pine is overtop- 

 ping it. At 120 years the white birch follows the aspen, 

 leaving the white pine in possession, with an understory 

 of hemlock now attaining a fair size. These two species 

 live together until they both reach old age. Should the 

 pine be cut out, the hemlock will usually die from undue 

 exposure to drought. If the forest is burned, the whole 



