Hill: Penobscot Vegetation 381 



in Figure 27. The wood may remain in this condition for 

 a considerable period of time, 40-60 years, according to 

 Dana (5), but sooner or later the conifers become dominant. 

 Recent studies of Tourney's (27) on the relation of gray 

 birch to the regeneration of white pine are of interest in 

 this connection. These studies suggest that a stand of 

 birch is never sufficiently dense to cause the death of the 

 pine by shading. The latter tree is able to come in and 

 show a fair amount of growth in all cases. The rapidity 

 of growth, however, is dependent upon the density of the 

 birch, a falling off of the growth rate of pine accompanying 

 an increase in the number of individuals of the birch. This 

 is due to competition for soil moisture and nutrients rather 

 than to shading. The growth of spruce and fir under 

 birches in the Penobscot Bay region may well be analagons. 

 The conifers are able to persist and show some growth but 

 are retarded from complete development by the root com- 

 petition. It is only when the shorter lived birches begin to 

 die that this competition is lessened enough to allow the 

 conifers to complete their growth and become dominant. 

 As these latter increase in prominence any popple, which 

 may be present, drops out, since it is very intolerant of 

 shade. Some of the birches, however, may persist and oc- 

 cupy a prominent place, not only in the coniferous forest, 

 but also in the climax forest which may follow. The ap- 

 pearance of conifers after a burn is entirely dependent on 

 the presence of seed trees in the immediate vicinity, since all 

 young growth is killed by the fire, and the trees do not 

 possess the ability to sprout from the roots. The birches, 

 on the other hand, not only have this ability, but in com- 

 pany with the popple, possess very light winged seeds which 

 can be blown great distances. As a result these trees are 

 invariably present on burned areas. The cherry, which is 

 widely distributed by birds, is also almost sure to be pres- 

 ent in such places. 



