Igor] GENETIC DEVELOPMENT OF FORESTS 319 
more unfavorable the humus conditions up to a certain limit, the 
more likely is the clearing to have a pine aspect. The humus 
in the soil may be so reduced as to be incapable of supporting a 
poplar-birch growth. fig. 15 shows a young jack pine growth. 

Fic. 14.—A hemlock -maple forest destroyed by fire. The rich growth of fireweec 
Aonapeel oe indicates the first stage in the life-history of a second- 
growth for, 
In this repeated fires have probably so reduced the humus con- 
ditions of the soil that the most xerophytic of the pines alone is 
able to gain a foothold. If favorable for the presence of both 
pine and poplar-birch growth (fig. 76), the pines in time over- 
top the poplars and birch, until a condition corresponding to the 
normal life-history is attained, viz., a pine society preceding the 
maple-beech society. 
