292 BOTANICAL GAZETTE [APRIL 
these two species are confined, their occurrence under lodgepole pine 
stands is not common. As soon as openings are made in the forest of 
mature lodgepole pines, due to causes other than fires, this undergrowth 
may spring at once into prominence and may come to occupy a place 
in the mature forests. At the same time the openings have made it 
possible for the existence of seedlings of such trees as western larch 
and Douglas spruce which demand light for germination. Of course 
lodgepole pine seedlings can germinate here also, and in the first 
generation or two they will still predominate; but each new generation 
will have fewer specimens of the latter species, for since it is not a 
long-lived tree, a canopy of the mature forest of the other species will 
in the long run crowd it out,by density of shade, or reduce its occur- 
rence to isolated trees here and there in the forests. In this way the 
forests will revert to the normal conditions. The time it will take to 
do this depends upon the nature of the conditions that the last fire 
left. If the lodgepole pine conditions had been established for a wide 
area, the time would be long, perhaps covering many generations of 
growth. If on the other hand the fire only partially destroyed the 
original forest, one or a few generations would suffice to permit the 
re-establishment of a forest similar to the original. Between these 
two extremes are all stages, some of which have been described. 
Indeed the present forest formations are a complex expression of the 
influence of fires upon them. | ; 
The general aspect of a lodgepole pine forest approaching matunty 
is somewhat different from that of other stands. It has already been 
shown that the growth in height is rapid. In dense stands of young 
growth the poles are long and spindling, hence the name lodgepole 
pine. The small diameter of the bole of the tree is very characteristic. 
A forest in which the average age of the trees is about one hundre 
years is seen in fig. 23. In this forest, where the trees are over @ 
hundred years old, the average diameter is probably 20°™, many — 
being only 15°™ in diameter. 
Compared with the other forests growing in like situations, the 
canopy that a lodgepole forest forms is not very heavy. This per- 
mits more light to reach the forest floor, hence there is 4 greater 
development of undergrowth. It has already been shown that seed- 
lings of silver pine, Engelmann spruce, and lowland fir can endure 
