212 BOTANICAL GAZETTE [MARCH 
humus more rapidly, for by fall of the dead limbs and leaves more 
organic material would be added than from the decay of lower forms 
of vegetation. The shade of the trees thus established would check 
the rapid oxidation of the organic material that takes place in the 
open. The ultimate effect would be the establishment of a forest 
in a place that was formerly occupied by a prairie. It is very probable 
that the whole yellow pine area just described was at one time 
occupied by a prairie vegetation. There is evidence that the present 
prairie island in this forest would become covered with forest trees 
if not interfered with by man, for along the roadside groups of young 
growth of bull pine seedlings are not uncommon. All around the 
border of the prairie young trees are present, and even in a timothy 
field there were noted seedlings that had escaped being cut by the 
mowing machine. Before the prairie island was settled it is very 
probable that surface fires swept it occasionally and thus checked 
the conquering of the prairie by the forest. Another prairie area 
similar to the one just described lies directly west of Echo Lake 
(map). Again, west of Nigger Prairie, just across the low range of 
Mission Mountains, is another ‘‘sand pocket” that has a prairie 
vegetation. These however border on the prairie climate. 
The question arises whether the presence of bull pine will in any 
way change the conditions sufficiently to make them capable of sup- 
porting other species of trees. It was shown that the humus content 
of the soil increases more rapidly when trees are present than when 
they are absent. With the gradual increment of organic decay, 
it is very possible that the water-holding capacity of the soil may 
be increased to such an extent that it will be rendered capable of 
supporting other species of trees. 
A bull pine seedling does not tolerate shade; the young tree must 
have broad open places where the sun strikes the soil a greater por 
tion of the day. An opening in the forest made by the removal - 
one or two trees does not allow sufficient light and heat to enter; 
thus young stands are excluded from anything like a tolerably thick 
growth of these trees. This is not the case with western larch and 
Douglas spruce; it has been shown that the former does not tolerate 
shade, but grows in slight openings in the forest; the latter 1 iti 
similar in this respect to western larch. If humus has accumulate 
