300 BOTANICAL GAZETTE [MAY 
occupy its territory. In the young pine forest near the tension 
zone there are often found scattered trees of the white birch 
(Betula papyrifera), oak, and the poplars (P. tremuloides and P. 
grandidentata). As the forest reaches maturity, these are usually 
crowded out, the birch being the last to disappear. 
Thus, with time and with the gradual increase of humus, the 
pine forest society is established. As a rule, the white pine 
seems to occupy the tension zone between the coniferous and 
the deciduous forests. If the climate be unfavorable for a higher 
type of forest tree, however, a stage beyond the white pine for- 
est is not reached. If the forest be not too open, and if surface 
fires are absent, the deep shade established by the pines means 
that the edaphic and atmospheric factors have become favorable 
for establishing the climax forest of the region. 
MAPLE, BEECH, AND HEMLOCK FOREST. Beginning with the 
middle beach we have seen that in each successive society a 
gradually increasing quantity of humus has been added to the 
soil, until a white-red pine forest is established. The pines add 
their quota of vegetable decay to the humus. The xerophytic 
soil thus becomes more and more mesophytic, and pari passu it 
has become more and more shaded. The humus means that the 
almost sterile soil of the beach has become richer in organic 
compounds, and by the aid of fungi and bacteria more nitrates 
are added. The capacity of the soil to hold water is likewise 
increased. The better edaphic and atmospheric conditions 
make a natural nursery for the growth of beech (Fagus fer- 
ruginea) and maple (Acer saccharinum Wang.) seedlings. The 
white pine does best in the richer soil conditions, but its seed- . 
lings require more light than they can obtain in the shade 
of the forest. As the maple and beech attain maturity, they 
come to occupy a prominent place in the forest, and a mixed 
pine-deciduous forest is the result. The broad-leaved decidu- 
ous trees produce a still deeper shade, so that if any pine seed- 
lings are able to survive in the pine forest, the increasing shade 
will finally make it impossible for a new generation of pine trees. 
The development of beech and maple seedlings, however, is not 

