1918] HUTCHINSON—FOREST TREES 489 
roughly outlined by a semicircle, a fact which contributes evi- 
dence that Thuja has migrated radiately from a limited area. The 
method of reproduction is such that it does not migrate rapidly; 
that a great proportion of seeds fail to develop is of importance in 
this connection. It would seem that the migration of this form 
has lagged behind changes in ecological conditions. With respect 
to its range of tolerance and its position in the scale of competitors 
under mean conditions, Thuja resembles Larix. These characters, 
together with the limiting action of time, account for most of the 
facts of its distribution. 
Pinus BANKsIANA.—The tolerance of this form toward low 
temperatures, dry conditions, and soil poor in humus, together with 
its limited range toward the other extremes, place it in a unique 
place among the trees of the northwestern region. It is practically 
eliminated from the low lying lands to the south and west of 
Hudson Bay and James Bay, water being the limiting factor. 
The inconsistencies in accounts of its northward distribution in 
this region are the result of its occastonal presence where there 
are higher lands between rivers. It extends northward to 56° N. lat. 
on the dry uplands east of Hudson Bay. Farther south, also, it is 
to be found only on dry rocky or sandy soil containing little humus. 
It is one of the pioneer forms and survives where it can avoid 
competition by enduring severe conditions. 
Pinus Strospus.—This species is also a pioneer among the coni- 
fers. Seedlings are seldom found except where there is a high 
light intensity and well drained soil. Its ranges of tolerance with 
respect to temperature and water do not include the extremes 
which characterize P. Banksiana. The northern limit follows a 
yearly isotherm (33° F.) very closely. It would seem that in this 
case temperature acts as a limiting factor. Because of its longevity 
and its towering height individuals or groves of mature trees often 
persist in a region where seedlings have long been eliminated by 
other forms which are higher in the competition scale. The pine 
forest is normally succeeded by such forms as Tsuga or Acer whose 
seedlings tolerated shade, the time factor; hence its perpetuation 
depends upon the maintenance of or reversion to pioneer condi- 
tions. 
