1918] HUTCHINSON—FOREST TREES 487 
conditions more nearly approximating the mean of the range of 
tolerance of these competitors, thereby contributing toward the 
elimination of Abies. 
The problem which arises by the appearance of Abies in swamps 
‘ south of its “normal” range may be explained by regarding soil 
rather than temperature as the limiting factor. Soil changes have 
been slower in the undisturbed humus and rock soil layers of the 
swamp than on the weathered uplands. Soil conditions approxi- 
mating the mean of the range for Abies have been maintained, 
hence this form has persisted; also, the presence of abundant 
water inhibits the attacks of parasitic fungi, thereby permitting the 
growth of Abies. 
PICEA MARIANA AND P. CANADENSIS.—Although these species 
are closely related morphologically, they are quite different eco- 
logically; in this respect P. canadensis is quite closely associated 
with Abies balsamea. P. mariana has a wider range of tolerance 
than either, Low states, “In Labrador (and northern Quebec) 
the white spruce grows on rich intervale grounds or near the shores 
of lakes and rivers. The black spruce is found on hills and in cold 
swamps. The two kinds have the same geographical range north- 
ward.”’ Soil development and soil water frequently become 
limiting factors, separating these species. The status of tempera- 
ture as a factor in distribution is demonstrated by the differences 
existing between Picea mariana and P. canadensis. Although they 
have practically the same temperature range, the latter is not found 
throughout a vast area of the region lying between Hudson Bay 
and Labrador. Available accounts and the evidence given by its 
habitat in other regions indicate that soil development is the limit- 
ing factor. In this respect Abies balsamea takes a position inter- 
mediate between these two species of Picea. Picea has previously 
been referred to as the chief competitor of A dies. 
LARIX AMERICANA.—This species has a very wide range of 
tolerance toward temperature, water, and soil conditions. BELL 
(2) states ‘That it has an equally thrifty growth in the country , 
to the south of James Bay and westward toward Lake Winnipeg. 
In this great region it attains its greatest perfection in the dry up- 
lands and in good soil near the rivers, but smaller trees with small 
