1918] HUTCHINSON—FOREST TREES 467 
Temperature factor 
The northern limits of many tree species are undoubtedly the 
result of low temperatures. WARMING (24) states, “It is clear that 
conditions as regards heat determine the boundaries of the dis- 
tribution of species on the earth.” The effect of temperature is 
emphasized by the fact that “the appropriate temperature for the 
growth of a number of species, such as Picea and Abies, is carried 
far to the south of their normal latitudes along the elevated parts of 
the continent, especially the Alleghanies and Rocky Mountains” 
(BELL 2). In such regions the tree species are in most cases 
identical with those found farther north. However, it is more diffi- 
cult to account for the southern limits of trees on a basis of minimum 
temperature. Bray (5) finds difficulty in explaining the occurrence 
of boreal (Picea, Abies) associations in the bogs of regions domi- 
nantly austral. ‘‘The question arises as to whether the factor of 
temperature plays a réle in the occurrence of these bogs,” 
again, “the extremely irregular boundary between the boreal co- 
nifer forests and the temperate hardwood forests of New Eng- 
land, for example, can hardly be explained by temperature alone”’ 
(itkeees 14). 
The lines representing the limits of Picea nigra, Larix americana, 
and Betula papyrifera follow yearly isotherms very closely from the 
mouth of the Mackenzie River across the continent until they reach 
the coast of Labrador, where they swing southward, here following 
a course almost parallel with the coast line. There is reason to 
believe that temperature is the limiting factor throughout a great 
area, while a second factor is active along the Labrador coast. 
From the fact that the same order in the limitation of these species 
is retained, even in the Labrador region, it would seem that the 
limiting factors are similar throughout. Excessive loss of heat 
energy due to the air currents so prevalent in this region has the 
same effect as the loss of heat energy due to excessively low tempera- 
tures. Similarly in southern Ontario, where latitude and lake 
influence together result in a region of a relatively high yearly 
temperature average, the limits of trees such as Juglans nigra and 
Castanea dentata are parallel with isotherms. Here also the evi- 
dence would indicate that temperature is the limiting factor with 
