1918] HUTCHINSON—FOREST TREES 485 
ABIES BALSAMEA.—Physical factors‘ Moisture and tempera- 
ture are the main factors influencing the distribution of Abies 
balsamea; it requires a cold climate and a constant supply of mois- 
ture at its roots; a mean annual temperature not exceeding 40° F. 
with an average summer temperature of not more than 70° F. and 
a mean precipitation not less than 25 inches evenly distributed 
throughout the year are the necessary conditions for its growth” 
ZON (27). The maximum of its range of temperature tolerance is 
high, very closely approximating that of Picea; the minimum is 
lower than has generally been conceded, other factors having prac- 
tically eliminated it from the warmer regions of its normal tempera- 
ture range. While Abies balsamea normally has a wide water 
range, it seldom thrives except in a moist soil because this hin- 
ders the growth of a fungus which in a drier soil’attacks the roots 
(“ground rot’). Southward Abies balsamea “attains its best 
growth and largest sizes on flats the soil of which is usually a moist 
deep sand loam” (Zon 27), while ‘“‘southwest of Hudson Bay it 
grows only in the warmest and best soils and is entirely wanting in 
the cold swampy tracts” (Low 18). A. balsamea demands com- 
paratively high light intensity; seedlings are seldom found except 
in clearings caused by windfall, or otherwise. Generally, Abies 
has a wide range of tolerance. 
Competition factor—Northward Picea is the chief competitor 
of Abies; their ranges of tolerance are similar, the maxima and 
minima of Picea generally being more extreme; consequently 
Abies under most conditions would be secondary were it not for 
the fact that near the mean of their ranges Abies grows more rapidly 
than Picea. Southward the chief competitors are Acer and Tsuga. 
These forms have the advantage of being more shade tolerant, 
and hence they gradually encroach upon and finally exterminate 
Abies, which demands greater light intensity (fig. 7). 
Time factor —The northward migration of Abies balsamea is 
conditioned by temperature, and since the magnitude of tempera- 
ture changes is dependent upon time, it is evident that the time 
factor has a bearing upon distribution. The distribution south- 
ward is affected by competition of such forms as Acer and Tsuga. 
Time is necessary for the environmental changes which produce 
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