16 
and freedom from incomplete or stunted rings, such as often 
occur in areas subject to drought, was noted in all cases. This 
uniformity of ring width is presumably correlated with the fairly 
uniform mean monthly temperatures, precipitation or ground water 
being sufficient to meet the very limited demands of plant growth. 
Wind velocity at Churchill exerts a pronounced effect upon 
tree growth and, in combination with low temperatures, may be 
sufficient to explain the stunted growth of trees there and at 
other localities such as the coast of Labrador. The most frequent 
direction of wind, according to Chart 3—10 of Thomas (1953), 
is from the northwest, and the next most frequent direction is 
about equally from the north, west, and south. A transverse sec- 
tion of a black spruce from Churchill was found by Williams 
(1950) to have growth rings dating back to the year 1650. It had 
an average diameter of 10 inches, taken 56 inches above the 
ground from a tree 22 feet tall with a basal diameter of about 
14 inches. The roots rested on permafrost, and surface seepage 
provided abundant moisture during the growing season. The one 
variable was obviously temperature, but few rings were incom- 
plete and no double rings were recognized, indicating that 
thermal efficiency was adequate. On the other hand, the sweep 
of the branches and top reflected the direction of the prevailing 
westerlies, as did also the fact that the central pith was located 
off-centre at a position almost exactly one-third of the diameter 
from the side of the tree that had faced west, the growth rings 
being consistently wider on the east side. Wind velocity is un- 
doubtedly a factor to be taken account of in regions where **it 
takes two trees to make a Christmas tree.” The explanation may 
be that, although supplied with sufficient moisture for the pur- 
poses of normal transpiration, the increased transpiration on the 
exposed side as opposed to the sheltered side results in enough 
desiccation, combined with low temperature, to kill exposed 
branches, reducing the food supply to that part of the tree. 
According to Cheney and Beckel (1955), windchill, or the loss 
in kilogram calories as influenced by wind velocity, is fre- 
quently well above 2000 and as high as 2500 at Churchill when 
the temperature is below — 30°F., compared with average wind- 
chills in January of 780, 1200, and 1450 on airfields at Vancou- 
ver, Ottawa, and Winnipeg, respectively. 
