

a = 
THE TREE. | 7 
feet the species will thrive with as little as 16.6 inches. In general, 
however, the indications are that no stand of large extent is likely 
where the annual rainfall is less than 20 inches. In Taos Canyon, 
with only 12.81 inches of rainfall, there are scattered individuals, but 
the presence of even these few trees is probably due to the moisture 
and shade from the canyon walls. 
It is the variability of annual precipitation in the Southwest, 
coupled with drying winds, which make conditions for tree growth 
particularly trying. Droughts are periodic and severe, and un- 
doubtedly impair the vitality of mature yellow pine and curtail the 
local distribution of the species. At Prescott, near which are com- 
mercial stands of yellow pine, the average annual rainfall for 35 
years was 17.4 inches. Yet during that period there were 7 years 
with less than 12 inches, and two periods of 4 years each with an 
average annual rainfall of but 11.5 and 13.3 inches, respectively. 
Increase in altitude brings, as a rule, increase in rainfall and 
climatic conditions in general more favorable to tree growth. 
Western yellow pine, however, is seldom found in pure stands of any 
extent above 8,500 feet, since it can not compete with the shade- 
enduring species which grow at the higher altitudes. It is present, 
in mixture with Douglas fir, at 10,300 feet, on the Gila National 
Forest. At the other extreme, it is not found ordinarily in any 
quantity below 6,500 feet, though in the sheltered beds of canyons 
and on favorable north slopes it grows at altitudes as low as 6,000 
feet, and at one place at least,-along Oak Creek in the Coconino 
National Forest, is abundant at 5,300 feet. Its presence here, how- 
ever, 1s due to the moisture and partial shade from the canyon walls. 
The heaviest extensive stands of yellow pine in Arizona and New 
Mexico are on the comparatively moist, rolling plateaus at elevations 
of from 7,200 to 7,800 feet. Probably the heaviest stand, though of 
small extent, in the two States, is on a well-watered flat north of 
Bellemont, on the Tusayan National Forest, at an elevation of 7,400 
feet. Here 2 acres of unusually tall, clean-boled timbers scaled 
72,000 board feet. 
From April to July the heavy and continuous southwest winds, 
which lessen the air moisture, increase evaporation, and bake the 
surface of the soil, are particularly trying to tree growth. Even at 
Flagstaff, where the climatic conditions are considered favorable, the 
average wind velocity for the past five years has been 7 miles per 
hour, and the average humidity for the same period, 62 per cent. 
SIZE AND LONGEVITY. 
Yellow pine in the Southwest does not attain large size. Seven 
average logs are often necessary to make a thousand board feet, and 
on one section in the Tusayan National Forest the average was 10 
