THE GROWING SEASON OF WESTERN YELLOW PINE 1 
By G. A. Pearson 
Director , Southwestern Forest Experiment Station , Forest Service , United States 
Department of Agriculture 
Observations at the Southwestern 
Forest Experiment Station, at Flag¬ 
staff, Ariz., over a period of 14 years 
have shown that the shoots of western 
yellow pine, Pinus scopulorum, begin 
to grow about the middle of May, 
elongate most rapidly in June, and prac¬ 
tically finish their growth by July 1. 
The major height growth occurs during 
the driest month of the year, although 
the lower soil strata are always moist 
from the heavy winter precipitation. 
Curiously enougli, shoot growth is not 
resumed during the summer rainy sea¬ 
son except occasionally in very vigorous 
seedlings. The needles continue to 
grow well into August. Does diameter 
growth take place at the same time as 
height growth or later? This, unlike 
the elongation of stems and needles, is 
not subject to ocular observation. 
During the summer of 1923 records 
of the diameter growth of two western 
yellow pines were maintained at the 
Southwestern Forest Experiment Sta¬ 
tion by means of MacDougahs den- 
drograph. Both of the trees are young 
and thrifty, one being 5.4 inches and 
the other 15.8 inches in diameter. 
The larger tree (No. 1) is in the edge 
of a group on a moderate slope with 
southeasterly exposure; the smaller one 
(No. 2) is more isolated and stands 
in a practically level situation near the 
north brow of a level bench about 500 
feet distant. 
For the benefit of those not familiar 
with the MacDougal dendrograph 2 
its essential features are here briefly 
described without going into mechanical 
details. The instrument is held in 
place by a belt of wooden blocks 
drawn tightly around the trunk of the 
tree. Above this, supported by stiff 
wires, is a floating frame of invar , a 
metal of low temperature coefficient, 
encircling the trunk, but touching 
it only at two points. One of these 
points is an adjustable screw; the 
other, on the opposite side of the tree, 
is a quartz rod which slides freely in 
and out with every movement of the 
bark. On the outer end of this rod 
rests the short arm of an L-shaped 
lever whose long arm carries a pen. 
As the tree expands the pen is 
raised and as it contracts (a daily 
occurrence) the pen falls. The pen, 
in contact with a ruled sheet of paper 
on a revolving drum driven by clock¬ 
work, makes a continuous record of the 
most minute expansion or contraction 
of the tree. 
Figure 1 shows the graph produced 
during a week of rapid growth. The 
strong diurnal movement does not 
reflect actual growth, but rather a 
swelling and shrinking which MacDou¬ 
gal has found to be intimately associ¬ 
ated with the water content of the 
trunk. It will be noted that the max¬ 
imum rise occurs at night or early in 
the morning, when transpiration is low. 
Actual growth or deposition of wood is 
indicated by the general upward trend 
of the graph. Thus, on June 4, the 
high point reads 31 mm. and on June 
11, it reads 37 mm. Since the instru- 
1 Received for publication June 30, 1924—issued January, 1925. 
2 MacDougal, D. T.— growth in trees. 41 p., illus. Washington, D. C. 1921. (Carnegie Inst., 
Wash., Pub. 307.) 
J ournal of Agricultural Research, 
Washington, D. C. 
(203) 
Vol. XXIX, No. 4 
Aug. 15, 1924 
Key No. F—20 
