204 
Journal of Agricultural Research 
Vol. XXIX, No. 4 
ment in this case was adjusted to give 
an amplification of 18, the growth 
amounts to about one-third of a milli¬ 
meter in six days. A pronounced and 
sustained swelling, apart from actual 
growth, is recorded during long periods 
of rainy weather and a corresponding 
shrinkage in dry weather. Allowance 
must be made for these movements in 
computing cur¬ 
rent growth. 
In this study 
the writer was in¬ 
terested primarily 
in determining at 
what time of the 
year diameter 
growth begins and 
ends. This is a 
matter of obvious 
importance in 
making periodic 
forest measure¬ 
ments with the 
ob j ect of ascertain- 
ingthe annual rate 
of diameter 
growth. The re¬ 
lation of current 
growth to external 
factors such as 
temperature, 
moisture, and me¬ 
chanical injuries 
offers an attract¬ 
ive field for re¬ 
search; but such 
a study would re¬ 
quire vastly more 
attention than 
could be devoted 
to it by the writer 
at this time. 
Moreover, these 
relationships are 
beinginvestigated 
by Doctor Mac- 
Dougal, of the 
Carnegie Institu¬ 
tion. 
The instru¬ 
ments were in¬ 
stalled on May 14. 
Tree No. 2 be¬ 
gan to increase slightly in diameter on 
May 16, but No. 1 showed no signs of 
increase until June 1 (fig. 2). The reason 
for . this difference is not apparent 
unless No. 2, because of its smaller 
size, was able to respond more quickly 
to the rising temperature. No. 1 
continued to grow until about August 
27, and No. 2 until about September 
10. Thus the approximate periods 
of diameter accretion were 88 days for 
the larger tree and 117 for the smaller 
one. In both trees active growth was 
practically over by September 1. 
It is of interest to compare the 
behavior of western yellow pine and 
other species in various localities of 
the Southwest. According to Mac- 
Dougal, a Douglas fir, Pseudotsuga 
taxifolia, on the slopes of Pikes Peak, 
began growth before June 17; but a 
yellow pine, Pinus ponderosa (probably 
scopulorum), in the same locality, 
made no appreciable growth until 
August. The Douglas fir continued 
to grow until August 22, and the pine 
until September 4. Pinus chihuahu- 
ensis in the Santa Catalina Mountains 
of southern Arizona showed slight 
enlargements in April and the latter 
part of May, but made the major 
growth during the summer rainy 
season in August. Pinus arizonica 
and Pinus strobiformis, in the same 
locality, exhibited slight growth in June, 
followed by a cessation until the rains 
were in progress after the middle of 
July. 
From the foregoing it appears that 
only in the case of j^ellow pine in 
northern Arizona and Douglas fir on 
Pikes Peak did active diameter growth 
begin in advance of the summer rains. 
Failure of yellow pine on Pikes Peak 
and of all the species in the Santa 
Catalina Mountains to grow in diam¬ 
eter during June is probably attribu¬ 
table to deficient moisture. This ex¬ 
planation, however, is not wholly 
adequate because in both of these 
regions, as well as in northern Arizona, 
the major height growth takes place 
prior to July 1, regardless of drought. 
Why the moisture should be insuffi- 
ceint for diameter growth when it 
apparently is sufficient for height 
growth is not clear. This problem 
requires further investigation. 
In studying growth by periodic 
measurement of standing trees, it is 
important that the trees, as far as 
diameter growth is concerned, be in a 
quiescent state at the time of each 
measurement. In Arizona and New 
Mexico, where growth plots have 
been remeasured every five years, it 
has been the custom to make these 
measurements in the fall, after Sep¬ 
tember 1, assuming that the seasonal 
growth was finished by that date. 
The present study shows that this 
assumption was essentially correct. 
Investigations in southern Arizona 
and in Colorado indicate that in those 
regions, and probably throughout New 
Mexico, the species occurring below 
the Douglas fir type may also be 
measured in the spring and early 
summer until the summer rains are 
well in progress. In northern Arizona, 
however, measurements in the spring 
should not be made after May 15. 
Growth-mmxifi 
O -&• © 
Fig. 2—Diameter growth 
of western yellow pine, 
Southwestern E x p e r i - 
mentStation,1923. Tree 
No.2 D.B.H., 5.4inch¬ 
es. Total growth dur¬ 
ing season, 4.7 mm. 
