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Voi/. XXIV Washington, D. C., Aprii. 14, 1923 No. 2 
PHYSIOLOGICAL REQUIREMENTS OF ROCKY 
MOUNTAIN TREES' 
By CAR1.0S G. Bates ^ 
Silviculturistf Forest Service, United States Department of Agriculture 
I. INTRODUCTION 
The most casual observer ascending any mountain range in the western 
United States can hardly fail to be impressed with the fact that there is a 
sharp line of demarcation between the grassy plains at the base of the 
mountain and the wooded slopes of the mountain proper. As one ascends 
farther, gradual or sometimes very abrupt changes are noted in the char¬ 
acter of the forest cover. With increase in altitude the forest generally 
becomes more dense, trees of greater stature are observed, and those who 
are able to distinguish note the occurrence of new species in each alti¬ 
tudinal zone. To a lesser degree the same differences in the forest cover 
may be noted on opposing slopes at the same elevation—that is, slopes 
facing the south bear forests similar to those at lower elevation, while 
those facing the north have the character of higher altitudes. The 
vegetation of ridges is always different from that of ravines at the same 
altitudes. 
The laymen will recognize that these differences in the forest cover are 
the result of different “growing conditions'* at different elevations, just 
as a person who had lived in the South would recognize, almost instinc¬ 
tively, that the growing conditions of the mountain valleys of Colorado 
could not possibly be suited to the cultivation of com or cotton. 
While the intensive study of the relations of plants to the soil and 
climatic conditions which comprise their environment goes under the 
formidable name “ecology," ecological knowledge is not confined to 
scientists and, in fact, has been common property for ages. The writer 
has had opportunity to observe the first impressions of a great many 
people who were visiting the western mountains for the first time, and 
has been impressed by the amount of lo^c exhibited in relating cause 
and effect in the matter of forest distribution. Why does this southerly 
exposure bear an open, scrubby forest of yellow pine, and that northerly 
exposure, directly opposite, bear a much more dense stand of vigorously 
growing firs? Often the people who know no botany and much less 
ecology take in the situation at a glance, at least so far as it is possible 
to do so from superficial evidence. 
To the forester such questions are of the utmost practical importance. 
Not only is it the forester’s business to know the trees with which he 
^Acwpted for publication July 6, 1921. . , , j i. r »i 
* This project has been under the direction of the writer since its inception in 1910, and he assumes full 
responsibility for the quality of the work done and for the conclusions deduced from the data. It is, how- 
evCT a pleasure to acknowledge the great efforts which have been required of a number of observers in the 
accumulation of the records. The records furnished by the Weather Bur^u are duly acknowledged and 
the efforts of all those numerous and changing observers who have created these records. The records of the 
Waeon Wheel Gap and Fremont Experiment Stations are the result of the concerted effort of many regular 
obs^ers all of whom deserve credit. The original installation of instruments at the Wagon Wheel G^ 
Experim^t Station was made, and the records were obtained for two years, under the direction of B. C. 
Kadel, of the Weather Bmeau. 
Journal of Agricultural Research, 
Washington, D. C. 
aav 
Vol. XXIV. No. 3 
Apr. 14, 1923 
Key No. F-8 
30616—23-i 
(97) 
