Apr. 14,1933 Physiological Requirements of Rocky Mountain Trees 101 
have, in general, been employed, the summarizing of data having been 
started at that time. However, some records particularly needed have 
been added since 1918. The period of operation, as well as the equip¬ 
ment of the several other stations, is given under the respective descrip¬ 
tions. 
method of study 
As has already been indicated, the primary data to be presented in 
this study are records of climatic and soil conditions in different forest 
types, and the main object of such presentations must be to show that 
differences in climatic and soil conditions, between the forest types, 
either do or do not exist in sufficient degree to account for the varying 
phenomena of occurrence and growth. 
Since the special data collected by the Forest Service in no case cover 
periods long enough td establish the ''normal” conditions of any of the 
forest types (and by this we mean the average conditions for a period 
of at least 10 years), and since even "normal” conditions as established 
by 10 years of observation are certain always to be changed by the 
addition of further records, it is necessary that we adopt short-period 
records as a basis for comparison. Such adoption can not be fraught 
with any serious dangers when the forest types to be compared are in 
the same general locality, that is, in the path of the same air currents, 
storm centers, etc. Any considerable separation of the stations, how¬ 
ever, especially in a rugged mountainous region, is likely to introduce 
temporary variations in certain conditions which are not "normal,” and 
particularly in those factors which are most directly influenced by the 
paths of storm centers. Thus, at the moment of writing this statement, 
it appears that the storm centers have for some time been passing con¬ 
siderably to the north of the Pikes Peak region, giving that locality 
rather unusual westerly winds and leaving it with a dearth of moisture, 
so that at the end of May an unprecedented shortage of water exists. 
Scarcely 100 miles to the north, unusually large accumulations of snow 
are reported at the same moment. Again, the moisture factor is most 
variably influenced by the restricted character of many of the summer 
showers; especially are the heaviest downpours in a given locality likely 
to affect only a very small area. 
Temperatures are generally not so directly affected by local conditions. 
Thus the month of December, 1917, was not only an unusually warm 
month at the Fremont Station but showed the same character over a 
large part of the western United States, and January, 1918, was, like¬ 
wise, generally cold to an unusual degree. 
We may, therefore, feel safe in comparing the records of any two 
near-by stations for short periods, whatever the factor under considera¬ 
tion, and we shall demand an increasing period as the distance between 
stations increases. 
Fortunately, the dozen or more stations located in the vicinity of 
Fremont all come under the same general, influences. This is true of 
the entire area from the plains to the summit of Pikes Peak, with the 
exception that summer rains frequently fall in one part of the area 
without wetting other parts. Winter snows may also be so localized, 
but usually in conformity with altitudinal zones. It is true, two stations 
not 100 yards apart may on a given day have temperatures varying by 
a couple of degrees in one direction and on the next day varying in the 
