lOO 
Journal of Agricultural Research 
Vol. XXIV, No. s 
therefrom the full reasons for the presence or absence of a given species 
on the various sites were so far from satisfactory that the need for con¬ 
trolled experiments to indicate the comparative reactions of the several 
species has long been apparent. The amount accomplished in this 
line is not insignificant, and, since the results aid very materially in 
interpreting the field conditions, we have chosen to present, first, all of 
the physiological data that are available. In other words, we shall first 
consider the manner in which the several species are affected by more 
or less controlled conditions in the laboratory or greenhouse; then, per¬ 
haps, we may weigh more precisely the importance of each condition 
encountered in the field, and especially avoid the almost universal and 
inevitable error of placing reliance upon the measurement of a condition 
which has not really been measured or expressed in the proper terms. 
Of the meteorological records which are used in this report the larger 
share have been obtained by the Forest Service at the Fremont Experi¬ 
ment Station, near Manitou, Colo., and by the Weather Bureau and 
Forest Service, cooperating in an intensive project at the Wagon Wheel 
Gap Experiment Station, near Wagon Wheel Gap, Colo. It has been the 
aim to conduct the meteorological work at both these stations on a high 
plane of accuracy, with equipment as complete as was thought necessary. 
At both places the observations have covered elevations from the station 
headquarters at about 9,000 feet to timber line at 11,000 or 12,000 feet. 
In both cases, too, soil moisture and soil temperature data have been 
secured during most of each period of atmospheric observations. 
In addition, the Forest Service has equipped and operated from the 
Fremont Station three subsidiary stations at which the Weather Bureau 
had previously started the usual observations of precipitation and air 
temperature. These stations were manned by forest officers, and a 
very diligent effort has been made to obtain continuous records for them. 
The stations in question are at the Monument Nursery, in the yellow pine 
type near Monument, Colo., at the Foxpark Ranger Station, lodgepole 
pine type, at Foxpark, Wyo., and in the Nebraska sand hills, where the 
planting of yellow pine and jack pine has reached such large and successful 
proportions. The first station is only about 20 miles north of the Fremont 
Station and for all practical purposes may be considered as belonging in 
the Pikes Peak altitudinal series. 
Although the Weather Bureau records of precipitation and air tem¬ 
perature, secured through cooperative observers, were available for a 
large number of stations in the region covered by this report, and many 
sudi stations are within the mountain forest zone, only limited use has 
been made of such records, and for the following reasons: 
(а) Many such observation points are situated, with the small towns, 
in deep valleys where the conditions met with, especially those of air 
drainage and soil composition, are not at all the conditions of the adja¬ 
cent forested slopes. In fact, nearly all such valleys, especially if they 
possess a distinct flood plain, are devoid of forest cover, and to use their 
weather records in this study might lead to very erroneous conceptions 
though the temperature conditions alone are probably not responsible 
for the absence of forests. 
(б) The records obtained at all such mountain stations are only those 
of precipitation and air temperature. No soil data whatever are secured. 
The special work in this project dates from January i, 1910, when the 
control station at Fremont was equipped. Records obtained up to 1918 
