[Illustration: Figure 30. Forest officer ascertaining the amount of evaporation from a free water surface. Fort Valley Forest Experiment Station, Flagstaff, Arizona.]
Experimental observations which have been conducted since 1908 at the various Forest Experiment Stations have shown that the forest exercises a decided moderating influence upon temperature extremes, wind motion, and evaporation. Likewise, the presence of a forest cover retards the melting of snow in the spring, and in this way huge snowbanks in the forests feed the nearby streams until late in the summer. Forests therefore have been shown to conserve the water supply and also causing this water to run off slowly rather than in sudden floods. Studies have also been conducted on determining the effect of cutting timber upon the climate within the forest.
_Meteorological Observations._ The climatic requirements of forest types have been studied at the Fremont Experiment Station since January 1, 1910, through experimental observations, and other stations have taken up the same problem since that date. The first step in this work at the Fremont has been to obtain a complete meteorological record as a basis for determining what climatic conditions are most important in limiting the natural range of such important species as Yellow pine, Douglas fir, and Engelmann spruce. The data collected so far have shown that soil moisture and soil temperature are the controlling factors in determining the existence of the three forest types. It has also been shown what climatic conditions each of the three types of forest must have in order to succeed. This work has since been extended to include other types of forest and a meteorological station has been established at timber line on Pikes Peak. This station, which is at approximately 11,500 feet, is equipped with self-recording instruments to measure the climatic factors which obtain at that elevation and which mark the uppermost altitudinal limit of tree growth in that locality.
Such studies as these, based upon systematic meteorological observations, have an important bearing on all other forest problems. The data secured in this way especially assist the technical foresters in solving the various problems in forest management, reforestation, fire protection, and land classification, besides giving positive knowledge of the environment in which our trees live and of the factors affecting their growth and reproduction. These systematic observations are of prime importance if we ever hope to have a science of American Forestry.