FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 139 



Precipitation measurements are obviously unsatisfactory for the 

 close analysis of contiguous forest types, since these may receive 

 equal amounts and yet be radically at variance in their moisture 

 conditions. With the precipitation known, measurements of evapora- 

 tion near the ground should give a very good idea of the extent to 

 which the moisture supply is dissipated, or conserved for the uses of 

 vegetation. It has been shown that the higher forest types have 

 somewhat less evaporation than those at low elevations, for the 

 year as a whole, and that northerly exposures are much less rapidly 

 dried out by insolation, than those which receive the sun's rays more 

 directly. 



The measure of the moisture of the soil gives most directly a 

 knowledge of the supply available for the trees of any forest type. 

 It has been shown, with the limited number of stations at which this 

 measure has been taken, that the absolute amounts of moisture in 

 the soil, during either the average growing season, or the driest part 

 of an unusually dry season, are commonly greatest in the spruce sites 

 and the north-slope Douglas fir sites. The yellow pine, limber pine, 

 and lodgepole pine sites have considerably less. Only on strong 

 southerly exposures, however, and near the lower limit of the yellow 

 pine type, where good precipitation is experienced during the grow- 

 ing season, is the depletion of the supply very considerable, and even 

 in these situations the wilting coefficients of the respective soils have 

 not been closely approached at any time during the period of observa- 

 tions. In fact, the higher moisture percentages of the spruce and 

 fir soils do not indicate much more favorable conditions for growth 

 than the lower percentages in the pine types, because the soils of the 

 latter class are always more sandy and hence such moisture as they 

 retain is relatively more available for plants. 



Nevertheless, the differences between the types in soil moisture 

 content have a significant bearing. The fact that the minima 

 reached in the various types are so similar in physiological value is 

 in itself evidence that the density of the stand or number of individ- 

 uals per unit area tends always to adjust itself to the average amount 

 of moisture present, or perhaps even more closely to the absolute 

 minimum amount reached during a long period of years. Therefore, 

 even if the several species showed no differences in their moisture 

 requirements or absolute drought resistance, the moisture content of 

 the soil would strongly influence the character of each forest type 

 by limiting the density «f the stand, and thereby affecting the 

 insolation, the evaporation, the soil temperature, and all other con- 

 tions of the forest floor, which in turn influence the character of the 

 reproduction. This only shows how closely interrelated are the 

 several conditions of the environment. The amount of insolation 

 falling upon a site affects the conservation of the precipitation. 

 The conserved water, by determining the density of the stand in 

 turn, affects the amount of insolation, and so on, ad infinitum. 



The important difference between the moisture conditions of the 

 spruce sites at one extreme and the pine sites at the other is not so 

 much in their absolute water contents, considering the entire soil 

 reservoir of each as in the fluctuations of their surface conditions. 

 The well-shaded spruce soil may become very dry at the surface 

 when the atmosphere has been dry for a long time, but it does not 

 dry out severely between the rather frequent summer showers. 



