UTAH ACADEMY OF SCIENCES ~~" 117 
seasons the evaporating power of the air has been meas- 
ured by means of the Livingston porous cup atmometer 
and soil samples were taken at ten-day intervals to show 
the march of soil moisture on the five important sites 
under study: 
1 Manzanita ( Arctostaphyllos platyphylia ) associa- 
tion. 
2 Wild apple ( Peraphyllum ramosissimum ) associa- 
tion. 
8 Sagebrush ( Artemisia tridentata) flat. 
4 Sagebrush flat, denuded of all vegetation. 
5 Oakbrush ( Quercus gambelli) association. 
Empirical plantations of 2-1 western yellow pine 
(Pinus ponderosa ) transplants were also established on 
each site. 
The march of the evaporating power of the air and of 
the soil moisture were illustrated graphically. The suc- 
cess of the western yellow pine plantations does not 
correspond very closely with the evaporation intensity be- 
cause the latter does not take into account the question 
of the available soil moisture. The results secured to 
date would indicate that a satisfactory expression of 
these factors could be indicated by the ratio of evapora- 
tion to soil moisture. The evaporating power of the air 
and the available soil moisture appear to be the chief 
physical factors in limiting the growth and development 
of plants in the chaparral zone. 
In order to obviate the necessity of securing detailed 
instrumental records on every site considered for forest 
planting the native plants indicative of a favorable site 
should be determined in the course of the correlation of 
the important physical factors with plant growth. From 
present indications those plants having deep roots, good 
crowns, affording some shade but not dense enough to 
cause suppression and which have relatively high trans- 
piration rates should indicate the most favorable sites. 
Forest extension in the chaparral belt should be limited 
to those lands having an assured supply of available soil 
moisture and which are protected from excessive evap- 
oration either because of their topographic position or 
by the presence of a vegetative cover composed of a 
goodly representation of favorable site indicators. 
