FOREST AREAS AND SOIE WATER 
145 
found the average daily evaporation to be 19.0 c.c. on north slopes 
as compared with 27.7 c.c. for south slopes. 
Summary 
The results obtained from the present studies in northern Iowa 
may be summarized briefly as follows : 
Forest cover exerts a direct influence in checking evaporation. 
There is a regular decrease in the rate of evaporation with the 
succession from open to brushy sites, and in turn to timbered 
sites. 
The evaporation is uniformly greater at the top of the slopes 
than at the middle for all the sites. 
The evaporation is greater on south than .north slopes, except 
as influenced by the direction of the wind. 
Topography and wind direction, therefore, both have a direct 
influence on the evaporation rate. 
Forested areas have a distinct effect in equalizing the rate of 
evaporation as shown by the low minimum and maximum rates 
and the small margin or range of fluctuation throughout the 
season in the timbered area. 
SOIE MOISTURE 
Relation to Peants and Peant Associations 
It has long been recognized that the amount of moisture in 
the soil is an important factor in all plant growth and distribu- 
tion. Fuller,^® Weaver, Weaver and Thiel, and other writers 
have shown that soil moisture content bears a direct relation to 
the type of vegetation found in various regions and to the suc- 
cession of plant associations. 
On the other hand, it is recognized that all the moisture present 
in the soil is not available for plant growth, and that plants wilt 
before the entire moisture content becomes exhausted. In work 
by Clements,^® Livingston,^® Briggs and McLane,^^ and Briggs 
and Shantz,^^ the point at which this wilting occurs is shown 
further to be by no means constant for the various soil types. 
Briggs and McLane compared the moisture retaining power of 
16 Fuller, G. D., Bot. Gaz., 58, 213-233. 
17 Weaver, J. F., Univ. Studies, Univ. Nebraska, XVII, No. 1, 21-37. 
18 Weaver, J. F., and A. F. Thiel, Bot. Sur. Nebraska, New Series, No. 1, 15-26. 
19 Clements, F. F., Research methods in ecology, p. 334, 1905. 
20 Livingston, B. F., The relation of desert plants to soil moisture and to evapora- 
tion: Carnegie Institution of Washington, Publ. No. 50, p. 78, 1906. 
21 Briggs, F. J., and J. W. McFane, The moisture equivalent of soils: U. S. Dept. 
Agric., Bur. Soils, Bull. 45, 1907. 
22 Briggs, F. J., and H. F- Shantz, The wilting coefficient for different plants and 
its indirect determination: U. S. Dept. Agric., Bur. Plant Ind., Bull. 230, 1912; Bot. 
Gaz., 51 , 210-219, 1911; and 53 , 20-37, 229-235, 1912. 
