Southeastern Washington and Adjacent Idaho. 37 
affecting the establishment of the different plant communities. 
The progressive increase of the humidity of the habitat causes 
a corresponding increase in the mesophytism of the plant associa- 
tions. This change of plant population from the xerophytic to 
- the mesophytic type is a phenomenon called succession. 
The evaporation rates and the amount of soil moisture in the 
various communities vary in general directly with the order of 
their occurrence in the succession, the climax community being 
the most mesophytic in both respects. 
SOIL TEMPERATURE IN RELATION TO SUCCESSION 
The temperature of the soil gives in a general way a summation 
of the heat factors of the habitat. It combines the effects of 
direct insolation and radiation, cooling by the retention of snow 
or by the evaporation of moisture, and variations in temperature 
caused by the passage of currents of air. Since plant communi- 
ties react directly on temperature, a large number of soil tempera- 
ture readings were made in the various plant communities to de- 
tain weeks in mid-summer, even in well developed prairie, reaching 64 cc. 
(Gates highest evaporation rate on open grofind for a single week is 21.6 
cc. per day.) 
Students of succession should keep in mind that the evaporating power 
of the air affects the possibility of water intake as well as water outgo. 
High rates of evaporation rapidly deplete the surface soil of water as well 
as desiccate the plant. Mesophytes can not establish themselves under such 
conditions. It is onfy after a cover of xerophytes cuts down the great 
evaporating power of the air in the stratum in which seedlings develop, 
by shading and by inhibiting wind movement that mesophytic plants. can 
establish themselves. Mesophytes do not replace xerophytes in succession 
until the reaction of the latter on the habitat is such as to make possible 
the requisite conditions of soil and air humidity, 7. e., a condition where 
sufficient water is available in the soil and air, so that transpiration may 
not exceed absorption. That evaporation is further reduced by the meso- 
phytes is repeatedly shown in the preceding data. Conditions for plant 
life are then less severe and even more mesophytic plants can establish 
themselves and become dominant. 
This concept is in agreement with Fuller’s conclusion, which he reached 
after several years of study in the Chicago region (5). Further evidence 
is brought forward in a recent paper dealing with the prairie-forest prob- 
lem in Minnesota and Nebraska. (Weaver, J. E., and Thiel, A. F. The 
Bot. Surv. Nebr., N. S., 1: 1-60, 1917.) 
37 
