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 ground 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 only 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, . 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., i : 1-60, 1917.) 



37 



