22 A Study of the Vegetation of 



orating power of the air have been carried on in the various plant 

 communities and a relation has been established between the mois- 

 ture conditions and plant succession. 



Livingston's standardized porous cup atmometers were used to 

 determine the rate of the evaporating power of the air. They 

 were always placed with the evaporating surfaces 1723 centi- 

 meters above the surface of the soil, care being taken in all cases 

 to select spots which possessed the average amount of herbaceous, 

 shrubby, or tree vegetation characteristic of the community as a 

 whole. These cups wer.e almost invariably run in duplicate, 

 sometimes in triplicate; they were frequently checked and re- 

 standardized, and the readings reduced to those of the standard 

 atmometer. In the following graphs, the ordinates give the aver- 

 age daily evaporation during the intervals indicated by the ab- 

 scissas. A preliminary report giving a detailed statement of a 

 part of these results has already been published (16). 



Soil moisture determinations were made at the same stations 

 and at the same time that evaporation readings were taken. In 

 making the determinations, samples of 100-150 grams were in- 

 variably taken in duplicate and from separate holes about 3 feet 

 apart. These samples consisted of a core of soil to a depth of 10 

 inches. They were dried in an oven at a temperature of 100- 

 105 C. and the water content calculated in percentages on the 

 basis of the dry weight. 



In the following figures, the results are represented graphically, 

 the wilting coefficients being shown by lines of the same type as 

 the line showing the range of soil moisture for a particular com- 

 munity. The ordinates represent the percentages of soil mois- 

 ture and the abscissas indicate the intervals between readings. 

 The intervals between the graphs and the lines denoting the wilt- 

 ing coefficients (if these exist above the latter) give the amount 

 of available water. 



During the summer of 1913 a group of stations were estab- 

 lished near Colfax, Washington. Here the south fork of the 

 Palouse River has cut a canyon in the basalt to a depth of 300 

 feet. Along the north bank of this canyon the Agropyron -con- 

 sociation is well developed. The sheltered south bank is covered 



22 



