EVAPORATION AND PLANT SUCCESSION IN SOUTH- 

 EASTERN WASHINGTON AND ADJACENT 



IDAHO 



JOHN ERNST WEAVER 



State College of Washington, Pullman, Washington 



During the summer of 1913, while a study of the ecological 

 conditions in Southeastern Washington and adjacent Idaho 

 was being made, measurements of the physical factors of the 

 habitat, including the evaporating power of the air, were made 

 in many of the more important plant associations. It is the 

 purpose of this preliminary paper to point out briefly the 

 sequence of succession, and to give the comparative rates of 

 evaporation in each of the major associations. A detailed dis- 

 cussion of the development and structure of the various stages 

 in the succession has been reserved for another time. 



The physiography and geology of the region in wh'ch these 

 investigations were carried on will be outlined, since both play 

 an important role in influencing plant distribution. Pullman, 

 the base station, in Whitman County, Washington, lies near 

 the eastern edge of the great lava sheet which forms the Colum- 

 bia Plains. Fifteen miles eastward, in Idaho, the lava rests 

 upon the flanks of a low range of mountains known as Thatuna 

 Hills. From these mountains two chains of buttes extend 

 westward into the lava, each for a distance of about eighteen 

 miles. These, with the mountains, enclose a peninsula of the 

 igneous rock approximately two hundred square miles in area. 

 The base station occupies a position near the neck of this penin- 

 sula. The enclosed area, and that extending northwestward 

 for fourteen miles, together with the buttes and mountains, 

 comprise the region under consideration. It has an average 

 altitude (exclusive of the mountains) of about 765 m., and lies 

 mostly in the life area knowii as the Arid Transition. Thatuna 



. 273 



THE Pr.ANT WORLD, VOL. 17, NO. 10, 1914 



