228 JOHN ERNST WEAVER 



acterized by a very open type of Bunchgrass vegetation. 1 Pull- 

 man, Washington, the base station, where these studies were 

 carried on, lies 85 miles south of Spokane and in the midst of 

 the best developed type of prairie. 



THE FACTORS OF THE HABITAT 



Since the supply of water in this region is the chief limiting 

 factor to plant growth, we shall first consider the total amount 

 of precipitation, with its seasonal distribution (which is of greater 

 importance than the total), after which the water content of the 

 soil will be considered. 



Hemmed in on all sides by mountains, and especially cut off 

 from the moist winds of the Pacific by the Cascades, the Colum- 

 bian Plateau has a very low annual precipitation. In much of the 

 area it is less than 10 inches, and even where the prevailing 

 southwest wind, cutting through the mountain gap of the Colum- 

 bia River and rising over the great High Plains, loses much of 

 its moisture near the high eastern border, the annual precipita- 

 tion is but 21 inches. In this semi-arid region where evaporation 

 rates are very high, a knowledge of the distribution of rainfall 

 and humidity is very important, for it is well known that scanty 

 rainfall throughout the year, or relative dryness of the air and 

 soil during the growing season, favors a sparse vegetation and 

 the development of xerophytic forms. Since vegetation is not 

 only an expression of present conditions, but to a greater extent 

 a record of conditions that have obtained during a period of 

 years; and since this record is not likely to be altered greatly 

 by a year or two in which conditions may depart from the normal, 

 a study of the precipitation in table 1 is instructive. 



This table gives the mean monthly precipitation at Pullman 

 covering a period of 22 years, and is very representative of 

 conditions in the High Prairies. 



It may be seen at a glance that over two-thirds of the pre- 

 cipitation occurs during the non-growing season, and that the 



1 The writer has under preparation a paper dealing in detail with the vegeta- 

 tion of the prairies of southeastern Washington together with the plant associ- 

 ations of the adjoining mountain-woodland formation. 



