THE PRAIRIES OF THE PACIFIC NORTHWEST 31 



III. THE PRAIRIES OF THE PACIFIC NORTHWEST. 



The prairies of southeastern Washington and adjacent Idaho, 

 where the following studies were carried on, represent an extreme 

 westward extension of the great grassland formation lying east of the 

 Rocky Mountains. Agropyrum spicatum, Festuca ovina ingrata, 

 Kceleria cristata, and Poa sandbergii are dominants. Stipa is entirely 

 absent, as are also the late-blooming grasses, such as Andropogon, 

 Bouteloua, and others of the eastern prairie. The absence of these 

 late-maturing grasses may be accounted for by the peculiar distribu- 

 tion of the precipitation. Only about one-fourth of the annual 21 

 inches of moisture falls during the growing season. Except for the 

 extremely retentive silt-loam soil, the region would be almost a desert. 

 This is an extremely important fact to keep in mind while consider- 

 ing root systems. The important role played in the eastern prairies 

 by Andropogon scoparius is here taken by Agropyrum spicatum, its 

 ecological representative. Like Andropogon, it presents the bunch 

 habit in drier soils, but becomes a sod-former with well-developed 

 rhizomes under more favorable moisture conditions. Eastward, these 

 prairies adjoin the main area of grassland lying east of the Rocky 

 Mountains. At Missoula, Montana, for example, the Agropyrum- 

 Festuca community of Washington and Idaho (Weaver, 1917) meets 

 and intermingles with such eastern prairie species as Stipa comata, 

 Aster multiflorus, etc., while Agropyrum spicatum, Kceleria cristata, 

 Solidago missouriensis, Achillea millefolium, and others occur through- 

 out both regions. 



The writer has presented elsewhere (1917) a detailed description of 

 the extent, successional relations, and floristic composition of these 

 western prairies. A glance at plate 17, b and c, may give some impres- 

 sion as to their general character. Hence we will proceed at once to a 

 discussion of root distribution. 



THE ROOT SYSTEMS OF THE GRASSES. 



Over 60 individuals of the four dominant grasses were excavated 

 and examined. Three, Koeleria cristata, Poa sandbergii, and Festuca 

 ovina ingrata, are shallow-rooted, the bulk of the absorbing system 

 lying above the 18-inch level, while Agropyrum spicatum penetrates to 

 a maximum depth of 4 feet 10 inches. 



Agropyrum spicatum. — This is the dominant bunchgrass in eastern Wash- 

 ington. It has its best development westward of the high upland prairies 

 of extreme eastern Washington and along the rim-rock through the eastern 

 part. The bunches are often 10 inches in diameter and reach a height of over 

 3 feet. The plant blossoms in June and dries out in early July, only to take 

 on renewed growth after the autumn rains and to remain green all winter. 



This grass has coarser roots than any of the other three important native 

 grasses. These coarse, fibrous roots have many short laterals. Some of the 



