PLANT COMMUNITIES OF SANDY SOIL 319 



The herbaceous plants of the shrub association are not numer- 

 ous and no one of them is especially common or characteristic. 

 Agropyron smithii is somewhat abundant; some of the other 

 species are Tithymalus robustus, Lepidium diver si florum, Peri- 

 toma serrulatum, Eriocoma hymenioides, Sophia incisa. In many 

 parts of the shrub association there are no herbs at all in the 

 spaces between the shrubs. 



Probably the high temperature at the soil surface greatly 

 limits the possibilities of establishment of shallow-rooted forms. 

 On September 1, 1919 a thermometer pressed into the sand 

 horizontally just below the surface showed a temperature of 

 40°C. At a depth of 2 dm. the temperature was 24°C. These 

 readings were taken at 10:00 a.m. with the sun shining directly 

 on the place where the observations were made. 



b. Wheat-grass association. This association of Agropyron 

 smithii is well known on the plains (2, 9) , especially in Wyoming 

 and Montana where it covers large areas. In Colorado it is 

 represented chiefly by a strip running north and south along the 

 mountain front and by isolated patches on the plains and in 

 valleys of the lower foothills. Commonly it serves as an indi- 

 cator of excellent soil for agriculture. In our present area of 

 study the wheat grass grows chiefly in rather loose sand, becoming 

 replaced by short grass as the soil gains in compactness. It does 

 not develop here a close stand, there being always a large amount 

 of bare ground. Wheat grass is a good soil binder behaving 

 much as beach grass on sand dunes of ocean shores. To one who 

 knows only the typical wheat-grass growth at the mountain 

 front this activity as a sand binder seems quite a new role. The 

 writer is familiar, however, with a similar behavior of the species 

 in sandy soil of the plains at Globeville, near Denver, and at 

 Wliite Rocks a few miles east of Boulder. 



Very few subordinate species are present with the wheat grass. 

 Most frequent are Tithymalus robustus, Anogra albicaulis, Oreo- 

 carya virgata, and Chrysopsis viscida. Often a square meter 

 quadrat has no plants other than Agropyron. If the soil has an 

 admixture of disintegrated granite there may be some Stipa and 

 perhaps various flowering herbs in small numbers, as Geranium 



