216 CLIMAX FORMATIONS OF NORTH AMERICA. 



The blow-outs may aJso pass directly into bunch-grass, or indirectly, 

 through the blow-sand stage. The sand prairies are then invaded by the 

 black-jack oak (Quercus marylandica), with which usually occur Q. velviina 

 and Hicoria microcarpa, and the shrubs Rhus aromatica, Amorpha canescens, 

 and Salix tristis. Finally, the black-jack may yield to more mesophytic 

 forests of Quercus macrocarpa and Q. alba. 



Gleason (1910) has made a comprehensive study of succession on the inland 

 sand deposits of lUinois, in which prairie and forest are the climax stages. An 

 adequate abstract of his account is impossible here, but the developmental 

 relations are shown by the diagram (fig. 7) on page 215. (133) 



Shantz (1911 : 62) has summarized the development of the three primary 

 adseres which end in the BuUnlis-Bouleloua climax of Colorado and the Great 

 Plains generally: 



"All the associations thus far described are stages in one or the other of 

 two great successions of vegetation. One of these begins with the appearance 

 of lichens on the rock outcrops and ends with the full establishment of a pure 

 short-grass cover. The other, beginning with bare sand on which plants of 

 the blow-out association first appear, leads to the establishment of the sand- 

 hills mixed association or of the bunch-grass association, and from either of 

 these may pass on through the wire-grass asssociation to a pure short-grass 

 cover. 



"In the first succession the lichens become established on disintegratiag 

 rocks. As soon as the rock is broken down into finer particles and soil has 

 been formed, plants of the Gutierrezior-Artemisia association begin to establish 

 themselves, and gradually the land is occupied by this association. The 

 presence of many rock fragments on the surface of the soil brings about a 

 condition favorable to the downward percolation of water and unfavorable 

 to its loss by evaporation. The greater number of the characteristic plants of 

 this association are without surface-feeding roots, yet the conditions are 

 favorable for the growth of the short grasses, and grama grass gradually 

 becomes estabUshed. This shallow-rooted grass takes up much of the water 

 in the surface layers of the soil, and consequently diminishes the supply which 

 can penetrate to a depth where it becomes available to the deeper rooted 

 plants of the Chdierrezia-Artemisia association. In the course of time the 

 rock fragments become thoroughly disintegrated, forming a true soU, and the 

 short grasses become dominant; the deeper rooted plants are slowly killed 

 out and the pure short-grass vegetation is established. 



"The succession which begins with the blow-out is much more complex. 

 The moving soil is first held by plants which constitute the blow-out associ- 

 ation. Th£ gives way to the sand-hills mixed association without the inter- 

 vention of any factors other than the appearance of the plants themselves and 

 the resulting increased stability of the soil. The vegetation may remain in 

 this condition for a long period or it may pass over into one of the modifications 

 of the sand-hills mixed association, or it may gradually give place to the btmch- 

 grass association. 



"In a general way we may say that from the lichen formation on imdisinte- 

 grated rock the vegetation passes gradually through a nimiber of well-marked 

 stages to the short-grass cover, and that from the first vegetation on bare sand 

 soil (for example, in blow-outs) it passes through an even greater number of 

 stages to the bunch-grass cover. By the aid of fires and grazing this bunch- 

 grass cover in time often passes over gradually to the pure short-grass type. 



