222 CLIMAX FORMATIONS OF NORTH AMERICA. 



Elmore (1901 : 29) studied the colonization of a dried-up mill-pond along the 

 Blue River in southeastern Nebraska. Of the 40 species which entered the 

 first year, but 10 grew on the adjacent banks, the other 30 having come from 

 a greater distance; 4 of the 10 were trees found on the adjacent banks, viz, 

 Ulmus americana, Salix nigra, Acer saccharinum, and A. negundo. The only 

 other tree present was Populus deltoides, which had migrated from a more 

 distant place. The most abundant species, Eragrostis reptans, Acnida tuber- 

 culata, and Polygonum lapathifolium, were not found on the adjacent banks. 

 Of the total number, 17 were annuals, 4 biennials, and 19 perennials; 13 were 

 anemochores, 3 zoochores, and 26 had no migration device. The five trees 

 present were represented by 100 to 200 plants, giving a clear indication of the 

 brevity of the subsere. 



Shantz (1906 : 190; 1911 : 65) gives the following summary of the subsere in 

 Colorado: 



"Wherever short-grass land is broken and then abandoned it is first covered 

 by a growth of weeds, after which the type of vegetation that immediately 

 preceded the short grass in this particular place regains possession. If, for 

 example, we break short-grass land which has been derived from the Gutier- 

 rezia-Artemisia association and consequently offers conditions favorable to 

 that association, the land will become occupied by the latter association. The 

 Gutierrezia-Artemisia vegetation will in turn gradually give way to the short 

 grasses, which will be fully reestablished within a period of 30 to 50 years. An 

 area of short-grass land in which the physical conditions approach more 

 nearly those of wire-grass land, if broken, will be possessed after the prelimi- 

 nary weed stage by plants of the wire-grass association, and will then gradually 

 return to the short grasses, the time required being 20 to 40 years. 



"Breaking done on wire-grass land will result in the establishment of a 

 vegetation such as usually characterizes a still lighter type of soU. Many 

 plants from the bunch-grass and the sand-hills mixed associations enter, and 

 in the early stages of this succession bunch-grass itself quite generally occurs. 

 The vegetation will ordinarily return to the wire-grass type in 15 to 30 years. 



"Wherever land characterized by the bunch-grass or the sand-hills mixed 

 association is broken a blow-out may result. This, however, is unusual, 

 although there is great danger if the land is plowed in the fall of the year. 

 Usually the weed stage is most prominent the first year or so, but the native 

 grasses soon regain possession and the succession is completed in a much shorter 

 time than on the heavier types of land. 



"It will thus be seen that when the vegetation of each of the plant associa- 

 tions is destroyed by breaking it will be followed, after the weed stage, by 

 vegetation characteristic of a lighter type of soil, and also that this vegetation 

 is that characteristic of an earUer stage in the natural succession." 



Cook (1908) finds that extensive areas of former prairies in south Texas are 

 now covered with a dense growth of Prosopis, Opuntia, and other shrubs. 



Before the prairies were grazed by cattle the luxuriant grass accumulated 

 for several years imtil conditions were favorable for the spread of accidental 

 fires. Such fires were especially destructive to trees and shrubs and corre- 

 spondingly promoted the dominance of the grasses. The settlers in southern 

 Texas practiced burning over the prairies every year, partly for protection, 

 partly to give readier access to a fresh growth of grass. While the grass was 

 still abundant, these burnii^s were able to keep the woody vegetation in 



