214 CLIMAX FORMATIONS OF NORTH AMERICA. 



[ "1. Humus little harmed. — The coniferous element is destroyed, but the 

 1 birches survive in their underground portions and sprout abundantly from the 



stvunp, usually producing a nearly piu"e forest of birches growing in cltmsips. 



Seedling birches and aspens are usually present also. Conifers gradually 

 I return, finally bringing about the reestablishment of the climax. 

 i " 2. Humus destroyed; bare rock exposed. — The reestablishment of the climax 

 I follows closely along the hne of the rock shore succession, but progress is 

 • usually more rapid because of the presence of soil materials and numerous 

 V^ invaders, and frequently protection from wind." 



THE PRAIRIE-PLAINS CLIMAX. 



PRISERE. 



While the work of Bessey (1887, 1895, 1897, 1900, 1901) and of Rydberg 

 (1895) had a distinct bearing upon successional processes and migrations in 

 the prairie region, the first studies of succession proper were made by Clements 

 (1897) and Pound and Clements (1898, 1900) in the revegetation of old trails 

 across the plains, and in the blow-outs and sand-draws of western Nebraska. 

 Since the xerosere is the more important in the semiarid prairies and plains, 

 it will be considered first. While it is probable that the prairies of Illinois 

 belong to the deciduous climax, their development is essentially identical with 

 that of those further west, and they will accordingly be considered here. 



XEROSERE. 



Pmmd and CUm^ts (1898^ : 392; 1900 : 366) found that Redfieldia flexuosa 

 was regularly the first pioneer in the blow-outs of the sandhills of western 

 Nebraska: 



It is usually followed by Muhlenbergia pungens, and this by Eriocoma, 

 Calamovilfa, and Eragrostis trichodes. As the sand becomes stabilized by the 

 grass-roots, as well as the mats and bunches, herbs such as TradescarUia, Erio- 

 gonum annuum, Meriolix, Laihyrus ornatus, Phaca longifolia, Euphorbia peta- 

 laidea, and Hymenopappus enter, and form a very open cover. After several 

 seasons the sand is held firmly, and Andropogon scoparius and A. hallii take 

 possession. The herbs of these consocies soon follow, and the reclamation of 

 the blow-out is complete. Ultimately Aridida enters and dominates as the 

 wire-grass consocies, but is in its turn slowly replaced by Stipa or Bouteloua. 

 In sand-draws the development consists of but few stages, and it might well 

 pass for a subsere if it were not for the persistence of extreme xerophytic con- 

 ditions and the slow colonization. The pioneers are Polanisia trachysperma 

 and Cristatella jamesii, with which are later associated Psoralea lanceolata, 

 Euphorbia, Collomia linearis, etc. The first grass is usually Munroa sguarrosa, 

 though Eragrostis major or Triodia purpurea may replace it. Gradually other 

 species enter, such as Meriolix, Chrysopsis, Argemone, etc., and these are 

 followed by Andropogon, Calamovilfa, and Aristida. 



Shantz (1906 : 187) has described the initial stages of the primary sere on 

 rock in the plains grassland about Colorado Springs. 



The pioneer lichens of lime ridges are Staurothele umbrina and Lecanora 

 privigna, followed by the small foliose forms such as L. rubina and L. nMna 

 opaca, and then by the foliose Parmelia conspersa. On the rocky mesa the 

 latter is the dominant, though Rinodina oreina and Lecanora calcarea are also 

 common. As crevices form and the rock weathers into soil, low herbs, such 



