Arctostaphylos and becomes dominant, completing the upgrade 

 series and one cycle. There are environmental gradient changes 

 associated with each phase. In reality the phasic cycle in the dwarf 

 Callunetum may consist of four, three, or two phases. The three- 

 phase cycle consists of the usual Calluna phase, a downgrade series 

 to the Cladonia and the bare soil phases, and then the upgrade 

 series to the Calluna phase, without that of the Arctostaphylos. The 

 two-phase cycle consists of the Calluna phase, a downgrade change 

 to the Cladonia one, and then an upgrade change to the Calluna 

 phase again to complete the cycle. 



A cyclic change system occurs in patchy grassland in western 

 North Dakota. In many areas there is a pattern of pitted patches 

 which are bare or covered with a thin stand of weeds and low 

 shrubs or grasses. The normal soil has a cover of blue gramagrass 

 (Bouteloua gracilis), needle-and-thread {Stipa comata), sedges (Carex 

 Jilifolia, C. stenophylla), and western wheatgrass {Agropyron 

 smithii).^^^-^'^'^ In the downgrade series, in the first stage, a saline 

 soil (solonchalk) is developed by an excessive concentration of 

 soluble salts in the upper layers. In the next stage, with an im- 

 provement in drainage, excess soluble salts are removed and an 

 alkali soil (solonetz) develops. Then a soloth is developed which 

 has a reduced ion exchange capacity, the A2 horizon may be 

 washed or blown away, and the vegetation is destroyed. Bouteloua 

 gracilis usually persists as a dominant through the saline and alkali 

 stages. In the saline stage, if the salts become too concentrated, 

 B. gracilis may be replaced as a dominant by Agropyron smithii, 

 desert saltgrass {Distichlis stricta) or alkaligrass {Puccinella nuttal- 

 liana), depending upon the concentration of the salt. 



The significant processes in the development of normal soil in 

 the upgrade series are calcification and reduction of sodium. The 

 vegetation stages of the upgrade series are the forb and low-shrub 

 stages, first grass stage, second grass stage, and the final stage; dis- 

 tinctive soil characteristics are associated with each upgrade 

 stage. In the forb and low-shrub stages the exposed B2 horizon is 

 most commonly invaded by knotweed {Polygonum erectum), pepper- 

 weed {Lepidium densiflorum), the salt bushes {Atriplex nuttallii and A. 

 argentea), plantain {Plantago elongata), prickly pear [Opuntia frag- 



144 * Dyzaanmics of Connxnisxmities 



