His), Iva axillaris, glasswort {Salicornia herbacea), desert saltgrass, 

 broom snakeweed {Gutierrezia saraothrae), gum weed (Grindelia 

 squarrose), Suaeda depressa, Russian thistle {Salsola kali), Agropyron 

 smithii, and Eriogonum multiceps. The first grass stage is character- 

 ized by perennial grasses, the more important being wheatgrasses 

 {Agropyron molle, A. albicans, A. smithii), Puccinellia nuttalliana, tum- 

 blegrass {Schedonnardus paniculatus) and Distichlis stricta. In the 

 second grass stage buffalograss {Buchloe dactyloides) is dominant; 

 other grasses and forbs are of minor importance. As mentioned 

 previously the dominants in the final stage are Bouteloua gracilis, 

 Stipa comata, Carex spp., and Agropyron smithii. 



It is important to note that cyclic replacement change, an 

 intracommunity change, might be confused with directional 

 change, an intercommunity change, which will be discussed in 

 detail later. The upgrade series of the cyclic replacement change 

 is rather conspicuous and might be confused with a successional 

 change, whereas the downgrade series, especially since it often 

 results in bare soil, may be confused with retrogression, i.e., from 

 the more complex to the less complex. In either case, the upgrade 

 or the downgrade cyclic series might be misinterpreted as repre- 

 senting the trend of the community as a whole rather than as the 

 normal internal dynamics of the community. 



In summary, replacement change may vary from a simple 

 process to a relatively complex one. The noncyclic change is 

 a simple process on an individual plant basis, while the phasic 

 cycle is a complex one in which several alternative cycles may 

 operate within a given community, forming patches. The replace- 

 ment change may be operative within the climax or within direc- 

 tional, fiuctuational, and intercommunity cyclic changes. In 

 general, the rate varies with the life-span of the component species, 

 especially for the noncyclic replacement change. The rate tends to 

 vary also in the respective series of the cyclic replacement system. 

 The upgrade cyclic series may be confused with a successional 

 change and the downgrade series with a retrogression change, but 

 the cyclic series must be interpreted as the internal dynamics of 

 the community rather than as a directional change of the com- 

 munity itself. 



Habitat Pattersms, Chazmges, axmd Clinmax • 145 



