where the communities have considerable stabiHty,^^ and diffi- 

 culties are often encountered in deciding if succession is actually 

 taking place. Furthermore, classification by successional status in 

 many instances cannot avoid a considerable degree of assump- 

 tion. Consequently, this criterion is not as sound as others. As has 

 been stated before, the analysis and classification of stands do not 

 require precise determination of the successional position, for, 

 without minimizing the importance of this position, the kinds 

 and rates of change that are occurring in a community can best 

 be treated in the description following the sociological analysis. 

 It is not desirable to inject successional stages into a plant socio- 

 logical classification, or, in other words, to mix the criteria of 

 successional status and floristic composition.-^^ 



Physiognomy 



Physiognomy is dependent upon a number of qualitative and 

 quantitative characteristics, especially life-form, as well as upon 

 structure and dominance. This basis of classification is very use- 

 ful in distinguishing and delimiting major types of communities 

 such as coniferous forest and deciduous forest, or shortgrass and 

 tallgrass communities. The use of physiognomy is important in 

 the preliminary reconnaissance of an area, to ascertain the major 

 types of vegetation, for mapping purposes, and as a start for sub- 

 dividing the major types into the constituent communities. For 

 example, in the mountain-front region of northern Colorado the 

 vegetation consists of zones and mosaics of stands of various 

 kinds, mostly grassland and shrub, chiefly the mixed mid- and 

 shortgrass type {Stipa comata-Bouteloua gracilis-B. curtipendula), the 

 medium tall grassland (Andropogon scoparius), the mixed perennial- 

 annual grassland {Agropyron smithii-Bouteloua gracilis- Bromus tec- 

 torum), and shrub types [Cercocarpus montanus, Rhus trilobata, Sym- 

 phoricarpos sp., and Physocarpus monogynus).'^^^ 



Classification based chiefly or solely on physiognomy is most 

 valuable in the first studies in a region where little is known 

 about the vegetation, and in aerial surveys, but even in such 

 studies whenever it is possible, the various communities con- 

 stituting each physiognomic group should be characterized 

 analytically and synthetically. As an example of this criterion, the 



Bases and Units * 185 



