Figure 6-3. This grassland under moderate grazing is ap- 

 proaching through succession the terminal grassland stage. 

 Western yellow pine {Pinus ponderosa) and Douglas fir 

 {Pseudotsuga taxifolia) are shown in the background. July, 

 Pikes Peak Region, Colorado. (U. S. Forest Service.) 



with narrow ecological amplitude, and from observations (or 

 rather crude measurements) of habitat conditions. Segregation of 

 communities on the basis of their location on gradients involves 

 difficulties, for usually there are gradients in a number of factors, 

 such as soil moisture, soil salts, pH, and temperature, each in- 

 fluencing the vegetation simultaneously (Figure 6-2). If the con- 

 trolling influences in the gradients are known and can be 

 measured, and if equilibrium between vegetation and the environ- 

 mental complex exists, then it would appear possible to establish 

 the composition of the community as a function of the position 

 on the ecological gradient. However, "the ecological gradients of 

 importance to vegetation are unknown at the beginning of an in- 

 vestigation of the vegetation of an area and cannot be guessed 

 beforehand," ^2. p 79 gQ ^j^^ ordination of communities according 

 to ecological gradients^^ is very difficult. There appears to be 

 general agreement that the basis for securing understanding of 

 nature must be the study of directly observable characters, and 



Bases and Units • 183 



