PROCESS INDICATORS. 97 



ones, and Atriplex nuttallii and A. corrugata, very poor ones. Throughout the 

 desert scrub, Larrea is an index of good roads, Prosopis of poorer ones, and the 

 saline subclimax of the very poorest, except where the presence of sand makes 

 some improvement. 



Physiographic indicators. Plant communities owe their significance as 

 indicators of physiography or physiographic processes to the influence of the 

 latter upon the direct factors, especially water and solutes. It is clear that the 

 indicators of factor-complexes, such as slope-exposure and altitude, have a 

 distinct physiographic correlation also. However, the basic relation between 

 physiography and indicators is through such processes as erosion and deposi- 

 tion which directly control the soil and its water-content. Since physio- 

 graphic processes are the universal causes of primary bare areas, their indi- 

 cators occur in successional communities that mark the progressive change of 

 the area from the initial condition to one of relative stabiUty. As has been 

 emphasized elsewhere (Plant Succession, 35), causes other than physiography 

 may produce similar bare areas and initiate the same sere, the successional 

 movement being due to the reaction of the conmiunities alone, or to this and 

 physiographic processes working together. In the great majority of primary 

 areas, however, physiographic causes or processes are so important or con- 

 trolling that the serai indicators are readily correlated with them and their 

 changes. 



The most outstanding and best-known series of indicator communities of 

 physiographic processes is that of ponds and lakes. In these, physiography 

 is normally the initial cause of the body of water, and deposition the process 

 which controls or promotes the serai movement. The primary stages of the 

 process are marked by the well-known associes of submerged plants, floating 

 plants, reed-swamp, and grassland, or scrub. Pearsall (1917 : 189) has 

 recently pointed out that still other associes should be recognized, and these 

 would serve as indicators of somewhat smaller changes. Finally, each con- 

 socies indicates a more or less definite set of conditions within the associal 

 stage. The succession in dunes, sandhills, and blowouts is almost equally 

 well known. In these the physiographic processes are very active, and the 

 indicators of the different degrees of reaction or stabilization well-marked 

 (Cowles, 1899; Gleason, 1907; Pool, 1914). The indicators of sandhills, and 

 of river and coastal dunes have received much attention during the studies 

 of the past five years. The dominants and serai communities are identical or 

 similar throughout the West, except along the Pacific Coast, where a very 

 different flora is concerned. During the same period, a special study has been 

 made of succession in Bad Lands, and this has permitted the correlation of a 

 large number of indicators with erosion and deposition, and the resulting 

 differences in water-content and salt-content. Similar though less extensive 

 investigations have been made of the indicators of saline bolsons and playas, 

 and of the geyser and mud-volcano areas of Yellowstone Park. Finally, the 

 serai indicators of cliffs, rock-fields, and gravel-slides have been worked out for 

 the central Rocky Mountains in particular (Clements, 1905 : 270; 1916 : 225). 



Climatic indicators. The value of climax communities as climatic indicators 

 has already been emphasized. Formation, association, consociation, and 

 society are correlated with different cUmates or cUmatic subdivisions, and 



