154 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



fact that Artemisia tridentata haa been found in 314 of the 416 localities where 

 the community has been studied. 



The ecological unity of the climax is due especially to the fact that all of the 

 17 dominants are half -shrubs or bushes. They range in height from an average 

 of 3 to 5 feet in Artemisia tridentata, A. calif omica, Chrysothamnus nauseosus, 

 Atriplex canescens, Eriogonum fasciculatum, and Salvia meUifera, while in 

 Artemisia trifida, Gutierrezia, and Eurotia the range is 1 to 2 feet. They are 

 typically deep-rooted, and this, with their bushy habit and perennial woody 

 stems, accounts for their success in competing with other dominants. For 

 their region, most of them have a further advantage in being able to endure 

 a more or less highly saline soil. The close similarity in their nature and 

 requirements is shown by the fact that they are replaced by such turf-forming 

 grasses as Agropyrum, BoiUeloua, and Hilaria, wherever conditions permit 

 the formation of a sod. All of the dominants, without exception, are marked 

 xerophytes in which transpiration is decreased by reduction of the leaf -surf ace, 

 a hairy epidermis, or succulence. This similarity in habit is confirmed by their 

 association. While the sagebrush in particular forms pure stands, mixed 

 communities are the rule. Of the 416 communities studied, 145 contained 

 two dominants and 143 contained three; in 44 there were four, in 5 five, and 

 in 1 six, or a total of 339 mixed communities in contrast with 75 pure ones. 



Successionally, the sagebrush climax is perhaps more uniform than any 

 other formation. This is due to the low rainfall, the high evaporation, and the 

 rapid drying-out of bodies of water. Since its area is a great intermountain 

 basin composed of several smaller lake basins, the lakes and ponds are either 

 saline or they form salt marshes as they dry out. As a consequence, the 

 typical succession is the halosere, originating in salt marshes, or on the saline 

 shales and clays, which are especially frequent. In fact, the successional 

 correlation of the serai and climax dominants is so fundamental that the 

 development of a local subclimax of Atriplex and Artemisia occurs widely 

 throughout the Bad Lands of the Great Plains and the prairies, in spite of the 

 fact that these are several hundred miles from the sagebrush climax proper. 



Range. It is impossible to draw the limits of the sagebrush climax with 

 accuracy, owing to the extent to which it mixes with contiguous formations. 

 The general tendency is to use the conspicuous dominants, such as Artemisia 

 tridentata and A. cana, to outline its area, but these extend far beyond the 

 limits of the formation proper. Since ecotones are areas in which dominants 

 meet on more or less equal terms, it follows that the limit of a particular 

 climax must be drawn at the line where it is still controlling. As a matter of 

 convenience, a formation is called dominant where it covers three-fourths of 

 a particular area or region. Outside of this climax mass occur many outposts 

 of the community as well as of individual dominants, but these are merely 

 expressions of topographic or climatic diversity in the area of adjoining cli- 

 maxes. For example, the erosion valleys of the Bad Lands of northwestern 

 Nebraska are covered with luxuriant sagebrush, but this is really subclimax 

 to the mixed prairie which ultimately replaces it (plate 32) . 



If the limits are set as indicated above, the sagebrush chmax will include 

 all of Nevada except the southeast, practically all of Utah, Colorado west of 

 the Continental Divide, central and southwestern Wyoming, a part of south- 

 western Montana, all of south-central Idaho, Oregon south of the John Day 



