124 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



extra-associational Bouteloua gracilis occupying the dry crests. The average 

 range of water-content for the Andropogons is 25 to 35 per cent, for Agro- 

 pyrum 20 to 30 per cent, for Stipa spartea and Koeleria 15 to 20 per cent, and 

 for Stipa comata 10 to 15 per cent. It is clear that these values will vary 

 greatly from the dry to the wet phase of the climatic cycle, and that their 

 efficiency will change with the factors controlling evaporation and transpira- 

 tion. 



While extensive quantitative studies will refine these values and will 

 definitize them for different regions, it seems clear that they will also further 

 verify the successional sequence indicated above. In applying the latter, it 

 must be borne in mind that the mixing of two or more of the dominants over a 

 certain area by no means invalidates the sequence. The requirements of two 

 successive dominants are more alike than different, and under minor dis- 

 turbances in the habitat-complex are actually or apparently equivalent, for 

 a time at least. These differences are modified by climatic fluctuations from 

 year to year. When variation of slope, exposure, and soil are taken into 

 account, it is readily understood why pure consociations extending over many 

 miles are impossible. The largest area of Agropyrum seen was in the valley 

 of Dog Ear Creek in South Dakota, but whenever the valley rose into hills, 

 Agropyrum gave way to Stipa comata or Stipa spartea. Likewise, on the rough 

 hills of the Pine Ridge reservation of South Dakota, Stipa comata appears 

 like fields of golden grain for miles in every direction, but the lower valleys, 

 swales, and roadways are characterized by Agropyrum. Stipa spartea shows 

 a similar behavior on a smaller scale. However pure the community appears, 

 it is regularly mixed with Koeleria or interrupted by Agropyrum or Andro- 

 pogon. 



The mixing of dominants within an association differs only in degree and 

 extent from the mixing of dominants at the edge of contiguous associations 

 or formations. But it would be a serious mistake to assume that the associa- 

 tions or formations concerned were essentially a unit because of the broad 

 ecotone that exists between thetia. There is a complete sequence of dominants 

 with overlapping ranges in the grassland from Andropogon furcatus on the 

 east to Bouteloua gracilis on the west. This corresponds with a gradual 

 decrease of rainfall from 30 to 40 inches to 10 to 15 inches. In spite of the 

 equalization brought about by physiography, the two species practically never 

 come in contact with each other as dominants. Between them lies the whole 

 region of the climax prairies, 200 to 400 miles wide, along which Andropogon 

 makes a broad ecotone on the east and Bouteloua on the west. A similar situa- 

 tion exists where grassland comes in contact with the sagebrush or the wood- 

 land climax. There is often a complete and more or less equal mixture for a width 

 of several to many miles. From the superficial evidence, the dominants 

 might well be placed in the same formation, but a study of the successional rela- 

 tions or a comparison of the ecotone with the formation proper on either side 

 will at once disclose the real facts. As a rule the careful study of the ecotone 

 will show that most of it can be referred to one or the other of the two forma- 

 tions or associations, and that the area of actual equilibrium is relatively 

 small. In fact, increasing familiarity with vegetation shows that most 

 transitions are due to disturbance or climatic cycles and are actually a part 

 of succession. 



