THE CHAPARRAL CLIMAX. 181 



Fendlera, Fallugia, Cowania, Coleogyne, Robinia, and Garrya, all but the last 

 belonging to the rose order. Of 35 species of dominants more than half range 

 from Saskatchewan, Manitoba, or the Dakotas to Texas or New Mexico, 

 thence to Arizona and California on the southwest and to Oregon, Washing- 

 ton, or British Columbia at the northwest. While only a few are major domi- 

 nants throughout this wide area, all are sufficiently important to show the 

 basic unity of the formation and the close relationship of the various associa- 

 tions, both climax and subclimax. 



Structure of the formation. — The studies of the last six years have revealed 

 several different regions in which the chaparral type of vegetation reaches 

 more or less complete expression. These are the Rocky Mountains, the 

 Pacific Coast, the Southwest, the Northwest, the Missouri Valley, and Texas. 

 In the last two, as in the mountains of the Pacific Coast, the chaparral is 

 subclimax. These communities do not belong to the formation proper and 

 are considered with it chiefly because of their contiguity and general relation- 

 ship. They are properly associes of a climax forest. Of the four climax 

 maxima, two stand out clearly, namely, the Rocky Mountain and Coastal. 

 The other two have been regarded tentatively as associations during the 

 course of the field work. In order to determine their real rank as well as the 

 relationship of the several communities, a summary has been made of all the 

 groupings of dominants recorded from 1900 to 1918, as well as in 1893, when a 

 botanical reconnaissance was made along the Missouri and Niobrara Rivers. 

 The summary comprises approximately 500 localities, of which 206 are in 

 the Rocky Mountain region, 39 in the Northwest, 38 in the Southwest, 45 on 

 the Pacific Coast, and 164 in the subclimax chaparral of the grassland forma- 

 tion. The occurrence of the dominants in the five regions is shown in the 

 table on page 182. No account is taken here of the Californian subclimax, 

 which is essentially different. 



Grouping of dominants.— The unity of the formation is readily seen from 

 the distribution of the genera especially. The first 7 genera occur in all the 

 five areas, 5 others occur in three, and 6 are found in two. Three of the 

 species are present in all five communities, 4 others in four of the areas, 5 in 

 three, and 9 in two. The differentiation of the maxima is revealed by the 

 presence of certain genera and species in one area and not in another, as well 

 as by their frequence. For example, Adenostoma and Quercus dumosa occur 

 only in the Coast chaparral, while Ceanothus cuneatus and Arctostophylus 

 pungens are of the first importance in it alone. Likewise Fallugia, Cowania, 

 Coleogyne, Robinia, and Fendlera are limited to the Southwest and the southern 

 part of the Rocky Mountain association. The differentiation of the subclimax 

 community is shown by Quercus macrocarpa, Symphoricarpus occidentalis, 

 Rosa arkansana, Elaeagnus argentea, Fraxinus viridis, Prunus americana, and 

 Rhus glabra, while a less distinct maximum in the Northwest is indicated 

 by Purshia, Opulaster, Philadelphus, Holodiscus, and Peraphyllum. 



The relationship of these five maxima is revealed by the frequence of the 

 dominants as shown in the table. The italic numbers indicate those which 

 occur in at least 10 per cent of the total number of localities visited. The 

 Rocky Mountain chaparral exhibits 12 dominants which occur in 10 or more 

 localities, and of these 8 are equally important in the Northwest, while but 

 2 are absent in the latter. The southwestern chaparral has 6 of the 12 most 



