178 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Unity of the chaparral formation. — In view of the exceptionally wide range, 

 the floristic unity of the chaparral is remarkable. The major dominants 

 belong to 10 genera, namely, Quercus, Ceanothus, Cercocarpus, Rhus, Prunus, 

 Amelanchier, Symphoricarpus, Rosa, Arctostaphylus, and Shepherdia. With 

 one or two exceptions, all of these occur as dominants in both associations, 

 and in the subclimaxes as well. The relationship is even better shown by such 

 species as Rhus trilobata, Prunus demissa, Arctostaphylus pungens, Cerco- 

 carpus parvifolius, and Ceanothus cuneatus, which are dominants in both asso- 

 ciations. Still other species, such as Amelanchier alnifolia, Holodiscus dis- 

 color, Symphoricarpus albus, and Philadelphus gordonianus occur as dominants 

 in one and are of secondary importance in the other. It is also a striking fact 

 that of 25 genera which play a considerable part in the formation, all but two 

 belong to the order Rosales or to the related Acer ales and F agates. This is 

 reflected in the appearance or physiognomy of the formation. The domi- 

 nants not only belong to the same vegetation-form, viz, shrubs, but also to 

 the same general growth-form. Instead of being tall shrubs, as a rule prac- 

 tically all of them assume the bush-form or produce several ste ns. The 

 latter is a consequence of the nearly universal habit of forming root-sprouts 

 to which the chaparral owes much of its success, especially in competition 

 with grassland. The regular occurrence of several dominants in mixture also 

 explains the general uniformity in height and habit, which so often gives 

 chaparral the appearance of a densely woven green carpet. 



One evident difference between the Rocky Mountain and the Coastal 

 chaparral lies in the fact that the former is deciduous, the latter evergreen or 

 sclerophyll. This difference is probably to be correlated with winter as the 

 most xerophytic period for the former and summer for the latter. While this 

 distinction is characteristic, it is not thorough and must not be given too much 

 importance. The most representative species of the Rocky Mountain chapar- 

 ral is Quercus undulata, which exhibits deciduous forms in the north and ever- 

 green ones in the south. Likewise, Cercocarpus parvifolius is a more northern 

 deciduous species and C. ledifolius a southerly evergreen one, but in spite of 

 this, both are found together over the foothills of the Wasatch Mountains 

 and elsewhere. Moreover, the evergreen Arctostaphylus pungens and Ceano- 

 thus cuneatus greggii are found from northern Arizona to central Utah in 

 intimate association with Quercus, Rhus, and Amelanchier. A similar condi- 

 tion is encountered in California, where most of the chief dominants are ever- 

 green, but they are often associated with deciduous species, such as Cerco- 

 carpus parvifolius, Holodiscus discolor, Prunus demissa, and others. As a 

 consequence, it must be recognized that there is nothing ontradictory in 

 having deciduous and evergreen dominants in the same ormation and even 

 in the same association. 



Climatic relations. — Geographically, chaparral is a western formation, 

 reaching its typical development on the foothills of the Rocky Mountans 

 and its numerous secondary ranges, and on those of the Sierra Nevada, Cas- 

 cade, and Coast Ranges of the Pacific Slope. This relation is strikingly shown 

 by its appearance in the Black Hills of South Dakota and the Wildcat Moun- 

 tains of western Nebraska at a distance of several hundred miles from the 

 main range. This is naturally to be explained by the climatic relations. As 



