THE SUBCLIMAX CHAPARRAL. 189 



The relative rank of the dominants is indicated by the figure placed after 

 each one, indicating the observed frequence. These apply chiefly to the central 

 and western portions of the area, and are less representative of the eastern and 

 southeastern edge. 



A number of other shrubs and bushes play some part, but most of these are 

 secondary or incidental. A few will doubtless take their place finally as domi- 

 nants. They are especially well represented in sandhill areas, such as those of 

 Nebraska, Kansas, and Oklahoma. The most important of these are Yucca 

 glauca, Artemisia filifolia, Ceanothus ovatus, Salix humilis, and Rhus radicans. 

 Sambucus canadensis and Cephalanthus occidentalis are more or less hydro- 

 phytic shrubs which persist with the usual dominants for some time. Cornus 

 asperifolia, C. amomum, and Corylus rostrata are layer dominants which some- 

 times occur outside the forest, while the shrubby forms of Q. breviloba and 

 Cercis canadensis, which occur in Texas, are probably to be regarded as true 

 dominants. 



Groupings. — Owing to the fragmentary nature of the community, many of 

 the dominants may occur in pure stands. This is most characteristic of the 

 bushes, such as Symphoricarpus, Rosa, Elaeagnus, Ceanothus, etc., which make 

 the closest approach to the grasses in their requirements. The shrubs and the 

 shrub-forms of the trees demand a higher water-content, and this permits the 

 mixing or intimate alternation of several dominants. Two dominants have 

 been found associated in 35 cases, three in 26, four in 29, and five, six, or seven 

 in 45 instances. As a result, the various groupings are too numerous to be 

 indicated, but the composition of the most common is indicated by the relative 

 sequence of the first 10 or 12 dominants. In the southern portion, Quercus 

 virens is the chief species, often with Q. undulata or Q. breviloba and more or 

 less Celtis, Cercis, Rhus, and Berberis trifoliata. 



Relations of the dominants. — The general sequence and factor relations of 

 many of the dominants have already been indicated. The subclimax chapar- 

 ral possesses a peculiarly wide range of adjustment, as suggested by the great 

 variation in the extent and complexity of the communities and the size of the 

 plants. Moreover, while it finds fair expression as far west as the isohyete of 

 20 inches, this is usually possible only where the evaporation is low (as toward 

 the north) or the water-content high (as in sandhills and broken plateaus). 

 As would be expected from its relation to forest, its best development obtains 

 between the lines of 30 and 40 inches of rainfall. The explanation of its con- 

 stant recurrence throughout the grassland climax is found partly in the higher 

 water-content of valleys, sandhills, and escarpments, and partly in the com- 

 petition relations between shrubs and grasses. It is highly probable that 

 shrubs and trees establish themselves in grassland during the wet phase of the 

 climatic cycle and are then able to persist during the dry phase by virtue of 

 their deeper root-systems. This appears to be the general explanation of 

 both tree and scrub savannah (Chap. VI) and the fragments of subclimax 

 scrub bear a similar relation to the dominance of the grasses. Once estab- 

 lished, such clumps of shrubs are practically permanent, since they can be 

 destroyed only by repeated fires or by the hand of man. 



While the shrubs modify the air and soil conditions in each thicket, their 

 growth is still controlled by the climatic factors, more or less affected by the 



