6 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



RELATIONS OF THE CHAPARRAL TO OTHER COMMUNITIES 

 OF SCRUB IN NORTH AMERICA. 



It will be well at this point to differentiate between the chaparral 

 and the other scrub communities of western North America. The four 

 great types may be characterized as follows, no attempt being made to 

 describe them minutely or to relate them to environmental conditions. 



The sagebrush type. — The characteristic species, Artemisia triden- 

 tata, always makes a large part and frequently grows in purity over 

 immense stretches. This is preeminently the type of the Great 

 Basin, though extending beyond it more or less in all directions. 

 It is sharply differentiated from the chaparral by reason of its abso- 

 lutely different leaf-type and by the fact that the two overlap very 

 sUghtly in range. 



The desert scrub. — This is made up of a multitude of species not 

 at all uniform superficially in ecological character. The succulents 

 are very important, opuntias of both the cylindrical and flat-jointed 

 forms being especially prominent. There are also thorny shrubs, 

 some of them with soft deciduous leaves. Others, with very small 

 leaves, have been well named microphylls. Finally, a few species 

 in aspect recall the chaparral, the most important of these being 

 Covillea tridentata. The desert scrub ranges from Arizona to Texas 

 and southward into Mexico. In ecological character it is sharply 

 set off from the chaparral, except for the superficial resemblance 

 of Covillea just noted. This species, although it ranges to the very 

 border of the chaparral country, mingles with that type practically 

 not at all. Cannon (20), moreover, states that "up to this time 

 .... all attempts to grow it at the Coastal Laboratory [Carmel, 

 Monterey County] have failed." He explains this on the basis of 

 less efficient soil temperature, but the value of that factor is of course 

 a direct result of climatic causes. There is thus some element in 

 the ecological make-up of the species which unfits it for life in the 

 chaparral region, in spite of its superficial likeness. Here is a warning 

 against the placing of too much reliance in the working out of ecologi- 

 cal relationships upon a single conspicuous character, or even upon 

 structural characters in general. Vital processes rather than struc- 

 ture are fundamental. 



The deciduous thicket type. — Most important here are scrub oaks 

 of various species, and the type is a widespread one, occurring from 

 the Atlantic to the Pacific and from Canada to Mexico. Great 

 stretches of foothills in the central Rockies are densely covered by it. 



This is the closest of the three to the chaparral, both floristically 

 and ecologically. In contrast to the first two, the transition between 

 this type and the chaparral is gradual. In the Sierras and in northern 

 California much of the scrub, dominantly evergreen, has nevertheless 

 a large deciduous element, and there are considerable thicket areas 

 which are composed entirely of deciduous species. 



