CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 269 



teaus may be marked by a zone of broad-leaved scrub. The zone is 

 widest and best developed in the southern Rockies, narrows and 

 becomes discontinuous in the central Rockies, and fades out en- 

 tirely farther north. The components of the community vary, but 

 oaks (Quercus gambellii, Q. gimnisoni, Q. undulata, Q. fendleri, 

 Q. emoryi, and others) are the largest (up to thirty-five feet) and 

 most conspicuous dominants in the south. North of the latitude 

 of Denver, Colorado, the oaks are spottily represented, and moun- 

 tain mahogany (Cercocarpus parviflorus, C. ledif otitis, etc.) is 

 dominant. Other important associates include Rhus triloba, Pur- 

 shia tridentata, Fallugia paradoxa, Amelanchier spp., and Sym- 

 phoricarpos spp., any of which may assume local dominance. The 

 vegetation does not form a continuous cover but occurs in dense 

 clumps, or even as individual plants, separated by areas of grass- 

 land or desert vegetation. 



The Black Hills — Although they are now isolated, the Black 

 Hills are geologically and ecologically related to the Rockies. 

 They deserve especial mention because of their mixture of eastern, 

 western, and northern species. Because the altitude is only a little 

 over seven thousand feet, the montane zone is chiefly represented. 

 There is no Douglas fir present. Instead, Picea glauca, which ex- 

 tends southward from Canada along the east slope of the Rockies 

 as an associate of Douglas fir, here is the only dominant on the high 

 slopes at the southern limit of its range. Paper birch from the 

 northern conifer forest is also present. Ponderosa pine dominates 

 most of the lower slopes, which include most of the area, and 

 lodgepole and limber pine in small numbers are additional repre- 

 sentatives from the Rockies. Species from the eastern deciduous 

 forest are ash, hackberry, elm, birch, and bur oak, of which only 

 the last attains any size. The scrubby appearance of the commu- 

 nity, as well as its distribution along the lower margin of the coni- 

 fer forest, suggests the oak-mahogany zone of the Rockies. 85 



Sierra Nevada Forest Complex.— The area here considered in- 

 cludes the southern portion of the Cascade Mountains and the 

 Sierra Nevada, which together extend from Oregon southward 

 along the eastern boundary of California as the innermost ranges 

 of the coastal mountain system. The long west slope of the Sierra 

 rises gradually to altitudes of 14,000 feet and more, but the east 



