Vegetation of Northwest Mexico 
[21 
this vegetation area coincides fairly well with the areas of 
mesothermal savannah climate (Civ of Koeppen). Two 
associations are dominant in this forest area which extends 
south for 800 miles, and averages 50 to 100 miles in width. 
1. Oak-agave-juniper: Commencing at an elevation 
of around 5,000 feet in the north, and somewhat higher in 
the south, the mesquite, creosote-bush, popotillo, and other 
shrubs that dot the basin grasslands are supplanted by 
Agaves , junipers, and oaks. The Yuccas , Nolinas, Dasyli- 
rions, and Opuntias continue upward for another thousand 
feet in this park landscape of mesothermal climate. Gramas 
continue to be the dominant grasses. Between 5,000 and 
6,000 feet junipers, oaks, and Agaves prevail. From 6,000 
to 7,000 feet, there is a pine-oak transition belt, and above 
7,000 feet is the pine country. The succession is from juni¬ 
per through oak to pine, with varying proportions of these 
trees and their associates. Neither soil nor slope seems to 
condition the vegetation as much as does mere elevation. 
Oaks are dominant in this foothill formation, Quercus 
emoriji (black live oak) prevailing at the lower elevations, 
and Quercus grisea (gray live oak) making up nine-tenths 
of the oak vegetation in the upper elevations—mainly above 
6,700 feet. On the Pacific coast side Quercus oblongifolia 
(white live oak) and Quercus Arizonicus (Arizona gray 
live oak) are dominant. Several other species of oaks, how¬ 
ever, are represented in this formation, which varies, in 
appearance, from chapparal through oak savannah to 
forest. Junipers (Juniperus pachyphloea and J. Me: r- 
icana) grow on the sunnier slopes and somewhat dry ridges, 
singly or in quite open groves, commonly interspersed with 
oaks and Agaves . The Agaves grow singly throughout the 
association area. 
On the Sonoran slopes the lower transition zone from 
basin grassland to oak-agave-juniper contains relatively 
few Yuccas , Nolinas , and Dasylirions, while Parkinsonia, 
Acacia , Prosopis, and Fouquiera are well represented. The 
