22 ] The University of New Mexico 
inferior limit of oaks is considerably lower than on the Chi- 
huahuan side, as oaks are common at elevations under 3,000 
feet. In Sinaloa the transition from tropical to mesothermal 
vegetation is marked by the complete absence of Yuccas , 
Nolinas , Dasylirions, ocotillo, and creosote-bush, and only a 
scanty representation of mesquite. These succulents and 
shrubs are replaced by a great number of Mimosas , Acacias , 
Cassias, Caesalpinias, figs, laurels, and other plants typical 
of the Sinaloa tropics. The transition zone in Sinaloa com¬ 
mences around 3,500 feet elevation, which is about the lower 
limit of oaks. 
2. Pine Forest: The district of pine dominion is on 
the plateau and mountain tops above 7,000 feet. Most of the 
forest consists of Pinus Arizonica , Pinus ponderosa , and 
Pinus montezumae —the first two dominating the north, and 
the Montezuma pine prevailing in Durango. Mixed with 
these three species are individuals of Douglas fir (Pseudot - 
suga mucronata ), Pinus lumholtzii , Pinus chihuahuana, 
Pinus reflexa , Pinus ayacahuite, etc. The Arizona pine and 
western yellow pine (Pinus ponderosa) are commercially 
most important. The growth is open, although there is some 
undergrowth of Ceanothus , Arctostaphylos , and Arbutus . 
Pinus cembroides and Pinus edulis (pihons) occupy the 
warmest and most arid slopes of meager soil. The grass 
cover is fairly good throughout the forest. Numerous moun¬ 
tain meadows also occur. The canyons carry species, espe¬ 
cially the scrubby forms, representative of the entire area. 
Sonoran Desert: The Sonoran Desert area possesses 
four association districts in addition to the scattered high¬ 
land regions having a vegetation of the Sierra Madre Occi¬ 
dental oak-agave-juniper type. These associations express 
changes in temperature and water supply as conditioned by 
elevation, river flood plains, and latitude. 
1. Colorado River Delta: The alluvial soil of the hot, 
nearly rainless Colorado River delta carries a rather dense 
vegetation of willow, cottonwood, Pluchea, mesquite, screw 
