NATURAL AREAS AND REGIONS 



563 



and the most complex topography of 

 the state is encountered. 



The southwestern half of Arizona is 

 a vast plain tilted toward the Colorado 

 and Gila Rivers, and studded with 

 numerous hills and mountains. The 

 floor of the plain descends from ele- 

 vations of 4000 to 5000 ft. near the New 

 Mexican boundary to near sea level at 

 Yuma. On the most elevated part of 

 this plain are to be found a number of 

 small and isolated mountains which 

 rise to elevations of 8000 to 10,500 ft., 

 including the Pinaleno (Graham), Chi- 

 ricahua, Huachuca, Santa Rita and 

 Santa Catalina ranges. Throughout 

 the remainder of the plain the moun- 

 tains are less numerous and not so high. 



It is often said in Arizona that a 

 bucket of water thrown out anywhere 

 in the state will flow past Yuma, a 

 remark which is as true with respect 

 to the drainage systems as it is facetious 

 with regard to the commonly prevailing 

 conditions of evaporation. 



The differences of altitude in Arizona 

 are accompanied by great differences in 

 temperature conditions. The growing 

 season ranges from 105 days at Flagstaff 

 (elevation 6886 ft.) to 310 days at Yuma. 

 The minimum temperatures range from 

 -20° at Flagstaff to 27° at Yuma, and 

 the maximum from 85° at Flagstaff to 

 115° at Yuma. The rainfall increases 

 with altitude from an annual average 

 of 3.36 in. at Yuma to 22.80 in. at Flag- 

 staff, and is greater in the vicinity of 

 large mountains than it is at the same 

 elevation in the open plains. 



II. ORIGINAL BIOTIC REGIONS (f. S.) 



Arizona may be subdivided into the 

 following natural landscape aspect re- 

 gions: Desert (Lower Sonoran Zone), 

 lying below 3500 ft. in the region south 

 of the Mogollon Mesa or below 5000 

 ft. to the north of it. Desert Grassland 

 (Upper Sonoran Zone in part), lying 

 above 3500 ft. and below 6000 ft., but 

 not of general occurrence between these 

 altitudes. Xerophytic Forest (Upper 

 Sonoran Zone in part), Ij^ing between 

 5000 and 7000 ft., its lower limit being 



lower on mountain slopes than in open 

 plains. Evergreen Forest (Transition 

 and Canadian Zones), occuring from 

 elevations of 6000 to 7000 ft. to the 

 summits of all of the higher mountains 

 except the San Francisco Peaks. 



A. Desert biota 



1. Vegetation {F. S.). The conspic- 

 uous perennial vegetation of this 

 region consists of a relatively small 

 number of species, although the total 

 flora, including ephemeral herbaceous 

 plants, is large. The animal life is also 

 rich, particularly in small mammals, 

 reptiles, birds, and insects. The most 

 abundant and widely distributed plants 

 are Covillea tridentata, Franseria diimosa, 

 Acacia paucispina, Parkinsonia micro- 

 phylla, Olneya tesota, Prosopis velutina 

 and numerous tj^pes of cacti, chiefly 

 of the genus Opuntia. The level plains 

 are covered with low open stands of 

 Covillea, or else of Franseria, Acacia 

 and Prosopis. The flood-plains bear 

 relatively dense forests of Prosopis 

 velutina or P. glandulosa, reaching a 

 maximum height of 30 ft., and accom- 

 panied by shrubby species of Zizyphus, 

 Celtis and Lycium. On coarse outwash 

 slopes there are open forests of Parkin- 

 sonia, Olneya and Prosopis, reaching a 

 maximum height of 20 ft. and accom- 

 panied by a large number of small- 

 leaved shrubs, such as Acacia, Lycium, 

 Jatropha, Krameria, and Simmondsia, 

 together with a great diversity of cacti, 

 including Carnegiea, Ferocactus, Opun- 

 tia and Echinocereus. The vegetation 

 of hills and lower mountain slopes is 

 sometimes heavier and richer than that 

 of the outwash slopes, particularly in 

 the case of hills with highly retentive 

 basaltic clay soil; or in other cases is 

 poor and open, as on hills with soils 

 derived from sandstone or rhyolite. 



8. Animals (C. T. V.). The most 

 abundant and conspicuous mammals of 

 this region are the desert jackrabbits, 

 the very interesting antelope jackrabbit 

 {Lepus alleni) (Limited distribution in 

 southern Arizona), Arizona cottontail, 

 small ground squirrels, kangaroo rats, 



