Vegetation of Northwest Mexico [ 17 
Durango. This elevated (3,000-7,000 feet) Basin and Range 
country falls into the arid climates (BWh and BSh of Koep- 
pen) due to low precipitation values, as the mean annual 
temperatures run under 20°C., excepting in the extreme 
northeastern portion. Xerophytic succulents (cacti, No - 
Unas , Dasylirions, Yuccas), microphyls (Prosapis, Larrea , 
Fouquiera, Acacia) and steppe grasses (Sporobolus, Boute - 
loua, Aristida, HUaria, Munroa) form the bulk of the vege¬ 
tation. In the more humid isolated mountain areas, oaks, 
junipers, and pines dominate the landscape. Along the 
river courses are gallery forests made up of cottonwood 
(Populus sp.), willows (Salix sp.), walnuts (Juglans sp.), 
hackberry (Celtis sp.), ash (Fraxinus sp.), and sycamore 
(Platanus sp.), in addition to thickets of Prosopis, Mimosa , 
Acacia , and other Leguminosae . 
1. Creosote-yucca-mesquite: This area falls into three 
subdivisions or vegetation districts. In northern Chihua¬ 
hua, from the mesa fronting the Rio Grande west to Laguna 
Guzman and the Santa Maria River and from the Potrillo 
Hills north of the International Boundary south to Bajio del 
Ojo Caliente, Laguna Patos and the Conchos basin, is a 
gravel- and sand-covered plain scantily vegetated with 
yuccas (mainly Yucca elata) t creosote bush (Larrea triden - 
tata) f and mesquite (Prosopis julifiora). The creosote bush 
is the dominant plant of this creosote-yucca-mesquite asso¬ 
ciation. Hundreds of square miles of arid basin land 
present to view the drab green of creosote bushes widely 
spaced over the gravelly plains. The yuccas, mesquite, and 
Koeberlinia spinosa are common in the sand dune areas 
where they often have contributed to the formation of the 
dunes. Occasional Crotons , chamiso (Atriplex canescens) 
and Zizyphus flank the dunes. On rocky ridges and the 
small isolated ranges Agaves , ocotillo (Fouquiera splen - 
dens) , Nolbias , Dasylirions , cacti, Yuccas , and mesquite vie 
with the ubiquitous creosote bush. Gramas (Bouteloua), 
needle-grasses (Aristida) and tobosa grass (Hilaria mutica) 
