May 24,1924 Significance of the Southwestern Desert Vegetation 
729 
many alternations of the creosote bush and desert-sage areas and the two mix 
to a large extent (fig. 3). 
The following are the principal plant communities in the Gila Valley: 
Plant communities 
Associations— 
Giant cactus and paloverde__ 
Creosote bush and bur-sage, _ 
Creosote bush_ 
Desert-sage_ 
Narrowleaf saltbush 
Seep weed_ 
Saltgrass_ 
Minor communities— 
Chamiso_ 
Atriplex fasciculata , 
Saltbush_ 
Mesquite thicket_ 
Dominant species 
Carnegiea gigantecf( Engelm.) Britton & Rose 
(giant cactus). 
Cercidium torreyanum (S. Wats.) Sarg. (palo- 
verde). 
Covillea glutinosa (Engelm.) Rydb. (creosote 
bush). 
Franseria dumosa A. Gray (bur-sage). 
Covillea glutinosa (Engelm.) Rydb. (creosote 
bush). 
Atriplex polycarpa (Torr.) S. Wats, (desert- 
sage). 
Atriplex linearis S. Wats, (narrowleaf salt¬ 
bush) . 
Dondia intermedia (S. Wats.) Heller (seep- 
weed). 
Distichlis spicata (L.) Greene (salt grass). 
Atriplex canescens (Pursh) Nutt, (chamiso). 
Atriplex fasciculata S. Wats. 
Atriplex lentiformis (Torr.) S. Wats, (salt¬ 
bush). 
Prosopis glandulosa Torr. (mesquite). 
The chamiso ( Atriplex canescens (Pursh) Nutt.) covers the sandy ridges and 
knolls occurring in the desert-sage areas. Towards the lower parts of the valley 
there is a gradual change from pure desert-sage to that mixed with narrowleaf 
saltbush or seepweed. Still lower these shrubs mix with the mesquite. The 
shrubs and seepweed grow on low hummocks, composed of a light soil piled by 
the wind in the open spaces between the mesquite trees. In low places where 
there is subirrigation the mesquite becomes a thicket and excludes the other 
shrubs. 
The lands along the rivers and creeks and the abandoned “seepland,” where 
the water table is high, are covered with saltbush (Atriplex lentiformis (Torr.) 
S. Wats.). 
In the vicinity of Casa Grande the vegetation is very similar to that in the 
Gila Valley. It differs most conspicuously from that in Coachella Valley in the 
absence of the large areas of pickleweed and arrowweed and saltbush which cover 
the wet saline lands. Near Casa Grande there are dry saline lands covered either 
with seepweed, narrowleaf saltbush or an annual Atriplex (A. fasciculata S. 
Wats.). 
The creosote bush and desert-sage are, by far, the most important types of 
vegetation in this section. While they occupy the same relative position as in 
the Coachella and Gila Valleys the lines between them are much more obscure 
than in the former, and slightly more so than in the latter locality. This is due 
to the very gentle slope of the land, which causes the soil conditions, texture, 
moisture, and salinity to change very gradually. These two types of vegetation 
alternate frequently and near the lines of contact mix to a considerable extent. 
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