506
BERNARD L. FONTANA
more clearly demarked. Greater erosion of mountains, brought on by
heavier rains, has done more to enrich the soil of bajadas and valleys,
and rich alluvial fans have built up at the mouths of washes. Central
Pimeria Alta is devoid of dry lake beds and land-locked basins, and the
greater rainfall and sloping plains ensure that the washes carry the water
away rather than allowing it to settle, thus avoiding large accumulations
of salts at the earth's surface. This is in spite of the fact that central
Pimeria also lacks perennial streams, and that all surface water flow tends
to be interrupted, intermittent, and ephemeral. Drainage is into the Gila,
Sonoyta, Altar, Magdalena, and San Miguel rivers.
c. Plant Life. Although most of the vegetation is characteristic of
Shreve's Arizona Upland province, the Baboquivari Mountains and the
area to the southeast of them actually lie beyond the Sonoran Desert altogether. The mountains here are within the Upper Sonoran life zone and
have an evergreen-woodland vegetation (oak-juniper-pifion) and the valleys have an Upper Sonoran desert-grassland vegetation. Neither of these
vegetation types is characteristic of a "true" desert (Lowe, 1964, pp. 36,
40-43, and 50-56).
The outstanding feature of Arizona Upland vegetation is that the foothill
palo verde, chollas, and prickly pears tend to dominate the scene in place
of creosote bush and bursage. This is also the heartland of the saguaro cactus; there are more, and larger, ironwoods, mesquites, and acacias; and
other succulents, such as the barrel cactus (especially Ferocactus
wislizenii), occur in comparative profusion. It is thus that the microphyllus
desert (Larrea-Franseria region) of the Lower Colorado Valley becomes
the crassicaulescent (stem succulent) desert (Cercidium-Opuntia region)
in the Arizona Upland.
In the plains and lower bajadas of the Arizona Upland, the creosote
bush continues to dominate, providing from 10 to 20% of the cover.
Acacias, mesquites, and chollas are also found here, as are occasional isolated plants of ocotillo, saguaro, and barrel cactus. In the upper bajadas,
which comprise half the intermontane plain area of this central region, is
the largest number of perennials per unit area outside of Sonoran Desert
flood plains. Creosote bush, palo verde, chollas, ocotillo, Lycium, gray
thorn (Condalia lycioides), netleaf hackberry (Celtis pallida), and mesquite are the dominant plants, with jojoba (Simmondsia
chinensis),
saguaro, ironwood, barrel cactus, and Mexican crucillo (Condalia spathulata) playing a conspicuous role. Smaller perennials include bursage,
brittlebush, fairy duster (Calliandra eriophylla), zinnia (Z. grandiflora),
and bush muhly (Muhlenbergia ported). Also not to be discounted are
the Thornber, or banana yucca (Yucca baccata), mountain yucca (Y.
schottii), small-flowered agave (Agave parviflora), desert agave (A.
BERNARD L. FONTANA
more clearly demarked. Greater erosion of mountains, brought on by
heavier rains, has done more to enrich the soil of bajadas and valleys,
and rich alluvial fans have built up at the mouths of washes. Central
Pimeria Alta is devoid of dry lake beds and land-locked basins, and the
greater rainfall and sloping plains ensure that the washes carry the water
away rather than allowing it to settle, thus avoiding large accumulations
of salts at the earth's surface. This is in spite of the fact that central
Pimeria also lacks perennial streams, and that all surface water flow tends
to be interrupted, intermittent, and ephemeral. Drainage is into the Gila,
Sonoyta, Altar, Magdalena, and San Miguel rivers.
c. Plant Life. Although most of the vegetation is characteristic of
Shreve's Arizona Upland province, the Baboquivari Mountains and the
area to the southeast of them actually lie beyond the Sonoran Desert altogether. The mountains here are within the Upper Sonoran life zone and
have an evergreen-woodland vegetation (oak-juniper-pifion) and the valleys have an Upper Sonoran desert-grassland vegetation. Neither of these
vegetation types is characteristic of a "true" desert (Lowe, 1964, pp. 36,
40-43, and 50-56).
The outstanding feature of Arizona Upland vegetation is that the foothill
palo verde, chollas, and prickly pears tend to dominate the scene in place
of creosote bush and bursage. This is also the heartland of the saguaro cactus; there are more, and larger, ironwoods, mesquites, and acacias; and
other succulents, such as the barrel cactus (especially Ferocactus
wislizenii), occur in comparative profusion. It is thus that the microphyllus
desert (Larrea-Franseria region) of the Lower Colorado Valley becomes
the crassicaulescent (stem succulent) desert (Cercidium-Opuntia region)
in the Arizona Upland.
In the plains and lower bajadas of the Arizona Upland, the creosote
bush continues to dominate, providing from 10 to 20% of the cover.
Acacias, mesquites, and chollas are also found here, as are occasional isolated plants of ocotillo, saguaro, and barrel cactus. In the upper bajadas,
which comprise half the intermontane plain area of this central region, is
the largest number of perennials per unit area outside of Sonoran Desert
flood plains. Creosote bush, palo verde, chollas, ocotillo, Lycium, gray
thorn (Condalia lycioides), netleaf hackberry (Celtis pallida), and mesquite are the dominant plants, with jojoba (Simmondsia
chinensis),
saguaro, ironwood, barrel cactus, and Mexican crucillo (Condalia spathulata) playing a conspicuous role. Smaller perennials include bursage,
brittlebush, fairy duster (Calliandra eriophylla), zinnia (Z. grandiflora),
and bush muhly (Muhlenbergia ported). Also not to be discounted are
the Thornber, or banana yucca (Yucca baccata), mountain yucca (Y.
schottii), small-flowered agave (Agave parviflora), desert agave (A.