The Amphibians and Reptiles of Sinaloa, Mexico - Smithsonian ...

54 UNIVERSITY OF KANSAS PUBLS., MUS, NAT. HIST.
by Brand (1936:24-25); the northern portions of the thorn forest,
as presented by Shreve (1937:608-611); the thorn forest as interpreted
by Gentry (1942:27-30); and part of the thorn forest as discussed
by Leopold (1959:35-36). Thorn woodland vegetation is
found from sea level to about 500 meters elevation throughout the
northern lowlands, southward along the coast to near the mouth of
theRioEIota(Fig. 7).
Thorn woodland vegetation is not uniformly distributed in Sinaloa
but is arranged in clumps, becoming nearly continuous in moist
habitats. Thorn woodland is found on the lowland basaltic bills
and mesas and to a lesser extent in the lowland valleys. The vegetation
of the mesas and slopes is uniform in type of growth with
close spacing of the individuals (Plate 1, Fig. 1). The vegetation
found along the margins of arroyos and in valleys is varied and
irregular in formation and individual spacing (Plate 1, Fig. 2). The
arroyos support a riparian vegetation composed of evergreens and
partially deciduous types. The valleys are often dominated by pure
stands of Prosopis chilensis (Gentry, 1942:29).
The belt immediately along the coast is more arid than the interior.
Likewise, the vegetation is reduced and more open (Plate 2,
Fig.l).
The average height of thorn woodland vegetation is about seven
meters. There is a strong influx of succulent and thorny plants
giving rise to a heterogeneous vegetation dominated by xeromorpbic
deciduous leaf types (Plate 2, Fig. 2). Creosote bush, saguaro, and
ocotillo, common to the north in Sonora, are virtually unknown,
being replaced by numerous species of Caesalpinia, Cassia, Acacia,
Cercidium, Jatropha, and Mimosa. Acacia cymbispina forms about
60 per cent of the cover in thorn woodland (Shreve, 1937:609).
Gentry (1942:30) mentioned Acacia cymbispina as the dominating
successionist in MUpa clearings and along roads and trails in the
valley of the Rio Mayo, Sonora. Species of Ficus, Enterolobium,
and Taxodium, and other tropical trees become increasingly numerous
in the river valleys. Thickets of the characteristic desert
plants including Cercidium, Encelia, and Opuntia have been reported
in the valley of the Rio Fuerte (Shreve, 1934:377). Other
species of common plants found in the thorn woodland are: Zizyphus
sonorenis, Pithecollobium sonorae, Cercidium floridum,
Pachycereus pecten-aboriginum, Franseria cordifolia, Karwinskia
humboldtiana, and Croton alamosanus.