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

AMPHIBIANS AND REPTILES OF SINALOA, Mexico 55
Tropical Semiarid Forest
The Tropical Semiarid Forest vegetation as used here is essentially
the same as; the semi-arid scrub, as described by Brand
(1936:25-27); the southern part of the thorn forest (Shreve, 1937:
608-610; Leopold, 1959:35-36); the short-tree forest (Gentry, 1942:
30-34); the tropical deciduous forest (Leopold, 1959:34-35). Semiarid
forest is found between 300 and 1000 meters elevation in
northern Sinaloa and continues south along the foothills to the
coast near La Cruz, and then into southern Sinaloa on the coastal
plain, Semiarid forest is bordered on the south by dry forest and
on the east by subtropical dry forest. In several places tropical
vegetation advances far into the Sierra Madre along deep barrancas
and river valleys that extend at low elevations for many miles inland
from the coastal plain (Fig. 7).
The semiarid forest is characterized by taller and more closely
spaced vegetation than thorn woodland. The change in physiognomy
is primarily due to the general increase in mean annual
precipitation. There is an overall increase in density, particularly
of shrubs and other undergrowth, which continues southward
through the formation. In the north the vegetation is more xeric
than along the slopes or in the southern lowland (Plate 3, Fig. 1;
Plate 3, Fig. 2). The prevalent height of the vegetation is about
10 meters, although in some areas it is often less.
The dominant plants of the semiarid forest belong to the family
Leguminosae. The subfamilies Mimosoideae and Caesalpinioideae
contain more than a quarter of the total tree and scrub species of
the formation, and possibly contribute three-quarters of the individuals
comprising the vegetation (Brand, 1936:26). Trees are
the dominant plants and tend to a uniform height. Succulents are
relatively unimportant except for the large columnar cactus, Packycereus
pecten-arboriginttm, which is an abundant and conspicuous
species in the semiarid forest (Plate 4). None of the desert shrubs,
such as Atriplex, Encelia, Franseria, and Viguiera is found in southem
Sinaloa. Acacia cymbispina, with its boat-shaped spines, dominates
the vegetation and frequently forms 90 per cent of the stand
in lowland areas (Plate 5). In certain regions the abundance of
Acacia cymbispina determines the level of the forest canopy, as is
the case in an extensive stand 40 kilometers south of Culiacan.
Terrestrial bromeliads and epiphytic species of Tillandsia are more
abundant in the semiarid forest than in the thorn woodland to the
north. The floodplains and valleys support Ficus, Enterolobium,