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

AMPHIBIANS AND REPTILES OF SINALOA, MEXICO 51
the state. A comparison of wind-direction, seasonal rains, and
migratory low pressure systems over western Mexico indicates that
northerly winds are associated with low pressure to the east and
southerly winds are associated with low pressure to the northwest.
The beginning of the rainy season coincides with the appearance
of southerly winds that carry moist air into the Gulf of California
and showers to the coastal lowlands. Although rain-laden clouds
are carried overland by southerly onshore winds during July through
September, the relief of the coastal plain is such that storm clouds
do not release their rain until a slight increase in elevation is encountered.
The coastal plain of Sinaloa is flat, and rain clouds often
pass over without dropping moisture—for example, in central Sinaloa
at Guamuchil and Badiraguato (Fig, 3). This contributes to the
xeric conditions on the coastal plain as compared to the foothills,
and is reflected in the composition of the flora and fauna.
Vegetation
The physiographic and climatic conditions in Sinaloa are reflected
in the composition of the natural vegetation and together with it
provide habitat for a rich and diverse amphibian and reptile fauna.
The nature of the transition between the northern temperate and
the southern tropical environments is gradual but clearly evident
in a comparison of the xeric thorn woodland vegetation of the arid
northern lowlands to the deciduous semiarid forest vegetation characterise
of the more mesic southern lowlands, or in a comparison
of the pine forests of the northern highlands to the subtropical and
tropical dry forests found at middle elevations in the southern portions
of the state.
The vegetation of northwestern Mexico, especially Sinaloa, has
been studied by only a few workers. Brand (1936) summarized
previous work and listed the more important references. Accordingly,
a historic sketch prior to 1936 need not be repeated here.
Brand recognized three different associations on the coastal plain
as expressions of temperature and precipitation and a fourth association
along the eastern border of the state. Shreve (1937) described
the vegetation of the lowlands of Sinaloa and discussed
the thorn forest with special reference to profile, density, foliation,
stratification, and components.
The vegetation of the Rio Mayo Valley, Sonora, was thoroughly
studied by Gentry (1942), He recognized four major divisions:
thorn forest from sea level to about 600 meters; short-tree forest in