Vegetation of Northwest Mexico [ 25 
southern river valleys higueras (Ficus sp.), huanacastles 
(Enterolobium), sabinos (Taxodium sp.), and other tropical 
trees are increasingly numerous. 
Sinaloa Tropical: The Sinaloa tropical district is tropi¬ 
cal in temperature values, but varies in moisture supply be¬ 
tween savannah and desert conditions. Four associations 
are prominent in this district, agreeing in distribution with 
the coastal lagoons, river flood plains, coastal inter-fluviatile 
uplands, and foothills of the Sierra Madre Occidental. Due 
to difficulties in mapping on a small scale map these forma¬ 
tions have not been differentiated on the vegetation map. 
Approximately between the Piastla and Quelite rivers there 
is a change from semi-arid to humid conditions with over 750 
mm. of precipitation annually. This change is marked by 
a taller and more closely spaced vegetation which attains 
to almost forest proportions in many of the river valleys. 
This, however, is more of a change in formation than in 
association. 
Along the coast, especially on the mud flats bordering 
lagoons, there is a dense shrub vegetation. This vegetation 
consists of mangrove (Rhizaphora 'mangle) in the tidal 
areas, and seagrape (Coccoloba uvifera), Pisonea sp., mora 
(Chlorophora tinctoria), numerous Caesalpinias , etc., along 
the shore. Various palms, especially Cocos nucifer'a are 
common near the beach. Dense and nearly continuous scrub 
forests of this coastal association occur south of Mazatlan, 
particularly from the mouth of the Rio del Presidio south¬ 
ward. North of Mazatlan the association is more open, 
lower, and separated by long stretches of sandy beach. 
The flood plains of the Sinaloa rivers and adjacent up¬ 
land slopes have been cleared and cultivated for many cen¬ 
turies, in parts going back even to pre-Spanish days, accord¬ 
ing to the evidence of archaeologic sites from the Culiacan 
river southward. The portions not cleared for cultivation 
have been selectively devegetated in accordance with de¬ 
mands for charcoal, timber, cabinet woods, and dye woods. 
