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Tropical Forest is determined by the absence of freezing temperatures and the occurrence of ample rainfall for at least part of the year. Some tropical forests have a dry season, while tropical rain forest is never stressed for water. Tropical deciduous forests have a dry season lasting from three to nine months, during which time many of the plants become deciduous. Many of the tree species flower during the winter-spring dry season while leafless. In the rainy season the dense vegetation grows luxuriantly and forms closed canopies of foliage. The upper canopy ranges from fifteen to thirty feet (4.5 to 9 m) above ground in dry forests, to 150 feet (45 m) in lowland rain forests. Almost all life forms are represented, though annuals are nearly absent from rainforest. Flowering epiphytes (plants that grow on other plants or rocks but are not parasitic) are almost completely restricted to tropical habitats, and are a major component of wet tropical floras.

To the south, the Sonoran Desert merges almost imperceptibly into thornscrub in central Sonora, and thornscrub in turn merges with the northern limit of tropical deciduous forest in the southern tip of that state. A major proportion of the Sonoran Desert’s biota evolved from ancestors in these tropical biomes; examples are noted in the species accounts.