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How does rainfall determine whether a tropical forest is evergreen or deciduous?

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Ecuador: rainforest — Tropical rainforest Rainforest vegetation grows along the northern coast of Ecuador.

In the context of tropical forest ecology, rainfall is the primary climatic factor determining whether a forest is evergreen or deciduous. This distinction is driven by both the total annual precipitation and the presence of a pronounced dry season.1

Evergreen Forests: These occur in regions where temperatures remain high and annual rainfall is consistently high, typically exceeding 1,800 to 2,500 mm (about 70 to 100 inches). Because this rainfall is distributed fairly evenly throughout the year, the trees do not face the water stress that would necessitate shedding their leaves, allowing them to remain evergreen year-round.12
Deciduous Forests: These develop in hot climates where annual rainfall is lower, generally between 800 and 1,800 mm, and is characterized by a distinct season of low rainfall. During this dry season, up to three-quarters of the trees shed their leaves to reduce water loss, classifying them as tropical deciduous forests.12

While rainfall is the principal influence, other factors such as soil conditions and human disturbance can also affect forest structure and distribution.1