The main types of tropical forest are distinguished by differences in the distribution of rainfall throughout the year, by elevation, and by soil type. Tropical forests that experience ever-wet conditions with no month receiving less than 100mm of precipitation are generally referred to as ‘tropical rain forests’, although a distinction is also sometimes made between tropical ‘moist forests’ and ‘rain forests’ in a strict sense that receive annual rainfall in excess of 4000 mm.
The two other main tropical forest types, ‘tropical dry forests’ and ‘semi-evergreen rain forests’, experience an annual dry period. In tropical dry forests (also called ‘monsoon forests’) the dry period is severe, and during this most trees drop their leaves in order to reduce water loss. In semi-evergreen rain forests the seasonal drought is less extreme, and a leafless period does not occur to the same extent.
Within these broad moisture regimes, tropical forests are subdivided on the basis of elevation and soil type, and corresponding differences in forest structure. The distinguishing structural characteristics include canopy height, crown layering and the presence (or absence) of different climbers and epiphytes. Tree buttressing, crown shape, leaf structure, and position of flower/fruit formation are other important physiognomic descriptors of tropical forests.
On a global basis the most important types of tropical moist forest include lowland evergreen rain forests, upper and lower montane rain forests, heath forests, peat swamp forests, freshwater swamp forests and mangroves.