Rainforest Division (Bailey)
Between the equator and lat. 10 N. lies a region classified as wet equatorial or rainforest climate. Average annual temperatures are close to 80°F (27°C); seasonal variation is virtually imperceptible. Rainfall is heavy throughout the year, but monthly averages vary considerably due to seasonal shifts in the equatorial convergence zone and a consequent variation in air mass characteristics. Trewartha classifies this climate as Ar, with no month averaging less than 2.4 in (60 mm) of rainfall.
The equatorial region has a rainforest or selva type of vegetation unsurpassed in number of species and luxuriance of tree growth. Broadleaf trees reach 100 to 150 ft (30 to 45 m) in height, forming a dense leaf canopy through which little sunlight reaches the ground. Giant lianas (woody vines) hang from trees. The forest is evergreen, but individual species have various leaf-shedding cycles.
The rainforest is home to small forest animals able to live and travel in the continuous forest canopy. Bird species are numerous and spectacularly plumaged.
Copious rainfall and high temperatures combine to keep chemical processes continuous on the rocks and soils. Leaching of all soluble elements of the deeply decayed rock produces Ultisols and Oxisols that are often especially rich in hydroxides of iron, magnesium, and aluminum.
Streamflow is fairly constant because a large water surplus throughout the year provides ample runoff. River channels are lined with dense vegetation. Sand bars and banks are less conspicuous than in drier regions. Floodplains have meanders and many swampy sloughs where river channels have shifted their courses. Although water is abundant, river systems carry relatively little dissolved material, because thorough leaching of soils removes most soluble mineral matter before it reaches streams.
Not all equatorial rainforest areas have low topographic relief. Hilly or mountainous belts have very steep slopes; frequent flows, slides, and avalanches of soil and rock strip away surfaces down to the bedrock.
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