The diversity of trees found in the tropical rain forests is quite extraordinary, and far exceeds that of any other kind of forest, sometimes with as many as 100 tree species per hectare. Equally remarkable are the heights and diameters of the trees, which while not the tallest or most massive on earth, still dwarf those of most other forest systems. For example, Khaya ivorensis, a member of the family of trees collectively known as African mahogany, can attain heights of up to 60 metres, and diameters of 1.8 metres.
Because of the constantly high temperatures and extreme humidity found in the tropical rain forests, bacteria and other micro-organisms thrive in the top layer of soil, feeding on the mass of decaying matter which falls from the canopy. The result is that there is little matter left to accumulate, so the top soil is very thin and poor in nutrients.
Under normal circumstances large trees send out masses of roots which can extend for several metres. These roots absorb nutrients from the soil and give the tree a firm foundation. rain forest trees have a large number of roots, but these can only extend a small distance into the thin soil, and cannot therefore provide stability. The solution is to employ buttress roots which work in much the same way as buttresses on citadels or Christian cathedrals, and these can grow up to 5 m up the trunk of a tree.
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