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Trees in tropical rainforests typically do not have deep tap roots because jungle ecosystems have evolved so living things decompose on the surface and plants absorb the nutrients before they sink deep into the soil.  The top 8 inches of jungle soil contains most the nutrients, so trees have very shallow roots that spread long distances to take advantage of this.  Since they don't have the good support and stability provided by a deep tap root, many rainforest trees develop large buttresses to give them stability, much like the fins on a toy rocket.