Even though it looks lush and fertile in tropical rainforest, what we are dealing with is a very shallow soil horizon.  Most of the nutrients in tropical rainforest are in the first few centimeters of the soil. 

High temperatures, high rainfall and high humidity all year round means a lot of the vegetable matter on the forest floor breaks down very quickly releasing nutrients in the first layer of soil and that gives us what's technically called a short nitrogen cycle. 

So all the trees you see around you are what they call "surface rooted", they don't have those deep anchoring tap roots that trees in more temperate regions have.

The roots of the trees in tropical rainforest run across the surface of the ground where all those nutrients are readily available to them.  That also explains why a lot of trees form those large planks, or buttresses at their base.

Those are simply mechanical means to help shore up and support the trees in shallow soils. Those buttresses can grow to an enormous size starting high on the trunk and growing out a considerable distance and can be very narrow and plank like. 

However in times of cyclones or similar storms, as many trees with buttresses will fall over as those without 'cos they only help the tree stand up under normal growing conditions.

The Aboriginal people of the rainforests used suitable buttresses to gouge out their shields. Using a sharp stone they simply gouged out an oval or circular section until eventually they removed an oval or round section of the buttress, dishes and oven lids were made from the same material.

So, if you ever wondered where they got the flat wood from they used to make these implements, they used suitable rainforest tree buttresses, clever eh?