A rain forest or most any other forest for that matter has very poor soil, just the top few inches are where the nutrients are.
The plants feed off of there own dead fallen leaves and animal droppings.
The tree provides a home for critters and they repay it with fertilizer.
My observations on the tundra the best looking plants and flowers were always growing around caribou dung.
Every year they and the birds would arrive to graze, have babies and fertilize the soil.
Many people dont make that very important connection.