The air near the forest floor is hot and humid.  Only two percent of the light that reaches the canopy can get through to the forest floor, so the forest floor is dim.  This dim light slows the growth of many light demanding plants.  In some of the deepest jungles the ground is a maze of scattered fallen leaves, roots, branches and twigs.  When a tree falls down everything is very different because the extra light allows lots of lianas, herbs and saplings to grow.  Rates of growth are incredible.