Below the canopy is the three-layered understory, about 9 to 15 m (30 to 50 feet) shorter than the canopy. The top layer of the understory has smaller mature trees, saplings. There are also suppressed juvenile canopy layer trees waiting for an opening in the canopy. Below the sub-canopy is the shrub layer of low-growing woody plants. Most of the time, the lowest growing (and most diverse) layer is the ground cover or herbaceous layer.