Humus, leaf litter and duff pile up and are sifted, sorted, decomposed and recycled by fungi and invertebrates in their mysterious unseen ways. Plants are arranged in horizontal layers. The tallest trees making the canopy top, younger trees and shorter species compose the understory below the canopy. Still younger trees, saplings and tall shrubs make up the next layer. Then there are small shrubs, seedling trees and the herbaceous plants on the forest floor, and finally the humus and duff layer on top of the soil with all its roots, microrhizae and organisms.
Light is the controlling factor.
Plants must adapt to its transient supply. Blooming and leafing out in spring progresses from the ground up. Flowers bloom first, using stored energy in their bulbs, before the shrubs, understory and canopy leaf out and eclipse the sun. Shrubs and trees often bloom before their leaves emerge and owe their survival to wind-pollinated flowers that produce seeds with wings that act like propellers for dispersing through the relatively open branches. In the summer, deep shade envelops the forest floor and only the largest of leaves, held horizontally to catch as much light as possible, can survive.