The next layer down is called the understory . This layer is composed of saplings of canopy tree species as well as smaller flowering trees such as dogwoods, redbud, shadblow, ironwood, and hop hornbeam in the eastern United States. The understory extends from 30 to 12 feet. The shrub layer is the lowest layer of woody vegetation. It occupies the area between 12 and 3 feet above the ground.
Shrubs grow in patches where light and space are sufficient.
A variety of forest birds, including vireos and some warblers, use the shrub layer for foraging and nesting. Many shrubs produce edible fruits that enable birds to lay on fat reserves in preparation for migration. The lowest above-ground layer of the forest, below 3 feet, is called the ground layer . Here, wildflowers, ferns, grasses, and sedges grow in often-spectacular assemblages. Plants in the ground layer also partition their environment vertically.