The understory is a segment of the woodland ecology that consists of those plants growing above ground level and below the forest canopy. It includes the groundcover of fungus, moss, forbs, graminoids, vines, and shrubs, and a good scattering of young saplings waiting for a break in the canopy. Understories may be very dense, in the case of rhododendron or bamboo forests, or comparability low-key, as in a birch forest.

The understory helps in maintaining a healthy watershed, stabilizing soil, providing shade and insulation for decomposers and helping to breaking down leaf liter, and providing habitat and food for wildlife. Plants adapted to living in the understory are often adapted to living in damper environments, as the layer of leaf litter and the shade from the canopy keep the forest floor cool and damp. They are generally shade tolerant, adapted to absorb light at the far red end of the spectrum, which is more likely to filter in through the canopy. While the large trees are likely to have heavy pigmentation and waxy leaf coatings to protect them from UV light and drying out in the sunlight, the understory species are more likely to have wide, thin, delicate leaves to maximize the use of what sunlight does reach them. In deciduous forests, understory species may sprout early or late in the year, to take advantage of additional sunlight when the larger trees are leafless.

Young saplings are often adapted in other ways; trees can remain as stunted juveniles for decades until there is an opening in the forest overstory, letting in sunlight and triggering them to grow to maturity. Other, smaller, tree species, such as dogwood, hemlock, and holly, rarely grow tall enough to join the canopy and remain as understory trees. Some trees, such as sugar maple, can go either way, sprouting up if the opportunity presents itself, but otherwise quite happily living out their full life-cycle as understory trees.

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