The understory (or understorey) is the layer of vegetation in a forest or woodland that grows beneath the main forest canopy but above the forest floor. Because the tall trees overhead block out most of the sunlight, the understory is a unique, shaded environment that requires plants and animals to adapt to low-light conditions.
Plants in the understory are specially adapted to thrive on limited sunlight, often receiving only one to 15 percent of the light that strikes the top of the canopy. This layer, which often contains the great majority of a forest's plant species diversity, varies greatly depending on the region. In lush tropical rainforests, it features giant ferns, thick woody vines and broad-leaved shade plants, while in cooler boreal or temperate forests it contains mosses, lichens and hardy shrubs.
Common to the understory are saplings, which are young canopy trees patiently waiting for a mature tree to fall and create a light gap that allows them to grow upward. This layer also hosts shade-tolerant shrubs, such as wild berries, huckleberries and rhododendrons, alongside true understory trees that naturally mature at shorter heights, such as flowering dogwoods, redbuds and Japanese maples. Additionally, this zone supports vines and epiphytes (non-parasitic plants that climb sturdier trunks) to reach more intense light.
Despite its darkness, the understory is one of the most active layers of a forest ecosystem. It provides wildlife habitat, including shelter, nesting sites and food (such as berries, seeds and insects) for many animals, including songbirds, bats, amphibians and small mammals. This dense vegetation also functions as an insulation layer, keeping the forest floor more humid, cooler in the summer and less windy than the open areas above.