Understory (or understorey) is the term for the area of a forest which grows at the lowest height level below the forest canopy. Plants in the understory consist of a mixture of seedlings and saplings of canopy trees together with understory shrubs and herbs. Young canopy trees often persist as suppressed juveniles for decades while they wait for an opening in the forest overstory which will enable their growth into the canopy. Understory shrubs, however, can complete their life cycles in the shade of the forest canopy. Some smaller tree species, such as dogwood and holly, rarely grow tall and are generally understory trees.
Forest understories receive less intense light than plants in the canopy. The wavelengths of light that are available are only a subset of those are available in full sunlight. Understory plants must thus be shade tolerant—they must be able to photosynthesize successfully with the limited amount of light available. They are often able to use wavelengths that canopy plants cannot. In temperate deciduous forests understory plants "leaf out" before the canopy trees do. This is important because it allows the understory plants a window in which to photosynthesize without the canopy shading them. This brief period (usually 1–2 weeks) is often a crucial period which allows the plant to maintain a net positive carbon balance over the course of the year.
Forest understories also experience higher humidity than exposed areas. The forest canopy reduces solar radiation, so the ground does not heat up as rapidly. Consequently, the understory dries out more slowly than more exposed areas do. The greater humidity allows fungi and other decomposers to flourish. This drives nutrient cycling, and provides favorable microclimates for many animals and plants.