Douglas Harper's Etymology Dictionary
Wiktionary
n. The layer of plants that grow in the shade of the canopy of a forest.
Wikipedia
Understory (or understorey, underbrush) in forestry and ecology comprises plant life growing beneath the forest canopy without penetrating it to any great extent, but above the forest floor. Only a small percentage of light penetrates the canopy so understory vegetation is generally shade tolerant. The understory typically consists of trees stunted through lack of light, other small trees with low light requirements, saplings, shrubs, vines and undergrowth. Small trees such as holly and dogwood are understory specialists.
In temperate deciduous forests, many understory plants start into growth earlier than the canopy trees to make use of the greater availability of light at this time of year. A gap in the canopy caused by the death of a tree stimulates the potential emergent trees into competitive growth as they grow upwards to fill the gap. These trees tend to have straight trunks and few lower branches. At the same time, the bushes, undergrowth and plant life on the forest floor become more dense. The understory experiences greater humidity than the canopy, and the shaded ground does not vary in temperature as much as open ground. This causes a proliferation of ferns, mosses and fungi and encourages nutrient recycling, which provides favorable habitats for many animals and plants.
Understory (founded in 2012 as WInstruments) is a company that forecasts weather and collects data using a grid of weather-sensing hardware that tracks weather from the ground level.
Usage examples of "understory".
There were great forests covering the temperate regions, where stands of giant sequoia shaded pine understories.
South Fossa's canyon floor was thick with mature trees, the giant sequoias standing over an understory of pines and junipers and, in the lower stretch of the canyon, aspens and canyon oaks.
The birds had evolved for short flights in the upper canopy, with short forays into the understories, but they were capable of extended journeys so long as they could make frequent stops.
They moved on through the woods and up the steep slope at an angle, a dense forest of tall candle-straight trunks, Douglas fir and western hemlock interspersed with brush and yew, the weed tree of the understory.