In a dense forest, less than one percent of the sunlight ever reaches the forest floor. Because of this dense shade, many herbaceous plants flower in the early spring before the leaves open on the trees of the canopy. At times it may be difficult to tell where the ground cover ends and the understory begins. In some forests these layers may be very distinct, while in other forests there may appear to be continuous vegetation from the floor to the canopy. The number of layers of vegetation above the soil varies with the type of forest.
Tropical rainforests have up to six layers.
A mature deciduous forest usually has four layers and a coniferous forest three layers. The layers of vegetation result in differences in temperature, light, moisture, and wind speeds. The canopy receives the most light and undergoes the greatest changes in temperature. The direct sun and winds mean little moisture is available. The understory receives less light, and is cooler.