The temperate deciduous forests are only green for a part of the year. During the often harsh winters, many plants drop their leaves, and many organisms must go into hibernation or diapause. The temperature drops dramatically, resulting in snow and sometimes frozen water sources. When the forest is leafed, there is no real room for new plants to grow and succeed, because the tall trees and their leaves blot out the sun, starving sprouting plants. This means that the plants that do grow on the forest floor must be shade tolerant. However, the species that are able to survive struggle with overeating from the many herbivore species that call the forest home. They also need to grow extensive root systems to have a strong hold in the damp soil, especially in cases of heavy rains and floods. Sometimes organisms suffer from disease and parasites.