A savanna is a mixed woodland and grassland ecosystem that has sufficiently wide spacing of the trees that the canopy is not contiguous. This allows enough sunlight to reach the ground to support an unbroken layer of vegetation, mainly grasses. Savannas are often in a transitional zone between a forest and a grassland or desert and have the majority of their rainfall occurring in one season. They are found on all continents except Antartica and cover approximately one fifth of the earth's land area.