Litterfall, plant litter, leaf litter, tree litter, soil litter, or duff, is dead plant material, such as leaves, bark, needles, and twigs, that have fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer or O horizon ("O" for "organic"). Litter has occupied the attention of ecologists at length for the reasons that it is an instrumental factor in ecosystem dynamics, it is indicative of ecological productivity, and may be useful in predicting regional nutrient cycling and soil fertility.