In eastern deciduous woods, the forest floor usually contains five to ten years’ worth of leaves and twigs not entirely decomposed, but the same surface layer in western coniferous forests, where summer drought slows decay, may hold the accumulated needle-, cone-, and twig-fall of fifty years. The upper horizon of a coniferous forest floor is properly called litter, and the lower, partly decomposed layer, duff, but duff is also used more broadly to mean all debris in a coniferous forest above the humus and mineral soil. Fires can smolder in duff, erupting days or weeks after surface flames have been snuffed. A Scots term for boiled flour pudding, duff may have been transferred to the stuff of forest floors because of its mixed composition and its soft and yielding quality. Forest duff may or may not be etymologically related to the informal term for the portion of the human anatomy that resting hikers apply to it.

John Daniel