Coarse woody debris (CWD) is a term used in ecology to refer to downed wood -- larger branches and tree trunks -- that have fallen to the ground in a forest or wetland. This is most often of interest to humans because the amount of CWD in a forest can help predict the severity of forest fires and soil erosion during heavy rains, and the accessibility to hikers and campers.
However, CWD also provides habitats for small animals, insects, lichen, fungus, moss, and bacteria. One effect of CWD is to provide damp, cool, shady habitats that can persist even in the summer heat, and to retain more humidity in the soil. CWD may provide ecological niches for up to 40% of species in many forests; this can be problematic in cases when it is removed to lower the chances of a forest fire or simply to be more attractive to visitors.
The level of CWD is usually estimated by random sampling, either by choosing random squares of woodland and estimating the biomass of CWD, or by placing random lines through the woodland and measuring the biomass that falls across that line.
Anything with a diameter of less than about 2.5–20 cm in diameter (it varies highly from one author to another) is not coarse woody debris but small woody debris; smaller bits, including small twigs and leaves are called plant litter or duff.
When it falls in the water, CWD is usually called large woody debris.