In forest ecology, a snag refers to a standing dead or dying tree, often missing a top or most of the smaller branches. In freshwater ecology it refers to trees, branches, and other pieces of naturally occurring wood found sunken in rivers and streams; it is also known as coarse woody debris. Snags provide habitat for a wide variety of wildlife but pose hazards to river navigation. When used in manufacturing, especially in Scandinavia, they are often called dead wood and in Finland, kelo wood.
A fir tree snag among living fir trees
A coast Douglas-fir snag provides nest cavities for birds
Successional stages of a snag from death of a tree to final decomposition.
Stage 5 snags, Wallowa County, Oregon
Coarse woody debris (CWD) or coarse woody habitat (CWH) refers to fallen dead trees and the remains of large branches on the ground in forests and in rivers or wetlands. A dead standing tree – known as a snag – provides many of the same functions as coarse woody debris. The minimum size required for woody debris to be defined as "coarse" varies by author, ranging from 2.5–20 cm (1–8 in) in diameter.
Coarse woody debris in Białowieża Forest, Poland
Huhu beetle larvae (Prionoplus reticularis) consuming CWD in New Zealand
Fungi sprouting from fallen log, Germany
Ensatina eschscholtzii, a species of salamander associated with CWD in western North America
