In forest ecology, a snag refers to a standing, partly or completely dead tree, often missing a top or most of the smaller branches, while in freshwater ecology it refers to trees, branches and other pieces of naturally occurring wood found in a sunken form in rivers and streams.
Standing snags provide critical habitat for many species, e.g., woodpeckers that feed on insects dwelling in decomposing wood. Snag persistence depends on two factors, the size of the stem, and the durability of the wood of the species concerned. The snags of some large conifers, such as Coast Redwood on the Pacific Coast of North America and Alerce in Chile, can remain intact for 100 years or more, becoming progressively shorter with age, while other snags with rapidly decaying wood, such as aspen and birch, break up and collapse in 2-10 years.
