Snag (ecology)

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.

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 Patagonia, 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.

Snag trees are referred to for various bird species. Water hunting birds like the Osprey or Kingfishers can be found near water, perched in a snag tree, or feeding upon their fish catch. The snag offers clear unobstructed movement for flight, as well as observation for predators.

In the freshwater ecology in Australia and the United States, the term snag is used to refer to the trees, branches and other pieces of naturally occurring wood found in a sunken form in rivers and streams. Such snags have been identified as being critical for shelter and as spawning sites for fish, and are one of the few hard substrates available for biofilm growth supporting aquatic invertebrates in lowland rivers flowing through alluvial flood plains. Snags are important as sites for biofilm growth and for shelter and feeding of aquatic invertebrates in both lowland and upland rivers and streams.

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