- Snag
In
forest ecology , a snag refers to a standing, partly or completely deadtree , 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.Forest snags
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 andbirch , break up and collapse in 2-10 years.Snag trees are referred to for various bird species. Water hunting birds like the
Osprey orKingfisher s 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.Freshwater snags
In the freshwater ecology in
Australia and theUnited States , the term snag is used to refer to the trees, branches and other pieces of naturally occurring wood found in a sunken form inriver s and streams. Such snags have been identified as being critical for shelter and as spawning sites forfish , and are one of the few hard substrates available forbiofilm 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.Also known as "deadheads", partially submerged snags posed hazards to early riverboat navigation and commerce. If hit, snags punctured the wooden hulls used in the 1800s and early 1900s. In the United States, the U.S. Army Corps of Engineers operated Snagboats such as the W.T. Preston in the
Puget Sound of Washington State and the Montgomery in the rivers of Alabama to pull out and clear snags.In
Australia , the role of freshwater snags has been largely ignored until recently, and more than one million snags have been removed from theMurray-Darling basin . Large tracts of the lowland reaches of the Murray-Darling system are now devoid of the snags that native fish likeMurray Cod require for shelter and breeding. The damage such wholesale snag removal has caused is clearly enormous, but is difficult to quantify (but see [http://www.blackwell-synergy.com/doi/abs/10.1046/j.1526-100X.2002.01043.x;jsessionid=bzJx9fH83X44rRfwaT?journalCode=rec] ). Most snags in these systems areRiver Red Gum snags. As the dense wood of River Red Gum is almost impervious to rot it is thought that some of the River Red Gum snags removed in past decades may have been several thousand years old.
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