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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. When used in manufacturing, especially in Scandinavia, they are often called "dead wood" and in Finland "kelo wood".
== Forest snags ==
Large standing snags provide critical habitat for many species both faunal and avifaunal such as woodpeckers that feed on insects dwelling in decomposing wood. Because snags are a result of decay, they are soft enough for woodpeckers and other cavity excavators to make entrance holes and cavities. Snags often lack branches and dense canopies which allow for unobstructed flight movements by birds during predation. This makes snags optimal habitat for woodpeckers such as the pileated woodpecker which create the majority of cavities used by secondary cavity users in forest ecosystems. Because snags are not resourceful for timber harvests, they are often left behind and help maintain the biodiversity of the area which may have otherwise been lost. In the Puget Sound ecosystem, research and observations have shown that the pileated woodpecker excavates cavities for more than 80 species and the stabilization of their population relies on snags being conserved during or after timber harvests.
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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