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".

Snags are an important structural component in forest communities, making up 10-20% of all trees present in old-growth tropical, temperate, and boreal forests. Snags and downed coarse woody debris represent a large portion of the woody biomass in a healthy forest.

In temperate forests, snags provide critical habitat for more than 100 species of bird and mammal, and snags are often called 'wildlife trees' by foresters. Dead, decaying wood supports a rich community of decomposers like bacteria and fungi, insects, and other invertebrates. These organisms and their consumers, along with the structural complexity of cavities, hollows, and broken tops make snags important habitat for birds, bats, and small mammals, which in turn feed larger mammalian predators.

Snags are optimal habitat for primary cavity nesters such as woodpeckers which create the majority of cavities used by secondary cavity users in forest ecosystems. Woodpeckers excavate cavities for more than 80 other species and the health of their populations relies on snags. Most snag-dependent birds and mammals are insectivorous and represent a major portion of the insectivorous forest fauna, and are important factors in controlling forest insect populations. There are many instances in which birds reduced outbreak populations of forest insects, such as woodpeckers affecting outbreaks of southern hardwood borers and Engelmann spruce beetles.

Snag creation occurs naturally as trees die due to old age, disease, drought, or wildfire. A snag undergoes a series of changes from the time the tree dies until final collapse, and each stage in the decay process has particular value to certain wildlife species. 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.

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