Snag (ecology)

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A fir tree snag among living fir trees SnagTree.jpg
A fir tree snag among living fir trees

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. Snags provide habitat for a wide variety of wildlife but pose hazards to river navigation. When used in manufacturing, especially in Scandinavia, they are often called dead wood and in Finland, kelo wood.

Contents

Forest snags

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. [1] [2] [3] Snags and downed coarse woody debris represent a large portion of the woody biomass in a healthy forest. [1] [2] [3]

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. [4] [5] 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. [6]

A coast Douglas-fir snag provides nest cavities for birds Pseudotsuga menziesii 01221.JPG
A coast Douglas-fir snag provides nest cavities for birds

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. [6] 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. [6]

Successional stages of a snag from death of a tree to final decomposition. Snag Stages.jpg
Successional stages of a snag from death of a tree to final decomposition.

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. [7] 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 Giant Sequoia and Coast Redwood on the Pacific Coast of North America, and the Alerce of 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.

Stage 5 snags, Wallowa County, Oregon Wallowa Lake - DPLA - 61c7e8e29a510a50b898c10457239fed.jpg
Stage 5 snags, Wallowa County, Oregon

Snag forests, or complex early seral forests, are ecosystems that occupy potentially forested sites after a stand-replacement disturbance and before re-establishment of a closed-forest canopy. [8] They are generated by natural disturbances such as wildfire or insect outbreaks that reset ecological succession processes and follow a pathway that is influenced by biological legacies (e.g., large live trees and snags downed logs, seed banks, resprout tissue, fungi, and other live and dead biomass) that were not removed during the initial disturbance. [9] [10]

Water hunting birds like the osprey or kingfishers can be found near water, perched in a snag tree, or feeding upon their fish catch.

Freshwater snags

In 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.

A Canada goose feeds near a snag in the Nisqually River Nisqually River 05894.JPG
A Canada goose feeds near a snag in the Nisqually River

In Australia, the role of freshwater snags has been largely ignored until recently, and more than one million snags have been removed from the Murray-Darling basin. Large tracts of the lowland reaches of the Murray-Darling system are now devoid of the snags that native fish like Murray cod require for shelter and breeding. The damage such wholesale snag removal has caused is enormous but difficult to quantify, however some quantification attempts have been made. [11] Most snags in these systems are river 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.

Maritime hazard

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 19th century and early 20th century. Snags were, in fact, the most commonly encountered hazard, especially in the early years of steamboat travel. [12] 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. Starting in 1824, there were successful efforts to remove snags from the Mississippi and its tributaries. [12] By 1835, a lieutenant reported to the Chief of Engineers that steamboat travel had become much safer, but by the mid-1840s the appropriations for snag removal dried up and snags re-accumulated until after the Civil War. [12]

Dead wood products

"Kelo wood" ("dead wood") used in Finland in the manufacture of rustic furniture products. Pinus sylvestris counter.jpg
"Kelo wood" ("dead wood") used in Finland in the manufacture of rustic furniture products.

In Scandinavia and Finland, snags, invariably pine trees, known in Finnish as kelo and in Swedish as torraka, are collected for the production of different objects, from furniture to entire log houses. Commercial enterprises market them abroad as "dead wood" or in Finland as "kelo wood". They have been especially prized for their silver-grey weathered surface in the manufacture of vernacular or national romantic products. The suppliers of "dead wood" emphasise its age: the wood has developed with dehydration in the dry coldness of the subarctic zones, the tree having stopped growing after some 300–400 years, and the tree has remained upright for another few hundred years. "Dead wood" logs are easier to transport and handle than normal logs due to their lightness.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Woodpecker</span> Family of birds (Picidae)

Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar and the extreme polar regions. Most species live in forests or woodland habitats, although a few species are known that live in treeless areas, such as rocky hillsides and deserts, and the Gila woodpecker specialises in exploiting cacti.

<span class="mw-page-title-main">Common goldeneye</span> Species of bird

The common goldeneye or simply goldeneye is a medium-sized sea duck of the genus Bucephala, the goldeneyes. Its closest relative is the similar Barrow's goldeneye. The genus name is derived from the Ancient Greek boukephalos, a reference to the bulbous head shape of the bufflehead. The species name is derived from the Latin clangere.

<span class="mw-page-title-main">Pileated woodpecker</span> Species of bird

The pileated woodpecker is a large, mostly black woodpecker native to North America. An insectivore, it inhabits deciduous forests in eastern North America, the Great Lakes, the boreal forests of Canada, and parts of the Pacific Coast. It is the largest confirmed extant woodpecker species in North America, with the possible exception of the ivory-billed woodpecker, which the U.S. Fish and Wildlife Service has proposed be reclassified as extinct. It is also the third largest species of woodpecker in the world, after the great slaty woodpecker and the black woodpecker. "Pileated" refers to the bird's prominent red crest, from the Latin pileatus meaning "capped".

<span class="mw-page-title-main">Downy woodpecker</span> Species of woodpecker

The downy woodpecker is a species of woodpecker, the smallest in North America. Length ranges from 14 to 18 cm. Downy woodpeckers primarily live in forested areas throughout the United States and Canada, with the exception of deserts in the southwest and the northern tundra. The bird nests in tree cavities and feeds primarily on insects, although it supplements its diet with seeds and berries. The downy woodpecker is very similar in appearance to the hairy woodpecker, although they are not closely related.

<span class="mw-page-title-main">Acorn woodpecker</span> Species of bird

The acorn woodpecker is a medium-sized woodpecker, 21 cm (8.3 in) long, with an average weight of 85 g (3.0 oz).

<span class="mw-page-title-main">Red-bellied woodpecker</span> Species of bird

The red-bellied woodpecker is a medium-sized woodpecker of the family Picidae. It breeds mainly in the eastern United States, ranging as far south as Florida and as far north as Canada. Though it has a vivid orange-red crown and nape it is not to be confused with the red-headed woodpecker, a separate species of woodpecker in the same genus with an entirely red head and neck that sports a solid black back and white belly. The red-bellied earns its name from the pale reddish blush of its lower underside.

<span class="mw-page-title-main">Red-naped sapsucker</span> Species of bird

The red-naped sapsucker is a medium-sized North American woodpecker. Long thought to be a subspecies of the yellow-bellied sapsucker, it is now known to be a distinct species.

Dead tree may refer to:

<span class="mw-page-title-main">Red-cockaded woodpecker</span> Species of bird

The red-cockaded woodpecker is a woodpecker endemic to the southeastern United States.

<span class="mw-page-title-main">Coarse woody debris</span>

Coarse woody debris (CWD) or coarse woody habitat (CWH) refers to fallen dead trees and the remains of large branches on the ground in forests and in rivers or wetlands. A dead standing tree – known as a snag – provides many of the same functions as coarse woody debris. The minimum size required for woody debris to be defined as "coarse" varies by author, ranging from 2.5–20 cm (1–8 in) in diameter.

<span class="mw-page-title-main">Old-growth forest</span> Forest that has developed over a long period of time without disturbance

An old-growth forest is a forest that has developed over a long period of time without disturbance. Due to this, old-growth forests exhibit unique ecological features. The Food and Agriculture Organization of the United Nations defines primary forests as naturally regenerated forests of native tree species where there are no clearly visible indications of human activity and the ecological processes are not significantly disturbed. One-third of the world's forests are primary forests. Old-growth features include diverse tree-related structures that provide diverse wildlife habitats that increases the biodiversity of the forested ecosystem. Virgin or first-growth forests are old-growth forests that have never been logged. The concept of diverse tree structure includes multi-layered canopies and canopy gaps, greatly varying tree heights and diameters, and diverse tree species and classes and sizes of woody debris.

<span class="mw-page-title-main">Bark beetle</span> Subfamily of beetles

A bark beetle is the common name for the subfamily of beetles Scolytinae. Previously, this was considered a distinct family (Scolytidae), but is now understood to be a specialized clade of the "true weevil" family (Curculionidae). Although the term "bark beetle" refers to the fact that many species feed in the inner bark (phloem) layer of trees, the subfamily also has many species with other lifestyles, including some that bore into wood, feed in fruit and seeds, or tunnel into herbaceous plants. Well-known species are members of the type genus Scolytus, namely the European elm bark beetle S. multistriatus and the large elm bark beetle S. scolytus, which like the American elm bark beetle Hylurgopinus rufipes, transmit Dutch elm disease fungi (Ophiostoma). The mountain pine beetle Dendroctonus ponderosae, southern pine beetle Dendroctonus frontalis, and their near relatives are major pests of conifer forests in North America. A similarly aggressive species in Europe is the spruce ips Ips typographus. A tiny bark beetle, the coffee berry borer, Hypothenemus hampei is a major pest on coffee plantations around the world.

<span class="mw-page-title-main">Fire ecology</span> Study of fire in ecosystems

Fire ecology is a scientific discipline concerned with the effects of fire on natural ecosystems. Many ecosystems, particularly prairie, savanna, chaparral and coniferous forests, have evolved with fire as an essential contributor to habitat vitality and renewal. Many plant species in fire-affected environments use fire to germinate, establish, or to reproduce. Wildfire suppression not only endangers these species, but also the animals that depend upon them.

<span class="mw-page-title-main">Forest ecology</span> Study of interactions between the biota and environment in forests.

Forest ecology is the scientific study of the interrelated patterns, processes, flora, fauna and ecosystems in forests. The management of forests is known as forestry, silviculture, and forest management. A forest ecosystem is a natural woodland unit consisting of all plants, animals, and micro-organisms in that area functioning together with all of the non-living physical (abiotic) factors of the environment.

Salvage logging is the practice of logging trees in forest areas that have been damaged by wildfire, flood, severe wind, disease, insect infestation, or other natural disturbance in order to recover economic value that would otherwise be lost.

<span class="mw-page-title-main">Tree hollow</span> Cavity in tree, often used as habitat

A tree hollow or tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree. They are found mainly in old trees, whether living or not. Hollows form in many species of trees, and are a prominent feature of natural forests and woodlands, and act as a resource or habitat for a number of vertebrate and invertebrate animals.

Variable retention is a relatively new silvicultural system that retains forest structural elements for at least one rotation in order to preserve environmental values associated with structurally complex forests.

<span class="mw-page-title-main">Dead hedge</span> Artificial barrier constructed from foliage

A dead hedge is a barrier constructed from cut branches, saplings, and foliage. The material can be gathered from activities such as pruning or clearing, and in traditional forms of woodland management, such as coppicing. Its ecological succession can be a beetle bank or hedge.

<span class="mw-page-title-main">Log jam</span> Accumulation of large wood in a stream or river, preventing movement downstream

A log jam is a naturally occurring phenomenon characterized by a dense accumulation of tree trunks and pieces of large wood across a vast section of a river, stream, or lake. Log jams in rivers and streams often span the entirety of the water's surface from bank to bank. Log jams form when trees floating in the water become entangled with other trees floating in the water or become snagged on rocks, large woody debris, or other objects anchored underwater. They can build up slowly over months or years, or they can happen instantaneously when large numbers of trees are swept into the water after natural disasters. A notable example caused by a natural disaster is the log jam that occurred in Spirit Lake following a landslide triggered by the eruption of Mount St. Helens. Until they are dismantled by natural causes or humans, log jams can grow quickly, as more wood arriving from upstream becomes entangled in the mass. Log jams can persist for many decades, as is the case with the log jam in Spirit Lake.

<span class="mw-page-title-main">Complex early seral forest</span> Type of ecosystem present after a major disturbance

Complex early seral forests, or snag forests, are ecosystems that occupy potentially forested sites after a stand-replacement disturbance and before re-establishment of a closed forest canopy. They are generated by natural disturbances such as wildfire or insect outbreaks that reset ecological succession processes and follow a pathway that is influenced by biological legacies that were not removed during the initial disturbance. Complex early seral forests develop with rich biodiversity because the remaining biomass provides resources to many life forms and because of habitat heterogeneity provided by the disturbances that generated them. In this and other ways, complex early seral forests differ from simplified early successional forests created by logging. Complex early seral forest habitat is threatened from fire suppression, thinning, and post-fire or post-insect outbreak logging.

References

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