In
forestA forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
ecologyEcology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, a
snag refers to a standing, partly or completely dead
treeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
, often missing a top or most of the smaller branches. In
freshwater ecologyLimnology , also called freshwater science, is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters...
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 debrisCoarse woody debris is a term used in English-speaking countries for fallen dead trees and the remains of large branches on the ground in forests. Some prefer the term coarse woody habitat . A dead standing tree is known as a snag and provides many of the same functions as coarse woody debris...
.
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
AlerceFitzroya is a monotypic genus in the cypress family.-Species:The single living species, Fitzroya cupressoides, is a tall, long-lived conifer native to the Andes mountains of southern Chile and Argentina, where it is an important member of the Valdivian temperate rain forests...
in Patagonia, can remain intact for 100 years or more, becoming progressively shorter with age, while other snags with rapidly decaying wood, such as
aspenPopulus section Populus, of the Populus genus, includes the aspen trees and the white poplar Populus alba. The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by...
and
birchBirch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...
, break up and collapse in 2–10 years.
Snag trees are referred to for various bird species. Water hunting birds like the
OspreyThe Osprey , sometimes known as the sea hawk or fish eagle, is a diurnal, fish-eating bird of prey. It is a large raptor, reaching more than in length and across the wings...
or
KingfisherKingfishers are a group of small to medium sized brightly coloured birds in the order Coraciiformes. They have a cosmopolitan distribution, with most species being found in the Old World and Australia...
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 ecologyLimnology , also called freshwater science, is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters...
in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, the term snag is used to refer to the trees, branches and other pieces of naturally occurring wood found in a sunken form in
riverA river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
s and streams. Such snags have been identified as being critical for shelter and as
spawningSpawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
sites for
fishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, and are one of the few hard substrates available for
biofilmA biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
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 19th century and early 20th century. In the United States, the
U.S. Army Corps of EngineersThe United States Army Corps of Engineers is a federal agency and a major Army command made up of some 38,000 civilian and military personnel, making it the world's largest public engineering, design and construction management agency...
operated Snagboats such as the
W.T. PrestonThe W.T. Preston is a specialized sternwheeler that operated as a snagboat, removing log jams and natural debris that prevented river navigation on several Puget Sound-area rivers. It is now the centerpiece of the Snagboat Heritage Center in Anacortes, Washington. It was designated a National...
in the
Puget SoundPuget Sound is a sound in the U.S. state of Washington. It is a complex estuarine system of interconnected marine waterways and basins, with one major and one minor connection to the Strait of Juan de Fuca and the Pacific Ocean — Admiralty Inlet being the major connection and...
of Washington State and the
MontgomeryThe Montgomery is a steam-powered sternwheel-propelled snagboat built in 1925 by the Charleston Dry Dock and Machine Company of Charleston, South Carolina, and operated by the United States Army Corps of Engineers...
in the rivers of Alabama to pull out and clear snags.
In
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, the role of freshwater snags has been largely ignored until recently, and more than one million snags have been removed from the
Murray-Darling basinThe Murray-Darling basin is a large geographical area in the interior of southeastern Australia, whose name is derived from its two major rivers, the Murray River and the Darling River. It drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural...
. Large tracts of the lowland reaches of the Murray-Darling system are now devoid of the snags that native fish like
Murray CodThe Murray cod is a large Australian predatory freshwater fish of the Maccullochella genus and the Percichthyidae family. Although the species is a called cod in the vernacular, it is not related to the northern hemisphere marine cod species...
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 are
River Red GumThe River Red Gum is a tree of the genus Eucalyptus. It is one of around 800 in the genus. It is a plantation species in many parts of the world, but is native to Australia, where it is widespread, especially beside inland water courses...
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.
See also
- Coarse woody debris
Coarse woody debris is a term used in English-speaking countries for fallen dead trees and the remains of large branches on the ground in forests. Some prefer the term coarse woody habitat . A dead standing tree is known as a snag and provides many of the same functions as coarse woody debris...
- Large woody debris
Large woody debris are the logs, sticks, branches, and other wood that falls into streams and rivers. This debris can influence the flow and the shape of the stream channel...
- Stream restoration
Stream restoration or river restoration, sometimes called river reclamation in the UK, describes a set of activities that help improve the environmental health of a river or stream. Improved health may be indicated by expanded habitat for diverse species and reduced stream bank erosion...
- Tree hollow
A tree hollow or tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree. These are predominantly found in old trees, whether living or not...