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. They are a prominent feature of natural forests and woodlands, and act as a resource or habitat for fungi and a number of vertebrate and invertebrate animals. Hollows may form as the result of physiological stress from natural forces causing the excavating and exposure of the

heartwood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

. Forces may include wind, fire, heat, lightning, rain, attack from insects (such as ants or beetles), bacteria, or fungi. Also, trees may self-prune, dropping lower branches as they reach maturity, exposing the area where the branch was attached. Many animals further develop the hollows using instruments such as their beak, teeth or claws. The size of hollows may depend on the age of the tree. For example,

eucalypt Eucalypt is any woody plant with Capsule (fruit), capsule fruiting bodies belonging to one of seven closely related genera (of the tribe Eucalypteae) found across Australia: ''Eucalyptus'', ''Corymbia'', ''Angophora'', ''Stockwellia'', ''Allosyn ...

s develop hollows at all ages, but only from when the trees are 120 years old do they form hollows suitable for

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, and it may take 220 years for hollows suitable for larger species to form. Hollows in fallen timber are also very important for animals such as

echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the Family (biology), family Tachyglossidae , living in Australia and New Guinea. The four Extant taxon, extant species of echidnas ...

numbat The numbat (''Myrmecobius fasciatus''), also known as the noombat or walpurti, is an insectivorous marsupial. It is diurnal and its diet consists almost exclusively of termites. The species was once widespread across southern Australia, but i ...

chuditch The western quoll (''Dasyurus geoffroii'') is Western Australia's largest endemic mammalian carnivore. One of the many marsupial mammals native to Australia, it is also known as the chuditch. The species is currently classed as near-threatened. ...

and many reptiles. In streams, hollow logs may be important to aquatic animals for shelter and egg attachment. Hollows are an important habitat for many wildlife species, especially where the use of hollows is obligate, as this means no other resource would be a feasible substitute. Animals may use hollows as diurnal or

nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...

shelter sites, as well as for rearing young, feeding,

thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

, and to facilitate ranging behaviour and dispersal. While use may also be opportunistic, rather than obligate, it may be difficult to determine the nature of a species' relationship to hollows—it may vary across a species' range, or depend on climatic conditions. Animals will select a hollow based on factors including entrance size and shape, depth, and degree of insulation. Such factors greatly affect the frequency and seasonality of hollow use. Includes a list of Victorian hollow-using animals. Especially in Europe, entomologists are interested in the use of hollows by

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s. One beetle associated with hollow trees, ''

Osmoderma eremita ''Osmoderma eremita'', the hermit beetle or Russian leather beetle, is a species of European beetle in the family Scarabaeidae. Adults measure between 21 and 32 mm in length. Distribution and taxonomy ''Osmoderma eremita'', when defined as ...

'', has been given the highest priority by the European Union's Habitat Directive.

Description

A tree hollow is a cavity in a living

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...

. Tree holes can be caused when an injury to the tree, such as breakage of a limb, creates an opening through the

bark Bark may refer to: Common meanings * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Arts and entertainment * ''Bark'' (Jefferson Airplane album), ...

and exposes the

sapwood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin that ...

. The sapwood is attacked by

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

and

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, which form a cavity in the bole of the tree. The resulting cavity can fill with water, thus becoming a type of

phytotelma Phytotelma (plural phytotelmata) is a small water-filled cavity in a terrestrial plant. The water accumulated within these plants may serve as the habitat for associated fauna and flora. A rich literature in German summarised by Thienemann (19 ...

. Therefore, there are wet and dry tree holes. Tree holes are important habitats for many animals, such as

Ceratopogonidae Ceratopogonidae is a family of flies commonly known as no-see-ums, sand flies or biting midges, generally in length. The family includes more than 5,000 species, distributed worldwide, apart from the Antarctic and the Arctic. A 2025 study fro ...

Chironomidae Chironomidae , commonly known as non-biting midges or chironomids , are a family of Nematoceran flies with a global distribution. They are closely related to the families Ceratopogonidae, Simuliidae, and Thaumaleidae. Although many chironomid ...

, the

common merganser The common merganser (North American) or goosander (Eurasian) (''Mergus merganser'') is a large sea duck of rivers and lakes in forested areas of Europe, Asia, and North America. The common merganser eats mainly fish. It nests in holes in trees ...

toucan Toucans (, ) are Neotropical birds in the family Ramphastidae. They are most closely related to the Semnornis, Toucan barbets. They are brightly marked and have large, often colorful Beak, bills. The family includes five genus, genera and over ...

woodpecker Woodpeckers are part of the bird family (biology), 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 ...

s, and

bluebird The bluebirds are a North American group of medium-sized, mostly insectivorous or omnivorous passerine birds in the genus ''Sialia'' of the thrush family (Turdidae). Bluebirds are one of the few thrush genera in the Americas. Bluebirds lay an ...

s. Tree holes can be important in the maintenance and spread of some diseases, for example La Crosse encephalitis. Hollows may be an adaptive trait for trees as animals provide the host tree with fertilizer.

Non-excavated hollows

Contrary to excavated hollows that are directly caused by animals such as woodpeckers, non-excavated hollows are hollows that are created after a tree has been damaged and decays because of fungal growth. This damage can be caused by insects, foraging birds, fire, lightning, snow, frost, and physical abrasion from rocks, falling trees, dendrotelms, and, circumstantially, large herbivores such as

red deer The red deer (''Cervus elaphus'') is one of the largest deer species. A male red deer is called a stag or Hart (deer), hart, and a female is called a doe or hind. The red deer inhabits most of Europe, the Caucasus Mountains region, Anatolia, Ir ...

(Cervus elaphus) or the

European bison The European bison (: bison) (''Bison bonasus'') or the European wood bison, also known as the wisent ( or ), the zubr (), or sometimes colloquially as the European buffalo, is a European species of bison. It is one of two extant species of bi ...

(Bison bonasus). In

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, woodpeckers play a keystone role creating holes for other birds or mammals. In

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

, data is deficient but here woodpeckers do not seem to be as much of a keystone group. In

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, woodpeckers are not seen as keystone species, as most cavities are non-excavated and these are where the majority of hole-nesting songbirds choose to nest.

Types of non-excavated hollows

*Knotholes: Created from a branch snapping off of a main stem *Chimneys: Created when a stem snaps and makes an upward-facing entrance *Cracks: Created when a trunk splits *Trunk holes: Created when a cavity forms in the main stem. If it is narrow and elongate, it is called a slit.

Artificial hollows

Animals have been found to use artificial structures as substitutes for hollows. For example,

pygmy possum The pygmy possums are a family of small possums that together form the marsupial family Burramyidae. The five extant species of pygmy possum are grouped into two genera. Four of the species are endemic to Australia, with one species also co-occu ...

s in the chute of a grain silo; or

pardalote Pardalotes or peep-wrens are a family (biology), family, Pardalotidae, of very small, brightly coloured birds native to Australia, with short tails, strong legs, and stubby blunt beaks. This family is composed of four species in one genus, ''Pard ...

s in the top, horizontal pipe of a children's swing. Purpose-built

nest box A nest box, also spelled nestbox, is a man-made enclosure provided for animals to nest in. Nest boxes are most frequently utilized for birds, in which case they are also called birdhouses or a birdbox/bird box, but some mammals such as bats ma ...

es, such as birdhouses and bat tubes, are also constructed for conservation and for wildlife observation. The size of the nest box, entry hole and placement height may be chosen in consideration of certain species. However, nestboxes have different microclimatic conditions and can therefore not be treated as direct substitutes. Natural hollows are generally preferred for

habitat conservation Habitat conservation is a management practice that seeks to conserve, protect and restore habitats and prevent species extinction, fragmentation or reduction in range. It is a priority of many groups that cannot be easily characterized in ter ...

. Actual tree hollows can be created artificially by cutting into trees with

chainsaw A chainsaw (or chain saw) is a portable handheld power saw, power saw that cuts with a set of teeth attached to a rotating chain driven along a guide bar. Modern chainsaws are typically gasoline or electric and are used in activities such as t ...

s and partly covering the resulting hollows with timber faceplates. These are readily used by arboreal animals including mammals and birds. Compared to nest boxes, they last longer and give better protection from external temperatures.

Around the world

Conservation of hollow-using fauna is an issue in many parts of the world. In North America, recovery of the

eastern bluebird The eastern bluebird (''Sialia sialis'') is a small North American migratory thrush found in open woodlands, farmlands, and orchards. The bright-blue breeding plumage of the male, easily observed on a wire or open perch, makes this species a ...

(''Sialia sialis'') has required nest boxes due to the loss of natural hollows. The scarcity of dead, hollow-bearing trees in

Scandinavia Scandinavia is a subregion#Europe, subregion of northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. ''Scandinavia'' most commonly refers to Denmark, Norway, and Sweden. It can sometimes also ...

n forests is a key threatening process to native bird life. In

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

, almost half of

red-listed The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological spe ...

species are dependent on dead hollow-bearing trees or logs.

Australia

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, 304 vertebrate species are known to use tree hollows in Australia: 29 amphibians, 78 reptiles, 111 birds, 86 mammals. Includes table of animal groups. Approximately 100 of these are now rare,

threatened A threatened species is any species (including animals, plants and fungi) which is vulnerable to extinction in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of ''critical depensatio ...

near-threatened A near-threatened species is a species which has been categorized as "Near Threatened" (NT) by the International Union for Conservation of Nature (IUCN) as that may be vulnerable to endangerment in the near future, but it does not currently qu ...

on Australian State or Commonwealth legislation, in part because of the removal of hollow-bearing trees. Threats to hollows include: native forest

silviculture Silviculture is the practice of controlling the growth, composition/structure, as well as quality of forests to meet values and needs, specifically timber production. The name comes from the Latin ('forest') and ('growing'). The study of forests ...

firewood Firewood is any wooden material that is gathered and used for fuel. Generally, firewood is not heavily processed, and is in some sort of firelog, recognizable log or branch form, compared to other forms of wood fuel like pellet fuel, pellets. ...

collection, rural dieback (such as from

inundation A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant concern in agriculture, civi ...

and

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

), grazing by

cattle Cattle (''Bos taurus'') are large, domesticated, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...

, and land clearing. Additionally, pest and introduced species such as the

common myna The common myna or Indian myna (''Acridotheres tristis''), sometimes spelled mynah, is a bird in the Family (biology), family Sturnidae, native to Asia. An omnivorous open woodland bird with a strong territorial instinct, the common myna has ada ...

and

western honey bee The western honey bee or European honey bee (''Apis mellifera'') is the most common of the 7–12 species of honey bees worldwide. The genus name ''Apis'' is Latin for 'bee', and ''mellifera'' is the Latin for 'honey-bearing' or 'honey-carrying', ...

(''Apis mellifera'') compete with native species for hollows; domestic and feral

cat The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

s and

black rat The black rat (''Rattus rattus''), also known as the roof rat, ship rat, or house rat, is a common long-tailed rodent of the stereotypical rat genus ''Rattus'', in the subfamily Murinae. It likely originated in the Indian subcontinent, but is n ...

s prey on hollow-using animals and have been damaging especially to island populations; and some native hollow-using species have increased population densities or expanded their ranges since European settlement, such as the

galah The galah (; ''Eolophus roseicapilla''), less commonly known as the pink and grey cockatoo or rose-breasted cockatoo, is an Australian species of cockatoo and the only member of the genus ''Eolophus''. The galah is adapted to a wide variety of m ...

common brushtail possum The common brushtail possum (''Trichosurus vulpecula'', from the Ancient Greek, Greek for "furry tailed" and the Latin for "little fox", previously in the genus ''Phalangista'') is a nocturnal, semiarboreal marsupial of the family Phalangeridae ...

and the

little corella The little corella (''Cacatua sanguinea''), also known as the short-billed corella, bare-eyed cockatoo, blood-stained cockatoo, and little cockatoo is a white cockatoo native to Australia and southern New Guinea. It was known as ''Birdirra'' amo ...

and compete with less common native species.

Russia, China, Korea

Asian black bear The Asian black bear (''Ursus thibetanus''), also known as the Asiatic black bear, moon bear and white-chested bear, is a medium-sized bear species native to Asia that is largely adapted to an arboreal lifestyle. It is distributed from southeast ...

s, also known as Himalayan bears (Lat.: ''Ursus thibetanus''), in northern parts of their range, such as Russian province Primorye, China, and both Koreas, prefer spend winter periods in large tree hollows, where females also give birth to cubs. Threats include massive deforestation in these countries, combined with direct poaching of wintering bears—with selective destruction of the best hollow trees. In Russia, attempts (sometimes successful) are made to restore such broken trees.''Denning trees of Himalayan bears are being restored in Primorye.''
(in Russian; Cyrillic Windows-1251) Unfortunately, only a small portion of all damaged trees can be restored in Primorye, where forests are basically logged without taking to account needs of large fauna.

Gallery

Image:Eucalyptus camaldulensis 01 Pengo.jpg, A

river red gum ''Eucalyptus camaldulensis'', commonly known as river red gum, is a flowering plant in the family Myrtaceae, and is endemic to Australia. It is a tree with smooth white or cream-coloured bark, lance-shaped or curved adult leaves, flower buds in ...

, with hollows. The younger trees surrounding it would generally not yet have developed hollows suitable for vertebrate species. Image:Dendrocopos minor mushrooms tree brok 1 beentree.jpg, Hollows excavated by the

lesser spotted woodpecker The lesser spotted woodpecker (''Dryobates minor'') is a member of the woodpecker family Picidae. It was formerly assigned to the genus '' Dendrocopos'' (sometimes incorrectly spelt as ''Dendrocopus''). Some taxonomic authorities continue to list ...

(''Picoides minor''). Also fungus, which may also help in the formation of hollows through the decomposition of heartwood.

References

External links

Life in a Tree Hole. Nature Bulletin No. 581. November 21, 1959. Forest Preserve District of Cook County

de:Baum-Mikrohabitat#Baumhöhlen Plant morphology Dead wood {{Forestry, state=expanded