Tree hollow

A tree hollow is a cavity in a living tree. Tree holes can be caused when an injury to the tree, such as breakage of a limb, creates an opening through the bark and exposes the sapwood. The sapwood is attacked by fungi and bacteria, which form a cavity in the bole of the tree. The resulting cavity can fill with water, thus becoming a type of phytotelma. Therefore, there are wet and dry tree holes. Tree holes are important habitats for many animals, such as Ceratopogonidae, Chironomidae, the common merganser, toucans, woodpeckers, and bluebirds. 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 (Cervus elaphus) or the European bison (Bison bonasus). In North America, woodpeckers play a keystone role creating holes for other birds or mammals. In Asia, data is deficient but here woodpeckers do not seem to be as much of a keystone group. In Europe, 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 Source: • 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 possums in the chute of a grain silo; or pardalotes in the top, horizontal pipe of a children's swing. Purpose-built nest boxes, 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. 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 (Sialia sialis) has required nest boxes due to the loss of natural hollows. The scarcity of dead, hollow-bearing trees in Scandinavian forests is a key threatening process to native bird life. In Sweden, almost half of red-listed species are dependent on dead hollow-bearing trees or logs. Approximately 100 of these are now rare, threatened or near-threatened on Australian State or Commonwealth legislation, in part because of the removal of hollow-bearing trees. Threats to hollows include: native forest silviculture, firewood collection, rural dieback (such as from inundation and salinity), grazing by cattle, and land clearing. Additionally, pest and introduced species such as the common myna and western honey bee (Apis mellifera) compete with native species for hollows; domestic and feral cats and black rats 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, common brushtail possum and the little corella and compete with less common native species. 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, 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 (Picoides minor). Also fungus, which may also help in the formation of hollows through the decomposition of heartwood. Image:Eastern gray squirrel in a New Jersey Fagus grandifolia.jpg|A hollow of an American beech inhabited by an Eastern gray squirrel. Hollows are commonly made into dreys by tree squirrel species. == See also ==