A birch is a thin-leaved deciduous hardwood tree of the genus Betula (/ˈbɛtjʊlə/),[2] in the family Betulaceae, which also includes alders, hazels, and hornbeams.

Birch species are generally small to medium-sized trees or shrubs, mostly of northern temperate and boreal climates.

[6] Distinctive colors give the common names gray, white, black, silver and yellow birch to different species.

The wood of all the species is close-grained with a satiny texture and capable of taking a fine polish; its fuel value is fair.

Once fully grown, these leaves[clarification needed] are usually 3–6 millimetres (1⁄8–1⁄4 in) long on three-flowered clusters in the axils of the scales of drooping or erect catkins or aments.

The scales of the mature staminate catkins are broadly ovate, rounded, yellow or orange colour below the middle and dark chestnut brown at apex.

Note: many American texts have B. pendula and B. pubescens confused, though they are distinct species with different chromosome numbers.

Dutch berk, Low German Bark, Danish birk, Norwegian bjørk), itself from the Proto-Indo-European root *bʰerHǵ- ~ bʰrHǵ-, which also gave Lithuanian béržas, Latvian Bērzs, Russian берёза (berëza), Ukrainian береза (beréza), Albanian bredh 'fir', Ossetian bærz(æ), Sanskrit bhurja, Polish brzoza, Latin fraxinus 'ash (tree)'.

The oldest known birch fossils are those of Betula leopoldae from the Klondike Mountain Formation in Washington State, US, which date to the early Eocene (Ypresian) around 49 million years ago.

They are regarded as pioneer species, rapidly colonizing open ground especially in secondary successional sequences following a disturbance or fire.

Birches are early tree species to become established in primary successions, and can become a threat to heathland if the seedlings and saplings are not suppressed by grazing or periodic burning.

Mycorrhizal fungi, including sheathing (ecto)mycorrhizas, are found in some cases to be beneficial to tree growth.

[17] Although its mechanism of action in helping to heal injured skin is not fully understood, birch bark extract appears to stimulate the growth of keratinocytes which then fill the wound.

[17][18] Preliminary research indicates that the phytochemicals, betulin and possibly other triterpenes, are active in Episalvan gel and wound healing properties of birch bark.

Birch paper (Sanskrit: भुर्ज पत्र, bhurja patra) is exceptionally durable and was the material used for many ancient Indian texts.

Proof of this adaptability is seen in its easy and eager ability to repopulate areas damaged by forest fires or clearings.

The leaves of the silver birch tree are used in the festival of St George, held in Novosej and other villages in Albania.

[citation needed] "Swinging" birch trees was a common game for American children in the nineteenth century.