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It is a small to medium size tree 15–20 metres (49–66 ft) tall with smooth grey bark even in old age, its life span being a maximum of 60–100 years. The leaves are matt green, ovoid, 5–11 centimetres (2.0–4.3 in) long and 4–8 centimetres (1.6–3.1 in) broad. The flowers are catkins, appearing early in spring before the leaves emerge, the male catkins pendulous and 5–10 centimetres (2.0–3.9 in) long, the female catkins 1.5 centimetres (0.59 in) long and one cm broad when mature in late autumn. The seeds are small, 1–2 millimetres (0.039–0.079 in) long, and light brown with a narrow encircling wing. The Grey Alder has a shallow root system, and is marked not only by vigorous production of stump suckers, but also by root suckers, especially in the northern parts of its range. The wood resembles that of the black alder, but is somewhat paler and of little value.
There are four to six subspecies, some treated as separate species by some authors:
- Alnus incana subsp. incana (Grey Alder). — Northern Europe and northwestern Asia, and central and southern Europe in mountains, mainly in the regions of the Alps, Carpathians and the Caucasus.
- Alnus incana subsp. hirsuta (Spach) Á. & D.Löve (=A. hirsuta Spach); Manchurian Alder. — In mountains of Northeast Asia and Central Asia.
- Alnus incana subsp. kolaensis (N.I.Orlova) Á. & D.Löve. — Subarctic northeast Europe.
- Alnus incana subsp. oblongifolia (=Alnus oblongifolia); Arizona Alder. — Madrean Sky Islands of southwestern North America, in Arizona, New Mexico, and Northwestern Mexico.
- Alnus incana subsp. rugosa (Du Roi) R.T.Clausen (=A. rugosa Du Roi); Speckled Alder. — Northeastern North America, including the Northeastern United States.
- Alnus incana subsp. tenuifolia (Nutt.) Breitung (=A. tenuifolia Nutt.); Mountain Alder, or Thinleaf Alder. — Western North America, including New Mexico to California and Alaska.
Alnus incana is a light-demanding, fast-growing tree that grows well on poorer soils. In central Europe, it is a colonist of alluvial land alongside mountain brooks and streams, occurring at elevations up to 1,500 metres (4,900 ft). However, it does not require moist soil, and will also colonize screes and shallow stony slopes. In the northern part of its range, it is a common tree species at sea level in forests, abandoned fields and on lakeshores. It is sometimes used for afforestation on non-fertile soils which it enriches by means of nitrogen fixing bacteria in its root nodules. Several species of Lepidoptera use Grey Alder as a food plant for their caterpillars. See List of Lepidoptera that feed on alders. In the Boreal forest area of Canada, A. incana is often associated with Black Spruce in the forest type termed Black Spruce/Speckled Alder.
- Flora of North America. 2009
- "Alnus incana subsp. tenuifolia". Jepson Herbarium: Jepson eFlora. Retrieved January 28, 2013.
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- "Alnus tenuifolia — Mountain Alder". PFAF Plant Database. Retrieved January 28, 2013.
- C. Michael Hogan. 2008
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- Stevenson, Matilda Coxe 1915 Ethnobotany of the Zuni Indians. SI-BAE Annual Report #30 (p. 80)
- Royal botanical Garden, Edinburgh. 2008. Flora Europaea: Alnus incana
- Flora of North America. 2009. Alnus incana
- C. Michael Hogan. 2008. Black Spruce: Picea mariana, GlobalTwitcher.com, ed. N. Stromberg