||The genus comprises about 30 species of
monoecious trees and shrubs, few reaching large size, distributed
throughout the North Temperate zone, and in the New World also along
the Andes southwards to Chile.
The leaves are deciduous (not
evergreen), alternate, simple, and serrated. The flowers are catkins
with elongate male catkins on the same plant as shorter female
catkins, often before leaves appear; they are mainly wind-pollinated,
but also visited by bees to a small extent. They differ from the
birches (Betula, the other genus in the family) in that the female
catkins are woody and do not disintegrate at maturity, opening
to release the seeds in a similar manner to many conifer cones.
The best-known species in Europe
is the Common or Black Alder (A. glutinosa), native to most of
Europe and widely introduced elsewhere. The largest species is
Red Alder (A. rubra), reaching 35 m (the tallest is 32 m) on the
west coast of North America, with Black Alder and Italian Alder
(A. cordata) both reaching about 30 m. By contrast, the widespread
Green Alder (A. viridis) is rarely more than a 5 m shrub.
The common name alder is derived
from an old Germanic root. The botanic name Alnus is the original
Latin name. Both the Latin and the Germanic derive from the proto-Indo-European
root el-, meaning 'red, brown,' which is also the ultimate root
for the English words elk and elm --a tree more distantly related
to the alders.
||The genus is divided into three
Trees. Shoot buds stalked. Male and female catkins produced in
autumn (fall) but staying closed over winter, pollinating in late
winter or early spring. About 15-25 species, including:
Alnus acuminata - Andean
Alder. Andes Mountains, South America.
Alnus cordata - Italian Alder. Italy.
Alnus glutinosa - Black Alder. Europe.
Alnus incana - Grey Alder. Europe & Asia.
Alnus oblongifolia (A. incana subsp. oblongifolia) - Arizona
Alder. Southwestern North America.
Alnus rugosa (A. incana subsp. rugosa) - Speckled
Alder. Northeastern North America.
Alnus tenuifolia (A. incana subsp. tenuifolia) - Thinleaf
Alder or Mountain Alder. Northwestern North America.
Alnus japonica - Japanese Alder. Japan.
Alnus jorullensis - Mexican Alder. Mexico, Guatemala.
Alnus nepalensis - Nepalese Alder. Eastern Himalaya,
Alnus orientalis - Oriental Alder. Southern Turkey,
northwest Syria, Cyprus.
Alnus rhombifolia - White Alder. Interior western
Alnus rubra - Red Alder. West coastal North America.
Alnus serrulata - Hazel alder, Tag or Smooth alder.
Eastern North America.
Alnus subcordata - Caucasian Alder. Caucasus, Iran.
Trees or shrubs. Shoot buds stalked. Male and female catkins produced
in autumn (fall) and expanding and pollinating then. Three species:
Alnus formosana -Formosan Alder Taiwan
Alnus maritima - Seaside Alder. East coastal North America, plus
disjunct population in Oklahoma.
Alnus nitida - Himalayan Alder. Western Himalaya.
Shrubs. Shoot buds not stalked. Male
and female catkins produced in late spring (after leaves appear)
and expanding and pollinating then. One to four species:
Alnus viridis - Green Alder.
Alnus viridis subsp. viridis. Eurasia.
Alnus viridis subsp. maximowiczii (A. maximowiczii). Japan.
Alnus viridis subsp. crispa (A. crispa). Northern
Alnus viridis subsp. sinuata (A. sinuata, Sitka
Alder or Slide Alder). Western North America, far northeastern