Alnus glutinosa, the common alder, black alder, European alder or just
alder, is a species of tree in the family Betulaceae, native to most
of Europe, southwest Asia and northern Africa. It thrives in wet
locations where its association with the bacterium Frankia alni
enables it to grow in poor quality soils. It is a medium size,
short-lived tree growing to a height of up to 30 metres (100 ft).
It has short-stalked rounded leaves and separate male and female
flower in the form of catkins. The small, rounded fruits are cone-like
and the seeds are dispersed by wind and water.
The common alder provides food and shelter to wildlife, with a number
of insects, lichens and fungi being completely dependent on the tree.
It is a pioneer species, colonising vacant land and forming mixed
forests as other trees appear in its wake. Eventually common alder
dies out of woodlands because the seedlings need more light than is
available on the forest floor. Its more usual habitat is forest edges,
swamps and riverside corridors. The timber has been used in underwater
foundations and for manufacture into paper and fibreboard, for smoking
foods, for joinery, turnery and carving. Products of the tree have
been used in ethnobotany, providing folk remedies for various
ailments, and research has shown that extracts of the seeds are active
against pathogenic bacteria.
3 Distribution and habitat
4 Ecological relationships
5 Cultivation and uses
5.2 Tanning and dyeing
5.3 Other uses
7 Details of
Alder structure and galls
9 External links
Male inflorescence (left) and mature cone-like flowers (right)
Alnus glutinosa is a tree that thrives in moist soils, and grows under
favourable circumstances to a height of 20 to 30 metres (66 to
98 ft) and exceptionally up to 37 metres (121 ft). Young
trees have an upright habit of growth with a main axial stem but older
trees develop an arched crown with crooked branches. The base of the
trunk produces adventitious roots which grow down to the soil and may
appear to be propping the trunk up. The bark of young trees is smooth,
glossy and greenish-brown while in older trees it is dark grey and
fissured. The branches are smooth and somewhat sticky, being scattered
with resinous warts. The buds are purplish-brown and have short
stalks. Both male and female catkins form in the autumn and remain
dormant during the winter.
The leaves of the common alder are short-stalked, rounded, up to
10 cm (4 in) long with a slightly wedge-shaped base and a
wavy, serrated margin. They have a glossy dark green upper surface and
paler green underside with rusty-brown hairs in the angles of the
veins. As with some other trees growing near water, the common alder
keeps its leaves longer than do trees in drier situations, and the
leaves remain green late into the autumn. As the Latin name glutinosa
implies, the buds and young leaves are sticky with a resinous
The species is monoecious and the flowers are wind-pollinated; the
slender cylindrical male catkins are pendulous, reddish in colour and
5 to 10 cm (2 to 4 in) long; the female flowers are upright,
broad and green, with short stalks. During the autumn they become dark
brown to black in colour, hard, somewhat woody, and superficially
similar to small conifer cones. They last through the winter and the
small winged seeds are mostly scattered the following spring. The
seeds are flattened reddish-brown nuts edged with webbing filled with
pockets of air. This enables them to float for about a month which
allows the seed to disperse widely.
Unlike some other species of tree, common alders do not produce shade
leaves. The respiration rate of shaded foliage is the same as well-lit
leaves but the rate of assimilation is lower. This means that as a
tree in woodland grows taller, the lower branches die and soon decay,
leaving a small crown and unbranched trunk.
Alnus glutinosa was first described by
Carl Linnaeus in 1753, as one
of two varieties of alder (the other being A. incana), which he
regarded as a single species Betula alnus. In 1785, Jean-Baptiste
Lamarck treated it as a full species under the name Betula
glutinosa. Its present scientific name is due to Joseph Gaertner,
who in 1791 accepted the separation of alders from birches, and
transferred the species to Alnus. The epithet glutinosa means
"sticky", referring particularly to the young shoots.
Within the genus Alnus, the common alder is placed in subgenus Alnus
as part of a closely related group of species including the grey
alder, Alnus incana, with which it hybridizes to form the hybrid
A. × hybrida.
Distribution and habitat
The common alder is native to almost the whole of continental Europe
(except for both the extreme north and south) as well as the United
Kingdom and Ireland. In Asia its range includes Turkey,
Kazakhstan, and in Africa it is found in Tunisia,
Algeria and Morocco.
It is naturalised in the Azores. It has been introduced, either by
accident or by intent, to Canada, the United States, Chile, South
Australia and New Zealand. Its natural habitat is in moist
ground near rivers, ponds and lakes but it can also grow in drier
locations and sometimes occurs in mixed woodland and on forest edges.
It tolerates a range of soil types and grows best at a pH of between
5.5 and 7.2. Because of its association with the nitrogen-fixing
bacterium Frankia alni, it can grow in nutrient-poor soils where few
other trees thrive.
Nodules on the roots caused by the bacterium Frankia alni
Galls on the leaves caused by the mite Eriophyes inangulis
The common alder is most noted for its symbiotic relationship with the
bacterium Frankia alni, which forms nodules on the tree's roots. This
bacterium absorbs nitrogen from the air and fixes it in a form
available to the tree. In return, the bacterium receives carbon
products produced by the tree through photosynthesis. This
relationship, which improves the fertility of the soil, has
established the common alder as an important pioneer species in
The common alder is susceptible to Phytophthora alni, a recently
evolved species of oomycete plant pathogen probably of hybrid origin.
This is the causal agent of phytophthora disease of alder which is
causing extensive mortality of the trees in some parts of Europe.
The symptoms of this infection include the death of roots and of
patches of bark, dark spots near the base of the trunk, yellowing of
leaves and in subsequent years, the death of branches and sometimes
the whole tree.
Taphrina alni is a fungal plant pathogen that
causes alder tongue gall, a chemically induced distortion of female
catkins. The gall develops on the maturing fruits and produces spores
which are carried by the wind to other trees. This gall is believed to
be harmless to the tree. Another, also harmless, gall is caused by
a midge, Eriophyes inangulis, which sucks sap from the leaves forming
The common alder is important to wildlife all year round and the seeds
are a useful winter food for birds. Deer, sheep, hares and rabbits
feed on the tree and it provides shelter for livestock in winter.
It shades the water of rivers and streams, moderating the water
temperature, and this benefits fish which also find safety among its
exposed roots in times of flood. The common alder is the foodplant of
the larvae of a number of different butterflies and moths and is
associated with over 140 species of plant-eating insect. The tree
is also a host to a variety of mosses and lichens which particularly
flourish in the humid moist environment of streamside trees. Some
common lichens found growing on the trunk and branches include tree
lungwort (Lobaria pulmonaria), Menneguzzia terebrata and Stenocybe
pullatula, the last of which is restricted to alders. Some 47
species of mycorrhizal fungi have been found growing in symbiosis with
the common alder, both partners benefiting from an exchange of
nutrients. As well as several species of Naucoria, these symbionts
include Russula alnetorum, the milkcaps Lactarius obscuratus and
Lactarius cyathula, and the alder roll-rim Paxillus filamentosus, all
of which grow nowhere else except in association with alders. In
spring, the catkin cup
Ciboria amentacea grows on fallen alder
As an introduced species, the common alder can affect the ecology of
its new locality. It is a fast-growing tree and can quickly form dense
woods where little light reaches the ground, and this may inhibit the
growth of native plants. The presence of the nitrogen-fixing bacteria
and the annual accumulation of leaf litter from the trees also alters
the nutrient status of the soil. It also increases the availability of
phosphorus in the ground, and the tree's dense network of roots can
cause increased sedimentation in pools and waterways. It spreads
easily by wind-borne seed, may be dispersed to a certain extent by
birds and the woody fruits can float away from the parent tree. When
the tree is felled, regrowth occurs from the stump, and logs and
fallen branches can take root. A. glutinosa is classed as an
environmental weed in New Zealand.
Cultivation and uses
Infrutescence and Achenes
The common alder is used as a pioneer species and to stabilise river
banks, to assist in flood control, to purify water in waterlogged
soils and to moderate the temperature and nutrient status of water
bodies. It can be grown by itself or in mixed species plantations, and
the nitrogen-rich leaves falling to the ground enrich the soil and
increase the production of such trees as walnut,
Douglas fir and
poplar on poor quality soils. Although the tree can live for up to 160
years, it is best felled for timber at 60 to 70 years before heart rot
On marshy ground it is important as coppice-wood, being cut near the
base to encourage the production of straight poles. It is capable of
enduring clipping as well as marine climatic conditions and may be
cultivated as a fast-growing windbreak. In woodland natural
regeneration is not possible as the seeds need sufficient nutrients,
water and light to germinate. Such conditions are rarely found at the
forest floor and as the forest matures, the alder trees in it die
out. The species is cultivated as a specimen tree in parks and
gardens, and the cultivar 'Imperialis' has gained the Royal
Horticultural Society's Award of Garden Merit.
The wood is soft, white when first cut, turning to pale red; the knots
are attractively mottled. The timber is not used where strength is
required in the construction industry, but is used for paper-making,
the manufacture of fibreboard and the production of energy. Under
water the wood is very durable and is used for deep foundations of
buildings. The piles beneath the
Rialto in Venice, and the foundations
of several medieval cathedrals are made of alder. The Roman architect
Vitruvius mentioned that the timber was used in the construction of
the causeways across the
Ravenna marshes. The wood is used in
joinery, both as solid timber and as veneer, where its grain and
colour are appreciated, and it takes dye well. As the wood is soft,
flexible and somewhat light, it can be easily worked as well as split.
It is valued in turnery and carving, in making furniture, window
frames, clogs, toys, blocks, pencils and bowls.
Tanning and dyeing
The bark of the common alder has long been used in tanning and dyeing.
The bark and twigs contain 16 to 20% tannic acid but their usefulness
in tanning is limited by the strong accompanying colour they
produce. Depending on the mordant and the methods used, various
shades of brown, fawn, and yellowish-orange hues can be imparted to
wool, cotton and silk.
Alder bark can also used with iron sulphate to
create a black dye which can substitute for the use of sumach or
galls. The Laplanders are said to chew the bark and use their
saliva to dye leather. The shoots of the common alder produce a
yellowish or cinnamon-coloured dye if cut early in the year. Other
parts of the tree are also used in dyeing; the catkins can yield a
green colour and the fresh-cut wood a pinkish-fawn colour.
It is also the traditional wood that is burnt to produce smoked fish
and other smoked foods, though in some areas other woods are now more
often used. It supplies high quality charcoal.
The leaves of this tree are sticky and if they are spread on the floor
of a room, their adhesive surface is said to trap fleas.
Chemical constituents of
Alnus glutinosa include hirsutanonol,
oregonin, genkwanin, rhododendrin
3-(4-hydroxyphenyl)-l-methylpropyl-β-D-glucopyranoside and glutinic
acid (2,3-pentadienedioic acid).
Pollen from the common alder, along with that from birch and hazel, is
one of the main sources of tree pollen allergy. As the pollen is often
present in the atmosphere at the same time as that of birch, hazel,
hornbeam and oak, and they have similar physicochemical properties, it
is difficult to separate out their individual effects. In central
Europe, these tree pollens are the second most common cause of
allergic conditions after grass pollen.
The bark of common alder has traditionally been used as an astringent,
a cathartic, a hemostatic, a febrifuge, a tonic and a restorative (a
substance able to restore normal health). A decoction of the bark has
been used to treat swelling, inflammation and rheumatism, as an
emetic, and to treat pharyngitis and sore throat. Ground up bark
has been used as an ingredient in toothpaste, and the inner bark can
be boiled in vinegar to provide a skin wash for treating dermatitis,
lice and scabies. The leaves have been used to reduce breast
discomfort in nursing mothers and folk remedies advocate the use of
the leaves against various forms of cancer. Alpine farmers are
said to use the leaves to alleviate rheumatism by placing a heated bag
full of leaves on the affected areas.
Alder leaves are consumed by
cows, sheep, goats and horses though pigs refuse to eat them.
According to some people, consumption of alder leaves causes
blackening of the tongue and is harmful to horses.
In a research study, extracts from the seeds of the common alder have
been found to be active against all the eight pathogenic bacteria
against which they were tested, which included
Escherichia coli and
methicillin-resistant Staphylococcus aureus (MRSA). The only extract
to have significant antioxidant activity was that extracted in
methanol. All extracts were of low toxicity to brine shrimps. These
results suggest that the seeds could be further investigated for use
in the development of possible anti-MRSA drugs.
Alder structure and galls
Alder carr in Germany
Trees in winter, Germany
Gall fungus, Taphrina alni
Alder Tongue Gall
Black alder in Ås, Norway
Black alder defies harsh conditions in the Swedish archipelago
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Wikimedia Commons has media related to Alnus glutinosa.
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