Alnus Barbata

''Alnus glutinosa'', the common alder, black alder, European alder, European black 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-sized, short-lived tree growing to a height of up to 30 metres (98 feet). It has short-stalked rounded leaves and separate male and female flowers 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 for 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 of 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.

Description

''A. glutinosa'' is a tree that thrives in moist soils, and grows under favourable circumstances to a height of and exceptionally up to . 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 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 gum.Flora of NW Europe: ''Alnus glutinosa''
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The species is monoecious and the flowers are wind-pollinated; the slender cylindrical male catkins are pendulous, reddish in colour and 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.

Taxonomy

''A. glutinosa'' was first described by Carl Linnaeus in 1753, as one of two varieties of alder (the other being '' Alnus 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, ''A. incana'', with which it hybridizes to form the hybrid ''A. ''× ''hybrida''. Both species are diploid. In 2017, populations formerly considered to be ''A. glutinosa'' were found to be separate, polyploid species: '' A. lusitanica'', which is native to the Iberian Peninsula and Morocco, and '' A. rohlenae'', which is native to the western part of the Balkan Peninsula.

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, Iran and 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 Africa, 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.

Ecology

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 ecological succession.

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 pustules.

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 lichen

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