Alnus glutinosa (L.) Gaertn.

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Credits

Tim Baxter & Hugh A. McAllister (2021)

Recommended citation
Baxter, T. & McAllister, H.A. (2021), 'Alnus glutinosa' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-glutinosa/). Accessed 2023-02-03.

Genus

  • Alnus
  • Subgen. Glutinosa, Section Glutinosae

Common Names

  • Common Alder
  • Black Alder

Synonyms

  • Betula glutinosa (L.) Lam.
  • Alnus vulgaris Hill
  • Betula alnus var. glutinosa L.
  • Betula alnus subsp. glutinosa (L.) Ehrh.

Glossary

article
(in Casuarinaceae) Portion of branchlet between each whorl of leaves.
hybrid
Plant originating from the cross-fertilisation of genetically distinct individuals (e.g. two species or two subspecies).
acuminate
Narrowing gradually to a point.
acute
Sharply pointed.
apex
(pl. apices) Tip. apical At the apex.
emarginate
Notched at the apex.
entire
With an unbroken margin.
glabrous
Lacking hairs smooth. glabrescent Becoming hairless.
hybrid
Plant originating from the cross-fertilisation of genetically distinct individuals (e.g. two species or two subspecies).
introgression
Incorporation of genes from one species into the genotype of another through repeated hybridisation or repetitive backcrossing between a hybrid and one of its parents.
obtuse
Blunt.
pedunculate
With a peduncle.
pendent
Hanging.
ploidy
Number of chromosomes.
pubescent
Covered in hairs.
retuse
Slightly notched at apex.
sessile
Lacking a stem or stalk.
subspecies
(subsp.) Taxonomic rank for a group of organisms showing the principal characters of a species but with significant definable morphological differentiation. A subspecies occurs in populations that can occupy a distinct geographical range or habitat.

Credits

Tim Baxter & Hugh A. McAllister (2021)

Recommended citation
Baxter, T. & McAllister, H.A. (2021), 'Alnus glutinosa' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-glutinosa/). Accessed 2023-02-03.

Tree to 35 m, crown pyramidal or oval, occasionally multi-stemmed. Bark dark grey-brown when young, darkening with age and becoming fissured. Young twigs usually glabrous, viscid, terete. Buds ovate with 2–3 scales, green, viscid, 2–3 mm, stipe 2 mm. Leaves obovate, elliptic, suborbicular or broadly ovate, 4–11 × 3.5–11 cm, apex rounded, obtuse or emarginate, base cuneate, truncate to rounded, margins lobulate and double toothed with 1–6 teeth between primary veins, blade glabrous adaxially, with hairy domatia abaxially in vein axils, 7–8 veins either side of midrib, craspedodromous. Petiole 20–50 mm, thin, ± glabrous. Male catkins in a lax raceme on a 7 mm peduncle, formed in autumn and exposed over winter, , 30–80 × 2–6 mm extending to, 80–120(–160) mm at anthesis, reddish or brownish, anthers red, loose. Female catkins formed in autumn and opening in spring, pendulous to semi-erect lax raceme, 4–6 × 1–2 mm, peduncle 5–15 mm long, angled and grey with warty appearance at maturity. Fruit borne semi-erect, dry-resinous, 8–12 × 6–20 mm, ovoid, apex and base truncate, green at first, dark brown or black at maturity. Seed very narrowly winged. Diploid, rarely triploid, 2n=28, 42. (Tutin et al. 1972; Vit et al. 2017; T. Baxter, pers. obs.).

Distribution  AlbaniaAlgeriaArmeniaAustriaAzerbaijanBelarusBelgiumBosnia and HerzegovinaBulgariaCroatiaCzechiaDenmarkEstoniaFinlandFrance including Corsica GeorgiaGermanyHungaryIranIrelandItalyKazakhstanLatviaLithuaniaLuxembourgNorth MacedoniaMoldovaMonacoMoroccoNetherlandsNorwayPolandPortugalRomaniaRussiaSerbiaSlovakiaSloveniaSpainSwedenSwitzerlandTunisiaTurkeyUkraineUnited Kingdom

Habitat Wet soils, stream and lake sides, wet flushes, fens and carrs. Common throughout most of its range to 1800 m asl.

USDA Hardiness Zone 3-7

RHS Hardiness Rating H7

Conservation status Least concern (LC)

Alnus glutinosa is the common alder found throughout Europe and Russia as far east as the Ural Mountains, and south into the Caucasus and northern Iran. It is variable across its extensive range and the latest taxonomic understanding is that there are four separate subspecies: subsp. glutinosa is the typical form throughout much of its range; subsp. barbata occurs in the Caucasus mountains, north-east Turkey and northern Iran; subsp. antitaurica occurs in northern Turkey and south-eastern Europe; subsp. betuloides occurs in a small area of eastern Turkey.

The typical form in Europe is a large tree with dark brown fissured bark, twigs that are sticky when young and hairless, obovate grey-brown buds on long (1–3 mm) stalks, its leaves are flat, matt, broader than long with an apical notch (retuse to occasionally emarginate), irregularly lobed at the margins, and female catkins pedunculate and somewhat pendent. It is easily distinguished from A. incana as this has hairy twigs, leaves that are acuminate to acute at the apex and often evenly lobulate, bark grey or yellow and relatively smooth, and sessile female catkins. A. matsumurae of Japan is also vegetatively similar but its leaves are always emarginate and glabrous, its buds elliptic with an acute apex, and its erect fruit are larger.

Within what has been treated as A. glutinosa, two new polyploid taxa have been named recently as A. rohlenae from south-east Europe and A. lusitanica from Spain (Vit et al. 2017). Tetraploid ‘glutinosa’ has been known since the 1980s with an anomylous count for plants originating from the Sperkios Valley at Ness (H. McAllister pers. obs.). The modern investigation and discovery of a clear pattern in distribution of ploidy levels does appear to support the separation of two new species.

Hybrids in the wild are not uncommon and can be almost impossible to determine accurately. These are generally always intermediate between the two parents. For example, Ball (1964) [PLEASE ADD REFERENCE RECORD] described the hybrid A. × pubescens (A. glutinosa × A. incana) THIS NEEDS AN ARTICLE which has young twigs and leaves pubescent (at least abaxially on veins) with obtuse to shortly acuminate apices, and female catkins shortly stalked. Such hybrids are variable and frequently occur wherever the parents grow together (Banaev & Bazant 2007). Hybrids are also known to exist with A. cordata (A. × elliptica q.v.) and A. incana subsp. rugosa (A. × silesiaca – THIS HYBRID DOESN’T APPEAR TO HAVE AN ARTICLE, IS THIS INTENTIONAL?). This situation gets considerably more complex in southern Europe, the Caucasus and Iran. Zare & Amini (2012) name several new species in this region, but these should be regarded with caution pending further work. There is some reluctance to name new taxa in regions where genetically compatible species overlap, especially in wind pollinated genera, unless there is clear evidence of biological separation rather than purely morphological differences. It is here suggested that hybridisation and introgression with the diploid species A. incana, A. orientalis, and A. subcordata may be involved. Polyploid forms of some of these taxa are already known to exist. The situation in Alnus taxa across this region is likely to be similar to that found in Betula (e.g. B. utilis subsp. occidentalis) (McAllister & Ashburner 2013), where there is considerable mixing of taxa where numerous compatible species intermingle as species spread out (mostly northwards) from refugia found throughout these regions. The correct taxonomic status of A. glutinosa subspp. barbata, antitaurica and betuloides remains uncertain pending further work [THIS APPEARS TO CONTRADICT THE OPENING PARAGRAPH – I THINK WE NEED TO SAY ‘THIS IS THE CURRENT UNDERSTANDING’ AND LEAVE IT AT THAT?]

In cultivation all forms of A. glutinosa grow into medium-large trees or upright multi-stemmed shrubs. It is a rather plain species apart from some of the cultivars with more attractive foliage; despite this it is still one of the most widely planted of all alders in parks, gardens and urban environments, not only within its native range but in North America too (at least in the guise of its many cultivars) (Jacobson 1996). In the wild it grows in wet soils, especially in riparian habitats. In cultivation it is moderately sensitive to drought and makes a useful tree for planting in wet places where few other trees would thrive. In drought-prone areas, provenances from SE Europe, Turkey and Iran (subspp. barbata, antitaurica and betuloides) should be more widely grown. A. glutinosa is commonly used in agroforestry for its nitrogen fixing ability, and it has been widely introduced outside its natural range for land stabilisation and amenity. However, it is also a moderate to serious non-native invasive species of wet sites in parts of North America, Australia and New Zealand (CABI 2018).

The tallest recorded tree in Britain grows at Audley End, Saffron Walden (32 m), with the largest a coppice stool at Furze Hills, Essex with a stool size of 14.75 m (Tree Register 2021). Plants are easy to raise from seed but hybrids do occur in cultivation, as noted above. Alder seed germinates in water (P. Bartlett pers. comm. 2015) but needs potting on as soon as the first true leaves appear. Seed tolerates immersion in salt water for at least two weeks and can be found germinating on drift lines [REFERENCE]. A. glutinosa is especially susceptible to Alder Leaf Beetle, Agelastica alni, and this beetle can strip the foliage from entire stands of trees. Trees appear to survive relatively unharmed but are likely to become more susceptible to other stress factors such as seasonal drought and Phytopthora alni.

The timber of A. glutinosa has many comtemporary and historic uses including for charcoal, gunpowder, and turning. Perhaps its best-known traditional use was for making clogs, especially in mill towns in the north of England. Its timber is not especially rot-resistant, except when submerged in water when it has a high degree of water resistance. It was frequently used for underwater piles and supports, most notably throughout Venice, and for lock gates on canals, mill wheels, and troughs. It has also been used for dye making, and its bark is astringent and has been used to treat sore throats and inflammation (https://www.wood-database.com/european-alder/). [FULL REFERENCING REQUIRED].


A × pubescens Tausch

A group-name for hybrids between A. glutinosa and A. incana, which are found fairly commonly where the two species meet. According to P. W. Ball in Flora Europaea, Vol. 1 (1964), such hybrids are usually characterised by downy young growths, leaves downy beneath, at least on the veins, blunt or shortly acuminate at the apex, and the female catkins shortly stalked (in the first parent they are stalked and in the second sessile).

'Aurea'

Leaves golden yellow. Raised in Vervaene’s nursery, Ledeberg-les-Gand, about 1860. Not so vigorous as the type.

f. incisa (Willd.) Koehne

Synonyms
A. g. var. incisa Willd

Leaves small, usually less than 1 in. long, rounded or ovate in outline, deeply cut into broad, toothed lobes, or even right to the midrib. The plant once grown at Kew made a dwarf, compact bush and was probably of the clone distributed by Loddiges’ Hackney nurseries, and by Booth’s nurseries, Hamburg, Germany, as A. glutinosa oxyacanthifolia – the thorn-leaved alder. A form with similar leaves, but making a large shrub or small tree, has also been described and may represent a different clone. Elwes and Henry mentioned such a one at Barton, near Bury St Edmunds, 44 ft high and 2 ft 8 in. in girth.The name A. glutinosa incisa has also been used, wrongly, for the clone ‘Laciniata’ (q.v.).

f. pyramidalis (Dipp.) Winkler

Branches erect. ‘birkiana’ is a clone of this character, once distributed by Späth’s nurseries.

f. quercifolia (Willd.) Koehne

Upper part of the leaves with triangular, toothed lobes, the deepest not reaching more than one-third of the way to the midrib. Plants of this form were distributed early in the nineteenth century by Loddiges’ nursery and are also found wild in Scandinavia.

'Imperialis'

Leaves deeply and pinnately lobed, the lobes lanceolate, slender, pointed, not toothed, reaching more than half-way to the midrib; stalks 1 to 1{1/2} in. long. Often a thin, rather ungainly tree, never of great size. A well-known garden clone, in cultivation since before 1859.

'Laciniata'

Similar to ‘Imperialis’, but not so deeply and narrowly lobed; lobes not toothed. In his review of the cut-leaf alders, Hylander accepts the statement made by Thouin (1819), and quoted by Loudon, that all the plants of this form descend from one grown in a garden near St Germain, ‘where the stool still remains from which all the nurseries of Paris have been supplied with plants, and, probably all Europe’. It makes a tree of some size, and reached 70 ft at Syon Park, Middlesex.It has been confused with f. incisa, but in that group the leaves are small and of rounded outline, with toothed lobes, whereas in ‘Laciniata’ the leaves are oblong and the lobes pointed and untoothed.


'Rubrinervia'

Leaves with red veins and stalks.

'Sorbifolia'

Leaves oblong or oval, deeply cut into about six pairs of lobes, which are oblong and coarsely round-toothed, the sinuses often widest at the base. One of the most distinct of the cut-leaved sorts. The tree itself is not a strong grower, and is of rather lax habit. The plant described is almost certainly of the same clone as the one to which Dippel gave the name A. g. var. sorbifolia in 1892. Similar forms are found wild in Scandinavia and the group name for them all is f. lacera (Mela) Mela (Hylander, op. cit.).