Alnus glutinosa (L.) Gaertn.

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].