Plant of the Week -5th October- Alder (Alnus glutinosa)

Alnus glutinosa (L.) Gaertn. is a native tree that can grow up to 30 m, found in damp woods and by the sides of streams. It has a few notable relatives. The most often seen is Grey Alder (A. incana) a naturalised neophyte in the British Isles, whilst A. rubra and A. viridis are aliens, introduced for ornament and amenity, and rarely seen in the wild. 

Alnus incana differs from A. glutinosa in having bark that is grey and smooth, leaves that are ovate, more pointed at the apex and more obviously biserrate than those of A. glutinosa (see the sketch in Stace 2019).  The two species have the same chromosome count, 2n=28, and sometimes they hybridise (Banaev & Bažan (2007).

All these species are monoecious and wind pollinated, with small seeds that are distributed mainly by water.  They have a symbiotic relationship with Frankia alni, an actinomycete that fixes nitrogen, and is mycorrhizal with basidiomycetes, especially with Alnicola (=Naucoria) species. 

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Alnus glutinosa is native to almost all of Europe, Turkey, Iran, Kazakhstan, Tunisia, Algeria and Morocco, and has been introduced to several other places around the world.  It is also one of the largest species of its genus, many Alnus species being shrubs.

Etymology:  Alnus is the old Latin name, glutinosa is the Latin for ‘sticky’, referring to the young twigs.  The common name alder evolved from the Old English word alor, which in turn is derived from Proto-Germanic root aliso (Online Etymology Dictionary).  The authority Gaertn. refers to Joseph Gaertner (1732 – 1791), a German botanist, professor of anatomy at Tübingen from 1760, and best known for his work on seeds, De Fructibus et Seminibus Plantarum (1788-1792).  Conrad Moench (Konrad Mönch), 1744 – 1805) was professor of botany at Marburg University from 1786 until his death.

The information in the following sections has been gleaned mainly from a series of seven vintage  papers in the Journal of Ecology (1955 et seq.) by Donald N. McVean¹, with further information from Rackham (2015).

Habitat and ecology: Alnus glutinosa thrives on riverbanks and flushes, but is also found in mixed woodland, including ancient woodland, along spring-lines and flushes and at the edge of forests.  More rarely, it is found, with other trees, beside water seeps on plateaux, for example at Bradfield Woods, Suffolk, where it forms mixed woods with Fraxinus excelsior (ash) and other trees.  In some circumstances it is the dominant tree species, as in Alder Carr (National Vegetation Type W5; however, Alder Carr is scarce in Scotland). The species can thrive in a variety of soils, and prefers a soil pH 5.5 – 7.2.  It can fix nitrogen by symbiosis with Frankia alni, but requires a reasonably good supply of available phosphate. Common companion (understorey) vegetation, in all its habitats, include Allium ursinum (Ransoms), Urtica dioica (stinging nettle) and Lysimachia nemorum (yellow pimpernel)

Reproduction:  It is monoecious and reproduces exclusively by seed: it produces no suckers.  Some plants are protandrous, others strongly protogynous, which encourages cross-fertilisation, but A. glutinosa has low self-fertility anyway (and is not apomictic).  The seeds are small (about 1.4 mg.), flattened and not adapted for wind dispersal: it is estimated that the typical distance seed is conveyed by wind is only 30-60 m, and seedlings are rarely found more than 20-30 m from the female parent unless carried by water.  But seeds are buoyant and waterproof, so that seed that falls into moving water may be carried a long distance, until landing on the stream bank. 

The reproductive process of alder takes two years, and each stage is susceptible to adverse weather. 

The viability of seed varies: low viability is almost entirely due to failure of embryos to form; seed with embryos has a high rate of germination.  The optimum temperature for germination is 26ºC and requires a high pO₂ and atmospheric humidity, but is unaffected by light levels, temperature fluctuations or soil pH. 

Alnus does not fruit every year, which some have ascribed to the phenomenon of masting, but the vagaries of weather and their effect on reproduction are sufficient to account for the irregularities of fruiting, which vary from place to place and from year to year.

Root system and adaptation to flooding and drought:  The principal roots are of two physiological types:  surface roots spread laterally above the usual level of the water table, and bear Frankia nodules.  The deep roots grow downwards, below the usual water table, and have anatomical adaptations of their xylem, similar to those of aquatic angiosperms, to supply them with oxygen at anoxic depths. 

A. glutinosa suffers from being flooded, but can survive it better than other tree species with which it may be in competition.  During prolonged floods, it develops adventitious roots on its trunk and special stellate lenticels which may assist the respiration of Frankia at these stressful times.  Trees that are likely competitors of Alnus cannot send roots below the normal water table, and therefore, in periods of drought when the water table falls, suffer severely from a shortage of water, but Alnus still has access to adequate supplies.  Alnus may therefore benefit, competitively, from alternating drought and flood.

History in the British Isles:  Pollen strata indicate that A. glutinosa arrived in what are now the British Isles soon after the last retreat of the ice, but does not seem to have been common.  There was, however, a sudden increase of Alder in the Atlantic period, 6200-3800 BCE.  Rackham 2015, p.72, remarks of this episode “The Atlantic Period began with a sudden rise of alder, which had previously been present throughout Britain and Ireland, and now became one of the dominant trees.  This is something of a mystery: it has been attributed to the climate getting wetter and the ground becoming sodden, but alder is a tree of flushed, not waterlogged, ground, and is not very dependent on high rainfall.  Moreover, if this were the explanation, the change should have been most marked in drier regions, which it was not”.  Tallantire 1992 and King and Ferris 1998 and 1990 are also interesting on the paleohistory of Alnus glutinosa. A possible explanation is that Britain experienced alternations of drought and flood at this time, which would have favoured alder.  It is predicted that current climate change will bring more droughts alternating with more floods, and it may be necessary to reserve sacrificial floodplains upstream of riverside towns, for protection.  In which case, we may perhaps see an increase in alder, and we should, therefore, perhaps be looking for more ways to utilise its wood; the traditional uses mentioned below are probably mostly no longer viable!

Uses and Economic significance:  It produces one of the most useful kinds of wood. Remarkably, the city of Venice is built on piles made of Alnus (Nakasako 2018).   The wood of certain alder species is often used to smoke various food items such as coffee, salmon and other seafood.  Alder bark contains the anti-inflammatory salicin, which humans metabolized into salicylic acid, and A. rubra has therefore been traditionally used medicinally by native Americans.  Alder wood (A. glutinosa) is durable under water (but not when dry), and has therefore been used to construct boats, sluices, and, in olden time, pipes. In Scotland it was used to make tubs to store butter in bogs. It coppices well, and makes excellent charcoal for gunpowder: an A. glutinosa coppice was established near the Royal Gunpowder Factory near Waltham Abbey for just this purpose (Rackham, p.167). It was used to make clogs and charcoal in times past, and had many other uses in mediaeval times for which it has now been replaced by the wood of conifers.  The wood is said to be preferred by woodworm, and used to be put in cupboards to attract egg-laying woodworm away from the wood of the cupboard.  Alder bark and wood contain tannin and are traditionally used to tan leather.  It is now used to make the bodies of the best electric guitars (including Fenders), veneers, pulp and plywood.  It turns easily, glues well, and looks much like birch. It seems to be under-utilized at present. In favourable conditions the tree grows fast and the wood may become a useful  source of bioenergy (Aosaar et al. 2002). 

References

¹Following the completion of his PhD on the ‘Ecology of Alnus glutinosa’ Donald McVean was employed by the Nature Conservancy in Edinburgh from 1953 until 1959. As well as publishing a series of papers on Alder in the Journal of Ecology, he collaborated with Derek Ratcliffe to write Plant Communities of the Scottish Highlands (1962) and with Jim Lockie to write Ecology and Land Use in Upland Scotland (1969).

Aosaar, J., Varik, M. and Uri, V. (2012).  Biomass production potential of grey alder (Alnus incana (L.) Moench.) in Scandinavia and Eastern Europe: A review.  Biomass and Bioenergy 45 11-26. https://doi.org/10.1016/j.biombioe.2012.05.013

Banaev, E.V. and Bažant, V. (2007).   Study of natural hybridization between Alnus incana (L.) Moench. and Alnus glutinosa (L.) Gaertn.  Journal of Forest Science 53(2) 66–73.

Bennett, K. D. and Birks, H.J.B. (1990).  Postglacial history of alder (Alnus glutinosa (L.) Gaertn.) in the British Isles.  Journal of Quaternary Science, 5(2) 123-133. https://doi.org/10.1002/jqs.3390050204

McVean, D.N., (1955) Ecology of Alnus glutinosa (L.) Gaertn.: I Fruit formation.  Journal of Ecology, 43(1), 46-60.

Nakasako, Eric. (2018) A Look at Venice: Past and Present. Illumin. University of Southern California.

Rackham, O. (2015). Woodlands.  William Collins (Harper Collins), London.

Stace, C. (2019).   New Flora of the British Isles, 4th edition.  C & M Floristics.  ISBN 978-1-5272-2630-2632.

Tallantire, P.A. (1992). The alder [Alnus glutinosa (L.) Gaertn.] problem in the British Isles: a third approach to its palaeohistory.  New Phytologist 122(4) 717-731.