Common alder: What sets species apart from other natives?

THERE’S no doubting the nativity of common alder (Alnus glutinosa), a versatile species with good eco credentials, able to grow good-quality timber, so why does the tree not have a more prominent place in forestry?

Common alder has historically faced the same fate as closely related common hazel (Corylus avellana), within the plant family Betulaceae (birch family). Both species have been cut down to their roots to become coppice stools instead of single-stem standard trees. Hazel is one of increasingly few native trees without a potential life-threatening pest or disease problem, while alder contends with what could end up becoming a survival-threatening pathogen called Phytophthora alni.

Pollinated and fertilised female flowers enlarge into barrel-shaped ‘false-cones’ (‘conelets’), green at first and becoming brown and woody at maturity. (Image: FJ)

Hazel is a tree of the wayside, whereas alder is a tree by the waterside. Alnus glutinosa is a lover of wet environments, with its botany closely tied to freely available fresh water.

However, not all wet habitats are favourable for alder, a tree which is not at all comfortable in very acidic peat marshes or distinctly calcareous soils. However, in heavy loams rich in nutrients and providing a constantly available supply of water, even waterlogged for lengthy periods, alder attains a full natural height of 20–30 m with a straight trunk and a lifespan of up to 150 years.

Everything about the form, structure and physiology of the alder tree is geared to survival in sodden environments that would drown most other native trees, with the exception of osiers, sallow and willows. Alder tolerates submergence and periods with roots standing in water up to 30 cm deep during the winter months. Firm anchorage in such soggy soils is secured by long tap roots extending five feet into the soil, enabling mature trees to survive falling water levels in summer.

Key to alder’s success in waterlogged soils short on plant-available nitrate is a symbiotic association with nitrogen-fixing bacteria (Frankia alni). The bacteria are found in nodules on the roots of alder trees and are equivalent to the Rhizobium bacteria typically found in the root nodules on leguminous plants. This supplementary nitrate is used by the alder trees, but some may also be utilised by other plants growing nearby. The soil fertility value of alder was apparently guessed long before scientists discovered the bacteria and established their precise function.

The alder whose fat shadow nourisheth,
Each plant set neere to him long flourisheth.
From ‘Forest Trees’ by William Browne (1591–1643)

Nitrogen-fixing bacteria convert gaseous nitrogen contained in the air into soluble nitrate through a series of chemical reactions, and supply this usable nutrient to the alder tree in return for water and carbon (sugar) resources. This, together with soil enrichment and replenishment from leaf fall, is sufficient for alder trees to grow in otherwise nutrient-poor soils. Leaf litter from the typically dense canopy of the alder tree plays an important part in the build-up of humus in the soil. Alder trees put more nitrate into the soil than is taken out, thus building up soil fertility and yielding excellent agricultural land like the fens of East Anglia, formed when the alder-dominated scrub is cleared and drained.

Alder roots can survive being bathed in water and will freely grow into rivers and lakes, provided the stem and tap root is firmly embedded bankside. Alder trees are commonly found growing right on the water’s edge, with dense masses of tough and cord-like red roots holding the bank firm against spates and the wash of passing boats. This large and concentrated root system is instrumental in alder’s ability to sprout rapidly after coppicing, with new spring (coppice shoots) growing up to 2 m in just one season.

Heavy waterside soils where alders thrive provide humid microenvironments to sustain the dense leaf canopy and moving water to disperse the buoyant seed-containing fruit.

Each fruit has a pair of wings at each margin, comprising hollow, air-filled tissue and called ‘floats’. Floats allow the fruit to stay buoyant (and viable) for many months before coming ashore, allowing the seed to germinate in mud along riverbanks. Seeds will only germinate on damp earth or mud. Common alder is an efficient pioneer species rapidly colonising damp ground provided there is no significant shade or competition from larger trees.

Male catkins of common alder are often borne in such large numbers that the otherwise bare branches are hidden during March before the leaves emerge in April. (Image: FJ)

With its adaptability to water-based environments, alder has become a true son of the soil. Trees grow the length and breadth of the land, from Caithness in the north of Scotland to the Norfolk Broads, happily tolerating waterlogged conditions during dormancy. 

Away from the waterside, the trees grow along spring lines in oak woods and damp hollows or on damp slopes in high-rainfall areas. Where drainage is neglected on wet meadows and set-aside land, alder will readily re-invade moist grassland. Alder prefers soil pH at or above 6 and tolerates infertile and drier sites but with greatly reduced height and longevity. Tolerance of wet soils gives rise to an alder-dominated scrub or woodland on predominantly alkaline (pH 7+) or very slightly acid (pH 6) soils, growing more or less permanently in wet peat called ‘fen’.  

On sites above the winter water level, free from standing water, alder forms established plant communities known in Northern Europe as ‘carrs’, a description derived from the Icelandic ‘kjarr’ meaning ‘fenwood’. Alder, with an age-old reputation as the best drainer of soil, is the prime mover in natural progression from the tree-less bogs that once covered much of the British Isles to established mixed-tree-species communities. Having pioneered the colonisation of wet soils and then dried them out sufficiently for other species, alder will gradually die out because seedlings do not compete well in shady woodland conditions.  

Once alder is established and the soil sufficiently drained, other species from the field and hedgerow will colonise. Alder buckthorn, birch, ash, hawthorn, holly and even oak (that would otherwise drown in the boggy soil) enter the alder scrub or carr to form a mixed fen-wood. 

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This can still be seen happening in East Anglia around the Norfolk Broads, which are not natural lakes but enormous mediaeval peat cuttings, which appear to have been abandoned because of flooding before the end of the 14th century. If man deflects this natural succession, as in the case of the famous Wicken Fen, where reeds (e.g. common Norfolk reed – Phragmites australis) and sedges (e.g. Fen sedge – Cladium mariscus) are harvested, then alder is inhibited.

BOTANY OF THE ALDER TREE

Dormancy is not mandatory for alder seed, although germination rate will increase following a period of chilling in moist conditions at 0.5°C for up to 10 weeks. Seeds are tiny, red-brown flakes with a quarter-million per kilogram. Seedlings grow rapidly in the nursery and may be planted out in the second year as monocultures or in mixed stands with oak and ash, providing the location is moist. 

From here on, extension growth is rapid, measuring an average of 0.5 m per annum for the first 30–40 years. Young bark is greenish-grey in colour, cracked into angled plates. Maturity is attained in 60 years and if untouched the reward is a fine-looking tree 20–30 m in height with a long, branchless bole supporting a dense foliar canopy with a distinctive pyramidal shape. Mature, untouched trees have a fine canopy structure and intensely fissured dark-grey bark which is not unlike that of English oak.

Winter buds on common alder are alternate, red in colour, vaguely conical and displayed quite close to the stem. Well-formed winter buds are seen in late November with leaves still very much green. Leaves are simple, broadly-round with dentate margins, prominent veins and a narrowing wedge-like base to give an overall shape resembling a tennis racket. They vary from 5–10 cm long and are borne alternately on short stalks along the stem, emerging in April and falling gradually and tardily, without developing marked autumnal hues, and right into December. 

Then wind with sudden rage, abrupt and blea,
Twirls every lingering leaf from off each tree.
Such is our lowland scene that winter gives
And strangers wonder where our comfort lives.
From ‘Winter in the Fens’ by John Clare (1793–1864)

Newly emerged leaves have tufts of whitish hairs on the abaxial (ventral or inferior) surface in the vein axils, which afford good protection against insects feeding on the soft leaf tissue. The leaf surface is gluey to the touch and maintains this feel into maturity – clearly why the word ‘glutinosa’ is the species name. 

Flowering occurs in March before the leaves appear, with separate male and female flowers borne on the same tree. Male flowers (catkins) are formed in autumn and remain erect until spring. They become pendulous at 3–6 cm long as the weather warms, with a distinctly orange hue owing to a blend of yellow pollen-bearing stamens and red scales. Male catkins are often borne in such large numbers that branches are hidden during March before the leaves emerge in April.  

Wind-pollinated female catkins are ovoid and stalked, remaining erect with fleshy scales covered by red-brown bracts of a woody nature that persists after the fruits have fallen out. They are strategically situated in clusters above the leaf buds with sticky stigmas to trap pollen drifting in the air. They enlarge into barrel-shaped ‘false-cones’ (conelets), 1–3 cm long, green at first, then brown and woody at maturity. Conelets contain the numerous winged fruits called nutlets, each with a single seed.  

Nutlets mature late in October, ripening and released over the next few weeks when scales on the conelet open. They are scattered into the air and dispersed to potential germination sites by wind and water. Empty wooden conelets will stay attached to the tree right through winter, borne together with next year’s prostrate male catkins. This gives leafless trees a dark-brown colour in winter and a rather cluttered appearance in early spring, with the new year’s lively catkins and last year’s dead conelets all crowded together on the same twigs.

The yellowy-red male catkins, carried in large numbers, provide an attractive amber-coloured offering in the low-angled sun of mid-March afternoons.

HISTORY, CULTURE AND FOLKLORE

Just like its tap root, the alder tree’s historical roots are long and secure. In some localities alder is still called Howler and Aller, the latter derived from its Anglo-Saxon names which were aeler, alr and aler.

The Welsh name for alder is gwern, giving rise to many place names like Llanwern, originally indicating low-lying pasture, while in the Scottish Gaelic language alder is called fearn (one hamlet in Easter Ross actually called Fearn).  

Similarly in Ireland, where in the Irish Gaelic language alder is called fearnog, you have Ferns in County Wexford and Glenfarne in County Leitrim, which translate respectively into ‘Place of Alders’ and ‘Valley of Alders’. 

Alder has given its name to many towns and villages in England, such as Aldershot, meaning ‘alder wood’, and Aldermaston, where a preponderance of alder wood may well have been used in the manufacture of gunpowder, while the area much later became famous as the site of the Atomic Weapons Establishment. 

The predominance of alder around insect-plagued ponds and on misty moorlands is largely responsible for its association with evil spirits, witches and the devil. Witches, it was said, used alder wood to summon up storms. According to many Northern European folk legends, alder casts out and absorbs all sorts of afflictions including fever, toothache and warts.

Alder has negative connotations in Irish folklore, including the conviction that passing an alder tree during a journey was bad luck. These malign vibes are thought to be due to the fact the timber, when cut, changes in colour from white to a vivid red, reminiscent of blood. It’s not surprising that alder is closely associated with war and death, or that the wood was used to make soldiers’ shields.

Increasing tree maturity sees bark on the common alder tree becoming more distinctly grey and cracked into angled plates. (Image: FJ)

On Good Fridays, during spring crop sowings, farmers placed small pieces of alder wood in their mouths or received the ‘alder sacrament’ through wearing a ring made of alder wood to deter sparrows from the fields. Rodents and moles were warded off by placing alder woodchips in all four corners of the newly sown field or the barn where the seed corn was stored.

Alder played a role in fertility potions due to the early spring appearance of the flowers, and its fame extends much further back than mediaeval times. Alder trees appear several times in Greek mythology, such as growing in the cave of the nymph loved by Odysseus who held him captive for seven years.

Alder is a centrepiece of ‘A Farewell’, written by the old romantic poet Alfred, Lord Tennyson, yearning for a stream as it meanders down to the sea and personified as a heroine worshipped by her trees along the way.

Flow down, cold rivulet, to the sea...
No more by thee my steps shall be...
But here will sigh thine alder tree,
And here thine aspen shiver.
From ‘A Farewell’ by Alfred, Lord Tennyson (1809–1892)

Another iconic poet, William Wordsworth, also penned a poem entitled ‘A Farewell’, but it is another of his works that features the alder tree. 
                                            
Thou thrush that singest loud – and loud and free,
Into yon row of willows flit,
Upon that alder sit;
Or sing another song, or choose another tree.
From ‘Tis said that some have died for love’ by William Wordsworth (1770–1850)