Alnus glutinosa (L.) Gaertn., Alder

Account Summary

Growth form and preferred habitats

Under ideal conditions, A. glutinosa trees can grow large, up to 25 m in height with a girth of around 3.5 m. However, in Fermanagh woodland and scrub they usually only reach around 6 to 10 m. The vast majority of individual trees have several stems, rather than a solitary trunk, and they are not long-lived (perhaps up to 100 years). As such, they might be considered as large shrubs, rather than proper trees. The seeds of the tree are chiefly dispersed in flowing water, and thus its prevalence in waterside habitats. The trees or shrubs often carry dead or dying die-back branches. A.glutinosa is readily identified even when minus its leaves, as the distinctive, small, barrel-shaped, woody female cones are constantly present. In addition, an examination of the bare twigs shows the presence of buds with a purple waxy bloom, borne on distinct stalks (Edlin 1964).

Writing in a British Isles context, Rackham (1980) points out that, "Of all familiar native trees, Alder is the most obviously restricted to particular habitats." Rackham (1980) recognised three types of alder woodland in Britain:

(1) Fen alder-woods, extensive on low-lying, level ground in the flood-plains of streams, rivers and around lakes, almost always containing alder mixed with a range of willows;

(2) Valley alder-woods, forming narrow fringes to streams, often within other types of woodland, notably ash-alder, mixed oak wood, and birch-alder. This category of alder-wood may sometimes climb up flushed slopes, especially in western parts of Britain and Ireland, the dominant tree species changing with altitude and exposure.;

(3) Plateau alder-woods, in which alder is a constituent of the mosaic of woodland trees on level uplands, sometimes with slight depressions, and often on a watershed which provides seeping irrigation, at least in winter.

Rackham's first two alder-wood categories are very common in Fermanagh, but there is very little or none of his plateau woodland involving alder anywhere in the VC. The upland plateau of W Fermanagh is instead occupied by modern conifer plantations on ground that previously supported either acidic blanket bog or peaty moorland pasture.

Fermanagh occurrence

Alder trees help form an important part of Fermanagh's landscape: on almost every streamside, river bank and lakeshore, the dominant species is Alder and it is present in 470 tetrads, 89% of those in the VC. In Fermanagh, A. glutinosa is most often found as a non-woodland tree fringing all types of watery habitats, and indeed it is a very good indicator of land liable to flooding. It is characteristic of shrubby woods fringing marshy lakeshores and along river banks. Alder ranks as the fourth most widespread woody species in the vice-county, and it is also the tenth most frequently recorded vascular plant in the county survey, which testifies to the damp mild climate and poorly drained soils of this part of western Ireland.

Alder ecology

Alnus is essentially a pioneer genus with the associated characteristics of heavy seeding, rapid colonisation of bare ground, fast initial growth, intolerance of shade and a short life span (White & Gibbs 2000). A. glutinosa is very sensitive to shading, the seedling less so, but still requiring a higher light intensity than seedlings of larger-seeded tree species. This fact alone explains why internal regeneration of Alder in woodland is practically impossible (McVean 1953).

Soil reaction

Alder is largely but not totally indifferent to soil reaction. It tends to be restricted to the unstable soils of stream and lake margins, recent alluvial river soils, flush soils and very wet soils where the water table is seasonally high, or where drainage is impeded. Alder roots are of great importance in helping to stabilise waterside soil banks, and the flow of water assists the tree by providing oxygen to the soil around its roots. Alders do not thrive around stagnant pools or where water flow is slight, although they may persist in a state of suspended animation for a long time after streams have been diverted (Mitchell 1996). A. glutinosa can grow on deep fen peat, or on acid peat of Molinia-Myrica bog, but not on blanket or raised bogs as noted above.

Effect of grazing pressure

Moderate grazing of marshes or fens prevents associated tall herb vegetation being kept from smothering and shading out the trees' seedlings and thus favours the spread of alder. Associated animal trampling also helps provide suitable sites for establishment by breaking the surface turf and the leaf litter layer. Cattle will trim young saplings and limit their growth to bush form if grazing becomes too intensive. Rabbits and hares seldom ring-bark saplings of alder in the way that they do with birch or plantation conifers. McVean (1953) suggested that alder does not establish itself as well in the 'natural' aquatic vegetation succession (ie primary hydrosere), as it does when that pattern of habitat development is disturbed or deflected by the activities of man and his farm animals.

Frost and drought tolerance

Mature alder trees are extremely frost hardy, and once deep taproots have been formed, individuals are also very drought resistant. It appears therefore that the plant's strong association with wet soils chiefly reflects its seedlings susceptibility to drought and shade.

Beneficial root associates

Mycorrhizal roots and roots bearing nitrogen-fixing nodules are formed; the nodules are of two sizes, many very small and few large, orange-coloured ones like tennis balls, which can live for a decade. The symbiotic organism contained in the nodules is an Actinomycete (generally strains of a genus called Frankia, one of a group of organisms related to filamentous bacteria). These nitrogen-fixing nodules confer considerable nutritional advantages to trees that bear them (Thomas 2000).

Low oxygen in waterlogged soils

Oxygen availability is often a limiting factor for organisms including trees that live in soils subject to flooding. Some oxygen will dissolve and diffuse down into the upper few cms of waterlogged soil, but beneath that shallow zone the substrate is devoid of oxygen (anaerobic). Woody roots can survive anaerobic conditions for a time when the tree or shrub is dormant, but finer roots quickly die, with a consequent decline in uptake of both water and mineral nutrients, directly affecting growth. Indirect effects include toxic substances produced in the soil (for instance, hydrogen sulphide) and toxins created by the tissues of the tree itself (Thomas 2000). Being able to exist temporarily without something (oxygen) that is constantly essential for your competitors is one of the great keys to ecological survival, the so-called 'anaerobic retreat' (Crawford 1989).

Tolerance of low oxygen

Alders tolerate waterlogged soil extremely well, and some of their roots live permanently submerged in water. Part of the mechanism that permits the tree to survive such conditions involves the passage of air downwards through the bark and within air-filled sections of the woody xylem tissues of the root. The bark- and wood-producing thin cambium layers were once considered impervious to gas, but it is now clear that this is not the case in wetland trees (Thomas 2000). Oxygen diffuses down and leaks out of the root into the surrounding mud, forming an oxygenated envelope around the root, permitting the root tissues to function normally. By the same route, toxic gases produced by incomplete anaerobic respiration can escape from the root, although many flood-tolerant trees also have biochemical adaptations to reduce the production and impact of these toxins.

British and Irish occurrence

Alder is abundant throughout most of Britain & Ireland, but is less common in the far N of Scotland and in the Central Plain of Ireland due to the extent of extremely wet, cold, acidic Sphagnum peat (ie raised and blanket bogland) that occurs in these regions, conditions which exclude all trees except birch (Betula spp.).

European and world occurrence

Alder extends throughout most of Europe except the Arctic, the Mediterranean basin and the Russian steppes, and it is also present in Asia Minor, W Siberia and N Africa (McVean 1953).

A new pathogenic disease attacking alders

In 1993, a lethal stem disease affecting A. glutinosa was first noticed and described in Britain. On affected trees the leaves are described as abnormally small, yellow and sparse, especially on the crown of the plant. The leaves frequently fall prematurely and leave the tree bare. The stem base of trees that are displaying severe crown symptoms often have black 'tar-like' spots on the bark up to 2 m from ground level. The spots indicate that the underlying bark is dead. The pathogen attacks the cambium layer of the root and lower trunk so that the tree dies from the base upwards. This new alder disease has been recorded throughout Europe and is caused by a fungus that infects trees through the action of water-borne zoospores, so it is not surprising that it appears, so far, most frequent and most virulent along riversides (Rackham 2006).

The causal organism was first considered to be a hybrid between two species of the genus Phytophthora, P. cambivora and P. fragariae, both of which are considered introductions to Europe. Neither of the Phytophthora species causes disease in alder on its own account (Brasier et al. 1999). More recent work has indicated that P. × alni was generated by hybridisation between P. uniformis and P. × multiformis (Ioos et al. 2006; Husson et al. 2015). The emergence of P. × alni highlights the dangers of sexual hybridisation between Phytophthora species resulting in novel phenotypes/genotypes that can be more damaging than either of the parental strains (Brasier 2000; Boutet et al. 2010; Érsek & Man, in: 't Veld 2013).

Die-back of alder trees attributed to P. × alni, P. uniformis and/or P. × multiformis has been reported in at least 12 European countries in both forests (11 countries) and in forest nurseries (seven countries) (Jung et al. 2016). The presence of the pathogen in forest nurseries implicates the horticultural trade in spreading the disease throughout Europe. Since the disease has become widespread in Europe it has focussed research throughout the European Community on the importance of alder, to the extent that an EC Concerted Action Plan for alder conservation was approved in 1998. Surveys have shown that disease severity varies in different regions, but it seems to be at its worst in parts of Britain and France where destructive epidemics appear to be developing with thousands of trees dying each year.

In Britain, Webber et al. (2004) estimated the disease had infected some 90,000 alder trees. Today (2017), it is estimated that P. × alni has infected between 16-20% of the riverside alder trees in Britain, including introduced exotic alder species. The worst areas for the disease at present (ie in 2017) are in SE England and along the border with Wales, although the pathogen is now also present along Scottish riverbanks.

The first record of the pathogen P. × multiformis in Ireland was reported in 1999 in Co Dublin from Alnus bark and from water baits in a riparian zone in the county (Clancy & Hamilton 2001). While the distribution of the pathogen in Ireland is not yet understood, symptoms of alder die-back and confirmation by isolation has been made in five recently planted alder stands (5-20 years old) in counties Cork (H3-H5) and Kildare (H19) (O'Hanlon et al. 2016), as well as in a mature Alnus in a riparian zone near infected recently planted alder trees in Co Cork [VC location not disclosed]. DNA sequencing of these isolates revealed them to be P. × alni and P. uniformis. Fortunately, so far there have been few cases reported of the disease in Ireland, although plant pathologists have been alert to the possibility of infection (A. McCracken, pers. comm., 2001; https://www.teagasc.ie/crops/forestry/advice/management/phytophthora-ramorum/ accessed December 2018). However, because so little is known about the disease, there is no way of predicting the effect on alder populations 10 or 20 years from now. Basic research on the nature of the disease is currently being pursued to discover how the pathogen attack on alder may be arrested, but the P. alni complex appears to be a growing threat to native Alnus stands in both Northern Ireland and the Republic of Ireland (O'Hanlon et al. 2016).

Ecological importance

There is growing concern that loss of significant numbers of the only British Isles native alder to disease may have far-reaching environmental consequences. Apart from providing habitat, food and cover for a variety of wildlife, aquatic, terrestrial and avian (eg the achenes are a very important food supply for finches), alders also play an extremely important role in riverbank stabilisation, erosion control, soil fertility improvement and the regulation of stream water temperature through shading. Alder is particularly important along headland streams where it supports dense populations of trout and salmon, directly by providing shading, cover and habitat, and indirectly by providing food in the form of insects associated with the canopy, or with leaf litter (Tipping 1999; White & Gibbs 2000).

Uses

Alder can be made into a fine, even-grained charcoal, and is used in the manufacture of top-quality gunpowder. The wood is noted for its durability under water and, as a consequence, has been used for pilings, sluices, pumps, troughs, boat bottoms and punts. Burr alder veneer is also greatly prized by cabinet-makers for its decorative grain (White & Gibbs 2000). Alder has a long tradition of being valued timber in Ireland. In an eighth century Irish document, Bretha Comaithchesa, the Laws of Neighbourhood, which protected trees and shrubs, alder ranked in the second order of importance, Aithig fedo - the commoners of the wood. The fine imposed for cutting a branch off a neighbour's 'commoner', was a sheep. In Irish folklore alder was regarded as unlucky - best avoided on a journey, and the tree was best left uncut, since the timber, although white when first cut, soon changes to red, like blood (Nelson & Walsh 1993).

Names

The Gaelic Irish name for alder is 'fearnög', 'fearn', or 'fern', which appears in place-names such as Ferns, Co Wexford and Ferney, Co Fermanagh. These names have no connection with the English vernacular plant name 'fern', for which the Irish equivalent is 'raithneach' (Nelson & Walsh 1993).

Threats

Importation of infected plant material carrying a newly recognised invasive fungus, Phytophthora alni, is a recently developed major risk to riverbank Alder populations in Britain. Unfortunately, the disease symptoms are very non-specific, making infection difficult to detect, but a careful eye must be kept on the possibility of the pathogen spreading in Ireland.

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