Northern Ireland's Priority Species

Frangula alnus – alder buckthorn


Distribution map

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Frangula alnus Miller
Family: Rhamnaceae

Sometimes referred to as Rhamnus frangula, and also called black dogwood or glossy or fen buckthorn, this is one of Ireland’s rarest native trees. Populations have declined, and probably have done so for several centuries, due to wetland drainage for agriculture and habitat loss from peat-cutting. The few stations that have managed to survive in Northern Ireland each support only one or a few individual small trees or shrubs.

In brief

  • It occurs around Lough Neagh and the Lower Bann area
  • It grows on a wide variety of moist soils, including both acid moss peat and alkaline fen sedge peat. It avoids only drought-prone and permanently waterlogged conditions, plus heavy shade in woodland
  • Best seen when in flower but has all year round interest
  • It has undergone rapid decline and is rare throughout Ireland
  • The main threats to the species are its isolation, nutrient enrichment, excessive drainage or peat-cutting
  • Bark of young shoots and fruit are both strongly purgative, and have long been used in herbal medicine.

Species description
Alder buckthorn is a deciduous, spineless, small tree or shrub growing to a maximum of about 6 metres but generally less. In the eastern Burren in County Clare, a prostrate form grows on rocky lake shores. Young shoots are green and downy. The thin, glossy, alternate, long-stalked leaves, 2-7cm long, are ovate and broadest above the middle, with untoothed, wavy margins. Leaf veins are parallel, usually 6 to 9 pairs each side of the midrib. Leaf surfaces are hairless above, somewhat downy brown beneath, especially when young. Small, yellowish-green, five-parted flowers are carried in clusters on one-year-old shoots. The globular fruit is green at first, then red, and finally a glossy dark purple (Webb et al., 1996).

Life cycle
Flowers and fruit at all stages of development are present from May to September. The tiny flowers are pollinated by bees, wasps and flies. The two- or three-seeded berry-like fruits are eaten and dispersed by birds and mice. Seed is the only means of increase and spread: it does not sucker or layer itself. Fruits float for two weeks in fresh water, assisting dispersal around lake shores and along rivers. Unlike many other woody species, light is essential for germination (Nelson and Walsh, 1993).

Similar species
Buckthorn, Rhamnus catharticus is superficially similar, but has opposite leaves with toothed margins, and strong, sharp thorns . The only other vaguely similar woody species is the Beech tree, Fagus sylvatica. In terms of scale, only saplings of this species could possibly cause confusion with Frangula. Alder Buckthorn always grows in wetter soil conditions than Beech.

How to see this species
Alder Buckthorn populations in Northern Ireland known to botanists are small and vulnerable so they really should not be visited. The species is widespread and much more abundant in England from Cumbria southwards, and in the Welsh borders.

Current status
The remaining populations here are in damp lakeshore scrub thickets and on peaty banks around Lough Neagh and the River Bann to the north (Harron, 1986). The only other site for the species is on the margin of a small cut-over bog near Newtownards, County Down. In Northern Ireland all wild plants are given some measure of protection under the Wildlife (NI) Order, 1985.

Why is this species a priority in Northern Ireland?

  • It has undergone rapid decline and is rare on an all-Ireland basis.

Threats/Causes of decline
It appears likely that the declining Irish populations, and especially those in Northern Ireland, for some reason are failing to set viable seed or to achieve seedling and sapling establishment. The species may simply have fallen below a minimum viable population size. Despite the wide tolerances of the species, habitat changes involving nutrient enrichment, excessive drainage or peat-cutting, may also oust the species due to increased competition from vigorous rival species or outright destruction of suitable growing conditions.

Conservation of this species

Current action

  • Implementation of the Northern Ireland Habitat Action Plan for Wet Woodland.

Proposed objectives/actions

  • The precarious state of this locally endangered species clearly requires positive management and a scientific program of restorative plantation.

What you can do
Records of new sites and estimates or counts of the sizes of populations are always valuable. Please do not pick or collect this plant anywhere in Ireland. However, if you think you may have found it, do photograph the plant. Send information to The Botanical Society of the British Isles, c/o Department of Botany, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU or CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU. Tel: 028 9039 5256, [at]

Further information

Northern Ireland Habitat Action Plan for Wet Woodland

Frangula alnus factsheet

Curtis, T.G.F. and McGough, H.N. (1988). The Irish Red Data Book. 1 Vascular Plants. Wildlife Service Ireland. Stationery Office, Dublin.

Harron, J. (1986). Flora of Lough Neagh. Irish Naturalists’ Journal Committee, Belfast and the University of Ulster, Coleraine.

Nelson, E.C. and Walsh, W.F. (1993). Trees of Ireland, native and naturalized. Lilliput, Dublin.

Preston, C.D., Pearman, D.A. and Dines, T.D. (2002). New Atlas of the British and Irish Flora. Oxford University Press, Oxford.

Webb, D.A., Parnell, J. and Doogue, D. (1996). An Irish Flora. 7th revised edition. Dundalgan, Dundalk.

Text written by:
Dr Ralph Forbes