Habitat account - Forests


91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae * Priority feature

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae).  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 91E0 Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae). Click image for enlarged map.

Description and ecological characteristics

 

Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae) comprises woods dominated by alder Alnus glutinosa and willow Salix spp. on flood plains in a range of situations from islands in river channels to low-lying wetlands alongside the channels. The habitat typically occurs on moderately base-rich, eutrophic soils subject to periodic inundation.

 

Many such woods are dynamic, being part of a successional series of habitats. Their structure and function are best maintained within a larger unit that includes the open communities, mainly fen and swamp, of earlier successional stages. On the drier margins of these areas other tree species, notably ash Fraxinus excelsior and elm Ulmus spp., may become abundant. In other situations the alder woods occur as a stable component within transitions to surrounding dry-ground forest, sometimes including other Annex I woodland types. These transitions from wet to drier woodland and from open to more closed communities provide an important facet of ecological variation. The ground flora is correspondingly varied. Some stands are dominated by tall herbs, reeds and sedges, for example common nettle Urtica dioica, common reed Phragmites australis, greater tussock-sedge Carex paniculata, and meadowsweet Filipendula ulmaria, while others have lower-growing communities with creeping buttercup Ranunculus repens, common marsh bedstraw Galium palustre, alternate-leaved golden-saxifrage Chrysosplenium oppositifolium and marsh-marigold Caltha palustris.

 

In the UK this Annex I habitat falls mainly within the following NVC types:

 

     

  • W5  Alnus glutinosa – Carex paniculata woodland
  • W6  Alnus glutinosa – Urtica dioica woodland
  • W7  Alnus glutinosa – Fraxinus excelsior – Lysimachia nemorum woodland
  • W2a  Salix cinerea – Betula pubescens – Phragmites australis woodland, Alnus glutinosa – Filipendula ulmaria sub-community

 

Riparian trees are excluded from the Annex I type except where these form part of a wider network of alluvial woodland and wetland communities.

European status and distribution

 

In general, these riverine and flush woods are widespread in Europe, but especially in the more intensively agricultural and long-settled lowlands they are much reduced in cover, often to just narrow strips or lines of trees.

UK status and distribution Click to view UK distribution of this habitat

 

Clearance of riverine woodland has eliminated most true alluvial forests in the UK. Many surviving fragments, as elsewhere in Europe, are fragmentary and often of recent origin. Residual alder woods frequently occur in association with other woodland types or with other wetland habitats such as fens.

Site selection rationale

 

Sites have been selected to take account of the wide ecological variability of the habitat type, the different topographic situations in which it occurs, and differing substrates, ranging from organic to shingle-dominated. The SAC series includes the few remnant extensive stands of alluvial alder woodland in the UK. Smaller examples have been selected to ensure adequate geographical representation.

 

The frequent occurrence of the habitat in association with other Annex I woodland types or with Annex I mire habitats is reflected in the number of sites where it is a qualifying feature, but not a primary reason for site selection.


Site accounts

Coed y Cerrig West Wales and The Valleys
Coed y Cerrig is a good example of alluvial forest in southern Wales. The valley-bottom woodland has a canopy dominated by alder Alnus glutinosa with ash Fraxinus excelsior, and a rich understorey that includes guelder-rose Viburnum opulus and bird cherry Prunus padus. The ground flora is characterised by abundant large sedges Carex spp., and a wide diversity of wet woodland species. The woodland is continuous with diverse ash-elm Fraxinus-Ulmus and oak Quercus spp. woodland on the valley sides.
Coedydd Derw a Safleoedd Ystlumod Meirion/ Meirionnydd Oakwoods and Bat Sites West Wales and The Valleys
Meirionnydd Oakwoods and Bat Sites includes probably the most extensive area of alder Alnus glutinosa alluvial forest in north Wales. The woodland occurs on a dynamic floodplain, allowing cyclical regeneration and decay of alder stands, and the development of a natural structure, rich in dead wood. There is a rich ground flora, with notable plant species including globe-flower Trollius europaeus and creeping-jenny Lysimachia nummularia. The woodland occurs in a mosaic with species-rich marsh and wet grassland, and is continuous with stands of 91A0 Old sessile oak woods with Ilex and Blechnum in the British Isles. The site is also important for wildfowl.
Conon Islands Highlands and Islands
Conon Islands, at the mouth of the River Conon in north-east Scotland, is an example of a relatively unmodified dynamic alluvial forest system – a rare situation in Europe. It provides one of the most complete examples in the SAC series of a transition from woodland through scrub and freshwater fen to saltmarsh communities. The upper part of the site supports alder Alnus glutinosa wood, which is subject to regular inundation and which gives way downstream to alder and willow Salix spp. woodland.
Hollymount Northern Ireland
Hollymount is an example of alluvial forests in the south-east of Northern Ireland. The site is a flooded inter-drumlin hollow. The wet woodland is characterised by a canopy of dense, mature alder Alnus glutinosa and grey willow Salix cinerea, with reed canary-grass Phalaris arundinacea and greater pond-sedge Carex riparia dominating the ground flora. Tussock sedge C. paniculata is also very conspicuous. There are natural transitions to oak Quercus spp. woodland on the drumlin slopes.
Kennet Valley Alderwoods Berkshire, Buckinghamshire and Oxfordshire
These, the largest fragments of alder-ash woodland on the Kennet floodplain, lie on alluvium overlain by a shallow layer of moderately calcareous peat. The wettest areas are dominated by alder Alnus glutinosa over tall herbs, sedges and reeds, but dryer patches include a base-rich woodland flora with much dog’s mercury Mercurialis perennis and also herb-Paris Paris quadrifolia. The occurrence of the latter is unusual, as it is more typically associated with ancient woodland, whereas the evidence suggests that these stands have largely developed over the past century.
Llwyn West Wales and The Valleys
Llwyn is one of the largest examples of alluvial forest in Wales. The woodland has formed on level ground on the floodplain of the River Clywedog, and has a canopy of alder Alnus glutinosa, with transitions to ash Fraxinus excelsior dominance on relatively drier ground. The structure is mature, with massive trees, abundant dead wood, and a rich understorey including abundant bird cherry Prunus padus and guelder rose Viburnum opulus, and occasional buckthorn Rhamnus cathartica and alder buckthorn Frangula alnus. The ground flora is diverse, including both wet woodland species, such as yellow iris Iris pseudacorus and remote sedge Carex remota, and those characteristic of drier ash-maple woodlands, such as ground-ivy Glechoma hederacea and moschatel Adoxa moschatellina. Although one of the largest examples of this woodland type in Wales, the total area is only modest, and the site is constrained by agricultural land and lack of natural dynamic disturbance.
Lower River Spey - Spey Bay Highlands and Islands
The Lower River Spey in north-east Scotland is unique within Britain in comprising an extensively braided channel along the whole length of the river. The active river channel provides a mosaic of substrates, and in more stable, damper situations large stands of valley alder Alnus glutinosa woods occur, along with willows Salix spp., ash Fraxinus excelsior and bird cherry Prunus padus. The ground flora includes both southern and northern elements such as wood speedwell Veronica montana and wood stichwort Stellaria nemorum.
Mound Alderwoods Highlands and Islands
Mound Alderwoods in north-east Scotland is the most northerly site selected and is the largest estuarine alder Alnus glutinosa wood in Britain. It provides examples of successional stages from estuarine mud to dense woodland and is representative of the more stable form of the habitat. A few dry ridges have an open growth of Scots pine Pinus sylvestris with a dry ground flora beneath. The alderwoods have both dry and waterlogged areas. In the former, characteristic plants include remote sedge Carex remota, Yorkshire fog Holcus lanatus and tufted hair-grass Deschampsia cespitosa. The swamp areas are generally richer and include species such as fen ragwort Senecio paludosus, marsh pennywort Hydrocotyle vulgaris, marsh bedstraw Galium palustre and meadowsweet Filipendula ulmaria.
Rea`s Wood and Farr`s Bay Northern Ireland
Rea’s Wood and Farr’s Bay is situated along the shores of Lough Neagh and is one of the best examples of alluvial forests in Northern Ireland. The site contains a complete successional sequence from open water through reedswamp to alder woodland. These habitats occur on permanently inundated and ponded areas that have developed as a result of successive lowerings of the Lough. The woodland canopy is dominated by alder Alnus glutinosa and willows Salix spp., with occasional ash Fraxinus excelsior. The ground flora is very variable and often consists of swamp species, including bottle-sedge Carex rostrata, meadowsweet Filipendula ulmaria, hemlock water-dropwort Oenanthe crocata, large bittercress Cardamine amara, yellow flag Iris pseudacorus and reed canary-grass Phalaris arundinacea. The swamp woodland grades to drier woodland inland.
River Eden Cumbria, North Yorkshire, Northumberland and Tyne and Wear
Throughout the length of the River Eden stands of alder Alnus glutinosa and willow Salix spp. occur associated with backwaters and seasonally-flooded channels. The least-disturbed stands are on the tributary River Irthing, where they occur on the shingle and gravels of actively-moving channels. The ground flora includes patches of common nettle Urtica dioica, butterbur Petasites hybridus and hogweed Heracleum sphondylium that grade into hollows with greater tussock-sedge Carex paniculata.
Shingle Islands Eastern Scotland
This site, on the Rivers Tay and Tummel in the eastern Highlands, comprises a series of extensive riverine shingle areas in various stages of colonisation from bare shingle to mixed woodland, and including old abandoned river channels. The woodland canopy is varied but dominated by alder Alnus glutinosa, with frequent willows Salix spp., ash Fraxinus excelsior, downy birch Betula pubescens and occasional wych elm Ulmus glabra. Hawthorn Crataegus monogyna, rowan Sorbus aucuparia, bird cherry Prunus padus and hazel Corylus avellana all occur in the understorey. The ground flora is also very diverse with many fen species in the wetter areas and more typical woodland herbs elsewhere. Small areas of drier woodland, dominated by ash Fraxinus excelsior with occasional pedunculate oak Quercus robur, and transitions to other shingle, scrub and grassland communities, further enhance the site’s diversity.
The Broads East Anglia
The complex of sites in the Broads of East Anglia contains the largest blocks of alder Alnus glutinosa wood in England. Within the complex complete successional sequences occur from open water through reedswamp to alder woodland, which has developed on fen peat. There is a correspondingly wide range of flora, including a number of uncommon species such as marsh fern Thelypteris palustris.
The New Forest Gloucestershire, Wiltshire and Bristol/Bath area, Hampshire and Isle of Wight
The New Forest contains many streams and some small rivers that are less affected by drainage and canalisation than those in any other comparable area in the lowlands of England. Associated with many of the streams, particularly those with alkaline and neutral groundwater, are strips of alder Alnus glutinosa woodland which, collectively, form an extensive resource with a rich flora. In places there are examples of transitions from open water through reedswamp and fen to alder woodland. The small rivers show natural meanders and debris dams, features that are otherwise rare in the lowlands, with fragmentary ash Fraxinus excelsior stands as well as the alder strips. In other places there are transitions to 9190 Old acidophilous oak woods with Quercus robur on sandy plains and 9120 Atlantic acidophilous beech forests with Ilex and sometimes also Taxus in the shrublayer (Quercion robori-petraeae or Ilici-Fagenion), for which this site has also been selected.
Upper Lough Erne Northern Ireland
Upper Lough Erne is the most extensive area of alluvial forests in Northern Ireland. The woodland occurs in scattered stands around the edges of the lough, where the shoreline is ungrazed or only very lightly grazed. Fluctuating water levels and variations in exposure, substrate and management have resulted in the formation of a wide range of wet woodland communities. These are generally characterised by a canopy in which species such as willow Salix spp. and alder Alnus glutinosa are dominant, with more notable species such as aspen Populus tremula, guelder-rose Viburnum opulus and buckthorn Rhamnus cathartica scattered throughout. The ground flora is often similar to that of the swamp and fen zone, with a rich variety of sedges and herbs. In places, there are well-developed transitions to drier woodland types, including 91A0 old sessile oak woods with Ilex and Blechnum.
Urquhart Bay Wood Highlands and Islands
Urquhart Bay Wood has developed on an alluvial delta at the confluence of the Rivers Enrick and Coiltie as they flow into Loch Ness. There are extensive stands of alluvial forests on the wetter ground associated with the river channels, with transitions on gradually rising land to stands of lowland broad-leaved woodland containing ash Fraxinus excelsior, alder Alnus glutinosa, wild cherry Prunus avium, rowan Sorbus aucuparia, wych elm Ulmus glabra, white willow Salix alba and bird cherry Prunus padus. There are also characteristic transitions to swamp and open freshwater.
West Dorset Alder Woods Dorset and Somerset
Mixed ash-alder Fraxinus excelsior - Alnus glutinosa woods are a characteristic feature of the sinuous valley woods developed along the headwaters of alkaline streams and seepages having their origin in the chalk downland and issuing from the underlying Upper Greensand at its junction with the Gault Clay. The woods vary from those with greater tussock-sedge Carex paniculata, remote sedge C. remota, hemlock water-dropwort Oenanthe crocata, opposite-leaved golden-saxifrage Chrysosplenium oppositifolium and alternate-leaved golden-saxifrage C. alternifolium, to transitions to drier oak-ash woodland with ramsons Allium ursinum. Several of the component sites are associated with valley mires with transitions to fen, reedswamp, fen meadow and acid grassland. Characteristic features of the woods are the shallow silty peats and tufa deposits which support an important assemblage of specialised invertebrates. The streams have natural meanders, back channels and debris dams, features that are otherwise rare in the lowlands. Ancient stands of ash-alder woodland have developed some ‘old growth’ characteristics with associated old forest lichens.

SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

Afonydd Cleddau/ Cleddau Rivers West Wales and The Valleys
Alyn Valley Woods/ Coedwigoedd Dyffryn Alun East Wales, West Wales and The Valleys
Ardgour Pinewoods Highlands and Islands
Breckland East Anglia
Calf Hill and Cragg Woods Lancashire
Coedydd Aber West Wales and The Valleys
Cothill Fen Berkshire, Buckinghamshire and Oxfordshire
Crymlyn Bog/ Cors Crymlyn East Wales
Drostre Bank East Wales
Exmoor and Quantock Oakwoods Devon, Dorset and Somerset
Gower Ash Woods/ Coedydd Ynn Gwyr East Wales
Insh Marshes Highlands and Islands
Loch Etive Woods Highlands and Islands
Loch Maree Complex Highlands and Islands
Loch Moidart and Loch Shiel Woods Highlands and Islands
Lower Derwent Valley East Yorkshire and Northern Lincolnshire, North Yorkshire
Norfolk Valley Fens East Anglia
North Pembrokeshire Woodlands/ Coedydd Gogledd Sir Benfro West Wales and The Valleys
Rhos Goch East Wales
River Camel Cornwall and Isles of Scilly
 

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.