UK Biodiversity Action Plan
Priority Habitat Descriptions
UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008.
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The definition of this habitat remains unchanged from the pre-existing Habitat Action Plan (http://www.ukbap.org.uk/UKPlans.aspx?ID=4) a summary of which appears below.
Wet woodland occurs on poorly drained or seasonally wet soils, usually with alder, birch and willows as the predominant tree species, but sometimes including ash, oak, pine and beech on the drier riparian areas. It is found on floodplains, as successional habitat on fens, mires and bogs, along streams and hill-side flushes, and in peaty hollows. These woodlands occur on a range of soil types including nutrient-rich mineral and acid, nutrient-poor organic ones. The boundaries with dryland woodland may be sharp or gradual and may (but not always) change with time through succession, depending on the hydrological conditions and the treatment of the wood and its surrounding land. Therefore wet woods frequently occur in mosaic with other woodland key habitat types (e.g. with upland mixed ash or oakwoods) and with open key habitats such as fens. Management of individual sites needs to consider both sets of requirements.
In terms of National Vegetation Classification (NVC) plant communities this habitat is characterised by W1 Salix cinerea - Galium palustre woodland, W2 Salix cinerea - Betula pubescens - Phragmites australis woodland, W3 Salix pentandra - Carex rostrata woodland, W4c Betula pubescens - Molinia caerulea woodland: Sphagnum sub-community, W5 Alnus glutinosa - Carex paniculata woodland, W6 Alnus glutinosa - Urtica dioica woodland, and W7 Alnus glutinosa - Fraxinus excelsior - Lysimachia nemorum woodland. Some birch stands classified as W4 are relatively dry and in management terms better treated alongside other extensive birch stands. As a provisional division, sub-communities W4a and W4b are better associated with Upland/Northern Birchwoods. Just as small wet woodland patches may be treated as part of a dry land mosaic, so dry land fringes of predominantly wet woodland areas are linked with the accompanying wet woodland. Wet flood plain forests of ash, elm and oak, lacking alder, are most likely to fall into W8 Fraxinus excelsior- Acer campestre - Mercurialis perennis woodland.
Many alder woods are ancient and have a long history of coppice management which has determined their structure, and in some situations it appears that this practice has maintained alder as the dominant species and impeded succession to drier woodland communities. Other wet woodland may have developed through natural succession on open wetlands (sometimes following cessation of active management) and structurally are little influenced by direct forestry treatments.
Notable concentrations of wet woodland on fens occur in East Anglia, Shropshire and Cheshire, while hill-side and plateau alder woods are more restricted to Wales, Cumbria and western Scotland. Fragments of ancient floodplain forest are rare, and the best examples are probably in the New Forest and northern Scotland. Bog woodlands of pine on bog are confined to Scotland, but fragments of birch bog woodland occur more widely in scattered stands across the UK.
Some wet woods include habitats identified under Annex 1 of the EC Habitats Directive, for example Residual alluvial forests and Bog Woodland.
There are no precise data on the total extent of wet woodland in the UK, but in the late 1980s the Nature Conservancy Council estimated the total extent of this type in ancient semi-natural woodland to be about 25,000 - 30,000 ha. The area of recent wet woodland may be at least as large again. Thus a crude estimate of the total wet woodland area in the UK is 50,000 - 70,000 ha.
Wet woodland combines elements of many other ecosystems and as such is important for many taxa. The high humidity favours bryophyte growth. The number of invertebrates associated with alder, birch and willows, is very large, although some are now confined to just a few sites, for example the biodiversity priority species beetles Melanopion minimun and Rhynchaenus testaceus. Even quite small seepages may support craneflies such as Lipsothrix errans and the endemic Lipsothrix nervosa. Dead wood within the sites can be frequent, and its association with water provides specialised habitats not found in dry woodland types - the fly Lipsothrix nigristigma for example is associated with log jams in streams. Wet woodland provides cover and breeding sites for otters Lutra lutra. While few rare plant species depend on wet woodland per se, there may be relict species from the former open wetlands on the site such as the marsh fern Thelypteris palustris.