Habitat account - Forests


91D0 Bog woodland  * Priority feature

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 91D0 Bog woodland.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 91D0 Bog woodland. Click image for enlarged map.

Description and ecological characteristics

 

Under certain combinations of physical circumstances in the UK, scattered trees can occur across the surface of a bog in a relatively stable ecological relationship as open woodland, without the loss of bog species. This true Bog woodland is a much rarer condition than the progressive invasion of bogs by trees, through natural colonisation or afforestation following changes in the drainage pattern which leads eventually to the loss of the bog community. The habitat type has not previously been well described in the UK, and consequently knowledge of its ecological characteristics is limited.

 

A few examples of this unusual habitat type are found in areas of Scotland where summer drying may permit the establishment and growth of tree roots in the upper peat layers. The structure and function of this habitat type is finely balanced between tree growth and bog development. Tree growth, however, is always slow (or the trees would take over the bog); the trees are likely to be widely-spaced (because much of the surface area is too wet for them to establish), and dead trees may be common even among the fairly small individuals (because their weight depresses the peat locally leading to waterlogging and death). Although stunted in form these trees may be of considerable age, with the oldest individuals in bog woodland in Scotland estimated at 350 years old.

 

The principal tree species in this form of Bog woodland is Scots pine Pinus sylvestris. Pine bog woodland types are likely to be intermediate in character between NVC type W18 Pinus sylvestris – Hylocomium splendens woodland and more open mire types such as M18 Erica tetralix – Sphagnum papillosum mire or M19 Calluna vulgaris – Eriophorum vaginatum blanket mire.

 

A birch Betula spp.-dominated variant of Bog woodland occurs where birch Betula spp. or willow Salix spp. occur in long-term stable combinations with bog vegetation. These birch/alder/willow types may be close to NVC type W4c Betula pubescens – Molinia caerulea woodland, Sphagnum sub-community or other wet woodland types, such as W2 Salix cinerea – Betula pubescens – Phragmites australis woodland or W3 Salix cinerea – Galium palustre woodland. Very small fragments occur on New Forest valley bogs and on the fringes of some peat bogs and mere sites in hollows within oakwoods, and other examples in Scotland have developed on M17 Scirpus cespitosus – Eriophorum vaginatum blanket mire vegetation.

 

Secondary birch woodland on degraded bogs, and woodland encroachment resulting from falling water tables, are excluded from the Annex I definition, but may in some circumstances be referable to 7120 Degraded raised bogs still capable of natural regeneration.

European status and distribution

 

Bog woodland is extensive in Fennoscandia but becomes increasingly rare through the lowlands of western Europe. However, it occurs through Germany, Poland, the Czech and Slovak republics, as well as having some outposts in the pre-Alps.

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

 

True Bog woodland is thought to be widespread but rare in the UK, but current knowledge on the distribution and extent of this habitat type is limited.

Site selection rationale

 

Selection of SACs for Bog woodland in the UK has been constrained by difficulties in interpreting the habitat definition and lack of survey data. The selected sites include the largest known examples of the habitat, and those showing good structure and function. They represent the ecological variation and geographical range of the habitat in the UK, and include pine-dominated forms in Scotland and birch-dominated forms in southern Britain. The key elements in evaluating habitat function have been (a) the relative ecological stability of the open woodland-bog species combination, and (b) the characteristic patterns of growth and survival of the tree species on the bog surface.


Site accounts

Ballochbuie North Eastern Scotland
The forest mires at Ballochbuie, including representative stands of bog woodland, collectively form one of the largest examples of this habitat in a native pinewood in the UK. Ballochbuie forms one of the largest continuous areas of 91C0 Caledonian forest in Scotland, situated on the slopes of the Dee valley below the foothills of the Lochnagar massif. Here, amongst extensive stands of native Scots pine Pinus sylvestris scotica, variation in drainage patterns has resulted in the local suppression of pine in favour of peat-forming species, allowing peat to accumulate in hollows and along some valleys. In contrast to the prevailing high-forest on the adjacent slopes, these mires are dominated by mixtures of bog-mosses Sphagnum spp., cottongrasses Eriophorum spp. and heather Calluna vulgaris, forming M18 Erica tetralix – Sphagnum papillosum mire and M19 Calluna vulgaris – Eriophorum vaginatum mire, with a sparse to open cover of variably stunted pine. The largest of the wooded mires is Mςine Chruinn in the Connachat valley, which comprises two areas of bog on either side of the valley, within a larger area of fen, all of which support bog pines. Smaller areas of open fen and swamp are also present at Mςine Chruinn, along and adjacent to the burn. Elsewhere, spatial transitions from bog communities to closed canopy high forest are evident at a number of localities. Protected from fire, these forest mires support a number of rare shrubs and bryophytes, including dwarf birch Betula nana, small cranberry Vaccinium microcarpum, and the rare mosses Sphagnum imbricatum, S. angustifolium and Dicranum undulatum. Small burr-reed Sparganium minimum is present at Mςine Chruinn.
Cairngorms Highlands and Islands, North Eastern Scotland
This site contains one of the largest areas of native 91C0 Caledonian forest in the UK, lying on gently-undulating glacial deposits in the foothills of the Cairngorms. Scots pine Pinus sylvestris Bog woodland has developed within the forest because the irregular glacial topography has led to marked variations in geomorphology and drainage pattern. The drier slopes and knolls support mature pine woodland and in the hollows between, wet mires with abundant bog woodland have developed. These stands are composed of mire vegetation, either M18 Erica tetralix – Sphagnum papillosum mire or M19 Calluna vulgaris – Eriophorum vaginatum mire, with a scattering of stunted pine trees and saplings. A good intact example of this community occurs at Mineral Well within Rothiemurchus forest. Recent peat stratigraphy shows evidence of a history of wooded bog on this site. The bog woodland appears to be stable, and the trees, although stunted, continue to grow. Other areas, including Inshriach, have been influenced by past management for commercial forestry, and recent restoration work has created the conditions required for wet woodland restoration. In total the hollows form an extensive area representing the largest example of Bog woodland in Scotland.
Dorset Heaths (Purbeck and Wareham) and Studland Dunes Dorset and Somerset
The Dorset Heaths contain small pockets of wet woodland within valley mires but most of these appear to be of recent origin. However, at Morden Bog a Bog woodland stand is of ancient origin, as shown by its pollen record and old maps. The woodland is dominated by downy birch Betula pubescens with a ground flora consisting of greater tussock sedge Carex paniculata and purple moor-grass Molinia caerulea. There is a rich epiphytic lichen assemblage, again indicating the persistence of this area of bog woodland.
Monadh Mor Highlands and Islands
Monadh Mor in northern Scotland is a complex area of ridges and hollows resulting from glacial deposition. Many of the hollows have become filled with peat and swamp, though larger depressions contain open water. Peat-coring suggests these bog woodland communities are relatively recent developments on previously cut-over mire. The ridges are free-draining and are largely wooded with Scots pine Pinus sylvestris and birch Betula spp., while more extensive hollows support bog on which stunted pine is abundant. These communities are classed as bog vegetation (M18 Erica tetralix – Sphagnum papillosum) with scattered trees. The pine-bog relationship appears to be stable, and the pines are still growing. This site represents one of the largest areas of Bog woodland in a single location in the UK.
Peatlands Park Northern Ireland
Peatlands Park is located within an extensive area of cut-over raised bog close to the southern shores of Lough Neagh in Northern Ireland. The Bog woodland appears to have developed through seral succession over a shallow, peat-bottomed lake. Downy birch Betula pubescens is dominant, with grey willow Salix cinerea one of the main associates. Bramble Rubus fruticosus agg. is locally abundant in the ground cover. However, most of the woodland floor is soft and spongy, with occasional quaking areas, dominated by acid fen communities. These are composed of a thick bryophyte carpet of Polytrichum spp., Scleropodium purum and a variety of bog-moss Sphagnum spp., through which grow a scattering of grasses, sedges and herbs.
Pitmaduthy Moss Highlands and Islands
This Bog woodland site in northern Scotland has, unusually, developed under relatively dry climatic conditions. The communities, which have strong affinities with Scandinavian bogs, are a complex of poor fen and bog. The Moss consists of a system of pools in a shallow hollow bounded by low ridges. Scattered stunted Scots pine Pinus sylvestris occur on the slightly drier areas within the hollow, with the characteristic slow growth pattern and in a stable relationship with the bog surface. The trees become more abundant and attain greater size on the drier ridges. Peat stratigraphy shows evidence of truncation in the peat layers, suggesting a history of peat-cutting at this site.
The New Forest Gloucestershire, Wiltshire and Bristol/Bath area, Hampshire and Isle of Wight
Within the New Forest, in southern England, birch – willow Betula – Salix stands occur over valley bog vegetation, with fringing alder Alnus – Sphagnum stands where there is some water movement. These stands appear to have persisted for long periods in stable association with the underlying Sphagnum bog-moss communities. The rich epiphytic lichen communities and pollen record provide evidence for the persistence of this association. The Bog woodland occurs in association with a range of other habitats for which the site has also been selected.

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

Borrowdale Woodland Complex Cumbria
Breen Wood Northern Ireland
Coedydd Derw a Safleoedd Ystlumod Meirion/ Meirionnydd Oakwoods and Bat Sites West Wales and The Valleys
Cors Caron West Wales and The Valleys
Kinveachy Forest Highlands and Islands
Loch Maree Complex Highlands and Islands
Owenkillew River Northern Ireland
Rhos Goch East Wales
Shortheath Common Hampshire and Isle of Wight
Strathglass Complex Highlands and Islands
 

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