Index Home Join IPCC

Bog Woodlands

Introduction
Many people will be familiar with the stands of low woodland and scrub on the margins and cutaway around the edge of raised bogs. Intact bogs in Ireland, however, are characteristically devoid of tree cover. This is in contrast to central and eastern Europe and parts of Scandinavia where wooded bogs are the norm rather than the exception and at first glance they may not appear like a bog at all to Irish eyes familiar with the typical, open landscape. However, there are exceptions in Ireland where small stands of trees occur in wet areas on the bog dome and these woodlands are some of the rarest and most interesting in the country. They are listed as a priority habitat for protection under the EU Habitats Directive.

The character and vegetation of bog woodlands
One of the first known accounts of such woodland is given by Osvald (1949) who describes an area of tall downy birch trees (Betula pubescens) with bracken (Pteridium aquilinum), narrow buckler fern (Dryopteris spinulosa) and royal fern (Osmunda regalis) on a bog close to Edenderry, Co. Offaly. Father J. J. Moore described a similar woodland on Pollagh Bog, also in Co. Offaly, but both these bogs have now been cut away and the bog woodland destroyed. Detailed descriptions were given by Douglas and Grogan (1985) for bog woodlands that they found in Galway and Roscommon and subsequent descriptions are given for other woodlands by Cross (1987, 1990) and Kelly (1993).

These woodlands are typically dominated by birch, usually downy birch but with occasional silver birch (Betula pendula) and sometimes scattered Scots pine (Pinus sylvestris). The trees form a light canopy 6-8m in height, occasionally over 10m, but sometimes much lower, under which occurs a shrub layer of willows (Salix cinerea ssp. oleifolia and S. aurita) and occasionally Scots pine and holly (Ilex aquifolium). A patchy dwarf shrub layer of ling heather (Calluna vulgaris), bilberry (Vaccinium myrtillus) and bog myrtle (Myrica gale), up to a metre in height, is typically present, sometimes intermixed with bramble (Rubus fruticosus agg.). Locally, the lower growing crowberry (Empetrum nigrum), cross-leaved heath (Erica tetralix) and bog rosemary (Andromeda polyfolia) may occur.

The herb layer is rather variable and depends on the hydrology. In the wettest areas it is typically characterised by purple moor-grass (Molinia caerulea), cotton grasses (Eriophorum vaginatum and E. angustifolium), bottle sedge (Carex rostrata) and soft rush (Juncus effusus). In drier areas bent grass (Agrostis canina) and Yorkshire fog (Holcus lanatus) may occur with scattered ferns, e.g. buckler ferns (Dryopteris dilatata, D. spinulosa), locally stands of bracken and a little tormentil (Potentilla erecta).

The most striking feature of these woodlands, however, is the mosses, particularly species of Sphagnum, which cover the ground to form a thick, spongy, hummocky carpet making walking difficult. The commonest species are the green or yellow-green Sphagnum palustre, S. recurvum, S. squarrosum and S. fimbriatum and the often red-tinged S. capillifolium. On the hummocks and in slightly drier areas a variety of other mosses occur, such as Hylocomium brevirostre, Polytrichum commune and Dicranum scoparium. Another moss, Aulacomnium palustre, indicative of slight water movement, is often present, and numerous smaller mosses and liverworts occur within these robust species. Sometimes this carpet of green is interrupted by patches of grey lichens, especially species of Peltigera and Cladonia, or scattered with the pink flowers or, in autumn and winter, the bright red berries, of cranberry (Vaccinium oxycoccus), which trails over the surface. The trees and shrubs are often festooned with grey-green epiphytic lichens, such as species of Ramalina and Usnea and mosses.

Stands of birch with sally (Salix cinerea ssp. oleifolia) also occur very locally in mineral-rich flushes on the edge of the Mayo blanket bogs in the vicinity of Bellacorick. These have been described by Cross (1987) and, in more detail, by Lockhart (1991). They are perhaps better described as scrub as the trees rarely exceed 2-2.5 m in height. The dwarf shrub layer consists of bog myrtle, ling heather and cross-leaved heath. Purple moor grass, black bog-rush (Schoenus nigricans) and devil's bit scabious (Succisa pratensis) are the most common herbs while the presence of bog bean (Menyanthes trifoliata) and reed (Phragmites australis) indicate the very wet nature of the terrain. A variety of mosses, including Aulacomnium palustre, Hylocomium splendens, Rhytidiadelphus squarrosus and the rare Homalothecium nitens also occur.

Similar types of birch woodland with well developed carpets of mosses occur occasionally among fen-carr vegetation. These probably represent a stage in the succession from fen to bog where ground water influence is declining. This is discussed in the chapter on fens.

Where are bog woodlands found?
Bog woodlands typically occur in very wet parts of a raised bog, which may seem surprising given the very acidic and nutrient-poor nature of the substrate. However, they are invariably associated with so-called flushes or 'soak systems' where a slight flow of water drains off the bog surface. Chemical analyses of the woodland on Clara Bog by Lara Kelly (1993) shows evidence of slightly increased levels of nutrients in the water, especially potassium, but no sign of ground-water influence. The slight flow of water also continually supplies the roots with a fresh supply of nutrients and oxygen, thus enabling the trees to survive. Typically, however, the trees do not grow very vigorously and even small individuals may be moribund while large ones often suffer from fungal attacks.

The stands of birch in the blanket bog flushes occur in very different types of situation. They are present in areas where water seeps up through the underlying mineral-rich substrate and then flows across the bog surface. The water is much less acidic than on the raised bogs and has higher levels of calcium and magnesium. If burning and grazing were removed these stands of birch might expand.

In general bog woodlands are small and usually confined to the uppermost part of flushes. The largest, c. 20 ha in extent, is that on All Saint's Bog, Co. Offaly, which was described in detail by Cross (1987). This site is probably drying out as a result of peat removal to the north and east, a process which may encourage expansion of the birch onto the drier peat but will lead ultimately to the loss of the species requiring wet conditions.

Only very few bog woodlands are known today but, as the evidence of Osvald and Father Moore indicate, it is probable that in the past they were more widespread. The following raised bogs contain bog woodlands: Addergoole, Corliska, Kilmore and Kilnaborris Bogs, Co. Galway; Clooneen Bog, Co. Longford; All Saint's and Clara Bog, Co. Offaly; Ballynamona, Cloonlarge, Shanville and Trien Bogs, Co. Roscommon. The birch stands in Mayo all occur in the Bellacorrick area.

Conservation importance of bog woodlands
These woodlands are important for providing vertical structure, shelter and warmth in an essentially open and exposed habitat as well providing habitats for a variety of additional plant and animal species. The trees themselves provide food and shelter for numerous invertebrates and cover for scrub and woodland birds in which to breed. The late Michael Feehan, Wildlife ranger in south Offaly, recorded Hooded Crow (Corvus corone cornix), Wren (Troglodytes troglodytes), Robin (Erithacus rubecula), Willow Warbler (Phylloscopus trochilus), Chaffinch (Fringella coelebs) and Redpoll (Carduellis flammea) in the wood on All Saint's Bog. John Wilson (1990) also recorded isolated breeding pairs of Bullfinch (Pyrrula pyrrhula) and Linnet (Carduelis cannabina) and suggests other woodland species may occur.

In early summer large numbers of green hairstreak butterflies (Callophyrs rubi), whose caterpillars feed on bilberry, among other plants, may be seen flying around the birch trees. Dead trees, both standing and fallen, are frequently present providing habitat for fungi and wood dwelling insects. O'Connor and Speight (1987) found a rare fly, Dictenidia bimaculata, which is recognized as part of the ancient forest fauna of Europe, in rotting wood in two bog woodlands (Clara and All Saint's Bog). This suggests that these woodlands have been present on the bog, or have occurred in close proximity to other woodlands, for a very long period. Evidence from the peat on Clara Bog, however, suggests that the woodland there has developed only within the last 150 years.

Conclusion
The total area of bog woodland in the country is very small and is probably under 200ha. This makes it almost certainly the rarest type of woodland in Ireland and also one of the rarest bog habitat types. The small fragments remaining, however, are of an importance out of all proportion to their size. As most raised bogs are under threat from peat cutting and associated drying out, it follows that the bog woodlands are also under threat. It is essential that everything possible is done to ensure their long term survival.

References
Cross, J. R. (1987). Unusual stands of birch on bogs. Irish Naturalists' Journal 22: 305-310
Cross, J. R. (1990). The raised bogs of Ireland: their ecology, status and conservation. Unpublished report to the Minister of State at the Department of Finance, Dublin.
Douglas, C. & Grogan, H. (1985). Survey to locate raised bogs of scientific interest in counties Galway (E) and Roscommon. Part II. Internal report. Forest and Wildlife Service, Dublin.
Kelly, M. L. (1993). Hydrology, hydrochemistry and vegetation of two raised bogs in County Offaly. Ph.D thesis, Trinity College Dublin.
Lockhart, N. (1991). Phytosociological and ecological studies of lowland blanket bog flushes in west Galway and north Mayo. Ph.D thesis, University College, Galway.
Moore, J. J. (1955). The distribution and ecology of Scheuchzeria palustris on a raised bog in Offaly. Irish Naturalists' Journal 11: 321-329
O'Connor, J. P. & Speight, M. C. D. (1987) Macrosiphum albifrons, Dictenida bimaculata, Callasidia defonscolombei and Xylapsis petiolata: insects new to Ireland. Irish Naturalists' Journal 22: 199-210.
Osvald, H. (1949). Notes on the vegetation of British and Irish mosses. Acta Phytogeographic Suecica 26: 1-62.
Wilson, H. J. (1990). Birds of raised bogs. In: Cross, J. R. (Ed.). The raised bogs of Ireland. Unpublished report to the Minister of State at the Department of Finance, Dublin.

Copyright Dr John Cross
Duchas
7 Ely PLace
Dublin 2