Liam Loftus
G00179936
28th February 2011
Blanket and raised bog formation with characteristic plant species associated with each habitat and adaptations that enable them to tolerate this acidic and nutrient deficient habitat.
Peatlands are a type of ecosystem in which organic matter is produced faster than it is decomposed. They occur in areas of impeded drainage and high rainfall. Waterlogged conditions reduce the amount of oxygen in the soil resulting in the accumulation of partially decomposed vegetation. The two main types of bog in Ireland are raised bogs and blanket bogs. Raised bogs are most abundant in the lowlands of central and Midwest Ireland where annual rainfall is about 800-900mm. They are the result of peat accumulation in shallow lake basins or topographic depressions. As peat continues to accumulate the Fig. 1 – Peatland distribution in Ireland
surface of the bog is raised
above the surrounding landscape, forming a ‘dome’ shape. Raised bogs are the oldest and deepest (up to 12m) bogs in Ireland. Their formation dates back to about 10,000 years ago when shallow lakes formed in poorly drained post-glacial basins. A process of terrestrialisation occurred as vegetation gradually encroached onto the open water from the lake margins. Lack of oxygen in the water
Fig. 2 – Raised bog formation
resulted in the partial decomposition and accumulation of plant litter. The arrival of Sphagnum mosses marked the beginning of peat formation. The ability of Sphagnum to retain water and to acidify the substrate brought about great changes in the vegetation. Acting like a candle wick, it drew water up to the surface and kept the bog permanently waterlogged. As older Sphagnum died it accumulated as a layer of peat which gradually raised vegetation above the water-table. Cut Fig. 3 – Sphagnum Moss
off from mineral-rich water below the bog became increasingly
reliant on rainwater as its sole source of nutrients. As a result plant species, capable of growing in the mineral-poor environments, began to colonise the bog. Today Sphagnum mosses still dominate the ground layer of raised bogs “forming living carpets over the entire surface of hummocks, lawns and hollows.”1 Hummock-hollow complexes allow for the formation of micro-habitats on the surface since the slightest change in water level can have dramatic changes in species composition. Hollows are shallow water-filled structures supporting aquatic plants such as Common cottongrass (Eriophorum Fig. 4 – Hummocks & hollows
angustifolium), Bog asphodel (Narthecium ossifragum) and Bogbean (Menyanthes trifoliata). Hummocks on the other hand lie are drier and tend to support heathers like Ling (Calluna vulgaris) and Cross-leaved Heath (Erica tetralix). Blanket bogs receive higher rainfall than raised bogs and generally occur on more mountainous and coastal terrain. Due to higher levels of rainfall blanket bogs are more acidic, having a higher pH of about 4.2. Blanket bogs are essentially manmade features, aided somewhat by climate. During the Neolithic Age, the first farmers cleared upland forests for agriculture. The exposed soils became vulnerable to a process of paludification. Heavier rainfall resulted in the leaching of minerals from the surface layers of the soil. These minerals were deposited lower down in the soil profile and formed an impermeable layer known as an ‘iron pan’. Rainwater was thus prevented from draining away and the soil became permanently waterlogged. Under these conditions peat began to accumulate, Fig. 5 – Blanket bog formation
covering the landscape like a blanket. Blanket bogs are extensive,
ranging from lowland landscapes in the case of Atlantic blanket bog to upland landscapes in the
1 1
C. O’Connell (1998) A Day on the Bog Field Studies Guide. Irish Peatland Conservation Council., p. 7.
case of Mountain blanket bog. Atlantic blanket bogs are found in low-lying coastal plains and valleys in mountainous areas of western counties. Mountain blanket bogs occur on relatively flat terrain in the higher mountains above 200m and are more widely distributed than Atlantic blanket bogs. Mountain blanket bogs can also be distinguished by it vegetation, particularly the absence of Black Bog-rush (Schoenus nigricans). It is characterised by the presence of Deergrass (Trichophorum caespitosum) and dwarf shrubs such as Ling (Calluna vulgaris) and Bilberry (Vaccinium myrtillus). Atlantic blanket bogs are largely confined to wetter regions along the western seaboard where the annual rainfall exceeds 1250mm. They have a typically ‘grassy’ appearance and are characterised by abundant Black Bog-rush (Schoenus nigricans), Purple Moorgrass (Molinia caerulea) and bog cotton. Wetter areas display a variety of surface drainage features which support aquatic plants such as bladderworts (Utricularia spp.) and Water Lobelia (Lobelia dortmanna). Many of the same plants that live in raised bogs are also present in blanket bogs, but overall, Sphagnum mosses do not dominate and as a result blanket bogs tend to have greater plant diversity. However, the challenging conditions of ombrotrophic bogs tend to limit the range of species found in them. Some plants species are well suited to life in the bog, but others have had to adopt various strategies in order to cope with the challenging conditions. Carnivorous plants such as sundews, butterworts and bladderworts are typical in nutrient poor ecosystems and supplement their diet by absorbing nutrients (especially nitrogen) from insects. Other plants obtain their nutrients through the capture of atmospheric nitrogen. Bog myrtle (myrica gale) for example, houses the nitrogen-fixing
Fig. 6 – Round-leaved Sundew
actinomycete in its roots which allows it to metabolise nitrogen gas from the air. Purple Moor-grass has “recycling mechanisms in which nutrients are efficiently transferred from older to younger leaves, and from leaves to overwintering below ground leaf-bases or buds.”2 In this way, 90% of its
2
M. Otte (2003) Wetlands of Ireland: Distribution, Ecology, Uses and Economic Value, p. 85.
nitrogen and phosphorous is recycled. Waterlogging also presents a serious problem for many peatland plants. Heathers which are usually found on drier bog hummocks have shallow rooting systems to avoid inundation. They also have reduced scale-like leaves, waxy cuticles, and in-rolled leaf margins in order to conserve water during the summer months. The internal system of Bog cotton (Eriophorum augustifolium) allows it to transfer oxygen to its roots which are embedded in deep waterlogged peat. In order for peatlands to function effectively they must have a high water-table. If the watertable is lowered, Sphagnum begins to disappear and the decomposition of peat is accelerated. Surface drainage directly affects the water-table and alters the species composition of the bog surface. The living surface layer of the bog is the key to peat accumulation. Once removed the functioning of the bog is
Fig. 7 – Hand-cutting Turf
destroyed. Peat extraction, through its systematic drainage and widescale removal of surface vegetation, provides the biggest threat to peatland destruction. Traditionally peat-cutting was largely confined to bog edges, “progressed relatively slowly and Fig. 8 – Turf drying
usually allowed the bog
flora to regenerate.”3 However, the use of modern machinery has accelerated the exploitation of peat for domestic fuel. Large-scale peat extraction companies,
Fig. 9 – Drainage & Industrial peat extraction
such as Bord na Móna produce about “5 million tonnes of peat annually (400,000 tonnes as briquettes; 1.5 million tonnes as horticultural peat; 3.1 million tonnes of milled peat for electricity generation.”4
3 4
J. Pilcher & V. Hall (2007) Flora Hibernica: The Wild Flowers, Plant and Trees of Ireland, p. 75. M. Otte (2003) Wetlands of Ireland: Distribution, Ecology, Uses and Economic Value, p. 97.
Afforestation presents a major threat to blanket bogs, since surface drains are “installed at 5m intervals across the peatland surface [and] destroy the integrity of the surface vegetation.”5 Dense conifer plantations exclude typical shade-intolerant bog species, while phosphate treatment makes peatland susceptible to colonization by invasive species. The use of fertilizer in agriculture coupled with drainage and land reclamation, serves to destroy Fig. 10 – Conifer plantation
peatlands and reduce their biodiversity. The
intensification of sheep grazing on blanket bogs also has a dramatic effect on the vegetation, as overgrazing can remove key upland species and lead to changes in composition. For example, ground-hugging species like mosses and liverworts help to create micro-habitats for other species. These plants are very sensitive to disturbance and once they are destroyed other plants soon disappear. The cumulative effect of overgrazing, coupled with high winds and heavy rainfall, can result in the gradual erosion of peatlands. Future changes in rainfall patterns, combined with centuries of habitat loss, are likely to have a major impact on peatland ecosystems. In recent years there has been a change in attitude to bogs. More and more people are recognising the greater value of peatlands beyond merely a resource to be exploited. Irish peatlands provide us with another kind of resource because they represent a unique habitat type. There is no other habitat type in the world where the rate of decomposition of organic matter lags behind the annual production. Peatlands are also a valuable ecosystem type and as such they are an essential part of the biosphere. They play a fundamental role in sequestering atmospheric carbon dioxide through peat deposition and bog restoration is one of the most cost-effective ways of avoiding greenhouse gas emissions. Peatlands are also an invaluable educational facility. They contain records of past environments within their layers of peat. As a result the “pollen and archaeological remains
5
Ibid, p. 98.
preserved in peat represent a most important archive for the history of man and the landscape since the Ice Age.�6 The waterlogged, acidic and nutrient-deficient nature of Irish peatlands also provide us with a good example of an extreme habitat, with communities and assemblages of species specifically adapted to these conditions and rarely found outside of them. Ireland’s extreme Atlantic environment means that Irish peatlands support combinations of plants and animals not found on peatlands anywhere else in Europe. The disappearance of the Irish bogs would have serious consequences for plant and animal species, as their last refuge would be destroyed. Ireland represents a very significant location in European and Worldwide study of this habitat type, and in conserving this resource, possessing almost 200,000ha of actively growing raised and blanket bogs which are of increasing European conservation importance.
6
C. O’Connell (1998) A Day on the Bog Field Studies Guide. Irish Peatland Conservation Council., p. 13.
Bibliography
Brooks, A., & Agate, E., (2004). Waterways and Wetlands: A Practical Handbook. British Trust for Conservation Volunteers. Cronk, J.K. (2001). Wetland Plants: Biology and Ecology. Lewis Publishers, U.S.A.
Fossitt, J., (2000). A Guide to Habitats in Ireland. The Heritage Council, Dublin.
Haslam, S.M., (2003). Understanding Wetlands: Fen, Bog and Marsh. Taylor & Francis Group, London. Keddy, P.A., (2000). Wetland Ecology: Principles and Conservation. Cambridge University Press, Cambridge. O’Connell, C., (1998). A Day on the Bog Field Studies Guide. Irish Peatland Conservation Council, Dublin. Otte, M. (2003). Wetlands of Ireland: Distribution, Ecology, Uses and Economic Value. University College Dublin Press, Dublin. Pilcher, J., & Hall, V., (2007). Flora Hibernica: The Wild Flowers, Plant and Trees of Ireland. The Collins Press, Cork.
Websites
IPCC – The Irish Peatland Conservation Council, online, retrieved February 2011, from http://www.ipcc.ie/ Coillte’s LIFE Project, online, retrieved February 2011, from http://www.coillte.ie/aboutcoillte/news/archive_pre_2010/news_2008/coilltes_life_project_restorin g_raised_bog_in_ireland/ Coillte, online, retrieved February 2011, from http://www.raisedbogrestoration.ie/
National Parks & Wildlife Service, online, retrieved February 2011, from http://www.npws.ie/en/PublicationsLiterature/BogResearchReports/
Bord Na Mona Website, online, retrieved February 2011, from http://www.bnm.ie/corporate/index.jsp?&1nID=93&pID=102&nID=391
Images Retrieved online February 2011 Fig. 1 - http://www.askaboutireland.ie/ Fig. 2 - http://www.wesleyjohnston.com/users/ireland/images/raised_bog_formation.gif&imgrefurl Fig. 3 - http://www.pitcherplant.com/images_2/red_sphag.jpg&imgrefurl Fig. 4 - http://www.gret-perg.ulaval.ca/uploads/pics/Tourbiere_3x.gif Fig. 5 - http://www.wesleyjohnston.com/users/ireland/images/blanket_bog_formation.gif Fig. 6 - http://www.mikebaker.com/plants/1280/DSC_0284.jpg Fig. 7 - http://www.ipcc.ie/cutturfcuttingbank.jpeg Fig. 8 - http://static.panoramio.com/photos/original/10480900.jpg Fig. 9 - http://bordnamona.wikispaces.com/file/view/P1010533SM.jpg/33347039/P1010533SM.jpg Fig. 10 - http://www.birdguides.com/i/articles/002315/forsinard2.jpg