Lowland raised bogs are peatland ecosystems, which develop primarily in lowland areas below 150m and are generally surrounded by mineral soils. They are a particular feature of cool, humid regions.
Much of the lowland landscape in Northern Ireland is dominated by drumlins derived from glacial boulder clays resulting in poorly drained soils along the major river valleys and inter-drumlin hollows and around the Lough Neagh basin. The consequent waterlogging provides the anaerobic conditions that contribute to the formation and accumulation of peat. So Northern Ireland has many lowland raised bogs.
However, much of the resource has been destroyed or damaged by a variety of means. Lowland raised bogs may develop from fen or, if the climate is sufficiently wet, by peat formation directly onto a bare substrate. A bog can only grow when the rate of accumulation of vegetable debris exceeds the rate of decay. Continued accumulation of peat elevates the bog surface above groundwater levels to form a gently curving dome, from which the term ‘raised’ bog is derived. Peat depths are variable, but can exceed 12 metres.
Historical info about Lowland Raised Bogs
Peat also preserves a unique and irreplaceable record of past human activity, ecology and climate. Archaeological research has revealed much evidence of human activity through the preservation of artefacts such as canoes and even human bodies. Bog butter has also been found at these sites, it has been said that during a second revolution of farmers, free farmers known as the bo-aire, preserved cattle to produce products such as milk, curds, cheese and butter. The butter would have been wrapped in a wooden vessel or sometimes in bark and buried in the bog to preserve it for winter time.
The peat sequence also holds information on past ecology and climate in the form of sediments and pollen. Several lowland raised bogs are also of international importance for volcanism over much of the Holocene (post 10,000 years before present) with volcanic glass shards (tephra) recorded in the peat column.
Species living in and around Lowland Raised Bogs
Lowland raised bogs support a variety of specialist plants and priority species. Plants found in these areas include Sphagnum bog mosses and vascular plants adapted to waterlogged conditions, such as the cotton grasses Eriophorum spp. Sphagnum mosses are the principal peat forming species on lowland raised bogs, and their dominance in the living vegetation layer gives a bog its characteristically ‘spongy’ surface. The ability of this layer to store water is important in keeping the bog surface wet during the summer.
Lowland raised bogs also support rarer plants such as the bog mosses Sphagnum pulchrum, S. austinii and S. fuscum as well as a number of higher plants which have become increasingly scarce, including Great Sundew Drosera anglica, Cranberry Vaccinium oxycoccos and Bog Rosemary Andromeda polifolia.
Lowland raised bogs also support a distinctive range of animals including breeding waders such as Curlew Numenius arquata, Skylark Alauda arvensis and a variety of invertebrates. Rare and localised invertebrates such as the Large Heath Butterfly Coenonympha tullia are found on some lowland raised bog sites.
Threats to Lowland Raised Bogs
The majority of the lowland raised bog resource has been damaged to some extent, with much of it drained, cutover or improved for agriculture.
Cutover bog describes any site where some of the peat has been removed by hand or more recently by mechanical means, mainly for fuel, leaving some depth of peat behind. In Counties Down and Armagh, there is little intact bog remaining with the majority of the resource cutover and/or reclaimed. Historically, the greatest decline of lowland raised bog has occurred through peat cutting, with 77.5% lost to hand-cutting for fuel.
More recently, mechanised peat extraction both for horticultural purposes and as a fuel has further increased the rate of peat loss. In the 1980s and 1990s, planning permission for the extraction of horticultural peat was granted for approximately 650 ha of lowland raised bog. In addition, the early 1980s saw the introduction of tractor-drawn auger machines that changed patterns of fuel peat extraction on lowland raised bogs with cutting taking place for both domestic and commercial purposes. In the future, it is possible that some bogs may gradually dry out as a result of prolonged cutting and past drainage activities resulting in a general lowering of groundwater tables.
There are three key conditions which have to be maintained if lowland raised peat bogs are to retain their characteristic features:
- Hydrology – any increase in water loss will destabilise the system.
- Nutrient inputs – because the intact bog surface is rain-fed, the nature of the peat is acid and nutrient-poor and supports a number of specialised plants. Significant increases in the base or nutrient-status of the system will alter the vegetation cover in favour of non-bog species.
- Surface vegetation integrity – the living layer of vegetation acts as a natural regulator for water loss. Destruction or alteration of the vegetation will have significant implications for the long-term stability of the ecosystem as a whole.
Factors which could disrupt the balance of these conditions, or which lead more directly to the destruction lowland raised bog habitats are described below.
- Peat cutting/milling- the extraction of peat by hand and more recently by machine for fuel.
- Mineral extraction – the extraction of underlying minerals beneath the peat deposits.
- Drainage – lowland raised bogs drained either directly or indirectly.
- Burning – lowland raised bogs are frequently subjected to burning on an ad hoc basis. Fires on raised bog habitats frequently burn out of control, destroying sensitive species, especially the cover of Sphagnum bog mosses.
- Agricultural improvement – a cutover bog that has been in-filled and topped with soil has frequently been converted to pasture in the past and can be of local significance.
- Grazing – rough grazing occurs on some lowland raised bogs and is frequently accompanied by drainage, trampling, burning and surface contamination with feed and dung.
- Waste disposal – cutover bogs have been used as designated waste disposal sites in the past and are frequently used for illegal dumping of agricultural and domestic waste. This can be of local significance.
- Pollution – contamination from adjacent landfill or agricultural drainage, fertiliser drift during its application, or localised dumping or fly-tipping, may be significant at certain sites.
- Planning developments – land reclamation for urban expansion, rural dwellings and development of the road network, may affect some lowland raised bogs with long term repercussions on the stability of the ecosystem.
References
http://www.ni-environment.gov.uk/lowland_raised_bog_web_version_april03.pdf
Mitchell and Ryan; Reading the Irish Landscape; 1998 edition; Townhouse Publishing; Dublin.