Mandy Tracey: A Tale of Two Bogs
The word bog comes from bogach the old Irish for soft ground which perfectly describes the wet spongy surface. Bogs are wetland ecosystems which produce peat and are generally low nutrient, acidic environments.
Bogs have two distinct layers called the acrotelm and catotelm. The lower layer of a bog termed catotelm is composed of peat, the partially decayed remains of organic material mainly of plant origin. This accumulation began roughly 7,000 years ago, 3,000 years after the end of the last glaciation event. Peat, sometimes referred to as turf in Ireland, develops when the rate of plant growth exceeds the rate of decay.
The catotelm is permanently saturated with water and as a result oxygen cannot freely pass through this layer of peat. This anoxic environment (absent in oxygen) inhibits the decomposition activities of micro-organisms and thus the dark brown peat soils develop. The acrotelm, the upper living layer of the bog, is largely composed of sphagnum mosses. The water table (surface level of ground water) fluctuates throughout the summer and winter months. When the water table drops below the acrotelm, the living vegetation is exposed to the air. This allows oxygen to move freely, promoting microbial activity, causing the partial decay of plant matter. During the cold and wet winter months the water table may rise to the surface of the acrotelm inundating the the partially decomposed remains of plants. This cuts off the source of oxygen, preventing microbes from further breaking down any plant matter. The process of peat formation has been well summed up by David Bellamy who describes bogs as inefficient compost heaps.
There are two principle types of bog in Ireland, raised and blanket bogs. blanket bogs are further sub-divided into mountainous and atlantic/lowland blanket bogs. Both types of bog are ombrotrophic meaning that they aquire their nutrients exclusively from precipitation. The two bog types may appear to look similar but in fact there are many differences between them but the key differences are in their distribution, the amount of rainfall each receives and how they formed.
Below is a map of the distribution of Irelands ombrotrophic bogs (raised and blanket).
Blanket bogs are predomninantly found in the west of Ireland and areas 150-300m above sea level were rainfall exceeds 1,250mm per year, cooler summers and high humdity provide the perfect climatic conditions.
The vast majority of lowland bogs occur in Mayo and Galway but extensive areas of lowland blanket bog are also found in Cork, Kerry and Donegal. Blanket bogs are named so because they appear to cover the underlying landscape like a large blanket. The distinct features of blanket bogs are the pools, flushes, flat and sloping areas aswell as swallow holes creating a patterned landscape.
The plants characteristic of lowland/atlantic blanket bogs are purple moor grass (Molinia caerulea) and black bog rush (Schoenus nigricans) which gives the surface of the bog a grassy appearance. Bilberry (Vaccinium myrtillus), Common heather (Calluna vulgaris) and crowberry (Empetrum nigrum) are the characterisitc plants of mountain blanket bog.
Raised bogs generally occur in the midlands and land below 130m which receives less rainfall (below 900mm per year) than the upland or Western bogs. Raised bogs are also found in the Bann river valley of Northern Ireland and the counties on either side of the mouth of the River Shannon.
The name raised bog describes the raised dome shape of the landscape. The accumulation of peat eventually elevates the surface of the bog above the surrounding landscape. As a healthy raised bog continues to grow it develops a series of hollows, hummocks and pools interspersed with flat areas called lawns.
Raised bogs can develop from former lakes or shallow depressions carved out by glaciers. Raised bogs may even develop over blanket bogs if the climatic conditions are right. Sphagnum mosses tend to be more abundant in raised bog habitats compared to blanket bogs. Other vegetation that dominates raised bog habitats are sedges such as members of the cottongrass family (Eriophorum) and species of the heather family (Ericaceae).
The video’s below present a nice simple animation of how blanket and raised bogs developed in Ireland preceeding the last glaciation event 10,000 years ago. Just to clarify that the end of the of the glaciation period in Ireland has been dated to about 10,000 years ago it was a further 3,000 years before climatic conditions favoured the development of Irelands bogs.
Raised bog development
Blanket bog development
Great bog of Allen swallow down
that heap of muck called Philipstown
and if thy mor can swallow more
Then take and relish Tullamore