The word "bog" is a colloquial term for what is more precisely referred to as an ombrotrophic mire

A mire is a wetland that supports a vegetation which is normally peat-forming.

An ombrotrophic mire is so called because its water supply, and also its nutrients, are derived directly from rainfall (or snow, hail etc.) alone. (Gk. ombros = a storm of rain, trophos = feeder).

This contrasts with fens, which are also peatlands, but which receive at least some of their water supply and nutrients from the mineral groundwater table of the surrounding landscape and are thus termed geotrophic (earth-fed).

Bogs, because they receive nutrients only from precipitation, are oligotrophic (poorly fed). As a result, they are also often very acidic environments. Indeed, ombrotrophic bogs represent one of the most nutrient-poor and acidic environments in the Welsh natural landscape.

There are various types of bogs or ombrotrophic mires, but two of the most important are raised bogs and blanket bogs.

Raised bogs are so called because of the domed profile of the peat - highest at the centre of the bog and gently sloping away at the edges, like an inverted saucer. Often they are formed over shallow lakes which became infilled with vegetation - climatic conditions then resulted in plant remains accumulating as peat eventually forming the bog as it exists today. The surface of a raised bog is not usually flat, but is often made up of a series of "hummocks" and "hollows".

One of the largest raised bogs in Wales is Cors Caron near Tregaron.

Blanket bogs are more typical of upland areas in Wales, and their shape much more reflects the contours of the landscape - overlaying them like a blanket!

Several sub-types of both raised and blanket bogs have been described. More information can be found in: Lindsay, R. Bogs: The Ecology, Classification and Conservation of Ombrotrophic Mires (Scottish Natural Heritage, 1995).

Mosses

One group of mosses in particular are associated with bogs - these are the Sphagnum mosses. They are of particular importance because of their moisture-holding properties, and because some of them are major contributors to the accumulation of peat. (More information on Sphagnum mosses ).

Sphagnum austinii

This moss has the typical structure of Sphagnum mosses generally. It is distinguished by the rather short and stubby branches, rather like little cigars, and its predominantly golden-brown colour,intermixed with some green and yellow. It was once a common species of raised bogs in Wales, and a major hummock-forming species. It has suffered a steady decline since the Middle Ages, for reasons which are not understood, and is now rare on raised bogs in Wales. In the UK it is classified as Nationally Scarce. Climate change, drainage of raised mires, and burning have all been suggested as possible causes of this decline.

Sphagnum capillifolium

This is one of the Sphagnum species associated with drier parts of bogs - the raised hummocks that occur in many bogs, for example. It varies in colour from green through to a deep wine-red, and grows in compact cushions.

Sphagnum cuspidatum

This is a species particularly associated with bog pools and flooded peat cuttings, where it grows submerged in water. In general appearance when wet it has been likened to a "drowned kitten". Green in colour, its branch leaves are notably long and narrow.

Sphagnum fuscum

This species is a rich brown in colour and grows in very compact, neat cushions on the tops of hummocks on raised bogs. It is rare in Wales, and known from only three sites, where it occurs in small quantities. In the UK it is Nationally Scarce.

Sphagnum pulchrum

The Latin suffix "pulchrum" means "beautiful", and indeed this is one of the most attractive of the Sphagnum mosses, with its regularly-arranged leaves and its deep orange-brown colour. It is exclusively a plant of raised bogs and occurs in wet depressions or pools, sometimes replacing the more common Sphagnum cuspidatum. Sphagnum pulchrum is classified as Nationally Scarce in the UK as a whole, and is known from only three sites in Wales. It can, however, be abundant where it does occur.

Sphagnum magellanicum

This is a robust species, a dark red in colour. Like Sphagnum tenellum it occupies a niche intermediate between the tops of hummocks and pools or depressions.

 

Sphagnum tenellum

This is a species which occupies an intermediate niche, not in the wettest, nor in the driest, parts of bogs. It is usually yellowish in appearance, and is characterised by very concave branch leaves which give something of the appearance of threaded beads, and the stem leaves which are large relative to the branch leaves and the plant as a whole.

Liverworts

Cephalozia connivens

This is a slender, pale-green to yellowish plant which grows over mosses or directly on peat. It has leaves which are bilobed, with the lobes pointed and turned towards each other.

Kurzia pauciflora

This is a tiny plant, dark-green in colour with its leaves divided into four finger-like lobes. It grows amongst Sphagnum mosses.

Mylia anomala

Superficially similar in some respects to the previous species, and often growing closely associated with it. This species is somewhat lighter in colour, and has very large leaf cells - easily visible with a hand lens.

Odontoschisma sphagni

This plant occurs as scattered stems or sometimes extensive wefts amongst Sphagnum mosses. It has two rows of round leaves, and is brown in colour.

Pallavicinia lyellii

This is a thallose liverwort found (in Wales) in wet hollows between tussocks of Purple Moor-grass on the edge of raised bogs. It is readily identified by its very prominent midrib, which is raised from the upper surface of the thallus. It is a UK Red Data Book species,and is currently known form only two sites in Wales.