|Bogs and fens are the two major types of peat land (defined as areas where the soil is covered with at least one foot of peat). The main difference between bogs and fens is their water supply. Fens depend on ample ground water, while bog surfaces have no connection to ground water sources. When rainwater is trapped in bog mosses (Sphagnum), their extraordinary capacity to retain water raises the water table to just a few centimeters below their heads. In bogs, vertical water exchange is very slow. Rainwater seeps through the upper peat layers (catotelm) at a rate of about one meter per day, and may take several weeks to reach the bottom of the bog.
Bog with pools
Bog with trees
|Bogs are rain fed (ombrotrophic). They need poorly-drained areas, a climate where precipitation exceeds evaporation, and a nutrient-poor environment that favors peat mosses in their ecologic competition against higher plants. Growth of higher plants is also curbed by peat mosses themselves because they bind available nutrients and render the bog water acidic, with a pH of 3-4. Bog peat consists almost exclusively of partly decomposed Sphagnum; its ash content is low (about 3%). Peat depth of bogs is 2-10 meters.|
Because the water surface is trapped among a dense network of Sphagnum stems and leaves, water movement is almost completely lacking, and temperature exchange between water and air is severely restricted. This results in an extreme microclimate, with temperature differences of more than 30º C between day and night. In summer, even if the sun heats up the plant cover, the water temperature hardly exceeds 10º C, and night frosts are common all year round. As compared to the surrounding areas, the vegetation period of bogs may be shortened by 2-3 months.
|Fens are a stage in the succession from open-water ponds or shallow lakes to ombrotrophic bogs. They are minerotrophic, i.e., their water supply is connected with mineral-containing ground water. Peat mosses and a variety of other plants grow in a slightly acidic, neutral or even alkaline (up to pH=8) environment, and generate peat with a relatively high ash content (about 10%). Peat depth of fens is generally less than 2 meters.
When a fen reaches the stage where its peat moss blanket loses contact to the ground water, climatic and environmental conditions determine its succession to a bog or to a terrestrial forest.
Fen with sedges
||Development of bogs is a very slow process that often started already at the end of the last glacial age (up to 10,000 years ago). Shallow lakes formed in glacier beds that were sealed by clay sediments. Under appropriate conditions, these poorly-drained basins were gradually covered with peat moss and typical fen vegetation, and eventually filled up with fen peat. In excessively wet climates, raised bogs were subsequently created by the continued growth of peat moss without any access to the ground water.|
Because of the different growth characteristics of peat moss species, bog surfaces are structured. Water-filled depressions (hollows) create small bog pools. Raised areas (hummocks) can unite to form ridges (strings). If these strings enclose hollows, the resulting bog pools are referred to as "flarks". Relatively dry tops of hummocks allow the growth of certain vascular plants and even stunted trees. At the bog margin, the peat moss growth can expand laterally by a rising water table, thus extending the bog area. This process is called "paludification". Alternatively, bog margins may consist of swampy areas (laggs) where bog water mingles with mineral-containing ground water. Enhanced drainage of these areas supports the growth of trees.
|Bogs in Austria|
|Approximately 220 km2, i.e., 0.3% of the country's area are covered with bogs. The map - which might not be complete, though - shows areas where bogs occur relatively frequently. Most Austrian bogs are small in size and remotely located in mountain areas, where they are protected from agricultural overfertilization and air pollution. The majority of the original bog areas in Austria has been destroyed by drainage.
In the East of the country, which is influenced by the warm, dry Pannonic continental climate, practically no bogs could develop.
Areas in Austria where bogs occur
|Vegetation: Peat Moss|
|The various Sphagnum species prefer specific environments. S. imbricatum, for instance, is a typical bog species, while
S. subnitens and S. squarrosum are restricted to fens. In Austria, S. girgensohnii and S. quinquefarium can be found in waterlogged wood areas. Another colonizing species is S. tenellum, which is also typical of bog lawns. It forms mats that often serve as base of hummock-forming species, such as S. magellanicum, S. capillifolium, and S. fuscum, the peat moss species with the highest capacity to raise the water table. Hummock-forming Sphagnum species generally grow tall and accelerate the production of peat. Typical species of hollows are S. flexuosum, S. fallax, and S. majus. S. cuspidatum and S. denticulatum even grow submerged in water, and can be found in bog pools or flooded peat cuttings. S. palustre, S. fimbriatum, and S. angustifolium prefer bog margins.|
|Other Bog Plants|
|Only a limited variety of highly-specialized higher plants can survive in bogs. They have to be acid resistant and able to take root in a spongy, extremely wet, oxygen-poor environment. Nutrients are scarce, and, in their competition for light, the plants have to adapt to the growth pattern of peat moss (zigzag banding from vigorous growth in summer followed by enormous compression of the moss structures in winter). Commonly known examples of vascular bog plants are heather (Calluna vulgaris) and the carnivorous sundew (Drosera rotundifolia). In the former, nitrogen uptake takes place in symbiosis with mycorrhizal fungi; the latter meets its nitrogen requirements by catching and digesting insects. For this purpose, sundew produces one of the stickiest known biological substances.
Other typical plants of Austrian bogs are wild rosemary (Ledum palustre), bog rosemary (Andromeda polifolia), cross-leaved heath (Erica tetralix), small cranberry (Vaccinium oxycoccos), bog bilberry (Vaccinium uliginosum), black crowberry (Empetrum nigrum), sedges (Carex spp.), cottongrass (Eriophorum spp.), deergrass (Trichophorum cespitosum), and stunted mugo pines (Pinus mugo). Green algae (mainly Desmidiales and Zygnematales) can be abundant in bog water. They even live within peat moss cells (hyalocytes).
Fen vegetation is much more varied. Floating mat-forming sedge species sometimes induce the first stage in bog succession.
|Animals in Bogs|
|Few animal species live permanently in bogs. There are neither fish in the acidic water, nor are there snails, mussels, crabs or other animals that require ample calcium supplies. However, bogs harbor a wide variety of insects, among them abundant dragonflies. Specialized bugs and butterflies, as well as their caterpillars, and several spider species can be found on the bog vegetation. Unicellular animals live in bog water or within hyalocytes of peat moss. Amphibians, particularly the moor frog (Rana arvalis), live and/or spawn in bogs; snakes enter bogs to hunt them. In Austria, bog visitors should beware of a poisonous adder (Vipera berus).|
Fens are home to a great variety of animals. Nowadays, they are important retreats for endangered bird species that have lost their original secluded habitats.
|Basics Morphology Life cycle Genetics Classification Bogs Links About me Home|