Mires are a special type of wetland, in which peat deposition occurs. They thrive in cold and wet areas with sufficient precipitation. Peat is composed of dead plant debris, poorly decomposing in the wet places with a lack of oxygen, which piles up. Plants therefore participate in creating a substrate, on which they grow and actively affect their living environment.
Several types of mires can be found in our natural habitats. Their dissimilarities primarily arise from the way they receive water, a key part of their existence. First, there are ombrotrophic mires (raised bogs), only dependent on precipitation water, which is extremely low in nutrients. By contrast, minerotrophic bogs (fens) are also saturated with ground or surface water with various mineral content, important plant nutrients. But there are various transitional forms, as is true for all things natural. Changes may even occur during the development of the individual peat bogs. For instance, large raised bogs, now only dependent on precipitation water, received groundwater in the early stages of their development and looked like fens.
Fens are minerotrophic bogs where various kinds of sedges and mosses grow. Two types – calcareous and non-calcareous – are distinguished based on the calcium content. Calcareous fens are common in the Pošumaví (Šumava surrounding foothills) area, where they are bound to crystalline limestone layers, while non-calcareous, moss fens are typical for Šumava. In most cases, they take the form of peaty sedge meadows, originating in deforested areas and maintained through traditional agriculture as meadows mowed for animal bedding. These are species-rich growths whose typical plants include, for instance, common sedge, carnation grass, western marsh orchid, and lousewort. Exceptionally rare and endangered species are bound, in particular, to sites saturated with mineral-rich groundwater. Such places enrich the Šumava flora with a number of rare species like dioecious sedge (Carex dioica) and alpine deer-grass (Trichophorum alpinum). Fens are also home to one of the rarest of Šumava orchids – Traunsteiner’s marsh orchid (Dactylorhiza traunsteineri).
Transitional are those bogs that are less saturated with groundwater, especially due to a thicker peat layer, and the influence of precipitation water is therefore more significant. They are more acidic, providing a good habitat for sphagnum moss that forms unbroken carpets on them. In Šumava, they most often take the form of poor fens with sparse growths of beaked sedge (Carex rostrata).
Bog spruce forests and waterlogged forests
A less prominent, but prevalent type of mire in Šumava, bog spruce forests often surround the spring areas, border the raised bogs, and constitute extensive stands in waterlogged depressions and valleys along the streams. Typical for them is an abundant moss layer, mostly comprised of sphagnum moss. Frequently found in the herb layer are blueberry, cranberry, and interrupted club-moss (Spinulum annotinum), but also lesser twayblade (Neottia cordata), a rare forest orchid.
The best-known and most prominent type of mires, raised bogs are distinguished by a peat layer, several metres deep, due to which they are raised above the surrounding landscape. Groundwater does not penetrate the peat body and raised bogs are therefore mainly saturated with precipitation water, creating an extreme environment different from the surroundings. In our country, they represent rare islands of arctic nature, relics of the last Ice Age.
Two basic types of raised bogs developed in Šumava. Mountain raised bogs with pools formed in the spring areas in the central mountainous part, at an altitude around 1000 metres. Typical for them is a rugged surface with mounds, flats, hollows, and pools. Rokytská slať (Rokytka Mire) is one of our most beautiful raised bogs.
Valley raised bogs developed in the valleys of larger rivers of the Vltava and Křemelná. In open areas, they are covered with non-forest vegetation with mounds and prevailing shrubs, or they are wholly overgrown with forest vegetation by the bog pine (Pinus rotundata). The largest peat bog in Bohemia – the 300-hectare Mrtvý luh in the Vltava valley – is a typical example of a valley raised bog. The largest one in the Křemelná dell is Hůrecká slať (Hůrka Mire).
Flora and fauna in mires
Highly adverse conditions prevail in the raised bogs, requiring special adaptation. Living organisms have to cope with permanent waterlogging, nutrient deficiency, a cold environment, as well as abrupt temperature fluctuations – severe frost and overheating. The temperature difference measured during a summer day (and night) can be an incredible 40 °C. This is due to the fact that local valleys are often frost hollows with considerable temperature inversion, where night frosts are not uncommon in the summer. Thanks to extreme conditions, raised bogs are home to many rare species of flora and fauna, not encountered elsewhere in the Czech Republic. Some of them have been preserved there since the Ice Age and are called glacial relicts.
The key organism in raised bogs is sphagnum moss, perfectly adapted to this environment. Sphagnum mosses have a special ability to continuously grow at the end, while the lower parts are dying, and after being compacted, they become part of the peat layers. They can impound a tremendous amount of water – they grow in dense clusters in which water rises through capillary action. Moreover, their bodies consist of two cell types – the small green cells serve for photosynthesis, while the large, hollow cells are reservoirs that impound water (hyalocytes).
Higher plants thriving in raised bogs include Ericaceae (heath) shrubs, for instance, bog rosemary, bog bilberry, small cranberry, and less frequent crowberry, while common heather is abundant in valley raised bogs. Wet sites are favoured by Cyperaceae plants. Bog lakes and hollows are overgrown with bog sedge (Carex limosa) and Rannoch-rush (Scheuchzeria palustris). Deer-grass forms continuous cover in the flats of Mountain raised bogs, the mounds are frequented by hare’s-tail cottongrass.
Carnivorous plants excellently accommodated to nutrient deficiency, adding insects to their diet. The best-known of them are sundews – the round-leaved sundew is relatively abundant, while the great sundew is much rarer. Another carnivorous plant is common butterwort, with massive, bright green leaves. The lesser bladderwort is a rare aquatic carnivorous plant.
Numerous rare and specifically adapted species can also be found among the invertebrates. These are frequently dietary specialists, where imagoes or developmental stages are bound to mire plants. For instance, caterpillars of the bog fritillary feed exclusively on cranberries, caterpillars of the moorland clouded yellow eat solely the leaves of the bog bilberry. A unique strategy developed in the European sundew moth, whose caterpillars eat the leaves of the carnivorous sundews from below to prevent being trapped by their sticky glands and become food themselves.
The predatory Ménétries ground beetle (Carabus menetriesi pacholei) is a remarkable mire insect. Being cryophylic, the beetle hunts in the night and overcomes the summer heat thanks to aestivation (dormancy). For winter dormancy, lasting more than six months, it crawls deep down into the peat moss.
Life is also colourful in bog lakes. Dragonflies and damselflies hunt in their surroundings, many of them living exclusively in the peat bogs. Their predatory larvae hunt in the dark bog pools. Small water fauna are hunted there by water bugs – back swimmers and water boatmen. The lakes are also home to the larvae of cryophylic arctic mosquitoes and amazingly variegated microscopic algae called desmids.
The vertebrates living in Šumava do not include any species living exclusively in the peat bogs. Frequent hosts include viviparous lizards and common European adders who like to bask in the sun-exposed forest-free areas. The specific environment of the raised bogs and surrounding peaty meadows is also favourable for the black grouse who finds enough sustenance and cover there, and uses the open areas as leks.