Fig. 3.7 .la. Near infra-red false-colour aerial photograph of the bog of la Vraconnaz on 31 July 1984.
Fig. 3.7.1b. Near infra-red false-colour aerial photograph on 20 October 1987, taken ca. one month after the bog burst. (Photos taken by the Co- ordination Centre for Aerial Photographs, Diibendorf).
3.7 A moving mire - the burst bog of la Vraconnaz Elizabeth Feldmeyer-Christe and Gilles Mulhauser Community: Sainte-Croix (canton Vaud)
Locality: La Vraconnaz Coordinates: 525-527/187-189 Elevation of the mires: 1,100 m Area of the raised bog: 31 ha Area of the fenlands: 25 ha
Area of the mire landscape La Vraconne: 200 ha
3.7.1 Highlight of the visit
This site is worth seeing because it was affected by a bog burst in 1987. Such an event, which occurs comparatively frequently in Scotland and Ireland, is very unusual in Switzerland.
3.7.2 General information
The bog of la Vraconnaz is located in the Jura Mountains (canton Vaud), to the north of Sainte-Croix and near the French border.
Situated in a large depression on a gentle slope north-west to south-west at a mean altitude of 1,090 m, the raised bog has a surface of about 30 ha surrounded by 25 ha of fen (Fig. 3.7.2). Most of the bog belongs to the Swiss League for Nature Conservation (SLNC).
Like most of the Jura bogs, the bog of la Vraconnaz was being cut for fuel by the 18th century; the peat was used for a smelting furnace near Mouille- Mougnon (1.5 km south-east of the bog's centre). The peat cutting stopped at the end of World War Two. In 1987 the bog suffered a bog burst (cf. Chapter 3.7.6).
Fig. 3.7.2. Location of the mires and the mire landscape of national importance in the area of La Vraconnaz (modified from DFI 1990, 1991b, 1991c).
1 Vantage point; 2 Bog burst area; 3 "Intact" bog area.
Scale of the map: 1 : 25,000; for key, see end-cover. Reproduced by courtesy of the Federal Office of Topography, Berne, 9 June 1992.
Sainte-Croix (1092 m) 6.00 1340 (11)
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Fig. 3.7.3. Climate diagram for Sainte-Croix (modified from W ALTER and LIETH 1960). For explanation see Fig. 1.5.4.
3.7.3 Geology and hydrology
Geologically, the bog is located in the Chain Jura (cf. Fig. 1.3.8). Most of the Jura in canton Vaud lies between two large, complex sinistral faults: the Vallorbe- Pontarlier running to the east and the Morez-Ies-Rousses running to the south- west. Dextral east-west to east-south-east to west-north-west conjugate faults are associated with both fracture zones. The kinematic relations between folding, thrusting and strike-slip movements are very intricate. Large, broad box-folds constitute the Vaud Jura, while the synclines show more complex structures. The bog lies on a large anticline. The bedrock is probably made up of impermeable marly rocks of the Cretaceous. The outcrops at the north and west edges are limestone of the Sequanian that forms the centre of the fold. The soils of the anticline in the east part consist of calcareous clay of the Callovian (mainly summarized from TROMPY 1980).
The water supply comes from rainfall and seeping water, which appears from several spring outflows. Because of its situation in a quite closed basin, the runoff water from the bog is unable to escape except into a range of dolines on the southern edge.
3.7.4 Climate
The climate data are supplied by the station located in Sainte-Croix -L' Auberson, 2 km to the south of the bog's centre (Fig. 3.7.3). The mean annual level of precipitation is between 1,320 and 1,480 mm with large variations. The mean annual temperature is between 4° and 5° C. Frost is possible in almost any month of the year.
3.7.5 Peatland types and vegetation
The bog at la Vraconnaz is a sloping bog. The undisturbed north-eastern part of the bog supports vegetation groups of Sphagnion magellanici (with a small and rare liverwort Loplwzia laxa), as well as mud sedge (Carex limosa), Rannoch rush (Scheuchzeria palustris) , feathery bog moss (Sphagnum cuspidatum) , and slender sedge (Carex lasiocarpa) in depressions. Most notable of all is the presence of string sedge (Carex chordorrhiza). The north-western part has vegetation groups of alkaline fens of the Caricion davallianae association. The parts which were exploited during the two world wars now present many regeneration complexes with a mixture of Sphagnion magellanici groups, CaUl/na heath and acidic fens of the Caricion nigrae group.
The bog is almost completely enclosed with pastures of the Cynosurion alliance, meadows of Polygono-Trisetion and woods with spruce (Picea abies).
In the north, the depression is lined by an Abieti-Fagetum forest.
3.7.6 The bog burst of la Vraconnaz
An exceptional climatic event caused the bog slide in September 1987. After three weeks of drought, heavy rainfall followed on 25 and 26 September; almost 180 mm of rain fell within one night. This has been the most extreme event ever recorded since the start of systematic climate measurements in the region 80 years ago (ROTHLISBERGER et al. 1991). It was so extraordinary that even the well-developed natural drainage system in the surroundings of the bog (Karst situation) may have been full to overflowing. Such a large amount of water could not be soaked up by the peat. A spring at the upper edge of the bog swelled so much that the peat body was torn away from the mineral sub-soil and slid downwards. The aerial photograph demonstrates that the mechanism of the peat slide was similar to the movements of a glacier (cf. Figs. 3.7.4 and 3.12.5).
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Fig. 3.7.4. Map of the area after the bog burst (Map by P. Thee 1989).
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Fig. 3.7.5. Cross-section A-A 'of the bog burst of la Vraconnaz (cf. Fig. 3.7.4). The vertical scale is exaggerated 20 x. (Diagram by P. Thee 1989).
Botanical succession
About 15 ha on the western part of the bog was affected by the slide. This area had been cut for fuel and still showed many scars. It was also well-known that this was an area of ground-water recharge during snowmelt. During the night, about 150,000 m3 of peat slid along the gentle slope for a distance of at least 300 m (Figs. 3.7.4 and 3.7.5). Many trees were carried with the peat-rafts and remained standing. The peat at the burst was eroded in places, until the grey mineral sub-soil was exposed. The two biggest dolines in the south were completely filled up with peat. A small pine forest (mainly Pinus montana) in the same area was entirely crushed. Because the residual water did not completely escape into the dolines, several new ponds were formed in the south-west and south-east corners.
This uncommon event immediately triggered a multi disciplinary programme of research in geophysics, vegetation science, pollen analysis, entomology, pedology and mapping (FELDMEYER-CHRISTE 1990a).
A photogrammetric map of the situation, at a scale of 1 : 1,000 (Fig. 3.7.4), was established after the bog slide by means of near infra-red false-colour aerial photographs (scale of 1:9,000 and 1:5,000) taken 3 years before and immedi- ately after the bog burst (cf. Figs. 3.7.1a and 3.7.1b). This map helped to provide an overview of the drastically disrupted landscape. Other maps were then produced, such as maps of drainage ditches, contour lines and longitudinal profiles (Fig. 3.7.5).
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3.7.7 Present land use
Zone of accumulated peat mass
Despite the SLNC owning a good part of the bog, it is still partially used for grazing. There has also been an increase of trampling by people attracted to the spectacular effect of the bog slide.
3.7.8 Research
After the bog burst in 1987, the bog became an interesting research area for development and regeneration studies.
The bog slide in the autumn of 1987 caused a complete change in the hydrologi- cal and soil conditions in the western part of the bog. Simultaneously, it offered an exceptional opportunity to study the development of the vegetation in such a disturbed environment.
Zoological successions
One hundred permanent plots were selected in 1988 to represent the greatest environmental diversity. Some of the plots are situated in representa- tive plant groups (Sphagnion magellanici, Caricion nigrae, Caricion davallianae or Pinus mugo forest) that seem at first sight little modified by the slide. The other plots are situated in different kinds of pioneer habitats like bare peat, new hollows, new pond banks or mineral soil.
Vegetation releves are made at different time intervals (every year, every two or five years) according to the speed of change in vegetation. The objectives of this survey are to study the different phases of plant settlement as well as plant successions. The behaviour and resistance of sphagna and other bryophytes to hydrological changes and eutrophication are also closely observed, as well as their sensitivity compared to phanerogams.
In a 3 year comparative study, the first results show a drying out in the groups ofSphagnion magellanici with an increase of heather (eal/una vulgaris).
The groups of alkaline Caricion davallianae and acidic Caricion nigrae exhibit a decline in characteristic species and the appearance of damp meadows species.
The changes in the pioneer permanent plots occurred more quickly. Out of 11 plots without vegetation in 1988, only 2 were still bare in 1990. The 11 plots in question all have a soil ofvery dry peat, strongly compressed and mineralized.
There is random settlement by pioneer plants in damp areas with competition beginning only after a lag period of some years.
For zoologists, the bog burst represented a special and profitable occasion to study the colonization of new habitats from the very beginning. The aquatic system was the most modified, so that numerous and varied aquatic habitats were created: ponds, streamlets, hollows and rivulets (MULHAUSER 1990).
Terrestrial parts were also carried down in the landslide, but with their original fauna. Thus, the problem of colonization does not really exist in this case (it is more a problem of reorganization linked to the evolution of vegetation).
Only some of the hundreds of aquatic species have been chosen, described and classified into 8 different types (MULHAUSER 1990). In three elements of each type, aquatic fauna (according to a standardized method) was captured every two years (1988, 1990, 1992), three times a year (June, July, September).
Data are presently being analysed; precise results are not yet available.
This landslide has brought a significant enrichment to the fauna. Compared with previous data, this is especially the case for beetles (Coleoptera), bugs (Heteroptera) and dragonflies (Odonata) whose number of species increased significantly. Beetles and bugs have shown a faster response, as have some Diptera families. Dragonflies seem to colonize more slowly; new populations of characteristic species (that were absent before the slide) are establishing themselves (MULHAUSER 1991).
The main goal of such a survey and analysis is to provide a description of the communities' succession of aquatic invertebrates in a specific habitat. This will allow biologists to know in advance which kind of fauna would colonize a new man-made pond. This information is also helpful in designing a specific environ- ment to promote certain taxa.
