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Rewetting bogs has positive effects

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Ariel Bergamini looks over from the forest edge at Hagenmoos, a bog near Kappel am Albis nearly four hectares in area. Hundreds of white feathery tufts are swaying in the wind in front of the WSL botanist: “Hare’s-tail cottongrass is a typical resident in this bog,” he says. “Since it was restored and the water table raised as a result, this plant has become more frequent here.” Visually inspecting the bog, Ariel discovers not only seven Sphagnum moss species, but also several other characteristic plants such as cranberries and bog rosemary.

Hagenmoos has been listed in the Federal Inventory of Raised and Transition Bogs of National Importance since 1991. It is one of the several thousand mires in Switzerland that were drained last century with a system of ditches and drain, often to remove peat. Since the Rothenthurm federal initiative was approved in 1987, mires have been protected, and some of them, including Hagenmoos, gradually rewetted. But there is still a lot to do.

Sphagnum mosses need wet feet

The strict protection of the remaining 551 raised and transition bogs in Switzerland is intended not only to conserve the few still intact raised bogs, but also requires the restoration of drained bogs. “This means, in particular, raising the water table far enough for the water-retaining Sphagnum mosses typically found in bogs to have permanently wet feet,” explains Ariel. Then new peat slowly forms and the bog starts to rise, on average by one millimetre per year, i.e. one metre every thousand years.

 

Raising the water table requires a lot of work and money. Water can be prevented from draining out of the bog in various ways. On the edge of Hagenmoos, for example, clay was used to block drainage ditches and small weirs were installed to control the water table in the bog. In other places, sheet piling, made of wood or steel and placed against the ditches’ direction of flow, prevent the loss of water.

Vegetation indicates typical wetland sites

As part of Monitoring the Effective­ness of Habitat Conservation in Switzerland, a project that the Federal Office for the Environment (FOEN) coordinates, researchers at WSL are investigating how the mires in the whole of Switzerland are developing, including those that have been restored. They record the frequency and locations of mosses, grasses, herbs, shrubs and trees on small, permanently fixed sample plots. Here the so-called indicator values of these plant species are important as they reflect site characteristics, such as humidity and nutritional content, as well as the pH values of the substrate and the proportion of humus it contains.

Rewetting Hagenmoos appears to have been successful: Sphagnum mosses and other plants characteristic of raised bogs are numerous. In some parts of the bog, a surface structure with hummocks and hollows has formed, consisting of small, moss-covered and slightly drier hillocks interspersed between lower areas that are often filled with water. “It is, however, a long road until a drained area can be called a bog again,” says Ariel. “Now, fifteen years after my last visit, I can see that the Sphagnum moss on Hagenmoos has developed well. This bog appears to be on the right path,” he says happily. (Reinhard Lässig, Diagonal 2/18)