Großes Fenn Nature Reserve: Analyzing the Post-Glacial Kettle Bog Landscape in Southwestern Berlin

The landscape history of Großes Fenn begins at the end of the Weichselian, the last glacial period in northern Europe. As the ice sheets retreated, isolated blocks of dead ice were buried in glacial sediments. When these blocks slowly melted, they left behind depressions, dead-ice holes, which subsequently filled with water and organic material, gradually developing into mires. Großes Fenn is one such kettle bog, with a mire basin covering about 1.9 hectares within a total reserve area of around 6.39 hectares.

For much of the Holocene the bog accumulated peat under naturally acidic, waterlogged conditions, building up the rootlet peat layers intermixed with sphagnum remains that still characterize the site today. The surrounding Düppeler Forest region was gradually forested with pine-dominated woodland, creating the landscape context in which the mire sits. As Berlin expanded in the nineteenth and twentieth centuries, the surrounding land use shifted but the bog itself remained relatively undisturbed within its forested setting.

The modern conservation history began on 5 December 1986, when the Berlin Senate designated Großes Fenn as a Naturschutzgebiet under sections 18 and 19 of the Berlin Nature Conservation Act in its 1979 version, as amended in 1983. By the 1990s, ecological surveys had documented rare mire plants and specialist invertebrates, lending the reserve additional scientific weight. However, the late twentieth and early twenty-first centuries also brought a fire in 1997 and accelerating drying, both of which disrupted the mire vegetation. Sporadic removal of encroaching woody plants has continued as part of ongoing management efforts.

The landscape of Großes Fenn is shaped by the contrast between a small, gently sunken mire basin and the surrounding pine-dominated forest plain of the Düppeler Forest. Topographically, the bog sits only one to two meters above the level of the Havel lowland mires further north, around thirty-one to thirty-two meters above sea level, Normalhöhennull, meaning the reserve is a subtle, almost imperceptible depression in an otherwise gently undulating glacial landscape.

Within the reserve, the core landform is the kettle-mire basin itself, flanked by slightly higher mineral ground where the sandy surface layer overlies the peat substrate. The ground surface reflects this structure: an upper sand horizon eight to seventeen centimeters thick resting on peat, and, in the bowl of the bog, deeper peat layers with shrinkage structures descending to roughly forty centimeters.

The broader visual character is one of quiet, enclosed forest rather than open mire. Pines have spread into the bog basin, both obscuring and reinforcing the site's isolation as a sheltered, acidic, damp hollow within a much larger woodland. The reserve sits just a few kilometers from the Wannsee lakeshore, giving it a regional setting of glacial sand plains, pine forests, and Havel floodplain mires characteristic of southwestern Berlin.

Ecologically, Großes Fenn is classified as a disturbed transitional mire, a so-called Zwischenmoor, dominated by cottongrass, pipegrass species, and stands of marsh reed grass. This vegetation reflects the historical position of the bog between lowland fen and raised bog types. Sphagnum mosses are still locally common, especially in the wetter microhabitats, and give the site its classic peatland character.

A short distance higher on the bog profile, more nutrient-marsh, Niedermoor, vegetation is also present, including species such as marsh violet, water pennywort, tufted loosestrife, marsh cinquefoil, stiff sedge, and grey sedge, all of which are listed on Berlin's Red List of threatened plants. Although many of the rare species historically recorded here, such as round-leaved and oblong sundews and certain cottongrass species, have not been seen in recent decades, the site still functions as a refuge for mire specialists that have largely disappeared from the surrounding intensively used landscape.

The mire vegetation is closely tied to hydrology, and changes in moisture strongly affect species composition. The site illustrates how even very small peatlands can concentrate botanical interest on a metropolitan scale.

The wildlife significance of Großes Fenn lies primarily in its invertebrate fauna and its specialist mire flora rather than in larger mammals or birds. Surveys have documented rare invertebrate species, including representative species of canopy spiders, Baldachinspinnen, that depend on the particular microclimate and vegetation structure of bog margins and adjoining pine stands.

Because the reserve is small and surrounded by dense pine forest, large mammals are not a central focus of its conservation identity. However, the structural diversity of open bog habitat next to forest edge creates conditions for a range of mire-associated invertebrates, spiders, and other small fauna typical of peatland transitions.

The loss of several bog plant species, including sundews and certain cottongrasses, since the 1990s is also a wildlife story: their disappearance mirrors the broader disturbance of the bog's water balance and the encroachment of pine trees into the mire basin. Conservation interest now centers on monitoring what specialist species remain and on understanding what management might allow certain groups to recover.

Großes Fenn has clear conservation significance as one of Berlin's few surviving kettle bogs and as a representative example of a small, post-glacial peatland embedded in an urban forest. Its designation as a Naturschutzgebiet under the Berlin Nature Conservation Act in 1986, and its classification as an IUCN Category IV Habitat/Species Management Area, reflect a management approach focused on maintaining specific habitats and species rather than preserving a broader landscape.

The reserve's conservation challenges are substantial. Decades of drying, intensified by evaporation from encroaching pines, have reduced the water-storing capacity of the peat and accelerated soil mineralization, leaving the bog increasingly vulnerable to further degradation. A wildfire in 1997 displaced much of the surviving mire vegetation, and the underlying causes of the site's disturbed water balance have not yet been resolved. Because there is no nearby groundwater extraction to manipulate and no active rewetting program in place, options for hydrological restoration, such as rewetting combined with topsoil removal in the mire center, remain theoretical rather than implemented.

Current management is limited to sporadic removal of woody regrowth. Despite these pressures, the site retains enough of its transitional mire character and its rare species records to make it a meaningful reference site for peatland conservation in the Berlin region and a focus for ongoing monitoring and potential future restoration.

The highlight of Großes Fenn is its combination of geological origin and ecological fragility: a tiny, post-glacial kettle bog that has survived into the twenty-first century within a major European capital. Few protected areas illustrate so clearly how a feature formed at the end of the last ice age can still be read in the modern landscape, with sandy overlays, sphagnum-rich peat, and a slightly elevated position relative to the surrounding Havel mires.

Its botanical and zoological interest, particularly the historical presence of sundews, cottongrass species, and rare invertebrates, gives the reserve a strong scientific profile despite its limited size. Combined with its protected status under the Berlin Nature Conservation Act and its IUCN Category IV classification, Großes Fenn stands out as a compact but characteristic example of Berlin's hidden peatland heritage.

Großes Fenn can be visited at any time of year as a quiet walk through part of the Düppeler Forest, but the experience of the reserve itself changes with the seasons. In late winter and early spring, before leaf-out and under cooler, wetter conditions, the structure of the bog basin is most visible: bare peat surfaces, frost patterns, and open views across the mire.

Late spring and early summer bring the most active growth of cottongrass, reed grasses, and other mire vegetation, making the site's distinctive plant communities easier to observe. Summer visits, while visually greener, also coincide with the driest period, when water tables are lowest and pine encroachment is most evident. Autumn offers cooler, damper conditions again and a quieter atmosphere in the surrounding forest. Because the reserve is small and ecologically sensitive, visits are best kept to brief observation along forest paths rather than entry into the mire itself.