"Wildes Offenland" - Die Bedeutung und Implementierung von "Störungen" für den Erhalt von Offenlandökosystemen in ansonsten nicht gemanagten (Schutz-)Gebieten

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Peringer, A.; Schulze, K.; Giesbrecht, E.; Stanik, N.; Rosenthal, G.
Bonn - Bad Godesberg
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We investigated fundamental questions regarding the role of natural disturbance regimes in German potential wilderness areas. Central to our research was, if endangered species that depend on open landscapes might find habitats in areas that develop according to their own principles and therefore without human interference. This question especially arises in land-scapes that currently undergo transitions from an open to forested state and where conserva-tion of open habitats is an equal aim to wilderness development and where management is not an option, e.g. in former military training grounds of the German Natural Hertiage Sites. We distinguished sites for which potential natural vegetation is assumed to be forest (zonal forest ecosystems) from places where frequent and severe disturbances or abiotic stress are known to create and maintain open habitats (floodplains, coastal areas, the high alpine zone and peatbogs). In the zonal forest ecosystems (mixed oak forest and heathlands, beech forest on limestone, subalpine mixed forest as examples), we investigated the potential of wild mega-herbivores to create and maintain open habitats following the megaherbivore theory. We compared the currently occurring herbivore community with a completed community to which European wisent was added. Wisent was used as a representative for intermediate foraging herbivores having strong effects on vegetation due to combined grazing and browsing activity. We used computer simulations to project future wilderness development in zonal forest eco-systems with current and completed herbivore communities and for climate change time scales, i.e. several centuries, and considered the specific disturbance regimes of forest eco-systems, i.e. wildfire, windthrow and bark beetle outbreaks respectively. For floodplains, coastal areas, the high alpine zone and peatbogs, we performed a literature survey. In all zonal forest ecosystems, natural disturbance was able to create open habitats from time to time, i.e. just after the disturbance impact. However, these open habitats were of poor quali-ty for nature conservation when grazing by herbivores was absent. Initial clear-cuts prior to long-term wilderness development showed the same. Without the grazing pressure of inter-mediate herbivores, vegetation development leads to fallow succession similar to abandonend fields. In the presence of intermediate herbivores, two effects occurred simultaneously: Their grazing stabilized open habitat and developed meadows and lawns. In the long-term, tree cover in the forest was thinned by browsing on regeneration. The establishment of light demanding tree species was facilitated towards the development of a species rich mixed forest community. Additional disturbance increased open habitats and structurally diversified the mosaic of open and forest habitats and interconnecting ecotones. In the case of wildfire scenarios in mixed oak forest, heathlands and beech forest, the devastating effect of fire was regulated by herbi-vores due to their consumption of biomass in the herb and sapling layer. In a mosaic to fre-quently burned heathlands and pine stands, so-called fuel breaks emerged that allowed the persistence of fire sensitive species and old (oak) forest stands. In the case of bark beetle scenarios, the promotion of mixed forest instead of spruce dominated subalpine stands by herbivores hindered outbreaks, because stem density of susceptible spruce was low even after windthrow. These self-regulating ecosystem dynamics increased habitat diversity and appeared to de-pend on complete landscape ecosystems regarding natural disturbance regimes and interme-diate herbivore communities. In scenarios, where one factor lacked (either herbivores or wild-fire, bark beetles respectively), comparatively simple landscape patterns emerged that were preconditioned by topography and soil, as it is known from forest-grassland segregation in wood-pastures and from fire-dominated landscapes and bark beetle outbreaks. Our results call for an ecosystem approach to the establishment of wilderness areas. Interme-diate herbivore communities in combination with wildfire point towards a solution for former military training areas with conflicting goals of nature conservation: conservation of FFH-protected heathlands and wilderness development. Wildfires alone did not succeed here, nor did herbivores alone satisfactorily for fire-dependent heath. Wilderness ecosystems without herbivores tend towards forest, which later on is prone to catastrophic disturbance, because regulation by herbivores lacks. We therefore argue to complete current herbivore communities with intermediates and to allow natural disturbances wherever possible in wilderness areas of sufficient size, shape and habi-tat conditions. Initial catastrophic events should be considered as a first step towards a novel sort of ecosystem dynamics, in which self-regulating feedbacks are on the way to develop. Large compact areas are a pre-requisite here, because free ranging herds and catastrophic events need to be bufferd to the surrounding landscape. Moreover, the emerging habitat mo-saics require heterogeneity in soil conditions, topography and initial conditions, which alto-gether form mosaics at a larger scale. Even stochastic disturbance impacts may not create habitat mosaics if areas are too small and will be entirely devastated by fire or bark beetles without leaving parts untouched. Especially herbivores need such refuge areas in times of disturbance. In floodplains, coastal areas, the high alpine zone and peatbogs altogether, the restoration of the natural disturbance regimes (floods and avalanches) or abiotic stress (high water tables) appears to be sufficient for wilderness development. The re-established natural ecosystem dynamics will create open habitats and mosaics of various succession stages as it did in the past before human intervention. Moreover, upcoming climate change is expected to increase disturbance frequencies and severity (floods) and to thereby promote landscape openness. In peatlands and floodplains, the completion of herbivore communities with wild grazers adapted to these environments has the potential to create and maintain open habitats similar as in the case of zonal forest ecosystems. However, large areas are required to ensure refuge areas in times of floods of long duration, which are characteristic to large lowland rivers.


Last updated on 2019-14-05 at 09:48