Melioration left many peatlands drained in Germany, which in turn influenced the goundwater tables within their catchment area. Additionally, the drainage of ditches caused a rapid overall drainage of the whole landscape. Rewetting of peatlands can contribute to groundwater enrichment. This requires that renaturation achieves a backwater effect in the wider area and that further discharge is slowed down (Joosten et al. 2013, Mauersberger 2010).
Melioration facilities from agriculture do not exist in Berlin. Through the close integration of peatlands with the urban area, water retention effects from rewetting are only of minor relevance. Hence, due to the proximity of peatlands to city settlement such rewetting would often have negative effects, e.g. the possible risk of damage to adjacent buildings caused by rising groundwater tables. (Verleger & Limberg 2013).
Yet the peatlands of Berlin make a positive contribution to the regional water balance: serving as natural water reservoirs they help to maintain typcial microclimatic conditions throughout dry summer periods, have positive effects on biodiversity, and that way, indirectly, also on people, whose health and wellbeing is enhanced through inner-city ‚oases‘ (SenStadtUm 2012).
In Berlin-Brandenburg, hot and dry summer periods are part of the natural weather pattern (e.g. Sommer 2003). According to predictions made by Pik (2009), such and more extreme wheather phenomena are to be reckoned with to occur more frequently due to climate change. Adverse effects due to severely dry weather following the „drought year 2003“ have been documented, for example, for the forest trees of Berlin (SenGUV, p.17).
The property of peatlands to retain water for a prolonged period of time during dry seasons is essential to maintaining typical wetland conditions with characteristic flora and fauna. High water levels and typical peatland vegetation together mean a higher evaporation effect compared to their environment (Cooling Services). For fens with a sinking water level, the evaporation rate also decreases. Evaporation cooling and the relatively low capacity of peat to conduct heat lead to significantly lower night-time temperatures in peatlands, compared to their environment. During the day, differences are marginal. The number of hours per day with high relative humidity (> 97 %) is significantly higher (Eggelsmann 1990) in a fen (6–14 h) compared to the terrestrial habitat (here: Geest, 0–6 h). The microclimate of wet peatlands has a positive effect on the biocenoses of a peatland and its immediate catchment area during dry periods, because damage from drought can be avoided (Luthardt 2014).
A very sensitive reaction is shown by many insect species which are at home in sparsely wooded transition bogs (or mesotrophic bogs) and dependent on low temperatures also at night time in summer, for example because they rely on the water from dew formation (verbally, Mitt. Gelbrecht 2014). Many plant species of the mesotrophic, transitional bogs (acidic to calcareous), too, are highly endangered from the long-term effects of dry periods, as more competitive plant communities tend to establish themselves and replace the native ones once the median water level has sunk (Clausnitzer & Succow 2001). Water retention thus is a self-serving purpose of a peatland, preserving its natural ecologic functions.
Pedogenetically unaltered peats have a large pore volume. This is generally indicative of a soil structure favourable for capillary rise and re-swelling when rewetting of the peat takes place. Contrastingly, the storage of precipitation in drained horizons is impeded. On the one hand side, dense, plate-shaped layers, which occur when backwater builds, may impede infiltration, on the other, shrinkage cracks (Ht-Horizon, see Ad-hoc-AG Boden 2005) and aggregate intespaces (Ha-Horizon, see Ad-hoc-AG Boden 2005) of degraded fens concur to effect a rapid drainage of rainwater. The soil passage of the degraded horizons is bridged very quickly, if there is no long-term filling of the pore spaces (volumes), by percolating water (Zeitz 2001). This leaves the water retention function severely impeded.
Pathways of effect for a positive impact of the biodiversity having well-working water retention services.