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For Berkshire, Buckinghamshire and Oxfordshire Wildlife Trust Introduction The hydro-ecology of Wildmoor Heath was examined in 1990 and 1998. The nature reserve, which includes Owlsmoor Bogs and Heaths SSSI, is an area of woodland, dry and wet heathland, and valley mire, that supports a diverse and rich fauna and flora. Part of this diversity is based upon the wide variety of geological, topographic, soil and hydrological conditions. The initial survey provided information to mitigate the effects of a proposed road route and the later survey was concerned with site management for the conservation of its critical biodiversity. Hydrology and Hydrochemistry Water enters the site as three streams. The two small north-east and north-west streams have neutral pH values and nutrient-rich (eutrophic to hyper-eutrophic) water typical of water from urban areas and farmland and not suited to supporting wet heathland. The main southern stream takes flows from a variety of sources and is neutral and phosphorus rich at source but becomes increasingly acidic and reduced in nutrients down stream as it takes drainage water off the valley mires. Water off Rackstraw Lane is slightly alkaline, has very high nitrogen levels, and is not suited to wetland habitats. Water within the mires and marshes is derived from active springs and seepages and which varies considerably in nutrient levels and pH. Phosphorus levels varied from negligible amounts to high values of up to 3ppm. pH varied from extremely acidic values as low as 3.7 to mildly alkaline values as high as 7.4. Conductivity ranged from moderate values of 130 EC units to high values of 480 EC units. Soil water regimes Nineteen distinct soils types were recorded on the reserve ranging from well drained to permanently waterlogged conditions. Woodland on higher land has well-drained flinty brown sandy soils, often podzolised with brightly coloured leached subsoils. Middle slopes have strongly leached podzols with impeded drainage leading to seasonally waterlogging. On footslopes, the soils tend to be slowly permeable, often with seasonally wet humose surface layers. The mires and valley floors have permanently waterlogged soils with peaty surface layers. Where wooded, these soils have humified peat up to 40cm thick, but under acidic valley mires, the peats are made up of raw Sphagnum bog-mosses and can be up to 70cm thick. The mires The seven mires and wetlands occur as springs and flushes at the junction of permeable over less permeable Tertiary strata and in areas with high groundwater. Owlsmoor Bog is formed over a strong foot slope seepage zone. Acidic nutrient poor spring water flows down through the bog allowing development of strongly calcifuge plant communities with Sphagnum, Narthecium and Molinia. On the lowest slopes, the vegetation becomes less acidic in character and calcifuge flora becomes restricted to the tussocks rising above the water level. Potamogetan polygonifolius occurs in the only mildly acidic lowest channels. These floristic changes reflect distinct changes in water chemistry with oligotrophic standing waters in the peat deposits and spring heads and eutrophic flowing waters in channels. Peat here was up to 40cm deep. Wildmoor Bog, in contrast, had mainly moderately acidic and nutrient poor (oligotrophic) water, but pH increased slightly with depth according to groundwater influence. Peat here was up to 57cm depth. Other mires and wetlands have a variety of water chemistry conditions, many are strongly acidic while others have more neutral conditions. General conclusions While many of the mires have strongly acidic nutrient poor waters typical of valley mires, some of the valley side seepages have water that is more neutral and nutrient rich. It is essential that these variations in water chemistry are taken into account if the mires and wetlands are to be effectively managed for their varied hydrological regimes. |
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