Securing Biodiversity in Breckland - European Commission

polygons of alternating calcareous ridges and deeper more acidic sand. Surface layers of windblownsand have also been deposited. The resulting soils are extremely complex and varied.The predominant soil types are sandy highly drained and drought prone and low in mineral clay ororganic content and thus potentially very infertile and low in nutrients.Calcareous sandy soils and even shallower chalky rendzinas derived from the chalk-sand drift, cover37% of the Breckland National Character Area (Table 2). These tend to occur on slopes where theoverlying weathered material has been removed by solifluction. Deeper, well-drained and leachedsoils that range from less calcareous to highly acidic, cover a further 41% of Breckland. These tendto occur on plateaus and range from sand over gravel, stony sand or strongly acidic podzols. Graveldeposits cover some high level plateaus, up to 7 metres in depth, and other gravels occur in valleyterraces and valley head deposits. Localised areas of loamy soils occur in the peripheral areas ofBreckland (Figure 3) but are less droughty and nutrient poor, with dry loams, and damp loams andclay soils, covering 5% and 4% of Breckland respectively.Where vegetation is grazed and not annually cultivated, a range of grass-heath vegetation developson these sandy soils. Different plant communities are recognised on the chalky or on the acidicsands. The relationship between soil type and the species composition of a spectrum of grass-heathvegetation, from lichen rich vegetation on raw chalk soil through chalk turf to acidiphilousgrassland, was initially described by Alex Watt (1940). These grassland types were later re-classifiedas part of the National Vegetation Classification (Rodwell, 1991; 1992) (see Table 3).However, although the extremes may be distinct, the species composition of these plantassemblages merges and intergrades along a gradient of pH and soil development from rendzinasto podzols. While some management units may be dominated by one or other type of grassland,these assemblages can also occur in intimate and complex mosaics at many sites. This range ofcalcareous grassland, acidiphilous grasslands and lowland heath assemblages all share commonecological characteristics, they are: droughted, stress tolerant vegetation. developed on dry mineral sands that are low in nutrients, particularly nitrogen (Davy andBishop 1984).For this reason they are all treated as variants of “grass-heath” vegetation (following Watt 1940;Dolman and Sutherland 1992; Rothera 1998). For the purposes of this audit, and particular inmaking management recommendations, we most often generalise across these acidic andcalcareous vegetation types.Wetlands, fens, fluctuating water bodies and pingosElsewhere in the landscape, where drainage is impeded, gleys and peats have formed, covering 12%and 1.4% of the Breckland National Character Area respectively (Table 2). At the periphery offloodplains, or in the lower reaches of dry valleys adjacent to the fens, seasonal water tables canlead to impeded drainage and the development of gley soils formed under anaerobic conditions. Onthe lowest ground where the water table is permanently at or near the surface, the sands andgravels are covered by anaerobic peaty and humose gleys. Gleys can support mesic vegetation29