Past Peat Cutting and Present Vegetation Patterns in an Undrained Fen in the Norfolk Broadland

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Abstract
(1) The upper deposits (0-3 m depth) of a Broadland flood-plain mire (the Catfield and Irstead fens) are similar to those elsewhere in the east Norfolk valleys. Estuarine clay deposits (Romano-British transgression) are intercalated within the peat except at the northern margin of the site. (2) Recent (19th Century), shallow (60-80 cm depth) peat cuttings (''turf ponds'') occupy about half of the fen surface. Several community types (Cladietum marisci, Cicuto-Phragmitetum, Phragmites-Sium/Typha communities, Peucedano-Phragmitetum cicutetosum and P.-P. caricetosum) are restricted to turf ponds. The Peucedano-Phragmitetum caricetosum is very species-rich and of great conservation importance, but occupies only small areas of the turf ponds. (3) Patterns of turf-pond recolonization (terrestrialization) are analysed, and four main seres are identified, mostly different from those around the broads. Only one (Cladium sere) leads to Peucedano-Phragmitetum caricetosum vegetation. It is restricted to some marginal turf ponds in places not over clay deposits. (4) Mechanisms of turf-pond revegetation are discussed. Initial recolonization may have been largely from root material situated beneath the turf-pond bases or from surface turves thrown back into the excavated pits. (5) The species-rich communities are temporary seral phases which probably gradually develop into vegetation similar to that of uncut peat surfaces. To maintain them will require peat excavation, in suitable sites. (6) There were clear differences in apparent water-level fluctuation (relative to the peat surface) between revegetated turf ponds and oncut surfaces. Examples of both were flooded to comparable depths (i.e. the surfaces were at similar altitudes) but the amplitude of fluctuation was less in the peat cutting sites, so that the water table rarely fell far below the surface, if at all, in low water periods. Lower, sub-surface, water tables were found in uncut peats at corresponding times. (7) The loose peat infill of the cuttings contracts when the absolute water table falls below the normal (i.e flooded) altitude of the peat surface, thereby reducing the amplitude of water-level change relative to the surface. (8) The former peat cuttings generally had lower oxidation-reduction potentials in the upper 30 cm of peat than the uncut sites; dissolved sulphide concentrations were also higher. (9) There was much variation in concentrations of some cations and anions in the peat water and peat within both the former cuttings and unworked surfaces, but there were no consistent differences between the two situations. However, Cladium was largely restricted to freshwater conditions in turf ponds while it also dominated oligohaline sites on uncut peat. (10) The floristic differences between the herbaceous vegetation of the former peat workings and the uncut surfaces are probably largely a result of their different hydrological regimes and associated differences in redox potentials and redox-related variables.

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