Grazing marsh and floodplain grassland habitat audit

Grazing marsh and floodplain grassland are concentrated in coastal levels and the floodplains of major rivers and are typified by gentle topography with impeded drainage. The habitat depends upon periodic inundation and grazing (or cutting). These assemblages are usually found on surface water gley, ground water gley and peat soils with a low to moderate fertility, usually underlain by clays and loams of mildly acidic to neutral reaction.

The principle factors that determine the species composition of wet neutral grassland are soil type, moisture and management (past and present). Floodplain grassland in London covers a wide range of communities; from the Yorkshire fog Holcus lanatus - tufted hair-grass Deschampsia cespitosa community, which can be floristically very poor, to the crested dog's tail Cynosorus cristatus - marsh marigold Caltha palustris community. The latter is a rare, species-rich type, particularly associated with old flood meadows.

There are approximately 416 ha of estuarine grazing marsh and 432 ha of floodplain grassland in Greater London. Grazing marsh and floodplain grassland habitat in Greater London is limited in distribution, extent and quality, reflecting the national declines of these habitats. Estuarine grazing marsh has been recorded from three London boroughs: Barking & Dagenham, Bexley and Havering. Havering also has the highest total recorded floodplain grassland (135 ha or 31% of the London total); followed by Hillingdon and Richmond upon Thames.

It has been suggested that the national area of wet grassland declined by more than 40% between the 1930s and the 1980s. Indeed, Greater London lost 85% of grazing marsh on the Thames estuary between 1935 and 1989. Although there are no figures available for the decline of floodplain grassland within Greater London it is thought to follow similar trends.

Floodplain grasslands support a wide range of plant, bird and invertebrate species, many of which are rare and declining. However, many floodplain grasslands in London are floristically poor and predominantly composed of Yorkshire fog and tufted oat grass. Other examples of floodplain grassland can be species-rich with damp loving plants such as sneezewort Achillea ptarmica and ragged robin Lychnis flos-cuculi. Typical bird species of this habitat type are yellow wagtail, sedge warbler and snipe, although the former is now a scarce breeder in Greater London and the latter occurs primarily as a winter visitor. Where there are ponds and ditches within the floodplain, great crested newts and grass snakes may be present. The majority of the dragonfly species recorded in London, including the emerald damselfly and the ruddy darter, also favour this habitat. Well-vegetated ditches can also support colonies of water vole. The rather scarce Daubenton's bat has a preference for feeding over rivers and associated bankside habitat.

London's remaining Thames-side grazing marsh supports nationally scarce plants such divided sedge Carex divisa and marsh dock Rumex palustris but is mainly characterised by large expanses of grassland interspersed with ditches supporting common reed Phragmites australis and sea club-rush Scirpus maritimus. On the north bank of the Thames these ditches provide habitat for the nationally scarce emerald damselfly. Grazing marsh is particularly important for birds such as waders and wildfowl. It provides breeding habitat for a number of species such as lapwing, redshank and skylark; in winter, grazing marsh is the favoured hunting territory for short-eared owl. The remaining grazing marsh on both sides of the Thames supports large populations of water vole.

Urbanisation has accounted for the greatest loss in grazing marsh, with residential, industrial development and land-filling accounting for 68% of loss. Creation of amenity open space and conversion to arable has also resulted in loss of grazing marsh. Industrialisation and urbanisation has also led to large historical losses of flood plain grassland in Greater London.

Many rivers and streams have been culverted or canalised as flood defence measures, thus eliminating seasonal inundation of the floodplain. As a consequence, much of the former floodplain associated with London's rivers and streams has been built upon.

The remaining areas of Thames-side grazing marsh require protection and management. There are few, if any, opportunities to restore or enhance Thames-side grazing marsh in London outside of existing sites. In addition to securing long-term protection, the re-instatement of appropriate grazing and hydrological regimes would greatly enhance the existing value of this habitat. Initial management on Wennington and Aveley Marshes (that part of the Inner Thames Marshes Site of Special Scientific Interest outside the Greater London boundary) has shown that appropriate grazing and flooding can significantly enhance the habitat for breeding waders and wildfowl. Partnerships between key players could also provide new opportunities for public access and appreciation of the Thames-side grazing marshes.

There is significant potential for enhancement and restoration of floodplain grassland habitats in Greater London by modification and alteration of existing flood defences. Many flood defences alongside rivers in London were installed without consideration of the impact on biodiversity or the impact on the natural dynamics of the river. As flood defences are refurbished or replaced there are opportunities to restore the natural dynamics of the river system where this would not increase the flood-risk to private property. Indeed restoration of floodplain grassland and other riverside habitats can reduce the flood risk by slowing and reducing the level of water in the main channel. Potential sites should and are being highlighted within catchment LEAPS (Local Environment Agency Plans).