London scoping - ukcip

Final Report
80
The most important hydrological controls on wetland plant communities appear to be the mean,
highest and lowest groundwater levels, together with inundation during the growing season
(Wheeler, 1999). Moisture availability is critical to other habitats too. For example, by using
the SPECIES model and simple water balance estimates for the future, the MONARCH study
showed a drying of heathland and progressive loss of climate range for the shallow-rooted
beech in southeast England under the UKCIP98 High scenario by the 2050s (Dawson et al.,
2001). This is entirely consistent with observed high percentages of poorly foliated beech trees
in years following the dry summers of 1987, 1989-1992, 1995 and 1997 (Cannell and Sparks,
1999). The 1995 drought stress has also been linked to an increase of deeper-rooted plants on
grasslands (Buckland, 1997). This highlights a danger of extrapolating regional modelling to
local impacts: the site-specific response of London’s 273 ha of fragmented wetland is ultimately
governed by the water level requirements of individual species as well as local water balance
changes.
5.6.3 Intertidal Habitats
Situated at the highly dynamic interface between land and sea, intertidal zones are some of the
world’s most diverse and productive environments. The lower Tidal Thames is no exception,
supporting as it does about 120 species of fish, 350 freshwater, estuarine and marine macroinvertebrate
species, and nearly 300,000 over-wintering water birds (EA, 2001b). Areas of
intertidal habitat are present along the entire length of the Tidal Thames, but the most extensive
reaches are below Tower Bridge where the flood defences are set further back from the main
channel (London Biodiversity Partnership, 2002). Other habitats, such as the reedbeds at
Barking Creek have become re-established following the curtailment of dredging. The Thames
estuary also provides a wide range of habitats such as shingle and mudflats, and salt marsh.
The biodiversity potential of the intertidal habitat largely depends on the local design, building
materials and positioning of flood defences. Where the defences comprise sloping revetments
there are opportunities for the establishment of saltmarsh (e.g., downstream of Tower Bridge);
where the river is constrained by vertical concrete and metal piled walls, only a narrow fringe of
foreshore is exposed at low tide (e.g., between Wandsworth Bridge and the Greenwich
Peninsula). Under the latter circumstances, there are limited opportunities for vertical
succession in the relative absence of ‘natural’ river banks. The Environment Agency is
particularly concerned about further encroachment of riverside development on the Thames
foreshore, and has suggested that unitary development plans for the London Boroughs adjoining
the River Thames should include policies prohibiting development on the foreshore (EA,
2001b). The Agency is also seeking to protect river corridors and to enhance their ecological
value through the planning process and best practice riverbank schemes. For example, at the
Millennium site, Greenwich, the tidal defences were installed 130 m inland to create an
additional 10 m of intertidal habitat, as well as an area of salt marsh with a series of terraces
between the site and existing flood wall (EA, 2001b). Similarly, the recently announced
reprieve from development for the western edge of the Rainham Marsh SSSI (FoE, 2002) is
consistent with the wider objective of regeneration for the Thames Gateway zone, including
riverside habitats.
The anticipated impacts of climate change and sea-level rise for London’s intertidal habitat
include increased levels of inundation and storm flooding; accelerated coastal erosion; sea water
intrusion into freshwater tributaries; changes to the tidal prism, tidal range, sediment supply and
rates of accretion; changes in air temperature and rainfall affecting growth of salt marsh plants
with secondary effects on sedimentation (Adam, 2002; Kennish, 2002; Moore, 1999; Nicholls et