- Chalky soil areas
- Acid soil areas
- Ancient Woodland
- Rivers, Canals and Wetlands
- Local Nature reserves and open spaces
- Ruderal habitats
These contain the highest percentage of native plants, but introductions and non-natives occur as well, especially near gardens, car parks and other disturbed places. The flora is grouped in communities reflecting the type of soil, degree of shading and hydrological characteristics.
Calcareous soils are present as a band overlying chalk across the southern edge of the circle and still contain good botanical diversity. The reserves owned by the Corporation of the City of London at Farthing Downs, Kenley Common and Riddlesdown (near Croydon) are among the best examples and include an interesting example of grassland restoration from former secondary woodland at Farthing Downs. Other smaller patches of calcareous sould are present near Harefield in the west (e.g. Harefield Pit), and near Hertford in the north. They include open downland, woodland and arable land, each with its typical species, both common and uncommon.
Acid soils occur over materials such as the Bagshot and Thanet sands, the pebbly Blackheath beds (e.g. Gilbert's Pit), and Claygate beds (e.g. Oakhill Pit). They occur widely across the circle. Relatively ‘natural’ tracts of this type are scattered across the circle in small patches, as in the north of Hampstead Heath (e.g. Sandy Heath), on Barnes and Wimbledon Commons, at Keston in Bromley, the Surrey Commons, and in Epping Forest. Lowland Heaths such as these are an internationally important habitat type, and the best-preserved examples in our study area are those in Surrey. See also the Royal Parks booklet "Acid Grassland".
Ancient WoodlandA few patches of ancient woodland, (defined as areas that have had tree cover since before 1600), still exist in otherwise urban areas, such as Oxleas Wood in the south-east, and Queen’s and Highgate Woods in the north. These are all on acid soils and contain indicator species such as Wild Service Tree and Great Woodrush as well as a good seasoning of introduced plants. Further out, on the western boundary of Middlesex around Ruislip, lies London’s largest tract of ancient oak/hornbeam woodland. Towards the southern edge of our circle are the woods of the steep chalky slopes of the North Downs dominated by Beech, Pedunculate Oak and Yew. Dormice can be found in some of these woods.
Rivers, Canals and other Wetland Habitats
The tidal Thames contributes a strip of saline habitats, from the muddy edge of the river itself to grazing marshes and their associated ditches on either side of the lower reaches of the river, although like most others they are being eroded by other interests. The most extensive remaining of these within the circle is Rainham Marsh. Most of the edge of the tidal river itself has been embanked, with only a few relict patches of the original margin at Dukes Hollow, in the grounds of Syon House, and in the Thames estuary near Swanscombe. The latter is sadly under threat of development.
Tributaries of the Thames such as the Rivers Fleet and Westbourne, nearer the centre of the city, have been culverted and built over, but the Colne and the Wandle in West London and the Lee in the East, with their associated water meadows still possess elements of the ancient flora. Naturally occurring waterways are supplemented with canals such as the Grand Union, Regents, Lee Navigation and Limehouse Cut, There are still many enclosed water bodies scattered across the circle, varying in size from small ponds everywhere to the much larger reservoirs of the Lee Valley and West London, which have much ornithological interest.
London is an international centre for waterfowl and many water birds take advantage of these water bodies. Some Duck species, such as Gadwall and Shoveler, are present in internationally important numbers (see this pdf). Other water birds such as Tufted Duck, Pochard, Goosander, Smew, Bittern, Great Crested Grebe and Cormorant are here in nationally important numbers.