Swamps are areas of mineral-based soil, normally flooded in the growing season and in this country usually dominated by tall emergent plants such as common or Norfolk reed (Phragmites communis). The few exceptions to reedswamp communities tend to grow in rather specialised circumstances or as a reed-marsh transition in areas of the north and west where the cool, rainy climate favours the buildup of peat and the development of acid bogs. Most swamps occur at the edges of open water in protected shallows where the current is slight. They also fill shallow, sluggish ditches and cover wet areas that were until recently open water or seasonal pools.
Most of Britain’s larger swamps have been drained, but a few still occur along the south and east coasts, for example at Arne in Dorset and Minsmere in Suffolk. The swamps and fens of the Kennet Valley are noteworthy because they are thought to have developed when beavers dammed the streams.
Reedswamp communities are typically ‘pure’, that is made up of single dominant or two co-dominant species with few if any associates. Reedswamp species usually have aerial shoots which, under favourable conditions, grow so tightly packed that competitors have little room. These species spread by sending out side-shoots from their root-balls or ‘rhizomes’. A single plant, arriving perhaps as a bit of broken root which sets, may spread to form a large colony. Rhizomes and roots of separate plants may interlock to create, for practical purposes, a single unit.
At its fringe, in deeper water where it is just becoming established or on drier ground where it is starting to lose its vigour, the stand becomes more open. Other species grow as associates or as plants of the ‘ecotone’ between the reedswamp zone and aquatic and terrestrial zones. Reedswamp often shows some internal floristic zonation as well, despite its uniform appearance, due to the varying preferences of species to such factors as depth of water and exposure to wave action.
Important reedswamp plants include, in addition to common reed, bulrush (Schoenoplectus lacustris), great reedmace (Typha latifolia) and branched bur-reed (Sparganium erectum). Reed sweetgrass (Glyceria maxima) commonly dominates swamps on the upper Thames and many of the East Anglian rivers. Lower plants, which do not compete very well with tall reedswamp species but which often fringe shallow ponds and ditches in lowland England, include the water plantains and their relatives such as arrowhead (Sagittaria sagittifolia).
Marshes
Marshes occur where the soil is waterlogged but where the summer level, while close to the surface, is seldom much above it. As with swamps, the soil has a mineral rather than peat base. Although Britain’s large marshes have been destroyed, in a natural state they covered vast areas of boulder clay around lowland glacial lakes and river valleys on alluvial gravels, sands and silts. Small marshes occur in drier surroundings below springs, dams or seepage lines where the soil is waterlogged. These may be very interesting, especially in highland regions where the ‘damp flush’ of water-borne nutrients allows a wide variety of plants to grow, in contrast to the surrounding leached-soil or bog vegetation.
The vegetation of both lowland and upland marshes is extremely varied. Often no dominant species can be singled out, in marked contrast to reedswamp communities.
Some marshes, like reedswamp, are dominated by almost pure local stands of rushes (Juncus spp), grasses such as branched bur-reed and dicotyledonous herbs such as great hairy willowherb (Epilobium hirsutum). Where the water is less than 150mm (6”) deep, lower-growing species such as horsetail (Equisetum ) and water dropwort (Oenanthe spp) may also form dense stands. Where the substratum is rocky or erosion prevents the growth of reedswamp- like communities, plants whose tough rhizomes can penetrate the hard surface may share dominance. These include common spike-rush (Eleocharis palustris) and reed Canary-grass (Phalaris arundinacea), accompanied by purple loosestrife (Lythrum salicaria) in the zone extending below summer water level and 75-150mm (3-6”) above it. The sedges (Carex spp) are important where the soil has a very high organic content. Where marshes are grazed or otherwise disturbed, a mixed community of sedges and rushes, none dominant, may grow along with the coarse, tussocky hairgrass (Deschampsia) and a number of short or creeping herbs.
Lowland marshes are usually invaded by such trees and shrubs as common alder (Alnus glutinosa), ash (Fraxinus excelsior), grey willow (Salix atrocinerea), alder buckthorn (Frangula alnus) and hairy birch (Betula pubescens). In slightly more acid and peaty conditions, sweet gale or bog myrtle (Myrica gale) may be the first invasive shrub species.
Upland marshes may have quite a different flora, although rushes and sedges continue to be important. There is often great variety, especially in rather open limestone areas, with many beautiful species such as globe flower (Trollius europaeus). In moderately acid soils, plants such as the butterworts (Pinguicula spp) indicate a transition towards mire conditions.
