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Based upon a world-wide literature review and a database, which refers to 250 publications and documents about 8500 phytosociological releves, the following questions are discussed: What are the common ecological parameters for temporary wetlands and which environmental conditions offer a niche for dwarf ephemerals? Which taxa have evolved and speciated within ephemeral wetland habitats? How do the relations between relief features, local hydrology and climatic conditions change in different parts of the world? Which global patterns In flora and vegetation do occur?

The review is restricted to ephemeral freshwater ecosystems with the following two properties: Above-ground plant cover is seasonal, and the habitats are water-saturated or submerged only part of the year. For a better understanding of large-scale patterns, the results of studies about small-scale zonation, variability in time (phenology and year-to-year dynamics), ecophysiology and life strategies are briefly reported. Finally, conservation aspects and trends of floristic globalization are considered.

1. Seasonal pools: They occur in semi-arid and subhumid climates at both sides of the Tropic of Cancer. The catchment areas are local or the ponds are purely rainwater systems.

2. Amphibic shorelines of permanent ponds, lakes and rivers: They concentrate in perhumid extratropical temperate zones and in orotropical climates. Along allochthonous rivers with extended catchments and with seasonal flood pulse, large temporary flood-plains also occur in semi-arid regions.

3. Ephemeral flush habitats: In the perhumid tropics and in the subhumid subtropics, ephemeral wetlands are linked to runoff-habitats like the slopes of inselbergs and rock outcrops and to interflow habitats along intermittent streams.

The distribution of some keystone taxa reflects the present climatic differentiation of the globe and to some extent also historical events (palaeogeography, speciation processes). Myosurus for example is linked to extratropical regions, Lilaeopsis to the New World and the Southern Hemisphere, Limnophila and Rhamphicarpa to the Palaeotropical. region. Vicariance patterns are a common phenomenon. Examples can be seen in Isoetes, Marsilea, Ophioglossum, Juncus (sections Tenageia, Ozophyllum and Caespitosi), Limosella, Crassula (section Helophytum), Bacopa, Hydrocotyle, Eriocaulon and Xyris. These genera speciated within this environment and evolved habitat equivalent species. The reduced size and the spatial isolation of the habitat reduce gene flow and favour allopatric speciation. The variability in time and small-scale ecological gradients stimulate sympatric speciation by temporal separation of the populations. Niche-equivalent taxa replace each other in different parts of the world. The niche of dwarf ephemeroid annuals is occupied by Centrolepidaceae in the Australian region, by Restionaceae in the Capensis, by Eriocaulaceae in the Australian region and East Asia, by Juncaceae in the holarctic kingdom, by Orcuttieae in the Californian phytogeographical sector, and by Cyperaceae, Crassulaceae, Gentianaceae, Elatinaceae and Apiaceae in all floristic kingdoms. Other predominant life forms are herbaceous perennials with the isoetid syndrome, geophytic ferns (Ophioglossum, Marsilea), carnivorous plants from the families Lentibulariaceae and Droseraceae, and polkilohydric vascular plants with the xyroid syndrome. The latter occur in the Tropics, with Xyridaceae (pantropical), Velloziaceae (neotropical region), Afrotrilepis, Craterostigma, Lindernia, Chamaegigas (palaeotropical region), Trilepis (neotropical region) and Borya (Australian region).

The floristic kingdoms of the world are also reflected in the phytosociological classification of the ephemeral wetland communities. They are further differentiated according to climate zones and altitudinal belts. High ranked syntaxa in the New World are the Beckmannio syzigachne-Rumicetalia salicifolii (temperate regions of North America), Downingio bicornutae-Lasthenietea fremontii (Californian phytogeographical province), Mayacetea fluviatilis, Xyridetea savanensis and Leptocoryphio-Trachypogonetea (neotropical lowlands from Cuba via the Guiana Shield to northern Argentina), and Limoselletea australis (Andean belt in the neotropical region, lowlands in the extratropical parts of South America). The communities in the Western Palaearctis belong to the Isoeto-Nanojuncetea. Within this class, two orders can be separated according to species combination, ecology, predominant life form and phenology: Isoetetalia in the Mediterranean area and Cyperetalia fusci in the temperate zone of Eastern and Central Eurasia. Amphibic vegetation on mud soils in East Asia belongs to the Lindernion procumbentis, on nutrient poor soils to Eriocaulion atratae (Northern Japan) and Eriocaulion hondoensis (Southern Japan). Seasonal waterlogged soils in arctic and subarctic climates, often kept open by cryoturbation, are colonized by Koenigia islandica-communities. In subsaharan Africa, observations are scanty for the montane and subalpine zones of Eastern Africa (Limoselletea africanae) and preliminary for Southern Africa. In West Africa, the vegetation of ephemeral flush sites on inselbergs and of seasonal ponds in plains is better known. Communities on meso- and eutrophic sites there belong to the Rhamphicarpo fistulosae-Hygrophiletea senegalensis, on oligotrophic sites to the Drosero-Xyridetea. The latter class ranges throughout inselbergs in Central and Eastern Africa. Seasonal waterlogged soils in Central African lowlands are colonized by the Microchloetea indicae. Southwestern outposts of this class occur in Namibia. A high degree of endemism characterizes the Centrolepidi aristatae-Hydrocolyletea alatae, distributed in the mediterranean climate of SW Australia, and the Crassulo sinclairii-Hydrocotyletea hydrophilae, which occur in New Zealand. Communities with Lilaeopsis polyantha are distributed from Tasmania to SE Australia.

Vegetation zonation according to water depth, periods of inundation and time of emergence is a characteristic feature of temporary ponds and emergent shorelines. Other differentiating gradients can be soil depth and water storage capacity (on rock outcrops), duration of seepage water flow (on inselbergs), wave respectively ice scour intensity (in the shoreline habitat), frost-heaving intensity (in subarctic and oreotropical climates) and salinity (in endorheic playa lakes in arid and semi-arid climates). These environmental gradients result in repetitive patterns of contact series of plant communities (= zonation complexes). Inundation experiments and seed bank analysis show that in a given year, only part of the seeds are stimulated to germinate. At a given place different plant communities can be awakened by different flooding regimes. The seed bank makes the temporary wetland ecosystems resilient to the interannual variability of ponding.

Seasonal dynamics and year-to-year variability are prominent attributes of ephemeral wetland vegetation. A high species turnover in the growing season allows to separate ecophases respectively chronocoenoses. Plant response to drought is individualistic and depends largely on the timing of meteorological events in relation to life-stages. Such a germination and sprouting ecology can explain the year-to-year fluctuations of vegetation zones. Interannual variability is greater on shallow soils and more accentuated in the summer ecophases respectively increases from the pool centre to the margins. The habitats are not submitted to progressive succession, when they are of primary nature and when the natural dynamic of the hydrological or geomorphological process continues.

Temporary wetlands shelter extremely rare and isolated taxa. The habitats are sensitive to human impact and they are threatened in many parts of the world. A strong decline can be stated for many species in Central Europe. The main reasons are missing dynamics of the river systems, abandonment of extensive pasturing and - in consequence - rare soil disturbance. Some species shift from primary to man-made habitats such as drained fish-ponds, arable land, rice fields and irrigated turfs. First tendencies of a globalization of the ephemeral wetland flora can be observed.