A review on habitats, plant traits and vegetation of ephemeral wetlands - a global perspective.
Based upon a world-wide literature review and a database, which refers to 250 publications and documents about 8500 phytosociological relevés, 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. Three habitat types can be distinguished according to relief, hydrology and climate: 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 floodplains 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 poikilohydric 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).