Eleocharis ovata and its alien allies in the Netherlands

A revision of Dutch herbarium specimens of Eleocharis series Ovatae shows that two exotic species from North America have also been recorded in the Netherlands since 2011– 2012: Eleocharis obtusa and E. engelmannii. The number of localities of these species is steadily increasing. The native species, E. ovata, nowadays only occurs on the Ewijkse Plaat along the River Waal, west of Nijmegen. This population appears to be viable, but it is in a highly dynamic environment close to the river, and might disappear again. The current Red list status ‘Sensitive’ with trend ‘unchanged or increased’ is due to confusion with these two exotic species, and may therefore be far too optimistic for this species. Although reported as absent from the Iberian Peninsula, the occurrence of E. ovata in Spain is reinstated. All three species are quite similar in terms of morphology and ecology, but can be distinguished from each other on the basis of the fruits (the width of the persistent stylopodium in relation to the dimensions of the achene), the length of the perianth bristles, the number of stamens and the number of stigmas on the style (2 or 3). Eleocharis ovata seems to be bound to large rivers and occurs in vegetations belonging to the Bidention tripartitae, but is also found on loamy to sandy and rather oligotrophic shores of shallow ponds. The two alien species are found in Bidention tripartitae, Nanocyperion flavescentis or Littorelletea uniflorae vegetations, or transitions between them, but the information on habitats for these two species is limited. For the time being they seem to be bound to slightly poorer soils than E. ovata.