Over de vegetatie met Elatine hexandra (Lapierre) DC. in de kleine plas van het Lonnekermeer

In this paper a description is given of the vegetation in a pool near Hengelo (prov. of Overijssel). In the dry summers of 1975 and 1976, large parts of the shores and shallows of this pool ran dry. On these places a vegetation developed consisting mainly of Eleocharis acicularis, Elatine hexandra and to a lesser extent Juncus bulbosus. Presumably the first species is present permanently; it only flowers if its habitat runs dry. The annual Elatine hexandra was seen from July to as late as January. While in the course of summer and autumn the water level was sinking, Elatine died off in the upper zones and appeared in zones recently run dry. Somewhat later than the former two species, Juncus bulbosus developed. Whilst J. bulbosus is usually a perennial, in this pool it appeared as an annual. Locally Littorella uniflora was seen, always in open vegetation. In 1975 Eleocharis acicularis and Elatine hexandra were also seen locally in shallow water; the latter species flowered and fruited under these conditions. The four species mentioned above are characteristic for amphiphytic communities belonging to the alliance Littorellion. The ecology of these communities has been studied extensively in the Netherlands for some 40 years (SCHOOF-VAN PELT, 1973). Because of the rare occurrence of Elatine hexandra, this species’ ecology is relatively little known. Formerly, it was recorded chiefly in the Isoëto-Lobelietum, although it occurred most often in pools with slightly ‘enriched’ water, where this association which is very vulnerable to eutrophication lost ground. In recent years E. hexandra has been found several times in the Eleocharitetum acicularis. The Eleocharis – Elatine community in our pool (table 1) can be reckoned to the last mentioned association. Whilst in the Netherlands E. hexandra is looked upon as a characteristic taxon of the alliance Littorellion, in southern Germany it occurs in the Eleocharitetum soloniensis (all. Nanocyperion) together with Eleocharis soloniensis, Limosella aquatica and other annuals. In fact, Elatine hexandra and other Elatine species are more or less intermediate between Littorellion and Nanocyperion. In sites periodically running dry communities of both alliances can form a mosaic pattern, the Littorellion association (e.g., Eleocharitetum acicularis) being present permanently, the Nanocyperion association (e.g., Eleocharitetum soloniensis) developing only on spots run dry. Elatine spp. constitute a transition group, occurring only in summer and autumn, but being able to germinate when inundated.