Voorkomen en standplaats van het kalkmos Entodon concinnus (De Not.) Par. langs de grote rivieren

While Entodon concinnus was considered almost extinct in the Netherlands, no less than six ‘new’ localities were discovered in 1990/91, all of them in elevated parts of outer-meadows along branches of the Rhine system (fig. 2). This moss appeared to thrive on somewhat open spots in the turf of dry pastures on lime-rich sand containing traces of clay, especially on mole-hills and on south-exposed slopes. All sites are grazed part of the year and most of them are situated in nature reserves. The vegetation in which Entodon occurs, belongs to the association Medicagini-Avenetum pubescentis (table 1). The 10 relevés show a striking mutual similarity: constant species (present in 7 or more relevés; Entodon itself being left out of consideration) have a presence share of 49%. The relations between the Medicagini-Avenetum and chalk grasslands (alliance Mesobromion, class Festuco-Brometea) are discussed. It is stated that while vascular plants characteristic of the Festuco-Brometea do not play a prominent role in the Medicagini-Avenetum, Entodon concinnus constitutes a link between both unities. This is especially true as it is accompanied in some of its stations by Homalothecium lutescens and/or Thuidium abietinum, which occur together with Entodon in chalk grasslands in several parts of Europe. As to the recent discovery of E. concinnus in the Dutch riverine area (apart from one 19th-century record), the question is asked whether it should be considered a recent arrival. This supposition is denied, because 1) the moss never shows fructification in this part of its area, nor does it produce gemmae, so that anemochorous long-distance dispersal is virtually to be excluded; 2) its stations are hardly ever reached by river water, making dispersal of entire plants unlikely; 3) the Medicagini-Avenetum in which it occurs, has more or less a relic distribution. It is concluded that bryologists did not search in the right habitat and that vegetation investigators (probably overwhelmed by the floweriness of these pastures) have overlooked the moss hitherto.