Discovery of the rare liverwort Haplomitrium hookeri in Belgium: relictualism or long-distance dispersal?

Haplomitrium hookeri, a rare liverwort in Europe, is reported as new to Belgium. This discovery fills a distribution gap between northern Holland (Friesen islands) and the French Alps in the previously known suboceanic distribution of the species in Europe. The Belgian collection solely consists in a few male plants. Haplomitrium hookeri occurs within a patch of Calliergonella lindbergii with other pioneer species on the upper shoreline of a dried pond. The latter had not been dried-out since 1960, suggesting that the existence of H. hookeri is not relictual but rather results from recent long-distance dispersal. The nearest Dutch localities are distant of about 400 km. The importance of dried ponds for the conservation of rare species is emphasized and a plea for the regular drying-out of this habitat is made.