Over de standplaats van Appelmos (Bartramia pomiformis Hedw.) en het voorkomen van enkele ‘bosmossen’ in het open duin

The moss Bartramia pomiformis, which has become very rare in Holland (see Fig. 1), was found in 12 localities in the sand dunes on the Isle of Texel. All localities are situated on steep, north-facing slopes that clearly showed terracing, which is caused by the activities of rabbits and grazing sheep. Bartramia grows on the steep banks of these terraces, in a vegetation that is dominated by bryophytes and lichens. Four releves of vegetations with Bartramia are presented in Table 1. It is pointed out that Bartramia requires a habitat that on the one hand offers sufficient protection against the desiccating influence of direct sunshine and wind, but which on the other hand still receives enough light to enable good growth of the moss. This kind of habitat is known as the ‘open shade’, which has been called ‘Blauschatten’ (‘blue shade’) by German workers, because of the fact that only diffuse, blue light, coming from the northern half of the sky, can reach such places. Since Bartramia typically occurs at such places as north facing slopes, wooded banks etc., and does not enter the deeper shade of the true forest, it can be regarded as a very good example of a ‘Blauschattenpflanze’. A remarkable feature of the vegetation of north facing slopes in the open dune landscape of the Wadden Sea Islands is the terrestrial occurrence of a number of bryophytes that are otherwise mainly restricted to woodlands in the Netherlands. Some of these also grow on the ground in woodlands, e.g. Hylocomium splendens and Rhytidiadelphus triquetrus, while others usually occur as epiphytes, e.g. Frullania tamarisci, Isothecium myosuroides, Dicranoweisia cirrata and Metzgeria furcata. The presence of such 'woodland bryophytes' as terrestrials in open dunes has in the past been regarded as the result of a presumed greater 'oceanicity' of the Islands, as compared with the interior. This supposition is probably wrong, since the climate on the islands is not significantly more oceanic than in the rest of Holland. Therefore the occurrence of these bryophytes must largely be attributed to the suitable microclimate that is offered on dune slopes, rather than to greater oceanicity. Finally some attention is paid to the disappearance of Bartramia pomiformis from many of its former localities. The decline of this moss in large parts of the Netherlands is probably connected to the effects of acid rain and the destruction of hedgebanks. The maintenance or reintroduction of sheep grazing as a management tool is recommended for the conservation of its localities, along with those of other interesting bryophytes, in dry sand dunes.