Along the east coast of Zealand, Denmark, several dune systems exist. They are all relatively small in area, and normally the coastal dunes are low in height [1] . They possess, however, often a valuable ecosystem dominated by lichens and bryophytes [2] [3] . Coastal ecosystems in Denmark are affected by the incresasing occurrence of the invasive Rosa rugosa Thunb. Ex Murray, an invasive shrub from East Asia to parts of Europe [4] [5] . The plant occurs mostly in coastal ecosystems, where it inflicts changes in biodiversity. However, little is known about the vascular and non-vascular plants associated to the invasive Rosa rugosa or its soil properties. Rosa rugosa may alter the diversity of the cryptogams, firstly by providing shelter during the growing season and the winter, which might mainly benefit some bryophytes, secondly by providing new substrates in the form of dead twigs for the epiphytes. Furthermore, the reduced trampling from man and animals in and close to the Rosa rugosa shrubs protect the slow growing lichen and bryophyte vegetation. Dying shrubs of Rosa rugosa thus provide improved growth conditions for epiphytic as well as epigeic cryptogams.

Rhynchostegium megapolitanum (Web. et Mohr) B.S.G. was previously observed by Hansen and Vestergaard [3] in man-made sand dunes in East Denmark. This rare bryophyte has been studied in Austria recently by Zechmeister, Moser and Milasowszky [6] , and it was hypothesized that the species is expanding, possibly due to climate change and increasing atmospheric nitrogen deposition. Only little is otherwise known of the ecological preferences of this species (see e.g. [7] ).

The aim of this paper is to contribute to the knowledge of the ecology surrounding Rosa rugosa on these dune systems; with a focus on the epiphytic cryptogam vegetation and on the epigeic vegetation below, in particular Rhynchostegium megapolitanum.

The bryophyte species at the site are mostly common and widespread, reflecting the pioneer stage of succession as well as the existence of more sheltered growth conditions, e.g. under the living Rosa rugosa shrubs. Aulacomnium androgynum (Hedw.) Schwaegr. and Oxyrrhynchium (B.S.G.) Warnst. sp. often occur under more shaded conditions, Pleurozium schreberi (Brid.) Mitt is frequent in open heath communities, Hypnum cupressiforme Hedw. has a wide range of habitats, mostly acid substrates, while the remaining species with the exception of Rhynchostegium megapolitanum, are characteristic of early succession stages in open, exposed habitats [9] . Ceratodon purpureus (Hedw.) Brid., Hypnum cupressiforme and Brachythecium rutabulum (Hedw.) B.S.G. were most common outside or at the edge of the Rosa rugosa shrubs. Ceratodon purpureus is a pioneer species typical of open habitats [9] , and may therefore have been suppressed by lack of light below R. rugosa. The common Dicranum scoparium Hedw. has a wide ecophysiological range and may form carpets on open sandy soils as well as in more shaded humus rich habitats [13] .

The finding of Rhynchostegium megapolitanum was unexpected, and the species is rare in Denmark. Dierssen [7] states, that Rhynchostegium megapolitanum may thrive well in more or less shaded habitats. The shade provided by Rosa rugosa may thus not inhibit the growth of this moss. On the contrary, the stable substrate and diminished disturbance within the open Rosa rugosa scrubs may favour the epigeic community as a whole. Even living scrubs of Rosa rugosa permit ample penetration of nearly full sunlight during fall, winter and spring, where the twigs are naked. Isermann [14] found, however, in dunes at the German North Sea coast a reduction in relative irradiance inside R. rugosa shrub to below 10% at about 45% - 50% cover of R. rugosa. The reduc- tion of irradiance inhibited growth of light-demanding species typical for dunes, which after some years were outcompeted in favour of less light demanding species.

The soil properties measured are consistent with a nutrient poor sandy soil, with some impact from the sea in terms of shell fragments and sea spray. The pH level of the sandy soil at the study site reflects this impact in particular; pH values between 5 and 7 was recorded. The low P content and organic matter indicates a substrate that only sustains a vascular vegetation with small productivity and little competitiveness. It is thus not surprising, that weakening of Rosa rugosa shrubs results in a plant community dominated by slowly growing cryptogams.

The regional value for East Zealand of atmospheric deposition of nitrogen compounds amounts to 14 kg N ha⁻¹∙yr⁻¹ [15] due to prevailing westerly winds and proximity of highways and industry. This value is close to the critical load for this coastal dry grassland system.

The epigeic lichens found were exclusively Cladonia spp. Most species were found outside or in the periphery of the rose scrubs, presumably due to the suboptimal light conditions at the soil surface below the roses. So, most species recorded are typically found in open habitats like dunes and heathlands. The species least influenced by R. rugosa, Cladonia macilenta Hoffm. and Cl. merochlorophaea Asahina, are the most organophilic species observed [16] .

The epiphytes, nearly exclusively Hypogymnia physodes, which is characteristic for acid bark and sandy substrates in dunes, reveal the nutrient poor conditions of the substrate and indeed the limited impact of atmospheric nutrients. The reason for the total dominance of Hypogymnia physodes on dead or dying twigs of Rosa rugosa is most probably, that Hypogymnia physodes occurred as a part of the epigeic cryptogam community before the invasion of Rosa rugosa.

This study by no means closes the chapter on the ecology of Rhynchostegium megapolitanum. Is the species always confined to early succession stages, or is it becoming an integral element in climax systems? [17] Due to the lack of Danish studies of the species, the national distribution is not well known, and the question of increasing distribution of the species, as suggested by Zechmeister, Moser and Milasowszky [6] , can not yet be verified. This study seems to indicate that the ecophysiological range of the species may be broader than previously envisaged. The species is often believed to be confined to relatively nutrient rich substrates with high pH, which clearly is not the case here. Further studies of distribution and growth conditions are needed, before the ecology of this rare bryophyte is understood.

I wish to thank Peter Vestergaard for inspiring collaboration during the field work, data treatment and synthesis. Also thanks to Karna Heinsen and Karin Larsen for assistance in the field with soil sampling, and for soil analyses.