Riparian bryophyte communities on Madeira: patterns and determinants of species richness and composition

Abstract

We studied 16 streams evenly distributed over the northern and southern slopes of Madeira in order to investigate the riparian bryoflora. Within each stream, three positions (upper, middle and lower reaches) were delimited and within each position two areas were selected. Within each area two plots (each composed of six microplots of 0.2 m²) were sampled, one in the within-stream habitat (submerged all year round), and the other in the stream-border habitat (submerged only in winter). We found that species composition of the riparian bryophytes is affected by the habitat and position in the stream, but not by the main aspect (northern versus southern slope). Concerning species richness, we found that the stream-border community was clearly richer than the within-stream community, upstream plots were richer than plots downstream, and plots on the northern slope of the island were richer than plots on the southern slope. Habitat type was the most significant factor in determining the richness of the threatened species with more species present in the stream-border habitat. Additive partitioning showed that the between-stream component contributed most to total species richness, especially to the richness of the infrequent and threatened species. However, for the common species, the lowest level, i.e. the within-area component, was the most important. Although northern upstream areas are climatically favourable for many bryophyte species due to their higher humidity, the clear effects found may not only be climate-induced, as these areas are also less disturbed and mostly covered by the natural laurel forest. In the southern, downstream parts only a few species were present. Human impacts are largest in the latter situations and probably contributed to the low species richness. As the streams differed considerably in terms of their bryophyte flora, and most of the species were rare, changes in the riparian areas can greatly affect the bryoflora. Therefore, in order to protect the riparian bryophytes as comprehensively as possible, we emphasize the need for careful monitoring of any changes.