Patterns of bryophyte diversity and rarity at a regional scale

Information on bryophyte diversity and rarity were combined with information on soil conditions and land use for Walloon Brabant (central Belgium, 1091 km(2)) in order to investigate which landscape features sustain the most rare and diverse species assemblages. Presence-absence of 325 bryophyte species was recorded in 87 grid-squares of 4 X 4 km. Species diversity was significantly correlated with forest cover (r = 0.71, P < 0.001), sandy soils (r = 0.61, P < 0.001), loamy soils, (r = -0.68, P < 0.001), and agricultural fields (r = 0.49, P < 0.001). The most diverse grid- squares possessed up to 182 species and were characterized by at least 10% forest cover and the presence of unique micro-habitats. Grid-squares with forest cover reaching at least 10% but lacking unique micro-habitats contained between 90 and 130 species. Below 10% forest cover, diversity ranged between 55 and 110 species per grid-square. However, even the least diverse cultivated areas included a significant amount of the regionally rare species. A number of the latter are characteristic in other areas for specific primary habitats lacking in Walloon Brabant but display an unexpected ability to disperse throughout hostile areas and colonize secondary habitats. The tendency of such species to occur in man-made habitats decreased our ability to predict species richness and rarity from landscape features and soil conditions.