A Leafy Liverwort Hydrosere on Yakobi Island, Alaska

The liverworts Nardia compressa (Hook.) Gray and Scapania paludosa K.M. are able, because of their aquatic nature and rank growth of intertwined stems, to dam a swift mountain rivulet, thus causing a series of terraced pools to form. Growth of the liverworts closes the pool surface, and vascular plants invade the liverwort mat, leading to the development of a Cladothamnus pyroliflorus Bong. physiographic climax community in an area occupied by late—melting snowbeds. Other hydrophytic bryophytes of the area form mats of insufficient strength to impound streams on steep slopes. The typical snowbed climax communities of this region are not, however, the result of liverworth hydrarch succession.