Size of Acer saccharum hosts does not influence growth of mature bryophyte gametophytes in Adirondack northern hardwood forests

Several epiphytic bryophyte species (Anomodon attenuatus, Anomodon rugelii, Brachythecium oxycladon, Leucodon brachypus, Neckera pennata, Porella platyphylloidea, and Rauiella scita) have diminished frequencies of occurrence and abundances in second-growth and partially-cut Adirondack northern hardwood forests relative to old-growth forests. The occurrence and abundances of these species, all of which exhibit mesophytic growth forms and two of which (A. attenuatus and A. rugelii) are known calcicoles, are positively correlated with diameters of sugar maple (Acer saccharum) host trees. The mechanisms causing their specificity for large, old sugar maples hosts are not known, but their distributions may reflect interactions of their respective physiological tolerances with changing microenvironmental conditions on sugar maple bark as the host trees age and mature. We tested this "gradient in time" hypothesis by transplanting mature gametophyte mats on sugar maple trees ranging from 20-85 cm diameter at breast height. Furthermore, we tested this hypothesis at two sites, a mesic and a xeric forest stand, in order to assess the influences of mesa-scale (stand-level) environmental factors on the micro-scale (individual tree-level) regulation of gametophyte growth. All but two of the 96 transplants remained alive after six years. Following an initial year of overall dieback, average cover increased 23% over the subsequent five year period. Tree diameter had no effect on expansion rates of any transplanted species. Only one species, Neckera pennata, exhibited significantly greater expansion at the mesic site than at the xeric site. Our data suggest that age-related changes in microenvironmental conditions of sugar maple bark do not have an important influence on the distribution of these "large-maple" epiphytes. Rather, factors such as dispersal limitations or establishment requirements of protonemata probably limit their distribution. Additional, well-replicated transplant experiments are required to determine the effects of micro- and meso-scale environmental variation on the establishment, growth, and reproduction of epiphytic bryophytes. We discuss implications of our results for restoration of epiphytic bryophyte communities in heavily disturbed forest landscapes.