Age Structure and Mortality Within a Group of Closely Related Mosses

The age distribution and mortality curves of six species of moss in the family Polytrichaceae were determined for each species in pure stand and in mixture at nine locations along an altitudinal gradient on Mount Washington, New Hampshire, USA. The shapes of the age distributions of adult shoots were found to be remarkably uniform both for species within site and for single species at different sites. There were no consistent differences in patterns of survivorship or mortality between species when found in pure stand and in mixture, suggesting that the presence of closely related species does not predictably affect the form of the age distribution. Each species does however exhibit an individualized pattern of response to interactions with closely related species. Polytrichum commune and Poolytrichastrum formosum exhibited decreased mortality in mixed populations while Polytrichastrum pallidisetum exhibited increased mortality. No differences were found between pure stands and mixtures in Polystichum juniperinum, Polytrichm piliferum or Polytrichastrum pallidisetum. Mortality increased throughout the adult life of these species approaching an asymptote late in life. Density influenced the form and properties of the age distribution, but it was impossible to clearly separate real effects from artifacts introduced by large differences in sampling size. Mosses appear to attain greater ages in high—density populations.