Terrestrial mosses as bioindicators of SO pollution stress : Synecological analysis and the index of atmospheric purity.

The turf structure of terrestrial mosses was analysed in the understory of white spruce associations along two SO stress gradients associated with a single SO source. Changes in coverage, turf depth, biomass and percent living moss were followed. SO stress initially caused changes in turf structure in the vertical plane (green turf depth, percentage and biomass) whereas changes in the horizontal plane (coverage) were most noticeable in areas of severe stress. The Index of Atmospheric Purity (IAP) reflected SO stress only in these latter areas. Intrusion of weeds and increased prominence of subordinate species occurred in lightly stressed areas, but these perturbations were not reflected by the IAP. The effect of Q , frequency, and coverage factors in the IAP are discussed and the criteria of the Index of Community Vigor are proposed.