Contrasts between bryophyte and vascular plant synecological responses in an SO2-stressed white spruce association in central Alberta.

Canopy coverage analysis was used to examine the synecological changes exhibited by vascular plants and terrestrial mosses in a white spruce association exposed to SO2 fumigation. Both these understory components were found to decline in coverage as SO2 stress increased, but mosses were more sensitive to SO2 in the more heavily stressed areas. This was observed along both an angle-dependent and a distance-dependent gradient of pollution stress. Diversity steadily declined with increasing SO2 stress along the angle-dependent gradient but some localized increases in diversity occurred with increasing stress along the distance-dependent gradient. This was due to invasion of openings resulting from attrition of SO2-sensitive species by weedy angiosperms and by vegetative growth of moss species more tolerant of pollution stress. Conclusions have been drawn about the reproductive strategy of vascular plants and mosses subjected to increasing concentrations of SO2. We have elucidated the ecological consequences for community structure of the systematic removal of pollution-sensitive understory species from an otherwise stable vegetation unit.