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

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.