Effects of sulphur dioxide pollution on a Californian coastal sage scrub community

Phytotoxic effects of sulphur dioxide emissions from an oil refinery (annual average of 0.09-0.17 ppm for a 25 year period) were studied near Santa Maria, on the rural central coast of California. Stands of coastal sage scrub along an SO(2) gradient downwind of the refinery were compared with stands in relatively pollution-free upwind sites. Physiological and growth changes in the dominant shrub, Salvia mellifera (black sage), were emphasised. Stomatal resistance of Salvia was found to be significantly lower on the polluted sites than on the pollution-free sites. Internodal stem length and heights of Salvia shrubs were also reduced. Significant reductions in photosynthetically active tissue of Salvia resulted from increased defoliation and decreased leaf size associated with SO(2) stress. Of particular ecological interest was the relative importance of species at different sites; abundance of perennial shrub species was significantly lower and abundance of annuals greater on the most polluted sites relative to the sites farthest from the SO(2) source. The influx of annuals and decrease in perennial cover resulted in greater species richness and reduced concentration of dominance similar to that observed in early post-fire sage scrub stands.