THE EFFECTS OF AMBIENT AIR ON A SEMINATURAL GRASSLAND COMMUNITY

The effect of filtering ambient air pollution on biomass production and population dynamics of a semi-natural grassland in rural southern Britain was studied over two successive seasons using open-top chambers. No effects were observed in 1988 when ozone concentrations were relatively low, but in 1989 when ozone levels were higher (17% of days with max. hourly concentrations above 60 nl l-1), total above-ground biomass was increased in filtered air at the end of the season. This increase was confined to the major grass species; forb species showed a decreased biomass in filtered air. This is the opposite of predictions based on the ozone sensitivities of individual species in fumigation experiments. Point quadrat data showed that the species composition in filtered and unfiltered air began to diverge in July/August 1989, after rain followed a prolonged period of drought allowing regrowth. It is suggested that the response of the forb species is primarily due to changes in light penetration through the grass canopy rather than direct effects of pollutant gases. This study indicates that the impact of air pollution on plant communities cannot be readily predicted from laboratory ozone treatments of individual species or artificial two-species mixtures.