Oxygen and carbon isotopic signatures reveal a long-term effect of free-air ozone enrichment on leaf conductance in semi-natural grassland

The effect of ozone on leaf gas diffusion was investigated by analyzing the stable oxygen isotopic signatures (δ18O) in leaves of Holcus lanatus L., Plantago lanceolata L., Ranunculus friesianus (Jord.), and Trifolium pratense L. grown in temperate, semi-natural grassland. Dried material from plants exposed to ambient or elevated ozone levels in a long-term free-air experiment was sampled in 2002 and 2003. A general increase in δ18O in elevated ozone indicated increased limitation to gas diffusion, which was strongest during the driest and warmest period in 2003. In three out of four species, the increase in δ18O paralleled an increase in δ13C measured earlier in the same samples, meaning that the dominant effect of ozone was on gas diffusion and not on CO2 fixation. Only in R. friesianus, ozone affected both processes simultaneously. It is concluded that elevated ozone not only affects productivity, but also the water status of important component species of grassland communities.