Plant responses to long term elevated ozone in a British upland semi-natural grassland

A novel free-air gas concentration enrichment (FACE) experiment exposed a species rich semi-natural upland grassland to ozone at average levels of ambient + 75 ppb, + 25 ppb and + 15 ppb over a 6 year period in the North Pennines. In addition to this, high exposures lasting 4 days of up to 309 ppb occurred in June 2012. Three main factors were investigated over the growing season of 2012 in relation to exposure to elevated ozone: the flowering behaviour of 11 species; the nectar volume and composition of the hemi-parasite Rhinanthus minor; and the CO2 exchange of the ecosystem.
Dactylis glomerata flower abundance significantly increased over ambient levels with exposure to + 15 ppb ozone, but not ambient + 25 ppb. Conopodium majus also exhibited significant increases in flower abundance, though the effect was seen in ambient + 25 ppb when compared to ambient air, but not ambient + 15 ppb. However, Ranunculus acris flower abundance reduced sharply in both ozone elevations. Peak flowering dates were shown to be two weeks later in two grass species, Festuca pratensis and Trisetum flavescens, when exposed to a long term background of ambient + 25 ppb, and short term exposure to a mean 140 ppb. Short term exposure to 77 ppb and 52 ppb ozone did not significantly affect nectar volume or composition in R. minor flowers; however there may have been an effect on sugar composition at 77 ppb exposure. There was no significant effect of long term ozone exposure at ambient + 75 ppb, ambient + 25 ppb and ambient + 15 ppb on ecosystem respiration, net ecosystem exchange (NEE), or gross primary production (GPP) relating to CO2; however data suggest NEE and GPP may have been reduced through foliar injury relating to acute ozone exposure to an average 309 ppb. The findings have wider implications for conservation of upland grassland species diversity, and also indicate the potential for implications for the wider grassland carbon sink during peak ozone episodes.