A flux-based assessment of above and below ground biomass of holm oak (Quercus ilex L.) seedlings after one season of exposure to high ozone concentrations

Young plants of Holm oak (Quercus ilex) were exposed in non-limiting water conditions to four different\r\nlevels of ozone (O3) concentrations in Open-Top Chambers during one growing season to evaluate\r\nbiomass losses on roots, stems and leaves in relation to O3 exposure (AOT40) and phytotoxical ozone\r\ndose (POD1) absorbed. The exposure-effect and dose-effect relationships for the total biomass were\r\nstatistically significant and indicated a reduction of 4% and 5.2% of the total biomass for each increase\r\nstep of 10000 ppb h of AOT40 and 10 mmol m 2 of POD1, respectively. The results indicate a critical level\r\nfor Holm oak protection of 7 mmol m 2 of POD1, which corresponds to 4% of total biomass reduction. The\r\nlinear regressions based on the POD1 were significant for roots and stem biomass losses, but not significant\r\nfor leaf biomass. The biomass loss rate at increasing POD1 was higher for roots than for stems and\r\nleaves, suggesting that stem growth under high levels of O3 is less affected than root growth. Because of\r\nthe scarcity of data from the Mediterranean area, these results can be relevant for the O3 risk assessment\r\nmodels and for the definition of new O3 critical levels for forests in Europe.