Biomass and physiological responses of Quercus robur (L.) young trees during 2 years of treatments with different levels of ozone and nitrogen wet deposition

Key message: The root biomass of oak young trees significantly decreased after 2 years of exposure to high levels of ozone, but increased nitrogen wet deposition tended to partly contrast this effect. Abstract: A 2-year Open-Top Chamber (OTC) experiment with young Quercus robur trees that were exposed to different levels of ozone (O3) and nitrogen deposition was performed in Curno (Northern Italy) for the FP7 Project ÉCLAIRE. The plants were exposed to four levels of ozone (−40 % of ambient ozone in charcoal-filtered OTCs, −5 % in non-filtered OTCs, and +30 and +75 % in O3-enriched OTCs) and two levels of nitrogen wet deposition (tap water and tap water +70 kg N ha−1 year−1). The stomatal conductance and A/Ci response curves were measured during the two experimental seasons, and in October, the plant dry biomass partition between the roots and stem was assessed. Oak plants were moderately sensitive to O3. After the second year of treatments, the dose–response relationships based on the O3 stomatal flux indicated a 4.6 % of root biomass loss and a 12.1 % of reduction of the number of leaves per 10 mmol O3 m−2 absorbed by plants grown with no nitrogen addition. Ozone also decreased both the stomatal conductance and the maximum carboxylation rate allowed by Rubisco (Vcmax) during the first year of treatments. However, the effect on Vcmax was lost during the second year, and the plants showed an uncoupling between leaf-level physiological responses and plant-level biomass responses. Increased nitrogen deposition enhanced the growth of plants and partially mitigated the O3 impact on biomass and physiology, but no significant effect of the interaction between the two factors was found. The data that were collected could contribute to the definition of the O3 dose–response relationships based on biomass losses for deciduous trees in Southern Europe climatic conditions and could improve the O3 risk assessment models by providing new information about the effect of increased nitrogen deposition on the ozone impact.