This is the longest continuous experiment where ethylenediurea (EDU) was used to protect plants from
ozone (O3). Effects of long-term ambient O3 exposure (23 ppm h AOT40) on biomass of an O3 sensitive
poplar clone (Oxford) were examined after six years from in-ground planting. Trees were irrigated with
either water or 450 ppm EDU. Above (51%) and below-ground biomass (47%) was reduced by O3
although the effect was significant only for stem and coarse roots. Ambient O3 decreased diameter of the
lower stem, and increased moisture content along the stem of not-protected plants (þ16%). No other
change in the physical wood structure was observed. A comparison with a previous assessment in the
same experiment suggested that O3 effects on biomass partitioning to above-ground organs depend on
the tree ontogenetic stage. The root/shoot ratios did not change, suggesting that previous short-term
observations of reduced allocation to tree roots may be overestimated.