Acclimation to ozone affects host/pathogen interaction and competitiveness for nitrogen in juvenile Fagus sylvatica and Picea abies trees infected with Phytophthora citricola

In a two-year phytotron study, juvenile trees of European beech (Fagus sylvatica) and Norway spruce (Picea abies) were grown in mixture under ambient and twice ambient ozone (O3) and infected with the root pathogen Phytophthora citricola. We investigated the influence of O₃ on the trees' susceptibility to the root pathogen and assessed, through a ¹⁵N- labelling experiment, the impact of both treatments (O₃ exposure and infection) on belowground competitiveness. The hypotheses tested were that: (1) both P. citricola and O₃ reduce the belowground competitiveness (in view of N acquisition), and (2) that susceptibility to P. citricola infection is reduced through acclimation to enhanced O₃ exposure. Belowground competitiveness was quantified via cost/benefit relationships, i.e., the ratio of structural investment in roots relative to their uptake of ¹⁵N. Beech had a lower biomass acquisition and captured less ¹⁵N under enhanced O₃ and P. citricola infection alone than spruce, whereas the latter species appeared to profit from the lower resource acquisition of beech in these treatments. Nevertheless, in the combined treatment, susceptibility to P. citricola of spruce was increased, while beech growth and ¹⁵N uptake were not further reduced below the levels found under the single treatments. Potential trade-offs between stress defence, growth performance, and associated nitrogen status are discussed for trees affected through O ₃ and/or pathogen infection. With respect to growth performance, it is concluded that O₃ enhances susceptibility to the pathogen in spruce, but apparently raises the defence capacity in beech.