Research Paper Abstract: In a two-year phytotron study, juvenile trees of Euro- pean beech (Fagus sylvatica) and Norway spruce (Picea abies) were grown in mixture under ambient and twice ambient ozone (O 3 ) and infected with the root pathogen Phytophthora citricola. We investigated the influence of O 3 on the trees’ susceptibility to the root pathogen and assessed, through a 15 N-labelling ex- periment, the impact of both treatments (O 3 exposure and in- fection) on belowground competitiveness. The hypotheses test- ed were that: (1) both P. citricola and O 3 reduce the below- ground competitiveness (in view of N acquisition), and (2) that susceptibility to P. citricola infection is reduced through acclima- tion to enhanced O 3 exposure. Belowground competitiveness was quantified via cost/benefit relationships, i.e., the ratio of structural investment in roots relative to their uptake of 15 N. Beech had a lower biomass acquisition and captured less 15 N un- der enhanced O 3 and P. citricola infection alone than spruce, whereas the latter species appeared to profit from the lower re- source acquisition of beech in these treatments. Nevertheless, in the combined treatment, susceptibility to P. citricola of spruce was increased, while beech growth and 15 N uptake were not fur- ther reduced below the levels found under the single treat- ments. Potential trade-offs between stress defence, growth per- formance, 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 O 3 enhances suscepti- bility to the pathogen in spruce, but apparently raises the de- fence capacity in beech. Key words: Ozone (O 3 ), European beech (Fagus sylvatica), Nor- way spruce (Picea abies), competition, pathogen resistance, N-15 labelling. Introduction Already, in 1973, Heagle stressed the importance of unravel- ling the interactions between plant responses to air pollutants and pathogen defence. Nevertheless, even after three decades of continued research (Manning and von Tiedemann, 1995; von Tiedemann and Firsching, 1998; Karnosky et al., 2002), such interactions have not been clarified to an extent that would allow understanding of ecophysiological principles re- garding effects, in particular, of ozone (O 3 ) and Phytophthora species on forest trees (Matyssek and Sandermann, 2003; Lau- rence and Andersen, 2003). O 3 is considered to be the potentially most phytotoxic air pol- lutant (e.g., Lefohn, 1992; Skärby et al., 1998). During the last decade, surface O 3 concentrations have increased on average by 1 –2% each year (Stockwell et al., 1997), and O 3 levels are expected to stay high during the forthcoming decades (Fowler et al., 1999; Fabian, 2002; Ashmore, 2005). Adverse O 3 effects on trees include foliar injuries, premature leaf loss, reduced growth (Matyssek and Sandermann, 2003; Matyssek and In- nes,1999), and limited belowground carbon allocation (Ander- sen, 2003; Samuelson and Kelly, 2001). At the same time, O 3 is known to elicit plant responses typically associated with pathogen defence, including biosynthesis of lignin, increased phenylalanine ammonia-lyase (PAL) activity, as well as accu- mulation of phenolic compounds and pathogen-related pro- teins (Heagle, 1973; Heller et al., 1990; Sandermann et al., 1998; Matyssek and Sandermann, 2003). Cahill and McComb (1992) compared defence responses of two Eucalyptus species of contrasting susceptibility to P. cinnamomi and concluded that inhibition of the pathogen in the resistant species was due to a distinct induction of PAL which, as a consequence, ini- tiated lignin synthesis and accumulation of phenolic com- pounds in the infected tissue. P. citricola is an oomycete known to cause root rot across a broad range of plant hosts (Erwin and Ribeiro, 1996), including European beech (Fagus sylvatica; Fleischmann et al., 2002, 2004; Werres, 1995). Although affecting beech on a regional scale (Jung and Blaschke, 1996), P. citricola has not yet been found to infect Norway spruce (Picea abies) in the field (Nech- watal and Oßwald, 2001). Nevertheless, under laboratory con- ditions, spruce seedlings infected with P. citricola die or sustain severe root injury (Nechwatal and Oßwald, 2001). Therefore, development of juvenile spruce trees beyond the seedling stage is likely to be affected by P. citricola infection. Since infection with P. citricola causes malfunctioning and rot of the roots, constrained belowground competitiveness of in- fected plants is to be expected. Likewise, O 3 causes trees to reduce their carbon allocation to the soil compartment and, Acclimation to Ozone Affects Host/Pathogen Interaction and Competitiveness for Nitrogen in Juvenile Fagus sylvatica and Picea abies Trees Infected with Phytophthora citricola G. Luedemann 1 , R. Matyssek 1 , F. Fleischmann 2 , and T. E. E. Grams 1 1 Department of Ecology, Ecophysiology of Plants, Technische Universität München, 85354 Freising-Weihenstephan, Germany 2 Department of Ecology, Pathology of Woody Plants, Technische Universität München, 85354 Freising-Weihenstephan, Germany Received: May 13, 2005; Accepted: September 5, 2005 Plant Biol. 7 (2005): 640 – 649 © Georg Thieme Verlag KG Stuttgart · New York DOI 10.1055/s-2005-872902 ISSN 1435-8603 640