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In the present experiment, the single and combined effects of elevated temperature and ozone (O 3 ) on four silver birch genotypes (gt12, gt14, gt15 and gt25) were studied in an open-air field exposure design. Above- and below-ground biomass accumulation, stem growth and soil respiration were measured in 2008. In addition, a  13 C-labelling experiment was conducted with gt15 trees. After the second exposure season, elevated temperature increased silver birch above- and below-ground growth and soil respiration rates. However, some of these variables showed that the temperature effect was modified by tree genotype and prevailing O 3  level. For instance, in gt14 soil respiration was increased in elevated temperature alone (T) and in elevated O 3  and elevated temperature in combination (O 3  + T) treatments, but in other genotypes O 3  either partly (gt12) or totally nullified (gt25) temperature effects on soil respiration, or acted synergistically with temperature (gt15). Before leaf abscission, all genotypes had the largest leaf biomass in T and O 3  + T treatments, whereas at the end of the season temperature effects on leaf biomass depended on the prevailing O 3  level. Temperature increase thus delayed and O 3  accelerated leaf senescence, and in combination treatment O 3  reduced the temperature effect. Photosynthetic : non-photosynthetic tissue ratios (P : nP ratios) showed that elevated temperature increased foliage biomass relative to woody mass, particularly in gt14 and gt12, whereas O 3  and O 3  + T decreased it most clearly in gt25. O 3 -caused stem growth reductions were clearest in the fastest-growing gt14 and gt25, whereas mycorrhizal root growth and sporocarp production increased under O 3  in all genotypes. A labelling experiment showed that temperature increased tree total biomass and hence  13 C fixation in the foliage and roots and also label return was highest under elevated temperature. Ozone seemed to change tree  13 C allocation, as it decreased foliar  13 C excess amount, simultaneously increasing  13 C excess obtained from the soil. The present results suggest that warming has potential to increase silver birch growth and hence carbon (C) accumulation in tree biomass, but the final magnitude of this C sink strength is partly counteracted by temperature-induced increase in soil respiration rates and simultaneous O 3  stress. Silver birch populations' response to climate change will also largely depend on their genotype composition.

Interactive effects of elevated ozone and temperature on carbon allocation of silver birch ( Betula pendula ) genotypes in an open-air field exposure