A one-year phytotron study was conducted to investigate the intra- and interspecific competitiveness of juvenile European beech ( Fagus sylvatica ) and Norway spruce ( Picea abies ) exposed to ambient and elevated O ₃ concentrations.

The following hypotheses will be tested:

(1) Enhanced O 3 exposure causes trees to reduce their carbon (C) allocation to below ground compartments (coarse and fine roots, mycorrhizae, soil microbes – cooperation with B9 Pritsch/Hartmann and B12 Munch/Schloter).

(2) Limited C allocation belowground reduces the competitiveness for soil resources.

In spring 2004, two- and three-year-old beech and spruce saplings were planted in mono and mixed cultures (Fig. 1) and subsequently acclimated to ambient (1xO ₃ ) and twice-ambient O ₃ concentrations (2xO₃ ) in a climate controlled glasshouse (GSF-National Research Center for Environment and Health). During the following growing season, plants were transferred to the four GSF-phytotrons where the O₃ -treatments were maintained. To trace the carbon (C) allocation in biomass and stem respiration, the d ¹³ C of the air was changed (from c. -10 to +106 ‰) for five subsequent days. Already few hours after the increase of d ¹³ C, the label shows up in the stem respiration signal (Fig.2). Simultaneously to the ¹³ C-labelling, ¹⁵ N-ammonium- ¹⁵N-nitrate was added to the soil to quantify the belowground competitiveness of the juvenile trees. At the end of the ¹³ C and ¹⁵ N labelling the plant systems were completely harvested.

Fig. 1: Collecting of CO₂from stem respiration with an open stem gas exchange system. Stem diameter growth is assessed with electronic dendrometers (red arrow) in parallel to the stem respiration, which is determined using aluminum covered stem respiration cuvettes (blue arrow).

Fig. 2: Increase of d ¹³ C in stem respiration of a juvenile beech tree after the increase of d ¹³ C in air. The arrow marks the change of d ¹³ C in the air from c. -10 to +106 ‰ . Five days later about 80% of the C respired in the stem is newly fixed C. Gray bars give night-time. The numbers represent the percent of newly fixed C which was found in stem respiration.