The present study compares the dynamics in carbon (C) allocation of adult deciduous beech (Fagus sylvatica) and evergreen spruce (Picea abies) during summer and in response to seven-year-long exposure with twice-ambient ozone (O₃) concentrations (2 × O₃). Focus was on the respiratory turn-over and translocation of recent photosynthates at various positions along the stems, coarse roots and soils. The hypotheses tested were that (1) 2 × O₃ decreases the allocation of recent photosynthates to CO₂ efflux of stems and coarse roots of adult trees, and that (2) according to their different O₃ sensitivities this effect is stronger in beech than in spruce.

Labeling of whole tree canopies was applied by releasing ¹³C depleted CO₂ (δ¹³C of −46.9‰) using a free-air stable carbon isotope approach. Canopy air δ¹³C was reduced for about 2.5 weeks by ca. 8‰ in beech and 6‰ in spruce while the increase in CO₂ concentration was limited to about 110 μl l⁻¹ and 80 μl l⁻¹, respectively. At the end of the labeling period, δ¹³C of stem CO₂ efflux and phloem sugars was reduced to a similar extend by ca. 3–4‰ (beech) and ca. 2–3‰ (spruce). The fraction of labeled C (f_E,new) in stem CO₂ efflux amounted to 0.3 to 0.4, indicating slow C turnover of the respiratory supply system in both species.

Elevated O₃ slightly stimulated the allocation of recently fixed photosynthates to stem and coarse root respiration in spruce (rejection of hypothesis I for spruce), but resulted in a significant reduction in C flux in beech (acceptance of hypotheses I and II). The distinct decrease in C allocation to beech stems indicates the potential of chronic O₃ stress to substantially mitigate the C sink strength of trees on the long-term scale.