How closely does stem growth of adult beech (Fagus sylvatica) relate to net carbon gain under experimentally enhanced ozone stress?

The hypothesis was tested that O3-induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy-level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O3 regime, as prevailing at the forest site (control), or under an experimental twice-ambient O3 regime (elevated O3), as released through a free-air canopy O3 fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O3. As this outcome only partly accounts for the decline in stem growth, O3-induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth. - Highlights: ► We model O3-induced changes in the photosynthetic carbon gain of adult beech trees. ► Elevated O3 decreases gross carbon gain but increases respiratory carbon loss. ► Reduction in net carbon gain only partly accounts for the decline in stem growth. ► O3 effects on the whole-tree allocation is crucial in addition to carbon gains. - Reduction in net carbon gain at the canopy level only partly accounts for the decline in stem growth under elevated ozone.