Carbon isotope composition of various tissues of beech (Fagus sylvatica) regeneration is indicative of recent environmental conditions within the forest understorey.

•  Here, the effects were assessed of local climate and canopy thinning on the carbon isotope composition (δ ¹³ C) signatures of different plant groups in the understorey of a beech (Fagus sylvatica) forest in southern Germany. The relationship between δ ¹³ C and environmental parameters, within different time integrals, was also studied. •  δ ¹³ C was analysed in different tissues of beech regeneration, and herbaceous and woody understorey vegetation, in thinned and untreated control stands differing in aspect and, hence, local climate, on three dates during the growing season. •  Generally, tissues were ¹³ C-depleted on the north-east, compared with the south-west aspect. Thinning had variable effects on δ ¹³ C, depending on plant group and sampling date. δ ¹³ C in beech leaves and roots in control stands was mostly influenced by mean soil water potential in 4- and 8-wk integrals before sampling, respectively, and in leaves, additionally, by mean radiation in a 4-wk time integral. Shoot water potential and transpiration influenced foliar δ ¹³ C of beech whereas δ ¹³ C in the beech wood was modulated by soil temperature integrated over a 4-wk period before sampling. Above-ground tissues of woody and herbaceous plants were ¹³ C-enriched in mid-summer; their δ ¹³ C was poorly related to environmental factors. •  δ ¹³ C of various tissues of beech regeneration appears to be indicative of recent environmental conditions within the forest understorey and, consequently, this easy-to-determine physiological parameter could be used widely to assess effects of silvicultural treatments.