Tracking phloem carbon isotopic composition to reconcile water-use efficiency estimates across scales in beech trees, under future climate change scenarios

Plant water-use efficiency (WUE) describes the intimate link between the carbon and water cycles. WUE can be estimated using multiple methodologies concerning different spatial and temporal scales, but empirical evidence has shown that these estimates do not always agree. Two of the most widely used methodologies to estimate WUE are measurements of the ratio of photosynthesis to stomatal conductance to water, using gas-exchange, and analyses of the carbon isotopic composition (δ¹³C) of plant material, most often measured on plant tissues (leaves and woody stems mainly), reflecting the signal of the plant physiological status all along the organ ontogeny. In addition, in tall trees, δ¹³C varies greatly among leaves and thus individual measurements cannot capture whole-tree physiological status. In contrast, analyses of the phloem δ¹³C collected at the base of the trunk should reflect the whole-tree physiological performance. To test this novel approach under contrasting climate change scenarios, we estimated WUE: from measurements of gas-exchange, from δ¹³C of leaf and woody tissues, as well as from δ¹³C of phloem samples collected along the whole plant pathway. We measured gas-exchange and collected samples for analyses of δ¹³C from European beech (Fagus sylvatica) saplings grown under controlled conditions and from adult trees in the field. Saplings were subjected to four climate change scenarios, resulting from a combination of two atmospheric CO₂ levels and two watering regimes. In the field, we measured WUE on adult beech trees during the unusually hot and dry growing season of 2022. Preliminary results show that phloem δ¹³C could serve as a good proxy of whole-plant WUE, provided that the internal leaf conductance is incorporated into the calculations.