title: Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech.
abstract: Combined δ(13) C and δ(18) O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A(n) ) between the investigated crown positions. Estimates of iWUE from δ(13) C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ(13) C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ(18) O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A(n)  between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions.
authors.affiliation: Department of Geobotany, University of Trier, Behringstraße 21, 54296 Trier, Germany. boegelein@unitrier.de
authors.forename: Rebekka
authors.initials: R
authors.lastname: Bögelein
authors.forename: Martin
authors.initials: M
authors.lastname: Hassdenteufel
authors.forename: Frank M
authors.initials: FM
authors.lastname: Thomas
authors.forename: Willy
authors.initials: W
authors.lastname: Werner
mesh_terms: D002247:Carbon Isotopes / Q000032:analysis; D029964:Fagus / Q000502:physiology; D008954:Models, Biological; D010103:Oxygen Isotopes / Q000032:analysis; D052585:Phloem / Q000502:physiology*; D010788:Photosynthesis; D018515:Plant Leaves / Q000502:physiology*; D054046:Plant Stomata / Q000502:physiology; D018526:Plant Transpiration; D028224:Pseudotsuga / Q000502:physiology; D014867:Water / Q000502:physiology
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D014867:Water