Differences in carbon isotope leaf‐to‐phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

• DOI: 10.1111/nph.15735
• Publication Date: 2019-02-11T09:49:10Z
• Authors: Rebekka Bögelein; Marco M. Lehmann; Frank M. Thomas
• Keywords: 0106 biological sciences; Carbon Isotopes; Time Factors; Chemical Fractionation; Phloem; 15. Life on land; 01 natural sciences; Pseudotsuga; Circadian Rhythm; Plant Leaves; Solubility; Fagus; Seasons; Organic Chemicals; Sugars; Cyclitols

Summary

While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport‐related fractionation mechanisms that influence phloem and subsequently tree ring δ13C are less investigated and may vary among species. We studied the seasonal and diel courses of leaf‐to‐phloem δ13C differences of water‐soluble organic matter (WSOM) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii). δ13C of individual sugars and cyclitols from a subsample was determined by compound‐specific isotope analysis. In beech, leaf‐to‐phloem δ13C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. 13C‐Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf‐to‐phloem δ13C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM. Trunk phloem WSOM at breast height reflected canopy‐integrated δ13C in beech but not in Douglas fir. Our results demonstrate that leaf‐to‐phloem isotope fractionation and δ13C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ13C.