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Method development for d18O analysis of phloem content.

PI: Zsofia Reka Stangl

The isotope ratio of the sugars in the phloem sap of a tree directly represent the fractionation during photosynthesis (Anet) and stomatal conductance (gs) and integrate the signal from the whole canopy. d13C of phloem content has been repeatedly used to estimate canopy water-use efficiency, i.e. Anet/gs. The simultaneous analysis of d18O and d13C would enable us to evaluate if differences in d13C are due to changes in photosynthetic capacity or in stomatal conductance (gs). This method has the potential to be used as an indicator of growth and survival under various environmental conditions, for example in response to changes in ambient temperature and water availability. Such a diagnostic tool could be useful in predictions of forest growth, changes in species composition under future climates or to screen for more resilient genotypes in forestry species. However, the analysis of d18O in phloem sugars is problematic. If sucrose breaks down during the extraction, the molecules exchange oxygen with the extraction medium, altering so the d18O of the sugars. In this project I propose to develop an optimised method for sampling and extraction of phloem content for oxygen isotope analysis, considering a set of extraction conditions, extraction time, sample preparation methods and sampling position along the stem. I will use phloem samples from mature Scots pine trees during the optimisation of the extraction method and finally compare Scots pine, Norway spruce and Silver birch when assessing the importance of the vertical sampling position along the stem. The method development will be executed in close collaboration with Jonas Lundholm at the SLU Stable Isotope Laboratory, who is an expert in analytical chemistry and oxygen isotope analysis. The field work will be done with the assistance from the staff at the Svartberget field station.

isotopic compositionstem profileisotope ratio