Site-specific responses to short-term environmental variation are reflected in leaf and phloem-sap carbon isotopic abundance of field grown Eucalyptus globulus

The carbon isotopic composition (δ13C) of plant material has been used extensively as an indirect measure of carbon fixation per volume of water used. More recently, the δ13C of phloem sap (δ13Cphl) has been used as a surrogate measure of short-term, canopy scale δ13C. Using a combination of δ13C physiological, structural and chemical indices from leaves and phloem sap of Eucalyptus globulus at sites of contrasting water availability, we sought to identify short-term, canopy scale resource limitations. Results illustrate that δ13Cphl offers valid reflections of short-term, canopy scale values of leaf δ13C and tree water status. Under conditions limited by water, leaf and phloem sap photoassimilates differ in 13C abundance of a magnitude large enough to significantly influence predictions of water use efficiency. This pattern was not detected among trees with adequate water supply indicating fractionation into heterotrophic tissues that may be sensitive to plant water status. Trees employed a range of physiological, biochemical and structural adaptations to acclimate to resource limitation that differed among sites providing a useful context upon which to interpret patterns in δ13C. Our results highlight that such easily characterized properties are ideal for use as minimally invasive tools to monitor growth and resilience of plants to variations in resource availability.