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We measured oxygen isotope ratios (δ 18 O) of xylem sap, phloem sap, leaves, wood and bark of  Eucalyptus globulus  Labill. growing in southwestern Australia. Carbon isotope ratios (δ 13 C) were measured in the dry matter of phloem sap, leaves and wood. Results were used to test several aspects of a mechanistic model of  18 O enrichment and provided insights into post-photosynthetic variations in dry matter δ 13 C. Xylem water δ 18 O varied little within the tree crown, whereas variation at the landscape-level was more pronounced, with plantations near the coast being enriched by up to 3‰ compared with plantations less than 100 km inland. Phloem water was significantly enriched in  18 O compared with xylem water in two of three sampling campaigns; mean enrichments were 0.5 and 0.8‰. Phloem sap sugars exported from  E. globulus  leaves closely reflected observed leaf water enrichment when diurnal variation in photosynthesis was taken into account. Photosynthetic rates were higher in the morning than in the afternoon, whereas leaf water  18 O enrichment increased to maximum values in the afternoon. A non-steady-state model of leaf water  18 O enrichment accurately predicted observed values through a full diel cycle. Mean estimates of the proportion of organic oxygen effectively exchanging with xylem water during cellulose synthesis were close to 0.40 for both leaves and wood. Carbon isotope ratios of nascent xylem tissues did not differ from those of phloem sap sugars collected concurrently, whereas nascent leaf tissues were depleted in  13 C by 2‰ compared with phloem sap sugars, suggesting that, in  E. globulus ,  13 C enrichment of sink tissues compared with source leaves does not result from an enriching process within the sink tissue.

Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus