Oecologia (2004) 138: 426–435 DOI 10.1007/s00442-003-1449-3 ECOSYSTEM ECOLOGY Margaret M. Barbour . John S. Roden . Graham D. Farquhar . James R. Ehleringer Expressing leaf water and cellulose oxygen isotope ratios as enrichment above source water reveals evidence of a Péclet effect Received: 6 March 2003 / Accepted: 30 October 2003 / Published online: 10 December 2003 # Springer-Verlag 2003 Abstract There is an increasing ecological interest in understanding the gradients in H 2 18 O enrichment in leaf water (i.e. a Péclet effect), because an appreciation of the significance of the Péclet effect is important for improving our understanding of the mechanistic processes affecting the 18 O composition of leaf water and plant organic material. In data sets where both source water and leaf water 18 O data are available, we can evaluate the potential contribution of a Péclet effect. As an example, we recalculate data published earlier by Roden and Ehleringer (1999, Oecologia 121:467–477) as enrichments in leaf water (Δ L ) and cellulose (Δ cell ) above source water. Based on these recalculations, we present support for the relevance of a Péclet effect in leaves. Further, we demonstrate that the subtle variations in Δ L and Δ cell caused by a Péclet effect may be masked in experimental systems in which variation in the source water oxygen isotope ratio is considerable. Keywords Oxygen-18 . Leaf water enrichment . Tree ring cellulose . Leaf gas exchange . Humidity Introduction The oxygen isotope composition (δ 18 O) of plant cellulose is of considerable interest to a number of different disciplines, including paleoclimatology, agriculture and ecology. Paleoclimatological studies use tree ring isotope ratios as correlative parameters to estimate the local prevailing temperature at the time when the cellulose was formed by assuming that δ 18 O of cellulose reflects δ 18 O of water taken up by the tree (e.g. Gray and Thompson1977). The oxygen isotope composition of soil water is normally closely linked to that of rainfall, and because δ 18 O of rainfall is known to vary with temperature, tree rings are viewed as “isotopic thermometers” (Libby et al.1976). Yet we also know that the isotopic composition of precipita- tion is a function of season (i.e. winter versus summer) instead of annual temperature, which restricts the potential of using 18 O as a simple thermometer (Ehleringer and Dawson1992; Lin et al.1996; Schwinning et al.2002). While variation in δ 18 O of water taken up by a plant is the baseline for variation in δ 18 O of cellulose, variation can also result from variation in evaporative enrichment of leaf water during transpiration. This evaporative effect has prompted plant breeders and physiological ecologists to suggest that δ 18 O of cellulose may represent an integrated record of environmental and stomatal regulation of water loss (Farquhar et al.1994; Barbour and Farquhar2000; Barbour et al. 2000a). Recent work (Helliker and Ehleringer2002) has dem- onstrated that when grass plants were grown in conditions of constant source water δ 18 O, variation in leaf water enrichment is recorded in δ 18 O of leaf cellulose. However, in studies with variable δ 18 O of source water, leaf enrichment effects might be less obvious. Barbour et al. (2003) suggested that whenever possible removing spatial and temporal variation in source water δ 18 O, by presenting cellulose δ 18 O as enrichments above source water (Δ 18 O), would allow the more subtle effects of leaf evaporative enrichment to be studied. This paper aims to highlight an advantage to presenting isotope compositions relative to the source water by recalculating previously published M. M. Barbour (*) Landcare Research, P.O. Box 69, 8152 Lincoln, New Zealand e-mail: barbourm@landcareresearch.co.nz Tel.: +64-3-3256700 Fax: +64-3-3252418 J. S. Roden . J. R. Ehleringer Stable Isotope Facility for Environmental Research, Department of Biology, University of Utah, Salt Lake City, UT 84112, USA Present address: J. S. Roden Department of Biology, Southern Oregon University, 1250 Siskiyou Blvd., Ashland, OR 97520–5071, USA G. D. Farquhar Environmental Biology, Research School of Biological Sciences, Australian National University, GPO Box 475, ACT 2601 Canberra, Australia