Interest in A18Op was first roused, in the late 1940s, because tree ring cellulose AlsO was suggested to be an 'isotope thermometer'. Recent work (e.g., Roden and Ehleringer, 1999; Barbour and Farquhar, 2000; Helliker and Ehleringer, 2002b) has provided firm evidence that the oxygen isotope composition of plant tissue contains a record of the leaf evaporative environment as well as the 5180 of source water. This means that temperature reconstruction from <5 l8C) of tree rings is not simple; the leaf evaporative environment (VPD) and plant regulation of water loss (gs) must also be taken into account. Recognition of these important effects on <518Op opens the door to a rather more interesting reconstruction of past climates, which includes relative humidity and water availability (via gs), as well as temperature, as part of that climate.

Recent work in which the wide annual rings of Pinus radiata grown in New Zealand were divided into very small sections, representing between 3 and 30 days of growth, has shown that <518Op may be interpreted in terms of climatic conditions (Barbour et al, 2002). In this experiment specific climatic events, such as drought and high rainfall, were recorded as peaks and troughs in tree ring cellulose <5180. As suggested by Barbour et al. (2002), multiple-isotope analysis (<5lsO, <513C, and <5D) of tree ring cellulose may allow a more complete picture of past climates, and plant response to environmental variation, to be drawn.

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