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Abstract
We reexamine the theory that trees do not fractionate water during uptake from soil or transport to the canopy. If fractionation occurs in stems, we expect heavy isotope concentrations to increase with height in the stem or to vary with aspect. Three species with different xylem anatomies (Pinus taeda, Liquidambar styraciflua and Quercus nigra) were sampled by height and aspect. We found no trends in isotope fractions with height or aspect, supporting the assumption that fractionation does not occur during xylem transport. We also evaluate the effect of cryogenic extraction system design on xylem water signatures. We extracted one set of samples each on a small and large volume cryogenic extraction system. We find that cryogenic extraction system design alters xylem water signatures, specifically large volume extraction systems yield fractionated signatures. We suggest that water extraction efficiency be over 99% for all studies drawing conclusions about water sources and sinks.