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Innards. A CT scan of an oak trunk, superimposed with a profile of water content.

Trees' Watery Secrets

T scanners aren't just for people anymore. Meteorites, fossils, animals, plants--they're all being probed. Now scientists have devised a scanning technique that will reveal fine details of tree trunks, and it's already shown that oaks pump more water than previously thought.

In the old days, botanists studied tree trunks by coaxing cross-sections to transport dyes. Now they rely on scanning technologies borrowed from medicine. The gold standard is nuclear magnetic resonance spectroscopy--which produces images based on how different compounds absorb radiation in a magnetic field--but NMR machines can hold samples no bigger than twigs. To see inside a tree trunk, you need a computed tomography (CT) scanner. The limitation is that CT scans, which generate images based on how samples deflect photons, have too low a resolution to study where the water is within the individual annual rinks of tree trunks.

Now, Jörg Fromm and his colleagues at the Technical University of Munich in Germany have overcome that hurdle. In order to focus on the water in the wood, they first scan a fresh slice of trunk, dry the wood to remove the water, and scan again. Comparing the two images allows them to look exclusively at the water concentration throughout the wood, without the distraction of cellular material. In the October issue of Plant Physiology, they report that this technique offers a resolution of 0.1225 mm3.

As a trial run, the researchers studied the movement of water in the oak trunk. The data revealed that the outer eight rings each have the same concentration, suggesting they all transport water. This challenges the going theory that sylvan aqueducts were restricted to the outer two rings in the sapwood of oak and comparable species. The team is currently following up on this observation in additional species.

Cornell University plant physiologist Roger Spanswick says that the new technique has very good resolution for objects as large as oak trunks. But he cautions that sawing the trunks may alter the distribution of water, so the results must be interpreted with caution.

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