Tree trunks swell in synchrony with tides.

Trees bloat and then shrink with the rhythm of the tides, Swiss and Italian researchers report.

Stem diameters of young spruces show this tidal pattern even when growing in separate containers in darkness and at constant temperature and humidity, says Ernst Zurcher of the Swiss Federal Institute of Technology in Zurich. He and colleagues from three other institutions describe their observations in the April 16 Nature.

The researchers measured stem diameter changes of several hundredths of a millimeter. The changes follow roughly a 25-hour cycle with two peaks, one higher than the other.

According to the measurements, this pattern shows up even in isolated chunks of stem that have been sealed to prevent water from going in or out. The pattern can persist for months, as long as the most actively growing layer, the cambium, survives. The rhythm also shows up in trees that are dormant or are grown in continuous light or reversed day-night cycles.

Such patterns had previously been reported by coauthor Maria-Giulia Cantiani of the University of Trento in Italy and her colleagues on the basis of data collected over 14 years. Recently, Zurcher noticed the curves' similarity to tidal patterns, prompting the tree-measuring team to superimpose tidal records on some of their data. "It was amazing to build up these curves," Zurcher says. "It was absolute synchronicity."

The curves from the two spruces presented in Nature haven't yet convinced plant physiologist Steve Pallardy of the University of Missouri in Columbia. "I would have to see more data," he says. He points out that the trees seemed to lead the tides slightly at one point and trail slightly at another.

Despite his hesitation, Pallardy notes that weirder correlations have survived scrutiny. Tree ring width, for example, really does vary with fish scale size, he says. Disturbances on land can stunt tree growth as well as affect bass in a stream.

Trees do thicken and then slim down, Pallardy says. In one well-known cycle, trees shrink a little from water loss during the day, when air holes in the leaves open and water evaporates. When the sun goes down, these holes swell shut, and the tree plumps up again, as water flowing in through the roots makes up for the day's losses.

Just how the tide correlation works is not clear, according to Zurcher. He suggests that some kind of miniature tidal sloshing moves water between the cytoplasm in living cells and the trunk's structural framework of dead cell walls.

Such a pattern might explain widespread folklore about cutting trees before a new moon to get the wood to dry faster, he says. He has heard this tree-cutting wisdom from India to South America. New moons, and their weaker coastal tides, may signal that less water flows into the hard-to-dry cell wall framework, he speculates. A full moon might indicate stronger tree "tides."

Zurcher sees the stem diameter correlation as fitting in with his earlier work showing that tree seeds germinate and grow at different rates, depending on the phase of the moon. However, not all the curves swooped up or down at the same point in the lunar cycles. "It is very complex," Zurcher cautions.