The Gizmodo article Undead Tree Stump Is Being Kept Alive by Neighboring Trees says:
Leuzinger and Bader stumbled upon the stump while out for a hike. The woody stub caught their eye because callus tissue could be seen growing over its dead and rotting parts. It was also producing resin, which indicated the presence of living tissue. This prompted a more thorough investigation in which the researchers measured water flowing through the tissues of the stump, and also its rate of respiration, which matched those seen in the surrounding trees.
“We measured water flow with ‘heat-ratio sap flow sensors’,” Leuzinger told Gizmodo. “They detect water movement in tissue by sending out very small heat pulses and measuring how quickly that heat dissipates.”
These measurements indicated that the kauri stump is inactive during the day when living trees transpire. But during the night and on rainy days, the tree stump becomes active, circulating water—and presumably carbon and nutrients—through its tissues. As the authors write in the new study, these results
"...indicate that such symbioses may be much more complex than previously assumed: by physiologically exploiting ‘‘downtimes’’ of transpiring trees during the night or rainy days with high water potentials in the root network living stumps seem to act partially autonomously, strategically tapping into resources rather than simply becoming part of the neighboring trees’ extended root networks."
I can get a general feeling for the principle. After a thermal pulse the local temperature of a probe will decay due to heat diffusion, but if there is movement of water (or sap) in and out of the area the decay will be faster...
But I have a few questions:
- How exactly would you analyze a temperature decay curve $T(t)$ in order to determine if there was water, sap, or any material moving in and out of the volume or not? Is there a distinctly different shape to the curve that rules out thermal diffusion alone, demonstrating that there must be mass flow as well?
- When used on trees, how is this instrument applied to the tree? Does it need to penetrate into live tissue to work, or can it work by proximity, perhaps using radio frequency or microwaves for heating and a surface contact thermometer?