An interesting note I came across on the internet:
“Ever heard a tree singing?” asks noted composer and bioacoustician Bernie Krause. “It's 70 kHz,” he adds as he reaches for a CD-R in his spartan Northern California studio.
“We were listening for the sounds of bats,” Krause continued, “which are up in the 47-plus kHz range. And we heard a steady signal, very unbiological in the sense of it being from a creature. As we moved closer to this cottonwood tree, the signal level increased. We drilled a little hole in the tree and put this hydrophone in. We had an instrumentation device with us that could record a frequency that high, and we got a signal coming from the trunk of the tree. We couldn't figure out what it was. Then we slowed it down by a factor of seven, to get it down within our hearing range.” As we listened to the tree's music, I was startled by the regularity of its pulse and the subtle rhythmic accents. It was as if we were hearing a recording of a virtuosic percussionist playing woodblocks.
“What we discovered was that during a drought, the cells in the xylem of the tree usually maintain a certain pressure from the water that comes into the trunk of a tree during normally wet seasons,” Krause explained. “When that pressure drops during a drought, the cells automatically fill with air, to try to maintain the osmotic pressure. And when they get too dry and they're pumping in air, they pop. When they pop, they die, and the dead cells form the tree's rings. So, when they pop, they make a noise: we can't hear it, but insects can. And when insects hear multiple cells popping, they're drawn to the tree because certain ones are programmed to expect sap. And when the insects are drawn to the tree, the birds are drawn to the tree to eat. It's all a microhabitat formed by sound.”