He discovered that the cells in the xylem and phloem of the tree fill with air to try to maintain the osmotic pressure that's usually produced by the sucking of water up through the roots. At a certain point the cells burst. Krause adds "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: The sound of popping cells." (Incidentally, when the xylem cells pop, they die and form the rings of the tree). Recordings are made at their natural high frequency (about 47 kHz!) with a hydrophone and then slowed down by about a factor of seven.
Bernie's done some fascinating work in the field of "biophony", which is based around the idea that every animal in an eco-system has its own acoustic territory, or bandwidth of sound that it vocalizes in. If something comes in and takes over a certain bandwidth (like the regular route of a noisy airplane) entire populations can suffer, or be forced to adapt.
You can find more of his recordings here
Sent from James' iPhone