The forest really does hum with life.
Though often too low or too high for human ears to detect, insects and animals signal each other with vibrations. Even trees and plants fizz with the sound of tiny air bubbles bursting in their plumbing.
And there is evidence that insects and plants "hear" each other's sounds. Bees buzz at just the right frequency to release pollen from tomatoes and other flowering plants. And bark beetles may pick up the air bubble pops inside a plant, a hint that trees are experiencing drought stress.
Sound is so fundamental to life that some scientists now think there's a kernel of truth to folklore that holds humans can commune with plants. And plants may use sound to communicate with one another.
If even bacteria can signal one another with vibrations, why not plants, said Monica Gagliano, a plant physiologist at the University of Western Australia in Crawley.
"Sound is overwhelming, it's everywhere. Surely life would have used it to its advantage in all forms," she told OurAmazingPlanet.
Gagliano and her colleagues recently showed corn seedling's roots lean toward a 220-Hertz purr, and the roots emit clicks of a similar tune. Chili seedlings quicken their growth when a nasty sweet fennel plant is nearby, sealed off from the chilies in a box that only transmits sound, not scent, another study from the group revealed. The fennel releases chemicals that slow other plants' growth, so the researchers think the chili plants grow faster in anticipation of the chemicals — but only because they hear the plant, not because they smell it. Both the fennel and chilies were also in a sound-isolated box.
"We have identified that plants respond to sound and they make their own sounds," Gagliano said. "The obvious purpose of sound might be for communicating with others."
Gagliano imagines that root-to-root alerts could transform a forest into an organic switchboard. "Considering that entire forests are all interconnected by networks of fungi, maybe plants are using fungi the way we use the Internet and sending acoustic signals through this Web. From here, who knows," she said.
As with other life, if plants do send messages with sound, it is one of many communication tools. More work is needed to bear out Gagliano's claims, but there are many ways that listening to plants already bears fruit.
Scientists first recognized in the 1960s that listening to leaves revealed the health of plants.
When leaves open their pores to capture carbon dioxide, they lose huge amounts of water. To replace this moisture, roots suck water from the ground, sending it skyward through a series of tubes called the xylem. Pit membranes, essentially two-way valves, connect each of the thousands of tiny tubes. The drier the soil, the more tension builds up in the xylem, until pop, an air bubble is pulled in through the membrane.
For some plants, these embolisms are deadly — as with human blood vessels — because the gas bubbles block the flow of water. The more air in the tubes, the harder it is for plants to pull in water, explains Katherine McCulloh, a plant ecophysiologist at Oregon State University.
But researchers who eavesdrop on plant hydraulics are discovering that certain species, like pine trees and Douglas firs, can repair the damage on a daily or even an hourly basis.
"These cycles of embolism formation and refilling are just something that happens every single day. The plant is happy, it's just day-to-day living," McCulloh said. "In my mind, this is revolutionary in terms of plant biology. When I learned about how plants moved water, it was a passive process driven by evaporation from the leaves. What we're beginning to realize is that's just not true at all. It's a completely dynamic process."