Give Fitbits (of Sorts) to the Trees

You might look at a tree swaying in the wind and see botanical tranquility—a hypnotic back and forth of life interacting with air. Scientists appreciate that too, but they also see something else: data in motion. It turns out that the way a tree moves says a lot about its biology, the local hydrology, and the landscape at large. And the best way to measure a tree’s swaying is to strap a fitness tracker to its trunk with waterproof duct tape. 

Well, a fitness tracker of sorts—the quantified self for plants. Using off-the-shelf accelerometers, researchers have been quantifying how trees sway differently over time: when they’re warmer or colder, hydrated or dehydrated, weighed down by snow or unburdened. “I like to call it a Fitbit for trees,” says University of Colorado Boulder urban ecologist Deidre Jaeger, who’s using accelerometers to study trees. “It’s high-resolution monitoring of tree activity, just like we have high-resolution monitoring of our activity as a human being that tells us metrics on how much energy are we burning? How much sleep did we get?”

One of the things researchers really want to monitor is how much water trees are capturing. Measuring precipitation, it turns out, isn’t as simple as tracking how much water falls out of the sky and soaks into the ground as liquid or becomes part of the snowpack. Trees actually “intercept” much of it, gathering rain and snow in their canopies. In fact, depending on the kind of forest, up to half of the snowfall gets stuck in the canopy. That means it sits there, baking in the sun and evaporating much of that water away—robbing the underlying environment of moisture. The snow that makes it to the forest floor, on the other hand, will be shaded, which slows its melting. 

Forest hydrological models struggle with these intricacies. But with accelerometers, scientists have a new way of measuring how much rain or snow a particular tree in a forest ends up intercepting. “How much of that actually gets to the ground is kind of a big question,” says Oregon State University hydrologist Mark Raleigh. “We can make measurements on the ground after it’s fallen down, but there’s a lot of interest in how we might predict that, especially if you’re trying to think of how you manage a forest for water resources.”

Raleigh’s own experiment began in 2014, when his team ventured into the wilds of Colorado and found two trees next to a tower that was already gathering data for other scientific projects. They sealed accelerometers in plastic baggies and taped them to the trees. Like your Fitbit, Apple Watch, or smartphone, the devices could measure minute movements, in this case the unique sway patterns that indicate how burdened the canopy is with snow.

The researchers took these measurements 12 times a second for six years, giving them an extremely detailed data set on how the two trees moved. “They basically oscillate when activated by the motion of the wind,” says Raleigh, lead author of a recent paper describing the work in the journal Water Resources Research. “The frequency at which a tree will sway not only depends on mass, but also how rigid the tree is.”