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Wednesday, April 17, 2024

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.”

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Both of these variables are constantly changing throughout the year. In the winter, the trees freeze, which stiffens them, and they are burdened with snow, which increases their mass. In the summer, the trees shed that snow mass and warm up and loosen. 

Because Raleigh and his colleagues also had temperature readings, they could track how rigidity and mass changed over the seasons. They further confirmed how much snow had accumulated on the branches by training cameras on the trees. “We're looking for drops in the tree sway frequency that tells us there's mass being added,” says Raleigh. Put another way: Trees weighed down with snow swayed more slowly, taking longer to complete a back-and-forth cycle.

Dom Ciruzzi, an ecohydrologist at the College of William and Mary, is using the same accelerometer technique for rain, to better understand how water flows through a forest ecosystem. Just as a tree stiffens in the cold, its water content influences how it sways: When there’s a shortage, a tree’s internal plumbing gets less pressurized, making the trunk more bendy. Dryness also decreases a tree’s mass, further changing how it moves. “You can see these daily signals in trees swaying one frequency at night, and then during the day, they sway at a different frequency when they're water-stressed,” says Ciruzzi. 

When it rains, the tree soaks up water through its roots, adding to its mass, and it gets more top-heavy as water accumulates on its leaves. Further complicating matters, as the rain stops and temperatures rise again, the tree dries out from the top (where it basks in the sun) to the bottom (where it’s shaded by surrounding trees). “So then there's this greater distribution of mass of intercepted water near the base of the canopy,” Ciruzzi says. “And that mass distribution lower in the canopy will make the tree sway a little bit faster than if that same mass was distributed higher up in the tree.”

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All of this shows up in accelerometer data. Sure, you could use a probe to monitor soil moisture, or a rain gauge to figure out what’s falling out of the sky. But, says Ciruzzi, “What an accelerometer, or just monitoring tree sway, can do in general is monitor multiple processes at once. So in the case of water stress, and how stressed-out a tree is, that's also an indicator of how much moisture is in the soil.” 

Over at the University of Colorado, Boulder, Jaeger is studying what accelerometers can reveal about urban trees. Just as an evergreen in a forest gains mass as it accumulates snow, deciduous trees in a backyard or park pile on the mass as they flower every year, then lose mass when they drop their leaves in the fall. Jaeger can see the same signals in how the sway of the tree changes over time. “There was a change in mass that was detectable with floral development, and then the mass decreased as the flowers opened and the pollen was released into the wind,” Jaeger says. “And so this is really exciting, because even when we do use remote-sensing tools, they have a hard time picking up any kind of plant flowering.”

Satellite imagery, for instance, can really only tell you when trees are getting greener. To fully keep tabs on how a tree is developing over a year, scientists have to visit them constantly and log changes to their foliage. But using accelerometers to detect mass changes would let them remotely determine when the trees might be flowering. “That would be super useful in pollen forecasting,” says Jaeger. It would also provide advance warnings about the arrival of invasive insects, like the emerald ash borer, which tears into trees and sickens them, driving a loss of mass that could show up in accelerometer data.

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Accelerometers strapped to trees won’t displace traditional sensors but could complement them. Ciruzzi and Raleigh are also starting to explore ways to spy on swaths of trees with video cameras. Video would give them data on the collective movement of many trees at once, whereas each accelerometer only tracks the motion of a single tree. “You can certainly retrieve the sway frequency from individual trees from video, but then you could look at multiple trees in a single scene,” says Raleigh. “What is the saying—‘Seeing the forest for the trees’? But it's kind of the opposite: Seeing the forest as what's going to matter as a whole.”

Better understanding forests is increasingly critical on a warming planet. The American West is suffering a historic drought and is heavily reliant on the snowpack for water. Being able to estimate how much of that snow is actually lost because it’s trapped in trees, and to model how climate change might influence this dynamic, could perhaps give water managers the data they need to change their strategies. “If we're trying to strategically modify our forest management decisions in order to try to maximize snow retention, that'd be a really important thing to know,” says Raleigh.

Jaeger’s urban trees, too, may be threatened by climate change, thanks to the heat island effect, which makes cities way hotter than surrounding rural areas. Outfitting the trees with accelerometers could help determine which species are more resilient to skyrocketing temperatures. “We want to know what trees are doing well under extreme weather,” Jaeger says. “Monitoring is going to play a really strong role in the future of our urban forests, and planning for the next generation of trees that are going to do well in a future climate.”


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