Have you ever looked at a tree sway in the wind? It looks so peaceful. It’s kind of a hypnotic back and forth of life interacting with air. Scientists get just as excited as lay people by a tranquil, beautiful tree. However, they also see data in motion. The way a tree moves says a lot about its biology, local hydrology and the landscape at large. The best way to measure a tree’s swaying is to strap a fitness tracker to its trunk with waterproof duct tape.
Using off the shelf accelerometers, researchers have been quantifying how trees sway differently over time. When they are warmer or colder, hydrated or dehydrated, weighed down by snow or unburdened, all of these factors must be taken into account. Deidre Jaeger, an ecologist out of the University of Colorado Boulder, likes to call these devices ‘fitbits for trees’. These devices provide a high-resolution monitoring of tree activity, just like we have high-resolution monitoring of our activity as human beings. These monitors, such as fitbits, give us metrics on how much energy we burn or how much sleep did we get?
Researchers are really interested in monitoring how much water trees are capturing. Measuring precipitation isn’t as easy 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 this water by 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 which means that it just sits there, baking in the sun and evaporating much of the water away. As a result, this robs the underlying environment of moisture. Snow that falls to the ground is shaded so it takes longer to melt.
By using these accelerometers, scientists have a new way of measuring how much rain or snow a particular tree in a forest ends up intercepting. Oregon State hydrologist, Mark Raleigh says, “how much of that (rain or snow) actually gets to the ground is kind of a big question.” He goes on to say, “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 and water resources.”
In 2014, Raleigh’s team went into the wilds of Colorado and found two trees next to a tower that was already gathering other scientific projects. They sealed accelerometers in plastic baggies and taped them to the trees. Like a Fitbit, these devices could measure minute movements such as 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 the researchers an extremely detailed data set on how the two trees moved. Raleigh says that they basically “oscillate when activated by the motion of the wind.” He adds, “the frequency at which a tree will sway not only depends on mass, but also how rigid the tree is.”
Both of these variables constantly change throughout the year. In the winter, the trees freeze, which stiffens them, and they are burdened snow, which increases their mass. In summer, the trees shed that snow mass and warm up and loosen. Raleigh’s team also had temperature readings so 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. They’re looking for “drops in the tree sway frequency that tells us there’s mass being added.” Stated another way – trees weighed down with snow swayed more slowly, taking longer to complete a back and forth cycle.
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 is the fact that, 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. As a result, there’s a greater distribution of mass of intercepted water near the base of the canopy. That mass distribution, if it’s lower on the canopy, will make the tree sway a little bit faster than if that same mass were distributed higher up on the tree.
All of this shows up in the accelerometer’s data. In fact, with the tree fitbit, you can monitor multiple processes at once. In the case of water stress and how stressed out a tree gets, that also indicates how much moisture is in the soil. Jaeger, for her part, is studying how accelerometers can provide similar data for 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 and lose it again when they drop their leaves in the fall.
Accelerometers strapped to trees won’t displace traditional sensors but could complement them. Some researchers are considering using video cameras to ‘spy’ on trees. Video would give researchers data on the collective movement of many trees at once which is something an accelerometer cannot do. Of course, the research is still in its infancy stages (well, relatively speaking of course). The important thing to note is that researchers are actively working on figuring out this puzzle.
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