Techniques that rely on adaptive silviculture can help trees become more resilient to severe storms
If a tree sways in the woods, what could it tell us about how likely it is to fall?
This is the question Amanda Bunce, a Ph.D. student in the Department of Natural Resources and the Environment, is working to address with the Stormwise Program in the Eversource Energy Center.
Stormwise is a collaboration between the Eversource Energy Center and researchers in the College of Agriculture, Health and Natural Resources (CAHNR). CAHNR researchers provide the Eversource team with vital information about roadside vegetation in the state to support informed decision making to improve the resiliency of Connecticut's power grid.
Bunce studies biomechanics, or how trees move in the wind, in forests across Connecticut. These measurements help Bunce determine how likely the tree is to be damaged or fall in a storm.
Recent destructive storm events in the Northeast have inspired utility companies like Eversource Energy, researchers, and other collaborators to work together on management approaches that will help adapt our forests to disturbances like increased storm activity, in an attempt to better prepare communities in the face of climate change.
"This is one element of the Eversource Energy Center's overall work to keep Connecticut's power on," says Robert (Bob) Fahey, associate director of the Eversource Energy Center and Cloutier Professor in Forestry. "Along with initiatives focused on sustainable energy, advanced outage prediction modeling, and workforce development, we're taking a holistic view to addressing these challenges."
The first measure Bunce looks at is frequency. This is a measure of how quickly the tree is moving back and forth. Tall, skinny trees have frequencies around 0.1 or 0.2. This means it takes 10 seconds to complete a sway from one side, to the other, and back again. Shorter, thicker trees have much higher frequencies. This means they complete a sway more quickly and could be seen to vibrate more than to sway in the wind.
Trees with higher frequencies tend to be more stable and less likely to fall during strong wind events. That stability seems to be mostly based on the stouter shapes of trees with higher sway frequency.
Bunce obtains these measurements using instruments called inclinometers. The device is kept in a weather-proof box and strapped to the tree. It then measures the incline of the tree in both the North-South and East-West directions.
Tree frequency has long been studied in plantation forests, primarily in Europe. This effort is the first-time researchers have applied these concepts to an area like Connecticut.
This has come with challenges, as Connecticut's forests are significantly different than timber plantations, which have a single type of tree that are all the same age. Nutmeg State forests are much more complex, both structurally, and with regards to biodiversity. The ways in which we use them are also more complex.
"Our forests are multiple-use," Bunce says. "Anywhere we grow trees for timber is also somewhere people hike, or gather mushrooms, or watershed protection, and it's a wildlife habitat, too. We have a lot more responsibility than just growing timber."
The other measure Bunce looks at is displacement. This determines how far the tree sways in the wind. While a tree's frequency is always the same, regardless of how strong the wind is, its displacement depends on wind strength.
"It's important to us, because how far the tree sways is a big deal in regards to if it falls down," Bunce says.
There has been much less research on this measurement due to the complications and variability in wind strength and speed.
Luckily there are methods to help trees become more resilient—they need to "work out." For instance, gradually exposing a tree to more wind develops its resistance and makes it more resilient in a storm.
To do this, the team has applied their Stormwise Prescription to test forests around the state. This prescription involves assessing a roadside forest and first removing trees that are growing into the road, over powerlines, or are "stressed" in some regard. Stressed trees include those that are hollow, infected with fungus, or ridden with pests.
Next, healthy, well-balanced trees are selected for retention, and the team removes trees around them to strategically open patches in the forest to let in more wind. This frees up resources for the remnant trees, allowing them to grow stronger. This kind of technique is known as adaptive silviculture, a complex process of controlling the growth of trees to improve the overall capacity of the forest to handle change and disturbances like storms.
"We've got a lot more science to do," Bunce says.
The art of the forest
Bunce, who studied art as an undergraduate, saw an important opportunity to help the community connect with the work she's doing through unconventional outreach.
She collaborated with the Digital Experience Lab at the Aldrich Contemporary Art Museum in Ridgefield to produce an artistic display of her work. For the exhibit, which was displayed from Oct. 2019 to Aug. 2020, Bunce attached an inclinometer to a tree and its information was transmitted to the Aldrich Museum. There, guests could watch a live digital feed of the tree's movement.
"The thing that always drew me to art is that you can use it to tell a story and using it to tell the story of science is brilliant and vital," Bunce says.
Bunce says she hopes outreach like this installation helps the community understand the value of her work.
"I want people to look around and see what's happening and work together," Bunce says. "We can do a tiny amount of adaptive silviculture. But what's really going to matter is if everyone gets on the same page."