Researchers Look For Breaking Point In Urban Tree Species

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Davey Tree Research Farm is surrounded by corn fields and woods, on a normally quiet stretch of Route 303 in Shalersville. Once every three years, researchers, including Jason Grabosky from Rutgers University, and volunteers from local tree trimming companies come here to mess with trees.

In one patch of the farm, a volunteer saws through tree roots. The idea is to find out how wind exposure affects their growth.

In another, a tool called an air knife exposes roots, using only forced air that’s traveling at two-times the speed of sound. This allows researchers to study the roots without damaging them. And there’s also some quiet observation going on. Tom Smiley is from the Bartlett Tree Research Laboratory in North Carolina. He’s here to examine how much a tree’s roots can be cut before it becomes unstable.    

"What we found is if you’re doing a linear cut for a water line or a sidewalk or a sewer, that you should measure the trunk diameter and multiply that by three and that’s as close as you’d want to get," says Smiley.

He’s also studying what happens when individual roots are cut entirely. He starts by cutting a single root. Then checks how much force is needed to pull the tree over. The answer he’s come up with: on average, every root cut decreases stability by 15%. Smiley’s back this year checking on roots that he’s cut during previous trips. He wants to know the additional effect of decay.

“And we’re looking at whether the trees increase or decrease in stability, I couldn’t tell you the answer to that, we haven’t looked at the data yet," says Smiley.

This is important information. When gas lines or water lines are installed or replaced, there’s likely to be tree roots in the way. Smiley says they were often cut too close to the tree in the past.

Alan Siewert is urban forester at the state’s Department of Natural Resources. He says researchers from several states, and as far away as Mexico, are at the Davey Tree Research Farm to answer some practical public safety questions.

“Something you don’t want to do in people’s backyards but here we can do that so we can find the limit of trees’ ability to withstand the environment and what we do to them and that’s what biomechanics is," says Siewert.

Wes Kocher is with the International Society of Arboriculture, which was founded in 1924. Kocher does educational outreach with arborists and says it takes a lot of work to change practices. Another example of hard to change techniques, according to Kocher, comes right at the start of a tree’s life.

“In the forest, a seed drops in the ground and the tree starts its life almost in the top inch of soil. Whereas in the city, they’re coming from a nursery, the tree’s been repotted or replanted numerous times, and every time it gets potted deeper and deeper and deeper and then when we finally put them in in the city, sometimes they can be a foot deep," says Kocher.

That’s why trees in the city look like a telephone pole stuck in the ground while trees in the forest have a natural wide base of partially exposed roots.

Guy Meilleur runs a tree trimming business in North Carolina. He’s here to test out different tree trimming techniques. The goal is to limit the decay that gets into a tree after it’s trimmed. And to prune trees in a way that encourages more compact regrowth.

“And we’re going to come back after three years and study the way the trees respond by sending out new growth below the cuts hopefully, we don’t want to see a lot of sprouting at the cut’s surface," says Meilleur.

Meilleur says they have a pretty good idea of how the tree will regrow, because they’ve seen it on the job.

“But out here, this gives us an opportunity to record data in a more systematic fashion and be able to publish it and that makes it more verifiable and easier for people to trust and follow," he says.

The research from this week, which was organized by the state’s Division of Natural Resources, will be available to the public. This is the third event like this held at Davey Research Farm. They plan to be back again in 2019.

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