Determining How Much Pressure a Root Can Generate when it Expands (Show Us the Data! 3)

Basic Vs Applied Research

In science, we often distinguish between basic and applied research questions. Basic questions are asked primarily to expand knowledge, without any immediate concern for practical application. There may never be a way to monetize the insight gained from estimating how many tree species have been cataloged across our planet (and how many likely remain undiscovered), but one could argue that the world is a better—or at least a more interesting—place for having answered it.

In contrast, Alyssa and I are applied researchers—answering questions that often come directly from the industry and focus on how trees cope in urban environments and how tree care professionals work with them. Questions like “How close can you trench near the base of a tree?” or “Does leaving a wire basket on a tree at planting really kill it?” are never going to earn us a Nobel Prize or a place in the journal Nature, but these topics tend to generate the most interest when presenting at local chapter conferences.

Can a Study be Both?

Andrew is currently wrapping up a project that is one of those rare studies bridging both basic and applied research. Funded by the Florida Chapter of the International Society of Arboriculture, the project asks: How much pressure can a tree root generate as it expands in diameter? The answer could one day find its way into a college tree physiology textbook or a botany journal. That said, it also has practical implications for designing sidewalks that resist lifting and pipe joints that withstand separation.

So How Do You Measure the Pressure Associated with Root Diameter Growth?

Fig. 1. Misra et al. 1986 used chalk sticks that were drilled out and milled to different diameters to measure how much radial pressure could be generated by expanding plant roots. Image adapted from their work.

Fig 2. Kolb et al. guided roots between photoelastic disks, which revealed the displacement caused by radial root expansion. (Image is a loose/artistic adaptation of their Figures 1 and 2).

This was the kind of puzzle that can be really fun to think about. There isn’t much research on the topic—most plant physiologists have been more concerned with the pressure roots generate during elongation, as this provides a better understanding of the limits of root movement through compacted soils. Far fewer studies have looked at the pressures associated with radial expansion, and those that did focused mostly on non-woody roots in tree and other seedlings. That said, we did find a few attempts to measure radial root pressure.

In one study, researchers drilled holes into chalk sticks—back before dry-erase markers and digital whiteboards made them largely obsolete. They then milled the sticks so that the shell walls had different diameters of known breaking strengths (see Fig. 1). Seedling roots were lined up to grow into the holes, and the researchers observed which chalk sticks broke. They then used logistic regression to determine the shell wall diameter at which breakage became unlikely. In another study, seedling roots were guided to grow between photoelastic disks. As pressure built up, the disks were lit up to show bands where displacement occured (Fig 2).

Our study had several key differences. First, we were interested in woody roots on mature trees—not the first root that emerges from a seed during germination. Second, we figured it might take a season or more of measurements to capture the root diameter growth we were looking for (the two studies noted above were relatively quick to run). So, we were pretty much left to our own devices when it came to building a root pressure measuring setup.

Flattening Roots

Fig. 3. Rendering of our root clamp design. We used aluminum bars to limit corrosion. Tension was applied using latex bands.

Fig. 4 Live oak root with root clamp installed. The orange chalk was used to determin the initial contact area with the clamp sides.

As some rather explosive research from Oregon demonstrated—the first sentence in the methods section literally lists the velocity of dynamite used to blast open rock faces—woody roots will flatten when they hit a barrier that exceeds the maximum pressure they can generate through radial growth. Building on this, along with insights from Norm Easey, my research partner Jason Grabosky and I developed a clamp to determine when roots begin to flatten under pressure (Fig. 3).

We sandwiched a root between two aluminum bars—kept aligned with guide rods—and used physical therapy bands to apply light tension to the setup (Fig. 4). Over one to two growing seasons, the roots expanded, gradually increasing the tension from the bands, until we saw visible flattening where the bars contacted the root. At that point, we cut the roots free from their irrigation valve box enclosures, measured the force needed to just lift the bar slightly off the root, and recorded the flattened contact area to estimate stress.

Speaking of stress, this was one of the more stressful studies I’ve ever worked on. A lot of what we do involves measuring trees out in the wilds of the urban forest or surveying professionals about their views and practices. With those kinds of observational studies, the risk is low—you know you’ll end up with some data, even if the findings are a little boring, commonsensical, or non-significant.

But with this root-flattening study, there were so many unknowns: our setup, root growth rates, whether everything would hold up over time—you name it. On top of that, it was funded by the Florida Chapter—a group of arborists I see many times throughout the year at various meetings, seminars, and conferences.

So when we first started seeing flat roots, I immediately called Jason… and then pretty much every collabafriend I could track down. The experiment was actually working!

What’s Next?

The project has wrapped up, and we have our data. We were able to use logistic regression—just like the folks with the chalk sticks—to determine a threshold for root flattening. I’m currently writing up the findings (this blog has been a nice diversion from the paper) and will be presenting them at the “What’s Up Doc?” session at next month’s Trees Florida Conference.

Once the paper is done, I’ll post a preprint (a non–peer-reviewed manuscript shared for early feedback) and highlight the key findings in another blog post.

All the Critters that Called our Root Enclosures Home…

Fig. 6. This corn snake was just one of the many critters to call our root enclosures home (image credit: Elise Willis).

One unforeseen hiccup in our study was just how attractive our irrigation meter box enclosures would be to the many insects, amphibians, and reptiles that call Florida home. And by “we,” I mostly mean my lead technician, Joe Leone, and the various students working on the project. We had cockroaches, toads, anoles (lizards), fire ants, snakes, and—at one point—over five pounds of bees take up residence. It was literally one jump scare after another. If that sounds like your kind of fieldwork, I’m always looking for new grad student assistants.

About this blog

Rooted in Tree Research is a joint effort by Andrew Koeser and Alyssa Vinson. Andrew is a Research and Extension Professor at the University of Florida Gulf Coast Research and Education Center near Tampa, Florida. Alyssa Vinson is the Urban Forestry Extension Specialist for Hillsborough County, Florida.

The mission of this blog is to highlight new, exciting, and overlooked research findings (tagged Tree Research Journal Club) while also examining many arboricultural and horticultural “truths” that have never been empirically studied—until now (tagged Show Us the Data!).

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