First-Person Science: Colleen Iversen on Belowground Ecology

In the First-Person Science series, scientists describe how they made significant discoveries over years of research. Colleen Iversen is a senior staff scientist at the Department of Energy’s Oak Ridge National Laboratory.

As a kid, I spent a lot of my family vacation pulled over on the side of the road looking at rock strata – my parents were both geologists. When I became a scientist, I decided to study the living things in the soil rather than the rocks themselves.

After I spent my childhood poking under rocks and in streams, I came to the University of Tennessee to do my PhD work. There, I got involved in a climate change experiment at the Department of Energy’s Oak Ridge National Laboratory. We looked at how trees respond when you give them extra carbon dioxide. Although we had hypothesized they would grow more mass above ground, we found that trees put all the extra growth into their roots. By investing in roots, the plants could get extra water and nutrients in the long term.

That’s when I got really fascinated with the world beneath our feet. I get to see things that nobody else sees and I get to understand them in a way other people haven’t thought about. Using special tubes and cameras that can see underground, I can see beautiful white fungal strands growing on a root and know that they’re working together to keep the plant alive.

So I think of myself as a cheerleader for the belowground world.

Peering Below the Surface

Because researching roots is hard, they don’t get as much attention as leaves. I tease people on our team who study photosynthesis that if they wanted to sample leaves in the same way that we sample roots, they should close their eyes and grasp around blindly to find a leaf.

Studying fine roots is even harder. Fine roots are less than 2 mm in diameter - that’s narrower than the cord that connects my phone with my earbuds. These little roots are tiny but mighty; they’re the ones that do all of the resource gathering for the plant.

As there’s no one best way to gather information on these fine roots, we use a lot of different techniques depending on the questions we are trying to answer.

People have been looking at roots forever because people have been growing food forever. The original version of a tool we use called a minirhizotron was first developed using a mirror and clear tube. These days, we still use clear tubes, but they instead have automated cameras. These cameras take pictures of what the roots are doing, when they’re born, how much they’re growing, and when they die. Some of them even have mini-robots that carry the cameras around the tubes. When a spider hitched a ride, we ended up with thousands of spider pictures. That was pretty funny.

But minirhizotrons only capture photos – I also want to measure the root’s chemistry. For that, we use traps called in-growth cores. Roots grow into a mesh cylinder filled with soil, allowing us to measure the root’s mass and chemistry. Taking soil samples helps us see how many roots there are at any given time and where they are growing.

Once we have these roots, we have to separate them from the soil. So we spend hours in the laboratory wearing these really goofy-looking jeweler’s glasses. Some of the roots we work with are as fine as a human hair. Getting miles and miles of them out of the soil can take a while.

Partnering to Create Virtual Worlds

Picking tiny roots out of the soil seems far away from the complex Earth system computer models my co-workers develop. But we both depend deeply on each other. As a graduate student, I hadn’t really thought about models. I would publish my papers and throw them over the fence assuming modelers would totally pick my data up and use it. But that’s not how we make advances.

Now, I know more about models than I ever thought I would. In Oak Ridge’s Climate Change Science Institute, where modelers and field researchers work side-by-side, we joke that we regularly “take a modeler to lunch.” But the informal conversations have led to interesting and useful discussions. Although the models can be kind of opaque to someone who’s not elbow-deep in programming languages, having modelers paint pictures of the virtual worlds they create helps me understand them better.

With this perspective, I started thinking about how what we know about ecosystems underground is represented in these computer models. Because models can’t be too complicated – they’d overwhelm computer systems – they necessarily have to simplify the world. But we need to simplify it in the most representative way possible.

While most models include roots in some way, they don’t really capture what’s happening in all of the corners of the world. They suggest without explicitly representing a lot of the aspects of underground systems. But if you’re not being explicit about those relationships and their mechanisms, then you can’t be explicit in your predictions of how they might change in the future.

For example, most models spread out nitrogen fixation – an essential process by which some plants pull nitrogen from the air – across all plants. But that doesn’t give you the right spatial distribution. Instead, we need to have a specific shrub represented in the model that plays that role.

Instead of complaining mightily about it, we have to put our money where our mouths are and provide data that can improve the models. We’ve been saying, “We don’t think this representation is right, but how do we think it should be? Where are the data that a modeler could use to improve their virtual world?”

A Database Named FRED

So in 2017, we created FRED – the Fine Root Ecology Database. My interns, A. Shafer Powell and Holly Vander Stel, originally named it the Fantastic Root Ecology Database, but we didn’t think that would fly in a publication.

FRED compiles datasets from around the world to help models better represent information about roots. It currently has more than 105,000 observations from 1,200 data sources. They include more than 300 root characteristics or traits, including root order (how close or far away the root is from the main one), diameter, and branching patterns. By documenting these traits, we have a better understanding of roots’ chemistry, relationship to microbes, decomposition, and role in the plant. FRED also includes data on the climate, soil, and growing conditions. Having all of the data in a common framework allows field scientists and modelers alike to compare data sets.

Assembling the data revealed some major gaps in how we think about plant characteristics and our research.

As we organized the data, we took a hard look at the idea of how plant ecologists categorize plants into groups. To make models simple enough to run, scientists sort the nearly 400,000 plant species into 14 categories, called plant functional types. The categories are based on where plants fall on the evolutionary family tree, the biome they appear in, and some above-ground traits. This division assumes that plants with similar traits play similar ecological roles. But we started wondering, “Are those divisions good enough to represent what’s happening below ground?”

From the FRED data, we saw the answer was “No.” In fact, the ratio of carbon to nitrogen in fine roots – a major trait – varied more within an individual plant functional trait category than between them.

In fact, the more we looked, the more we realized traditional plant functional traits seemed to be a poor way to categorize plants in general. Even for above-ground traits, these groupings didn’t always make a lot of sense. Looking at tundra plants, we found that the traditional functional groups

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